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In recent years, it has been found that magnetic dip caused by diamagnetic motion of injected plasma can provide an appropriate environment for excitation of electromagnetic ion cyclotron (EMIC) waves. These findings have been widely reported in the Earth's magnetic environment. However, it has rarely been reported in Jupiter's magnetic environment. This paper reports the characteristics of EMIC waves observed by Juno in the magnetic dip of Jupiter. Multiple‐band EMIC waves are observed in frequency range from 10−3 Hz to several Hz. The theoretical analysis shows that in this event both He+ band and O+ band EMIC waves can be constrained in the magnetic dip, which is consistent with the wave emissions observed inside the magnetic dip. Our result provides the first evidence that EMIC wave can be ducted inside a magnetic dip in Jupiter's magnetosphere. Plain Language Summary: The relationship between magnetic structures and electromagnetic waves in the Earth's magnetosphere has been widely reported. Recent studies have theoretically displayed the effects of the magnetic dip on the EMIC wave propagation in Earth's magnetic field. In this paper, we use the event observed by Juno in the magnetic environment of Jupiter to analyze the effect of the magnetic dip on the propagation of EMIC waves. As a result, the observed EMIC waves can be ducted inside the magnetic dip of Jupiter's magnetosphere. Since these ducted waves would influence the dynamics of energetic protons through longtime scattering effects, the magnetic dip is expected to play an important role in the dynamics of Jupiter's magnetosphere. Key Points: Electromagnetic ion cyclotron (EMIC) waves are observed during the magnetic dip by JunoTheoretical analysis about the effects of magnetic dips on the EMIC wave propagation in Jupiter's magnetosphere is performedBoth He+ and O+ band EMIC waves can be ducted inside the magnetic dip in Jupiter's magnetosphere [ABSTRACT FROM AUTHOR]

Both ground based magnetometers and ionospheric radars at Earth have frequently detected Ultra Low Frequency (ULF) fluctuations at discrete frequencies extending below one mHz‐range. Many dayside solar wind drivers have been convincingly demonstrated as driver mechanisms. In this paper we investigate and propose an additional, nightside generation mechanism of a low frequency magnetic field fluctuation. We propose that the Moon may excite a magnetic field perturbation of the order of 1 nT at discrete frequencies when it travels through the Earth's magnetotail ≈ ${\approx} $4–5 days every month. Our theoretical prediction is supported by a case study of ARTEMIS magnetic field measurements at the lunar orbit in the Earth's magnetotail. ARTEMIS detects statistically significant peaks in magnetic field fluctuation power at frequencies of 0.37–0.47 mHz that are not present in the solar wind. Plain Language Summary: Throughout history, the Moon has both captivated the human mind as well as had important practical consequences on the lives of people in coastal areas. Humans have attributed the Full Moon to all types of things, many of which do not stand up to scientific scrutiny. Recent spacecraft measurements have shown that the Moon has significant magnetic anomalies that are strong enough for producing a magnetic cavity in the solar wind plasma. The Moon also has a wake structure with a strong electrostatic potential which may act as a perturbation for the magnetic field lines. Here, by using ARTEMIS spacecraft observations in the magnetotail, we found surprisingly that the Moon may weakly perturb the Earth's magnetosphere when it travels through the magnetotail in about 4–5 days around the time of the Full Moon. This effect may have been stronger in the past, when Moon was closer to the Earth and its magnetic field was stronger. Key Points: A hypothesis for the Earth's magnetic field perturbation caused by the Moon in the magnetotail is proposedMagnetometer onboard ARTEMIS spacecraft detects the fundamental frequency corresponding to this theoretical predictionThese initial findings suggest the need for followup space and ground‐based statistical studies of the Moon‐Earth field line resonator [ABSTRACT FROM AUTHOR]

Through applying a 4‐MHz linear frequency modulation waveform, which has high range resolution and signal intensity, we successfully detected for the first time the ionospheric 150‐km echo enhancement at 430–450 MHz of the Ultra‐High‐Frequency (UHF) band using the newly built Sanya Incoherent Scatter Radar (SYISR). The obtained low signal enhancement (less than 0.5 dB) explains why previous UHF experiments did not detect them. We also found that our measured fine structure shows a much wider forbidden region than previous results and covers a much larger altitudinal and local time region. In comparison with recent upper‐hybrid instability theory and simulation, our results confirmed the predicted higher altitude occurrence, wider gaps between enhancements, the turn corner feature around sunrise, and perhaps the weak enhancement, which provide an independent evaluation of the newly proposed mechanism in UHF band. Future UHF experiments could further improve the physical understanding of 150‐km echo phenomenon. Plain Language Summary: In the altitude range of 130–170 km above the Earth's surface, the electrons ionized by the solar radiation effecting on the neutrals can generate enhanced radar echo with specific layered structures if the radar beam points perpendicular to or slightly off perpendicular to the geomagnetic field. Up to date, this phenomenon was only detected by lower frequency radars (30–60 MHz). Its physical mechanism has been a puzzle over several decades. Recently, due primarily to the advancement of numerical simulation technology and the improvement of computing power, this puzzle was resolved well by the newly proposed across scales energy transformation physical mechanism in several recent publications. However, these simulations also concluded that it is hard to be detected by higher frequency radars. In this paper, using our newly built Sanya Incoherent Scatter Radar, we applied a novel experimental setup to gain very high range resolution and signal intensity. We finally successfully detected this phenomenon in 430–450 MHz. By comparing our results with previous measurements and theoretical simulation, we can enhance current physical understanding from the perspective of observations. Key Points: Successful first detection of daytime 150‐km echoes in the Ultra‐High‐Frequency band with Sanya Incoherent Scatter RadarWe detected wider forbidden region and larger altitudinal and local time coverage than previous resultsNarrow layering features not following the zenith angle point to field‐aligned irregularitie echoes [ABSTRACT FROM AUTHOR]

How and when sedimentary rocks record Earth's magnetic field is complex. Most studies assume a time‐progressive lock‐in mechanism during sediment deposition called depositional remanent magnetization (DRM). However, magnetic minerals can also form in situ, recording a chemical remanent magnetization (CRM) that is discontinuous in time. Disentangling the two mechanisms represents a major hurdle, and differences in their recording efficiencies remain unexplored. Here, our theoretical solutions demonstrate that CRM intensities exceed DRM by a factor of six when acquired in the same magnetic field. Novel experiments growing greigite (Fe3S4) in sediments and subsequent redeposition under identical magnetic field conditions confirm the predicted difference in recording efficiency. Thus, if left unrecognized, CRM leads to overestimated paleointensity and deserves more attention when interpreting Earth's magnetic history from sedimentary records. Recognition of fundamental differences between CRM and DRM characteristics provide a way forward to distinguish the recording mechanisms through routine laboratory protocols. Plain Language Summary: Remanent magnetizations preserved in sedimentary rocks serve as a continuous record of Earth's magnetic field history and play a fundamental role in understanding the Earth system. It is commonly assumed that magnetic minerals align with the magnetic field as a particle settles through the water column, known as a depositional remanent magnetization (DRM). However, diagenesis can lead to chemical growth of magnetic minerals, known as a chemical remanent magnetization (CRM). CRM lacks stratigraphic continuity and can obscure or completely overprint the original magnetization any time after sediment deposition, leading to a magnetic record that is uncorrelated with the age of the rock. Yet, CRMs go largely unrecognized. Theory and experiments in our paper document that CRMs record the magnetic field six times more efficiently than DRMs. Our work provides a way to distinguish the two through routine laboratory protocols. Key Points: Recording efficiency of chemical remanent magnetization (CRM) is six times higher than depositional remanent magnetization (DRM)Undetected chemical remanences lead to overestimated relative paleointensity estimatesComparison of natural and laboratory magnetization and demagnetization behavior help identify chemical remanent magnetizations in sediments [ABSTRACT FROM AUTHOR]

Field Line Resonances (FLRs) are a critical component in Earth's magnetospheric dynamics, associated with the transfer of energy between Ultra Low Frequency waves and local plasma populations. In this study we investigate how the polarisation of FLRs are impacted by cold plasma density distributions during geomagnetic storms. We present an analysis of Van Allen Probe A observations, where the spacecraft traversed a storm time plasmaspheric plume. We show that the polarisation of the FLR is significantly altered at the sharp azimuthal density gradient of the plume boundary, where the polarisation is intermediate with significant poloidal and toroidal components. These signatures are consistent with magnetohydrodynamic modeling results, providing the first observational evidence of a 3D FLR associated with a plume in Earth's magnetosphere. These results demonstrate the importance of cold plasma in controlling wave dynamics in the magnetosphere, and have important implications for wave‐particle interactions at a range of energies. Plain Language Summary: Earth's space environment is home to electrons and ions across a wide range of energies, trapped in the region by our global geomagnetic field. Energy can be transferred to and from the trapped particles through oscillations in the magnetic field, and these processes are responsible for the extreme energization of trapped electrons to hazardous levels for local spacecraft. In this paper we explore a type of magnetic field oscillation termed Field Line Resonances (FLRs): standing waves on a field line analogous to the oscillatory motion of guitar strings. We use spacecraft observations to show that the direction of the field line oscillations changes significantly depending on the density of the background plasma. The results confirm previous modeling work, and are the first observational evidence of 3D FLRs at a plume. The findings have important consequences for how FLRs transfer energy between the electrons and ions. Key Points: We present the first observational evidence of a 3D Field Line Resonance at the sharp density gradient of a plume edgeThe observed polarisation change confirms magnetohydrodynamic modeling results and predictions made by Elsden and Wright (2022)The presence of 3D Field Line Resonances during storm times has impacts for how Ultra Low Frequency waves couple and interact with local plasma [ABSTRACT FROM AUTHOR]

The polar cap arc at 1500 MLT (15MLT‐PCA) has been considered as an auroral signature of the cusp's duskside boundary and been speculated to be caused by lobe reconnection. However, no observational evidence has been provided to support this speculation. Here we report a 15MLT‐PCA event occurred on 29 November 2017 using multi‐instrument observations. During the DMSP observed the 15MLT‐PCA, Cluster, with its footprints at the root of the 15MLT‐PCA, identified two FTEs in the southern hemisphere's lobe region, accompanied by an increase in electron and ion energy from hundreds of eVs to several keVs. AMPERE observed an increase in upward field‐aligned currents associated with the 15MLT‐PCA. SuperDARN observed a single cell convection with an enhancement of sunward plasma flow near the root of 15MLT‐PCA. We suggest that these observations provide the in‐situ observational evidence that the 15MLT‐PCA is generated by a lobe reconnection at the cusp's duskside boundary. Plain Language Summary: Auroras primarily occur within an oval‐shaped region centered on Earth's magnetic poles. Under specific conditions, they can also occur within the polar cap, where auroral structures usually appear as arc‐like or spot‐like. Auroras exhibiting arc‐like structures in this region are collectively termed polar cap auroral arcs (PCA). One such common phenomenon is the 15 magnetic local time‐polar cap arc (15MLT‐PCA), typically observed near the 1500 MLT sector in the summer hemisphere, extending from the poleward boundary of the auroral oval to the pole. Additionally, its occurrence shows a strong dependence on the By component of the interplanetary magnetic field (IMF). It is widely accepted that the generation of north‐south asymmetric auroral structures in the polar cap results from reconnection between the IMF and Earth's magnetic field lines at high latitudes. Based on the observational characterization of 15MLT‐PCA, it is currently believed that this structure is associated with lobe reconnection. However, direct in situ observational evidence for this process is currently lacking. Therefore, in this study, we combined ionospheric and magnetospheric satellites to validate a possible magnetospheric process for 15MLT‐PCA. Key Points: The observations show that the occurrence of 15MLT‐PCA was accompanied by FTEs on the dusk flank of the southern hemisphere's magnetopauseThe 15MLT‐PCA is accompanied by an enhancement of upward FACs and an increase in sunward plasma flow near its rootWe provided evidence that the 15MLT‐PCA results from lobe reconnection occurring at duskside boundary of the cusp [ABSTRACT FROM AUTHOR]

Whistler‐mode waves can modulate the electron distributions through wave‐particle interactions in the Earth's inner magnetosphere. In this paper, we have investigated the effects of the whistler waves on the electron distributions by a combination of electron magnetohydrodynamics and test‐particle simulations. Our simulations show a parallel whistler wave becomes gradually oblique during its propagation from the magnetic equator to higher latitudes. Such a whistler wave causes distinct flux increases of tens of keV electrons at large pitch angles to form electron butterfly distributions. This whistler wave traps numerous resonant electrons and transports them from lower latitudes to higher latitudes through nonlinear Landau trapping. Meanwhile, these trapped electrons are efficiently accelerated to higher energies contributing to the rapid formation of electron butterfly distributions at higher latitudes. Our study suggests nonlinear Landau trapping is a potential formation mechanism of the electron butterfly distributions observed in the Earth's inner magnetosphere. Plain Language Summary: Wave‐particle interactions are known to play an important role in regulating energetic electron distributions in the Earth's inner magnetosphere. In particular, whistler‐mode waves can efficiently modulate the electron distributions by cyclotron and Landau resonant interactions. In this paper, we used a 2‐D electron magnetohydrodynamics model to simulate a whistler wave in the dipole geomagnetic field. The simulation results indicate a parallel whistler wave becomes gradually oblique during its propagation from the magnetic equator to high latitudes. Furthermore, a test‐particle simulation shows that such a whistler wave can trap and accelerate electrons from the lower latitudes to higher latitudes by nonlinear Landau trapping, which leads to the rapid formation of electron butterfly distributions at tens of keV. Previous studies suggested that nonlinear cyclotron trapping can cause electron butterfly distributions at tens of keV observed in the Earth's magnetosphere. Our study provides a new potential formation mechanism of electron butterfly distributions in the Earth's magnetosphere. Key Points: Effects of whistler waves on electron distributions have been studied by a combination of electron magnetohydrodynamics and test‐particle simulationsA whistler wave causes distinct flux enhancements of 10s keV electrons at large pitch angles to form electron butterfly distributionsNonlinear Landau trapping contributes to the rapid formation of the electron butterfly distributions [ABSTRACT FROM AUTHOR]

The low‐latitude ionosphere is a dynamic region with a wide range of disturbances in temporal and spatial scales. The Giant Metrewave Radio Telescope (GMRT) situated in the low‐latitude region has demonstrated its ability to detect various ionospheric phenomena. It can detect total electron content (TEC) variation with precision of 10−3 TECU and also can measure TEC gradient with an accuracy of about 7 × 10−4 TECU km−1. This paper describes the spectral analysis of previously calculated TEC gradient measurements and validates them by comparing their properties using two bands. The analysis tracked individual waves associated with medium‐scale traveling ionospheric disturbances (MSTIDs) and smaller waves down to wavelengths of ∼10 km. The ionosphere is found to have unanticipated changes during sunrise hours, with waves changed propagation direction as the sun approached the zenith. Equatorial spread F disturbances during sunrise hours is observed, along with smaller structures moving in the same direction. Plain Language Summary: The Earth's ionosphere can limit exploring sub‐GHz frequencies of the sky and introduces an extra phase term that is difficult to calibrate. The same calibration data can be used to study the Earth's ionosphere more precisely than conventional probes. Radio interferometry is a technique for studying astronomical sources and Earth's ionosphere by measuring the spatial coherence function of multiple elements. The GMRT is a unique instrument for exploring the equatorial ionosphere region. This study used dual‐band observations of a bright radio source with the GMRT to explore the Equatorial Ionization Anomaly region. The GMRT can detect variations in total electron content and measure TEC gradient with high accuracy. Spectral analysis was performed on TEC gradient measurements to track individual waves associated with medium scales traveling ionospheric disturbances and smaller waves up to wavelengths of about ∼10 km. The results showed unexpected changes in the ionosphere during sunrise hours and observed large plasma irregularities and smaller structures moving in the same direction. Key Points: Giant Metrewave Radio Telescope (GMRT) can demonstrate an order of magnitude better sensitivity than GNSS‐based TEC measurements in characterizing ionospheric fluctuationsThe spectral analysis technique used with GMRT can detect multiple MSTIDs and smaller‐scale structures simultaneouslyGMRT can detect ionospheric variations as small as 10 km. The study also showed waves changing direction unexpectedly during sunrise time [ABSTRACT FROM AUTHOR]

We examine coupling of fluctuations in the solar wind with electromagnetic ion cyclotron (EMIC) waves in the magnetosphere using an advanced full‐wave simulation code, Petra‐M. Dipole tilt dramatically affects the coupling process. While very little wave power can reach the inner magnetosphere without tilt effects, a tilted dipole field dramatically increases the efficiency of the coupling process. Solar wind fluctuations incident at high magnetic latitude effectively reaches the ground along the field line and mode‐convert to linearly polarized field‐aligned propagating waves at the Alfvén and IIH resonances. Therefore, solar wind compressions efficiently drive linearly polarized EMIC waves when the dipole angle is tilted toward or away from the Sun‐Earth direction. Plain Language Summary: The solar wind is emitted near the ecliptic planes, and solar wind pressure is one of the critical sources of wave activities in the Earth's magnetosphere. Since Earth's magnetic field is tilted to the ecliptic plane, compressed solar wind fluctuations can be incident over a wide range of magnetic latitudes depending on seasonal and diurnal variations. When the solar wind fluctuations reach Earth's magnetosphere, incoming wave energy can be linearly transferred to another wave mode, called a mode coupling. This paper examines the role of Earth's magnetic tilt on this mode coupling between solar wind fluctuation and electromagnetic ion cyclotron (EMIC) waves using an advanced full‐wave simulation code, Petra‐M. Without magnetic tilt, solar wind fluctuations incident at a low latitude are almost totally reflected. In contrast, solar wind fluctuations incident at high latitude can propagate efficiently into the inner magnetosphere and reach the ground. Compressional fluctuations can also be converted to linearly polarized, field‐aligned propagating waves. These results suggest that solar wind compressions can drive the linearly polarized EMIC waves and that the wave occurrence can have seasonal and diurnal dependence, which is expected to maximize at the maximum tilt of Earth's dipole into the Sun‐Earth direction. Key Points: Wave mode coupling of solar wind fluctuation to electromagnetic ion cyclotron (EMIC) waves is examined using an advanced 2D‐full wave simulation code, Petra‐MWhen the dipole magnetic field is tilted, compressional waves can reach the ground near the cusp region via mode conversionThe solar wind can drive linearly polarized EMIC waves via mode conversion more efficiently when the dipole field is tilted [ABSTRACT FROM AUTHOR]

This paper presents observations of dayside auroral signatures resulting from a massive localized compression of the magnetopause, magnetosheath, and bow shock. On 10 December 2016, the Magnetospheric Multiscale Mission (MMS) spacecraft observed a compression of the entire magnetosheath and bow shock, which moved past the spacecraft in 1 min and 45 s. Shortly afterward, the resulting auroral signature was observed at the Kjell Henriksen Observatory located in Longyearbyen, Norway. The characteristics of this unique event have major ramifications for understanding dayside magnetospheric physics. Plain Language Summary: The aurora borealis and australis are the visible signatures of interactions between the Earth's magnetic field and the solar wind. When the solar wind's magnetic field interacts with the Earth's magnetic field, there can be large fluctuations in its positioning and shape. This paper presents observations from satellites of a very large compression of the magnetic field, resulting in unique ionospheric signatures directly over an all‐sky camera at Kjell Henricksen Observatory in Svalbard, Norway. One minute and 57 s after the compression at the magnetosphere, previously unstructured aurora began to intensify and organize, first propagating eastward and then brightening westward. The auroral arcs then began to spiral in a counterclockwise direction forming a spiral structure over the field of view. This defined shape only lasted for a few seconds before dispersing and moving poleward. The characteristics of this unique event have major ramifications for understanding dayside magnetospheric physics. Key Points: Analysis of rapid compression of the magnetopause, magnetosphere, and bow shockObservations of unusual auroral signature over Svalbard, Norway resulting from compression [ABSTRACT FROM AUTHOR]

150 kilometer echoes are strong, coherent echoes observed by equatorial radars looking close to perpendicular to Earth's magnetic field. Observations over a day show a distinct necklace pattern with echoes descending from 170 km at sunrise to 130 km at noon, before rising again and disappearing overnight. This paper shows that the upper hybrid instability will convert photoelectron energy into plasma wave energy through inverse Landau damping. Using parameters from a WACCM‐X simulation, the upper hybrid wave growth rates over a day show a nearly identical necklace pattern, with bands of positive growth rate following contours of the plasma frequency. Small gaps in altitude with no echoes are explained by thermal electrons Landau damping the instability where the upper hybrid frequency is a multiple of the gyrofrequency. This theory provides a mechanism that likely plays a crucial role in solving a long‐standing mystery on the origin of 150‐km echoes. Key Points: Kinetic theory is developed for photoelectron‐driven upper hybrid instabilityUsing WACCM‐X for background parameters, the instability growth rates follow the same morphology as 150‐km echoesLandau damping by thermal electrons creates observed gaps in altitude where the upper hybrid frequency is a multiple of the gyrofrequency Plain Language Summary: For decades, large radars have observed strong, unexplained echoes returning from altitudes of 130–170 km in the atmosphere. All radars work by reflecting radio waves off a target and measuring the returned signal. For atmospheric radars, the targets are free electrons within the plasma in the upper atmosphere. Since the free electrons are typically a disordered gas, the radio waves are reflected in random directions. This means some process is needed to create a coherent structure in the plasma for the radio waves to strongly reflect back in the direction of the radar. In this paper, we show that the region between 130–170 km in the upper atmosphere is likely to create and maintain a specific set of plasma waves that act as a coherent structure for radar measurements. We show that predictions of where the plasma waves are generated match well with the observed patterns of these "150‐km echoes." This is the first research to provide a specific explanation for what causes 150‐km echoes. In understanding this cause, we learned more about the Sun's influence on our upper atmosphere while expanding the capabilities of atmospheric radars. [ABSTRACT FROM AUTHOR]

The most important driver of geomagnetic activity is the north–south (Bz) component of the interplanetary magnetic field (IMF), which dominates the solar wind‐magnetosphere coupling and all solar wind coupling functions. While the east–west (By) IMF component is also included in most coupling functions, its effect is always assumed to be symmetric with respect of its sign. However, recent studies have shown that, for a fixed value of any coupling function, geomagnetic activity is stronger for By>0 than for By<0 in Northern Hemisphere winter. In Southern Hemisphere winter, the dependence on the By sign is reversed. In this paper, we use measurements of National Oceanic and Atmospheric Administration Polar‐Orbiting Operational Environmental Satellites to show that the flux of magnetospheric electrons precipitating into the atmosphere also exhibits an explicit By dependence. This By dependence is strong in the midnight and dawn sectors where it causes a related By effect in the westward electrojet and geomagnetic activity by modulating ionospheric conductivity. Key Points: Fluxes of energetic electrons precipitating into atmosphere are strongly dependent on the sign and amplitude of IMF ByThis IMF By effect on electron fluxes is strong around midnight and at dawn but weak at duskThis IMF By effect is expected to modulate ionospheric conductivity and to affect a similar effect in geomagnetic activity [ABSTRACT FROM AUTHOR]

The geomagnetic activity index Kp is widely used but is restricted by low time resolution (3‐hourly) and an upper limit. To address this, new geomagnetic activity indices, Hpo, are introduced. Similar to Kp, Hpo expresses the level of planetary geomagnetic activity in units of thirds (0o, 0+, 1−, 1o, 1+, 2−, ...) based on the magnitude of geomagnetic disturbances observed at subauroral observatories. Hpo has a higher time resolution than Kp. 30‐min (Hp30) and 60‐min (Hp60) indices are produced. The frequency distribution of Hpo is designed to be similar to that of Kp so that Hpo may be used as a higher time‐resolution alternative to Kp. Unlike Kp, which is capped at 9o, Hpo is an open‐ended index and thus can characterize severe geomagnetic storms more accurately. Hp30, Hp60 and corresponding linearly scaled ap30 and ap60 are available, in near real time, at the GFZ website (https://www.gfz-potsdam.de/en/hpo-index). Plain Language Summary: The geomagnetic activity index Kp is a measure of planetary geomagnetic activity, expressed in units of thirds (0o, 0+, 1−, 1o, 1+, 2−, ...9o). Kp is widely used in the space physics community, as it is known to be a good proxy of the solar‐wind energy input into the magnetosphere‐ionosphere‐thermosphere system. Kp has two important limitations. One is the temporal resolution. Kp is a three‐hourly index, so that temporal features within 3 hr are not resolved. The other is the upper limit of the index. Kp does not exceed a maximum value of 9o, so that under extremely disturbed conditions, geomagnetic activity is not accurately represented. We introduce a group of new geomagnetic activity indices Hpo that overcomes these limitations. Hpo is designed to represent planetary geomagnetic activity in a similar way as Kp but with higher temporal resolution and without the upper limit at 9o. This paper describes the production of 30‐min (Hp30) and 60‐min (Hp60) indices, and demonstrates their properties in comparison with Kp. Hpo indices since 1995, including near‐real‐time values, are distributed through the GFZ website (https://www.gfz-potsdam.de/en/hpo-index). Key Points: New Kp‐like planetary geomagnetic activity indices, Hpo, are presentedHourly (Hp60) and half‐hourly (Hp30) indices are available from GFZ websiteHpo indices are open‐ended without the upper limit at 9o [ABSTRACT FROM AUTHOR]

The eruption of the Hunga Tonga‐Hunga Ha'apai volcano on 15 January 2022 triggered atmospheric waves at all altitudes. The National Aeronautics and Space Administration Ionospheric Connection Explorer (ICON) and European Space Agency Swarm satellites were well placed to observe its impact on the ionospheric wind dynamo. After the Lamb wave entered the dayside, Swarm A observed an eastward and then westward equatorial electrojet (EEJ) on two consecutive orbits, each with magnitudes exceeding the 99.9th percentile of typically observed values. ICON simultaneously observed the neutral wind (90–300 km altitude) at approximately the same distance from Tonga. The observed neutral winds were also extreme (>99.9th percentile at some altitudes). The covariation of EEJ and winds is consistent with recent theoretical and observational results, indicating that the westward electrojet is driven by strong westward winds in the Pedersen region (∼120–150 km). These observations imply that the dynamo is a key mechanism in the ionospheric response to the Tonga disturbance. Plain Language Summary: The January 2022 Tonga volcanic eruption caused atmospheric impacts around the world. As a natural experiment, it can be used to test our understanding of how the lower atmosphere affects space weather. Researchers are only beginning to document the chain of events post‐eruption, and this paper focuses on its impact on the generator that drives electric fields in near‐Earth space, a key part of space weather. This generator is driven by the atmosphere pushing charged particles across Earth's magnetic field. This usually creates a strong eastward current above the equator. When the Swarm A satellite coincided with the wave from Tonga, it observed that this current strengthened dramatically, then reversed. Although reversals are not unusual, this was the strongest reversal observed by Swarm since its 2013 launch, except for one large geomagnetic storm in 2015. Another satellite, the Ionospheric Connection Explorer, was luckily at the right time and place to observe related motions of the upper atmosphere, which were similarly extreme. These observations are shown to be consistent with our theoretical understanding of the generator. This study is important because it represents a critical test of atmosphere‐space interactions and implies that the Tonga eruption caused a major space weather event. Key Points: Extreme thermospheric winds and ionospheric currents were observed in coordinated space/ground‐based measurements, 10 hours post‐eruptionThe westward electrojet current when the Lamb wave reaches the dayside is consistent with recent studies of the wind‐driven electrojetObservations of linked dynamo processes provide direct evidence of the space‐weather impacts of acute lower atmospheric forcing [ABSTRACT FROM AUTHOR]

Magnetopause diamagnetic currents arise from density and temperature driven pressure gradients across the boundary layer. While theoretically recognized, the temperature contributions to the magnetopause current system have not yet been systematically studied. To bridge this gap, we used a database of Magnetospheric Multiscale magnetopause crossings to analyze diamagnetic current densities and their contributions across the dayside and flank magnetopause. Our results indicate that the ion temperature gradient component makes up to 37% of the ion diamagnetic current density along the magnetopause and typically opposes the classical Chapman‐Ferraro current direction, interfering destructively with the density gradient component, thus lowering the total diamagnetic current density. This effect is most pronounced on the flank magnetopause. The electron diamagnetic current was found to be 5–14 times weaker than the ion diamagnetic current on average. Plain Language Summary: The solar wind represents a continuous outflow of charged particles from the Sun's upper atmosphere into the solar system. Upon reaching Earth's magnetosphere, the solar wind's dynamic pressure is balanced by the magnetic pressure of Earth's magnetic field in a boundary layer known as the magnetopause. This boundary layer represents the entry point of the solar wind's energy into Earth's magnetosphere and upper atmosphere, playing a crucial role in energy transport throughout the interconnected system. Plasma density and temperature differences across the boundary layer generate an electric current that supports the magnetopause. In this paper, we clarify the physical mechanism of the magnetopause current by using high‐resolution data from NASA's Magnetospheric Multiscale mission. We found a significant ion temperature contribution to the magnetopause current not identified in previous studies. Our results also indicated that the plasma electrons' contribution to the magnetopause current was significantly smaller than the ion contribution. Key Points: The magnetopause diamagnetic current is composed of opposing density and temperature gradient generated componentsThe temperature gradient contributes up to 37% of the ion diamagnetic current density along the magnetopauseThe temperature component typically opposes the classical Chapman‐Ferraro current direction [ABSTRACT FROM AUTHOR]

This paper reports a special midlatitudinal medium‐scale traveling ionospheric disturbance (MSTID) event, accompanied by a poleward surge of airglow depletion/enhancement and a bifurcation of depletion during the magnetically quiet period. These special structures were generated during a decreasing height of ionosphere with nighttime plasma density enhancement and increment of airglow emission intensity possibly caused by a passing‐by midnight brightness wave. We suggest that the interaction of the passing‐by MSTID and nighttime plasma density enhancement resulted in the poleward surge and bifurcation of airglow depletion. The nighttime plasma density enhancement could feed the high plasma density to interact with the airglow depletions of MSTID, causing the poleward surge and bifurcation of airglow depletion by the secondary gradient drift instability. The poleward surged airglow enhancement within two adjacent depletions of MSTID is also firstly reported in this study. Plain Language Summary: It was generally believed that the midlatitudinal ionosphere is fairly quiescent until the first report of airglow structure of the medium‐scale traveling ionospheric disturbance by Garcia, Kelley, Makela, and Huang (2000, https://doi.org/10.1029/1999JA000305). Kelley et al. (2000, https://doi.org/10.1029/2000GL000022) and Garcia, Kelley, Makela, Sultan, et al. (2000, https://doi.org/10.1029/1999JA000306) further reported magnetically storm‐time "intense mid‐latitude spread F" events accompanied by poleward surged bifurcations of airglow depletions, which caused the severe perturbations of GPS signal. Those two events were most likely the interactive result of airglow depletion and enhanced plasma density region from the equatorial ionization anomaly crest region driven by magnetic storms. In this paper, we report a very similar airglow event accompanied by a poleward surged airglow depletion/enhancement and a bifurcation of depletion but occurred at magnetically quiet time in Xinglong, China. We suggest that the interaction of the passing‐by medium‐scale traveling ionospheric disturbance and the nighttime plasma density enhancement/nighttime increment of airglow emission intensity from a possible source of midnight brightness wave resulted in the poleward surged bifurcation of aiglrow depletion, most likely driven by the secondary gradient drift instability. This event can broaden our knowledge on the dynamical process of midlatitudinal ionosphere and provide a new guidance for further investigation of the midlatitudinal special airglow events. Key Points: MSTID with a poleward surged airglow depletion/enhancement and a bifurcation of depletion occurred during the magnetically quiet timeSecondary gradient drift instability could have developed the poleward surged bifurcation of depletionThis event is suggested to be caused by the interaction between MSTID and NPDE [ABSTRACT FROM AUTHOR]

This paper is a statistical survey of polar cap patches in relation to solar and geomagnetic activity. Ten thousand six hundred eighty‐eight patches have been identified from in situ plasma observations of the Defense Meteorological Satellite Program F16 satellite for 14 years (2005–2018). These patches are divided into two groups: (a) cold patches, which consist of dense but cold plasma; and (b) hot patches, which consist of dense but hot plasma. The statistical results indicate that (a) the occurrence of cold patches is clearly dependent on solar and geomagnetic activity, but hot patches don not show such dependence; (b) both cold and hot patches preferably appear in the winter season; (c) the spatial size of both cold and hot patches decreases (increases) with solar (geomagnetic) activity; (d) the spatial size of cold patches appears larger than that of hot patches under similar solar and geomagnetic activity. Plain Language Summary: Polar cap patches are localized regions of enhanced plasma density in the polar ionosphere. They are associated with strong electron density gradients resulting in non‐negligible ionospheric scintillations for satellite communication and navigation systems. To characterize polar cap patches and investigate their statistical dependence on solar and geomagnetic activity, 10,688 patches have been identified from 14 years of in situ Defense Meteorological Satellite Program satellite plasma observations. These patches have been divided into cold and hot patches. We find that cold patches occur more frequently during solar maximum years. The spatial size of cold and hot patches decreases with solar activity (increases with geomagnetic activity). Furthermore, the spatial size of cold patches is larger than that of hot patches under similar solar and geomagnetic activity. Key Points: The occurrence of cold patches is clearly dependent on solar and geomagnetic activity, while hot patches do not show such dependenceIn NH winter, the spatial size of both cold and hot patches decreases (increases) with solar (geomagnetic) activityIn NH winter, the spatial size of cold patches appears larger than that of hot patches under similar solar and geomagnetic activity [ABSTRACT FROM AUTHOR]

The Cretaceous Normal Superchron (CNS), defined as a magnetic quiet zone spanning from ∼126 to ∼84 Myr, is often used to understand the Earth's geodynamo processes. However, whether there were polarity reversals within the CNS remains contentious. This paper presents the youngest detrital zircon U‐Pb age‐constrained magnetostratigraphic study of the Cretaceous Khorat Group for two sections (i.e., Ban Phonngam (BP) and Ban Nongpen (BN)) in Vientiane, Laos. Both sections are characterized by the prominent long normal‐polarity zones that are correlated to the CNS. Interestingly, six (four) short primary reversed horizons were identified within the BP (BN) section. Integrated with other reported reversed events, there likely at least five globally and two single reported reversed‐polarity events or clusters of events have happened within the CNS. The confirmation of frequent reversed polarities within the two parallel sections will provide a new comprehensive understanding of the Earth's geodynamic behavior during the CNS. Plain Language Summary: The Cretaceous Normal Superchron (CNS) was defined as a single normal‐polarity zone within the Cretaceous at ∼126–84 Ma. Geomagnetic field behaviors during this interval can help understand the geodynamic processes within the Earth's interior. But whether, when, and how often have the polarity reversals occurred within the CNS are still controversial yet fascinating. Here, we report a high‐resolution magnetostratigraphic study on the Cretaceous Khorat Group from two parallel outcrop sections (i.e., Ban Phonngam (BP) and Ban Nongpen (BN)) in the Vientiane Basin, Laos. Both sections show the distinct long normal‐polarity zones, which are correlated to the CNS under the constraints of youngest detrital zircon U‐Pb ages and regional geochronological evidence. It is interesting that six and four short episodes of primary reversed polarity were observed in the BP and BN sections, respectively. By summarizing the worldwide records of polarity reversals in marine, volcanic, and terrestrial rocks within the CNS, we believed that there likely at least five globally and two single reversed‐polarity events or clusters of events have taken place within the CNS. Our work provides new insights into the geomagnetic field characteristics during the CNS, which could guide the future geodynamo simulations. Key Points: Two parallel sections in the Vientiane Basin that are characterized by prominent long normal‐polarity zones are correlated to the Cretaceous Normal Superchron (CNS)The robust short reversed horizons indicate true short‐lived geomagnetic field reversals within the CNSAt least five globally and two single reported reversed‐polarity events or clusters of events have happened within the CNS [ABSTRACT FROM AUTHOR]

We present results of analysis of lightning observations and satellite measurements of ionospheric magnetic field fluctuations in the ultralow frequency (ULF) range. The fluctuations are measured by Swarm satellites, while the lightning observations are provided by the Geostationary Lightning Mapper (GLM) and the World ELF Radiolocation Array (WERA). We identify spatio‐temporal relationships that demonstrate a leakage of electromagnetic fluctuations caused by lightning into the upper ionosphere. Causal association between the two phenomena is evidenced by investigating relations between lightning and fluctuation properties. The presented results suggest that lightning generate ULF fluctuations in the ionosphere that can be detected by satellites, if the lightning‐satellite geographic distance is less than ∼5°. Typical properties of the fluctuations caused by lightning are described in the paper. To our knowledge, this is the first direct experimental confirmation of a link between lightning and magnetic field fluctuations in the upper ionosphere in the ULF range. Plain Language Summary: It has been a matter of interest, if electromagnetic fluctuations caused by lightning can propagate from the lower atmosphere higher into the ionosphere, which is an ionized and electrically conducting medium. This process requires conversion of the electromagnetic perturbation propagating in the neutral atmosphere into an ionospheric plasma wave, which for ultralow frequencies (ULF) can attenuate strongly the wave amplitude. The attenuation is considered to be rather significant, thus one could argue that it is virtually not possible to observe the ionospheric magnetic field perturbations caused by lightning using currently available satellite magnetometers. There are many remote observations of lightning and direct satellite measurements of ionospheric magnetic field fluctuations, that can be analyzed searching for a possible link between the two phenomena. Using Swarm satellite measurements and two types of lightning observations, we provide an evidence that lightning generate electromagnetic perturbations that propagate into the upper ionosphere and can be measured with magnetometers onboard low‐Earth orbit satellites, for example, by the Swarm constellation. We reveal links between lightning and ionospheric wave properties, that suggest a real causality relationship between the two phenomena. To our knowledge, this is the first direct experimental confirmation of such a relation in the ULF range. Key Points: Lightning can generate ultralow frequency (ULF) fluctuations leaking into the upper ionosphereObservations show that the fluctuation amplitude is proportional to the lightning charge momentThe ULF fluctuations generated by lightning can be detected by low‐Earth orbit satellites, for example, the Swarm constellation [ABSTRACT FROM AUTHOR]

The storm‐time ring current is formed by the inward convection of the near‐earth plasma sheet, so understanding the changing source of the plasma sheet is key for understanding ring current development. The ionospheric and solar wind sources can be distinguished by the charge state of the heavy ions; solar wind ions are highly iononized, while ionospheric ions are predominantly singly ionized. AMPTE/CHEM measurements are used to track the changing composition in the 6–9 Re plasma sheet as a storm develops in order to determine the fraction of the population that comes from each source. We find that prior to the storm, the solar wind source dominates. During the storm main phase, the solar wind contribution decreases, while the ionospheric contribution increases, making the source predominantly ionospheric. The source returns to predominantly solar wind plasma during the recovery phase. Plain Language Summary: The ions trapped in the earth's magnetic field have two possible sources: the sun or the earth's ionosphere. A stream of ionized particles from the sun called the solar wind is constantly moving outward, flowing by the earth. These particles can enter the earth's magnetosphere and become trapped. Ions can also be accelerated out of the ionosphere into the magnetosphere. The interplay between these two sources is not well understood. In this paper, we test how the source changes as a large geomagnetic storm develops. We find that the source switches from being dominantly solar wind, to dominantly ionospheric during the peak of the storm. Key Points: Ionospheric and solar wind sources to the near‐earth plasma sheet are tracked during one storm using the AMPTE/CHEM instrumentThere is a sharp change from a predominantly solar wind to ionospheric source during both intensifications in the storm's main phaseThe source changes back to predominantly solar wind during the recovery phase [ABSTRACT FROM AUTHOR]

Flux transfer events (FTEs), as flux ropes (FRs), are considered key agents for solar wind energy to enter the terrestrial magnetosphere. Recent observations identify entangled flux tubes that collide and pull against each other. Reconnection occurs to disentangle and produce a new pair of FRs with different connectivity. In this paper, we examine the entanglement interface and how such an entanglement process evolves in time by comparing 17 entanglements observed by the Magnetospheric Multiscale (MMS) mission. The By‐dominated interplanetary magnetic field (IMF) distribution of the 17 events agrees with previous findings. We have identified three evolutionary stages characterized by the magnetic field and pressure enhancement. Our study confirms the FR nature of these events and explains how a disparate pair of ropes is formed from two entangled flux tubes, each initially connected to a different hemisphere of the magnetosphere. Plain Language Summary: The Earth's intrinsic magnetic field deflects the solar wind flow at a boundary called the magnetopause. Near this boundary, twisted flux tubes are found. Such tubes may become entangled if they move toward each other, say, from the Southern and Northern Hemispheres. This study analyzes 17 such events and identifies three evolutionary stages as the entanglement proceeds. The results improve our understanding of not only the complex coupling between solar wind and the Earth's magnetic field but other similar processes in space plasma. Key Points: IMF clock angles associated with 17 flux tube entanglement events present peaks near horizontal directionThe entanglement process evolves in three distinguishable stagesMagnetic tension force contributes to the pressure balance at the entanglement interface [ABSTRACT FROM AUTHOR]

Simultaneous eastward and westward traveling surges were observed at Tjörnes, Iceland, and Syowa station, Antarctica, respectively. Several remarkable differences were identified. (1) The position of the initial bright spot was shifted by 1.7 to 2.3 MLT between both hemispheres. (2) The surges differ in traveling speed between the eastward traveling surge (6.5 km s−1) and the westward traveling surge (1.3 km s−1). (3) The Arase satellite was located on a geomagnetic field line connecting both ground stations and observed a significant excess in westward component of the magnetic field, which is consistent with the large shifts of the initial bright spots in both hemispheres. (4) The background Hall current flows eastward (Northern Hemisphere) and westward (Southern Hemisphere). The observed north‐south asymmetry of the traveling surges suggests that the ionosphere can play an essential role in controlling the fundamental spatiotemporal development of auroras in both hemispheres. Plain Language Summary: Occasionally, the aurora suddenly brightens in local areas of the Northern and Southern Hemispheres, causing explosive and widespread phenomena. At this time, it is known that a part of the aurora travels toward east or west from the point of the explosion; however, the formation mechanism of this traveling aurora is unknown. Recently, this traveling aurora has been reproduced by computer simulation, and it has been shown that the direction of its traveling is determined by the state of the ionosphere rather than by the projected motion of the source region in the magnetosphere. In this paper, we observed traveling auroras with two identical high‐speed cameras installed in the Northern and Southern Hemispheres simultaneously and clarified for the first time that the state of the ionosphere determines the direction of the traveling aurora. Understanding the mechanism of such connection between the space and the Earth is important for predicting the space environment required for various human activities and technologies using the space. Key Points: An auroral breakup was captured simultaneously by ground‐based all‐sky cameras at magnetically conjugate stations in both hemispheresThe auroral surge evolved asymmetrically regarding the longitudinal direction in both hemispheresThe surge evolved in the direction of the ionospheric Hall current, which is consistent with the expectation from a global simulation [ABSTRACT FROM AUTHOR]

Ducting Pc1 waves are observed over a wide latitudinal range by the low‐Earth orbit satellites as compressional mode. In this paper, we present the first observation of the modulation of ducting Pc1 waves by equatorial plasma bubbles (EPBs) based on the Swarm satellites. We show two ducting Pc1 events propagating across EPBs that occurred on 7 April 2016 and on 27 September 2017. We found that the EPBs modulate the Pc1 wave propagation by setting up reflection boundaries and leakage holes in the ionospheric waveguide. We also found that changes of Pc1 wave intensities generally follow the electron density variation and that the intensity is stronger at higher density region. From the comparison between Swarm‐A and Swarm‐C observations, we conclude that ionospheric plasma plays an important role for Pc1 waveguide even though their density is significantly low in EPBs. Plain Language Summary: Pc1 (or electromagnetic ion cyclotron) waves have been observed across the whole magnetospheric and ionospheric regions and on the ground with different characteristics. Typically, they propagate along the magnetic field line in the magnetosphere. But once the waves enter the ionosphere, plasma effects such as mode conversion and reflection complicate their propagation. One of the Pc1 wave propagation characteristics on the ionosphere is ducting (waveguide). Ionospheric ducting is a way radio waves can travel thousands of kilometers along an ionospheric layer. Ducting is controlled by various physical processes and ionospheric conditions, among which plasma density is one of the factors. In this study, we present the first observations of the ducting Pc1 wave controlled by ionospheric electron density, particularly by the equatorial plasma bubbles, which are localized density depletion near the geomagnetic equator. Our observation results show that the Pc1 wave intensity in the ionosphere strongly depends on the ambient electron density. Key Points: We report the first observation of the modulation of ducting Pc1 wave by equatorial plasma bubbles based on the Swarm satellitesEquatorial plasma bubbles modulate Pc1 wave propagation by setting up boundaries and holes in the ionospheric waveguideIonospheric plasma plays an important role as a Pc1 waveguide even though their density is significantly low in plasma bubbles [ABSTRACT FROM AUTHOR]

So‐called "Steve" subauroral purple emissions have recently been uncovered by auroral photographers and have rapidly become an intense subject of debate as to their origin. In some events, nearby periodic green emissions have also been uncovered and given the name "picket fence," owing to their appearance. The present paper advances our understanding of these phenomena by narrowing down the altitude extent of the Steve and picket fence emissions. Our determination is based on the event of 16 September 2017, which was simultaneously observed from two vantage points, allowing for a determination of the height range of Steve and picket fence through triangulation. We show that the picket fence extend between 95‐ and 150‐km altitude and is aligned with the geomagnetic field, while the Steve altitude spread is between 130 and 270 km. We also show the two phenomena to be on nearby or perhaps the same magnetic field lines. Key Points: The optical emissions of Steve range from 130 to 270 km in altitudeThe optical emissions of the green Picket Fence range from 95 to 150 km in altitudeSteve and the Picket Fence extend vertically along similar magnetic field lines [ABSTRACT FROM AUTHOR]

Data are analyzed from a Magnetospheric Multiscale encounter with a dayside magnetopause reconnection region on 29 December 2016. The uniqueness of the event stems from the small (~7 km) average spacecraft separation and the sequential sampling of an electron diffusion region with electron crescent distributions. We quantitatively investigate the earthward acceleration of magnetosheath electrons through the in‐plane null by the polarization electric field EN that points radially outward from the magnetopause. The results compare favorably with previous plasma simulations with one important difference that the reconnection electric field (EM) extends throughout the region of strong EN so that both fields energize electrons in the same region. This acceleration is quantified here for the first time. As the spacecraft penetrate deeper into the region of enhanced EN, the magnetic reflection of lower‐energy electrons produces a thinner crescent. Plain Language Summary: Reconnection causes the conversion of magnetic energy to particle energy and the breaking and reconnection of solar wind and geomagnetic field lines. In asymmetric reconnection at the dayside magnetopause with different plasma densities and magnetic field strengths on the Earth and Sun sides of an X‐line, this energy conversion is displaced to the earthward side of the current layer. An electric field pointing outward normal to the dayside magnetopause drives reconnection by accelerating solar wind electrons across the reconnection current sheet. These electrons take up dawnward‐directed meandering orbits, and they are ultimately the carriers of the reconnection current. This paper reports the first direct measurement of this acceleration. Key Points: Parallel and perpendicular energy conversion and electron acceleration are studied with data from four spacecraft with average separations of 7 kmThe magnetopause normal electric field (EN) accelerates magnetosheath electrons across the reconnection current sheet to form crescent distributionsMagnetic reflection causes the crescent energy peaks to become narrower with increasing depth into the diffusion region [ABSTRACT FROM AUTHOR]

This paper reports on new results in the determination of magnetic signals produced by oceanic tides as estimated from satellite magnetic measurements. We find that combining data from the past CHAMP (2000–2010) and the present Swarm (since 2013) satellite missions significantly improves the quality of the extracted tidal signals, in particular if along‐track and cross‐track magnetic "gradient" data are utilized. This allows us to determine the magnetic signature not only of the M2 tide but also of the much weaker N2 and O1 tidal constituents. To minimize disturbances from magnetospheric and ionospheric currents, we only use data from the nightside region during geomagnetic quiet conditions and remove core, crustal, and magnetospheric field contributions as given by the CHAOS geomagnetic field model. Despite their small magnitudes, all determined tidal constituents show sensitivity to the electrical conductivity profile of the underlying mantle, enabling imaging the upper mantle below the oceans. Key Points: Global models of the magnetic signals produced by the M2, N2, and O1 oceanic tides have been extracted from satellite magnetic measurementsCombining data from the Swarm and CHAMP missions as well as using field gradient plays key role in the robustness of the estimated modelsAll tidal constituents show significant sensitivity to the electrical conductivity of the oceanic mantle [ABSTRACT FROM AUTHOR]

A numerical study of the effects of seismically generated acoustic waves in the ionosphere is conducted using a three‐dimensional (3‐D) ionospheric model driven by an axisymmetric neutral atmospheric model. A source consistent with the 2011 Tohoku earthquake initial ocean surface uplifting is applied to simulate the subsequent responses. Perturbations in electron density, ion drift, total electron content (TEC), and ground‐level magnetic fields are examined. Results reveal strong latitude and longitude dependence of ionospheric TEC, and of ground‐level magnetic field perturbations associated with acoustic wave‐driven ionospheric dynamo currents. Results also demonstrate that prior two‐dimensional models can capture dominant meridional responses of TEC over latitude, even though dynamics at other longitudes are not resolved. Conclusions support that TEC and magnetic signatures can arise from nonlinear acoustic waves generated by strong earthquakes; simulations elucidate the comprehensive physics of their 3‐D ionospheric responses. Plain Language Summary: This paper reports new numerical model results on how the ionosphere—the ionized, conducting region of the upper‐atmosphere—responds to strong acoustic waves launched by earthquakes. The effects of these waves in the ionosphere can be observed from ground: They are measured as variations in Global Positioning System satellite signals received at the ground, which translate to variations in integrated electron content of the ionosphere. They can be detected also via small fluctuations in magnetic field, which are driven by ionospheric currents. The model results suggest that both measurement techniques can provide insight into the waves and their sources; new guidance for interpreting these measurements is provided. Further, results demonstrate a new capability for simulating these processes in three dimensions, which can be applied to more‐realistic case studies. Key Points: Earthquake‐driven TEC and magnetic perturbations are studied with a 3‐D modelMagnetic fluctuations driven by infrasonic acoustic waves are detectableLocal geomagnetic field geometry affects directionality of TEC responses [ABSTRACT FROM AUTHOR]

Pc3 range frequency (22–100 mHz) auroral pulsations often occur at daytime high latitudes, equatorward of the cusp/cleft and typically map to the dayside outer magnetosphere. In this paper we present simultaneous observations of compressional Pc3 magnetic pulsations in the dayside outer magnetosphere that occurred in direct association with daytime Pc3 auroral pulsations at South Pole Station (−74.4° magnetic latitude). The pulsations were almost identical at the two locations, and their correlation was clearest when the magnetospheric pulsations were highly monochromatic. Lower‐band chorus waves and keV electron fluxes were also modulated in the Pc3 range, likely by the compressional magnetic pulsations. The common Pc3 frequency in the magnetosphere and aurora matched well with the predicted frequency of upstream ultralow frequency waves. These results provide the first compelling evidence for the direct dayside outer magnetosphere‐ionosphere linkage between upstream‐generated compressional Pc3 waves, Pc3 range modulations of chorus waves and keV electrons, and Pc3 auroral pulsations. Plain Language Summary: South Pole Station, Antarctica (SPA), is a unique polar observatory, which can monitor dayside aurora from the ground throughout austral winter. High time resolution optical instruments at SPA frequently capture auroral pulsations with Pc3 frequencies (22–100 mHz) around the midday hours. Here we use unique conjunctions of ~2‐Hz SPA all‐sky camera and Time History of Events and Macroscale Interactions during Substorm (THEMIS) spacecraft to identify the origin of the periodicity of daytime Pc3 auroral pulsations observed at SPA and their driver in the magnetosphere. Simultaneous in situ measurements in the dayside outer magnetosphere observed identical Pc3 magnetic pulsations in the compressional component. Lower‐band chorus waves and keV electron fluxes were observed to be modulated also with the Pc3 range. The common Pc3 frequency in the magnetosphere and ionosphere matched well with the expected frequency of upstream ultralow frequency (ULF) waves generated in the foreshock region. The simultaneous SPA‐THEMIS spacecraft conjunctions provide, for the first time, direct evidence that upstream‐origin compressional Pc3 magnetic pulsations drive daytime auroral pulsations through wave‐particle interactions. One important implication of this study is that the high temporal cadence 2‐D observations of daytime auroral pulsations can be a useful tool for remote sensing the dynamics of outer magnetospheric Pc3 ULF waves. Key Points: Daytime auroral pulsations have a one‐to‐one correspondence with compressional Pc3 magnetic pulsations in the outer magnetosphereIn situ lower‐band chorus waves and keV electron fluxes are modulated by the compressional Pc3 magnetic pulsationsUpstream ULF waves are one of the most influential sources that control the frequency of the daytime auroral pulsations [ABSTRACT FROM AUTHOR]

Mesoscales, which couple small to large scales, and vice‐versa, are critical to the magnetosphere‐ionosphere coupling. Optical and radar measurements indicate that dynamical mesoscale features are present in the ionosphere, however quantifying their contribution to the overall dynamics remains a challenge. We use a new ionospheric data assimilation technique, Lompe (Local mapping of the polar ionospheric electrodynamics), to specify ionospheric electrodynamics using a wide variety of input data and a priori assumptions about the physical nature of the ionospheric electric field. We isolate the terms of the ionospheric Ohm's law and find that mesoscale structures in the FACs are driven by Hall gradients, while the larger scale patterns are associated with the divergence of the electric field. We calculate the relative contribution of mesoscales to the overall FAC patterns during a magnetospheric substorm, and find that in the nightside, mesoscale FACs contribute up to 60% of the total. Plain Language Summary: Calculating the amount of energy input into Earth's ionosphere, the upper layer of the atmosphere, is extremely important as it is the endpoint for the Sun's interaction with the Earth's space environment. The ionosphere is coupled to the magnetosphere, the protective cavity carved by Earth's magnetic field as the solar wind flows around, through electrical currents aligned with the Earth's magnetic field called field aligned currents, or Birkeland currents. These currents occur at various spatial scales, from small to regional to semi‐global. Studying the time varying and spatial structure of these field‐aligned currents is important for quantitative understanding of the ionosphere‐magnetosphere system, with implications for space weather impacts via changes in neutral winds and density, local electron density enhancements, and so on. In this study, we quantify the amount of ionospheric regional scale (∼10−1000 ${\sim} 10-1000$ km) structures in the field aligned currents, and show that they are related to structures in the ionospheric conductances of the same scales sizes. Key Points: The Lompe data assimilation framework accurately captures both large‐scale and mesoscale FACsMesoscale FACs are 30% of the total FACs, and 50–60% of the nightside FACs during the magnetospheric substorm studied here [ABSTRACT FROM AUTHOR]

The movement history of boulders is crucial for the reconstruction of paleo‐tsunamis. We report findings from viscous remanent magnetization studies of the boulders on Tongatapu Island, aiming to reconstruct their reworkings. Two boulders exhibited viscous remanence, whereas two larger boulders lacked viscous components but exhibited stable remanence. Both the viscous and stable components deviated from the geomagnetic field direction. These observations indicate that: (a) the boulders with a viscous component were reworked before the latest event, which could have reworked all boulders, and (b) the magnitude of the latest event was larger than that of an earlier event. The reworked timing indicated that the event occurred between 3,000 years ago and the fifteenth century. The difference in the wave height required to move boulders on the eastern and western coasts suggests that the source of the earlier tsunami was likely an eruption due to volcanoes along the Tonga Ridge. Plain Language Summary: This study explores prehistoric tsunamis on Tongatapu Island by examining large rocks transported during the past tsunamis. Upon analyzing the magnetic records of the rocks, we discovered evidence of multiple rock movements, shedding light on the recurrence and magnitude of past events. Magnetic measurements and previously dated latest movement ages revealed that a tsunami occurred between 3,000 years ago and the fifteenth century. Considering the mobility and immobility of the large rocks and their sizes, the data suggest a smaller paleo‐tsunami magnitude than that of the fifteenth‐century event that transported all the rocks; this expands our understanding of natural hazard history and has implications for assessing tsunami risks in the region. The approach adopted here provides a valuable model for investigating historical natural disasters and their impact on coastal areas, offering insights to enhance global tsunami hazard assessment strategies. Key Points: Viscous remanent magnetization of large boulders indicates multiple reworkings, enhancing our understanding of past tsunamis in TongatapuCombined magnetic records and published radiocarbon ages suggest tsunamis occurred between 3,000 years ago and the fifteenth centuryRemanence and wave height estimates show that the older tsunami was smaller than the fifteenth‐century event, with a possible volcanic source [ABSTRACT FROM AUTHOR]

Chorus waves induce both electron acceleration and loss. In this letter, we provide significantly improved models of electron lifetime due to interactions with chorus waves. The new models fill the gap that previous models have on some magnetic local time (MLT) sectors of the Earth's magnetosphere. This improvement is critical for modeling studies. The lifetime models developed using two different methods are valid for electrons with an energy range from 1 keV to 2 MeV. To facilitate the integration of these new models into different ring current and radiation belt codes, we parameterize the electron lifetime as a function of L $L$‐shell and electron kinetic energy at each MLT and geomagnetic activity (Kp). The parameterized electron lifetimes exhibit strong dependencies on L $L$‐shell, MLT, and energy. Simulations using these new models demonstrate improved agreement with satellite observations compared to simulations using previous models, advancing our understanding of electron dynamics in the magnetosphere. Plain Language Summary: There are a large number of energetic electrons trapped by our Earth's magnetic field in the near‐Earth space. The regions populated by these high energy electrons are called ring current and radiation belts. It is important to understand the dynamics of the energetic electrons because they can be dangerous to satellites and astronauts flying through these regions. Electromagnetic waves in these regions play an important role in the dynamic of ring current and radiation belt electrons. Among these waves, whistler mode chorus wave is an important wave that can cause both acceleration and loss of the energetic electrons. In our previous studies, we calculated diffusion coefficients to quantify the effect of chorus waves on the energetic electrons. Based on these diffusion coefficients, in this study, we estimate the lifetime of energetic electrons due to their interactions with chorus waves. To make this lifetime model more convenient to be used in different ring current and radiation belt models, we apply polynomial fits to the calculated lifetime. Our new lifetime model is more advanced than previous models, especially in the space coverage. We test the new models in simulations and the results agree better with satellite observations than the previous models do. Key Points: The new lifetime model provides extended space coverage in comparison to current widely used lifetime modelsSuch parameterized lifetimes are very significant for simulations of the dynamics of radiation belt and ring current electronsUsing the new electron lifetime model in simulations improves the agreement between the simulation results and the satellite observations [ABSTRACT FROM AUTHOR]

After days of intense solar activity, active region AR3664 launched seven CMEs toward Earth producing an extreme G5 geomagnetic storm commencing at 17:05 UT on 10 May 2024. The storm impacted power grids, disrupted precision navigational systems used by farming equipment, and generated aurora seen around the globe. The storm produced remarkable effects on composition, temperature, and dynamics in the Earth's thermosphere that were observed by NASA's Global‐scale Observations of the Limb and Disk (GOLD) mission and are reported here for the first time. We use synoptic disk images of ΣO/N2 and neutral temperature (at ∼160 km) measured by GOLD to directly link dynamics resulting from the storm with dramatic changes in thermospheric composition and temperature. We observe a heretofore unseen spatial morphology simultaneously in ΣO/N2, neutral temperature, and total electron content. Equator‐to‐pole temperature differences reach 400 K with high latitude peak neutral temperatures near 160 km exceeding 1400 K. Plain Language Summary: On Saturday 10 May 2024, the sun launched a wave of energized plasma toward the Earth. A large disturbance in the Earth's magnetic field associated with the solar wind resulted in an extreme geomagnetic storm. The storm impacted power grids, disrupted navigational systems used by farming equipment, and produced aurora seen around the globe. The storm produced remarkable effects in the Earth's upper atmosphere that were observed by NASA's Global‐scale Observations of the Limb and Disk (GOLD) mission. In this letter, we use images measured by GOLD to directly link atmospheric dynamics resulting from the May 10–12 superstorm with dramatic changes in composition, temperature, and global circulation in the Earth's upper atmosphere. We observe previously unseen structure in the upper atmosphere associated with equator‐to‐pole temperature differences exceeding 400 K. Peak neutral temperatures near 160 km exceed 1400 K at high latitudes. Key Points: GOLD disk images of ΣO/N2 and neutral temperature link storm time dynamics with changes in thermospheric composition and temperatureWe observe a previously unseen spatial morphology in ΣO/N2, neutral temperature, and total electron contentPeak equator‐to‐pole temperature differences exceed 400 K but relax to pre‐storm conditions well before ΣO/N2 [ABSTRACT FROM AUTHOR]

Earth experienced the strongest geomagnetic storm in 20 years over 10–13 May 2024. The Ap and Dst geomagnetic indices were 273 and −291.94 nT on 11 May. The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics satellite observed significant enhancement in thermospheric infrared emission at 15, 5.3, 4.3, and 1.27 μm. On 11 May the daily global power radiated by nitric oxide (NO) at 5.3 μm was 1.41 TW and by carbon dioxide (CO2) at 15 μm was 1.35 TW. These are the largest single day power values observed by SABER in 22 years and the first time the daily power radiated by NO exceeded that of CO2. The total infrared power (above background) radiated during the storm was 2.64 TW (2.28 × 1017 J). Significant enhancement in limb radiance observed at 4.3 μm (to 250 km tangent height) is likely indicative of NO + formation during the storm. Plain Language Summary: The layer of Earth's atmosphere above 100 km is referred to as the "thermosphere" and can be thought of as the boundary between the space environment above it and the atmosphere below it. In early May 2024, an active sunspot ejected a significant amount of charged particles (protons and electrons) within a strong magnetic field toward Earth. This "coronal mass ejection" or CME encountered Earth's high atmosphere 10–13 May, causing a significant "geomagnetic storm" that significantly heated the thermosphere and modified its chemical composition. The effects of this storm were observed by the SABER instrument which is flying on the NASA Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics satellite. Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) observes infrared light emitted by carbon dioxide, nitric oxide, molecular oxygen, and ionized nitric oxide produced during the storm. The 10–13 May storm caused the global average infrared radiation levels to increase by nearly a factor of 10 at some wavelengths in 1 day. The storm and the infrared radiation levels dissipated back to pre‐storm levels over the next 2 days. This storm was the third strongest in terms the power (2.64 trillion watts) radiated by the thermosphere in the past 22 years observed by SABER. The observed enhancements in infrared radiation at multiple wavelengths offer the opportunity to more fully understand how the Sun interacts with and influences Earth's highest atmosphere. Key Points: The daily Ap (273 nT) and Dst (−291.94 nT) indexes on 11 May 2024 were the largest since the launch of the Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics (TIMED) satellite in December 2001The power radiated 11 May by NO and CO2 exceeded 1 TW each and the NO power (1.41 TW) exceeded the CO2 power (1.35 TW) for the first timeTotal storm radiated power, above background, is 2.64 TW (2.28 × 1017 J), making it the third strongest storm during the TIMED era [ABSTRACT FROM AUTHOR]

Twenty‐two years (2002–2023) of infrared radiative cooling rate data derived from the SABER instrument on the NASA TIMED satellite are presented. Global daily and global annual infrared power (Watts, W) emitted by nitric oxide (NO) and carbon dioxide (CO2) illustrate the variability of the geospace environment on timescales from days to decades. The 11‐year solar cycle (SC) is evident in the global power data and in vertical profiles of infrared cooling rates (nW/m3). The global annual power radiated by NO and CO2 are larger in 2023 than at any time since 2003 and 2002, respectively. The to‐date peak in NO infrared power in SC 25 is larger than in SC 24, is comparable to SC 20, but is less than in SCs 18–19 and 21–23. Two geomagnetic storms in 2023 radiated more than 1 TW and are in the top 10 strongest storms observed by SABER. Plain Language Summary: SABER is an instrument on the NASA TIMED satellite, launched in December 2001, and is still operating nominally in 2024. SABER measures the amount of infrared energy emitted by the nitric oxide (NO) molecule and the carbon dioxide (CO2) molecule from Earth's thermosphere, the region of atmosphere above 100 km altitude. These measurements are used to derive the amount of power (Watts) radiated globally by these two molecules on daily to annual timescales. The infrared power data is then analyzed to determine its variability in the lower thermosphere from daily to decadal time scales. Of particular interest is the influence of the well‐known 11‐year cycle of solar activity which is readily evident in the infrared power data. SABER also observes the effects of geomagnetic storms, identifying two strong storms in 2023 that each produced over 1 TW of infrared power. Key Points: SABER on TIMED has observed infrared radiation from the thermosphere since January 2002, for 22 years, the equivalent of two solar cyclesGlobal annual infrared power radiated by NO and CO2 are larger in 2023 than any time since 2003 and 2002, respectivelyTwo geomagnetic storms in 2023 exceeded 1 TW of radiated energy, above background, and are in the top 10 strongest storms observed [ABSTRACT FROM AUTHOR]

On 24 April 2023, a Coronal Mass Ejection event caused the solar wind to become sub‐Alfvénic, leading to the development of an Alfvén Wing configuration in the Earth's magnetosphere. Alfvén Wings have previously been observed as cavities of low flow around moons in Jupiter's and Saturn's magnetospheres, but the observing spacecraft did not have the ability to directly measure the Alfvén Wings' current structures. Through in situ measurements made by the Magnetospheric Multiscale spacecraft, the 24 April event provides us with the first direct measurements of current structures during an Alfvén Wing configuration. These structures are observed to be significantly more anti‐field‐aligned and electron‐driven than the typical diamagnetic magnetopause current, indicating the disruption caused to the magnetosphere current system by the Alfvén Wing formation. The magnetopause current is then observed to recover more of its typical, perpendicular structure during the magnetosphere's recovery from the Alfvén Wing formation. Plain Language Summary: The solar wind applies pressure on the Earth's magnetic field, distorting it from a dipole into its compressed dayside and stretched tail configuration. However, this typical structure can be disrupted by eruptive solar events such as Coronal Mass Ejections (CMEs), which may cause the solar wind's pressure to drop low enough that it is no longer able to push the magnetosphere back to form a single unified tail. When this occurs, the tail splits into two separate structures, called Alfvén Wings. While this configuration is rare at Earth, it is common from interactions of the outer planets' magnetosphere's with their moons, where Alfvén Wing configurations have been studied and modeled. However, because the observing spacecraft lacked the necessary instrumentation, we have not yet directly observed the Alfvén Wing current structures. On 24 April 2023, a CME event led to the creation of an Alfvén Wing formation in the Earth's magnetosphere. We observed this event using the Magnetospheric Multiscale spacecraft, which enabled us to make the first direct observations of Alfvén Wing current structures. These currents were found to be mainly parallel to the local magnetic field, in contrast to typical magnetopause currents. Key Points: On 24 April 2023, the Magnetospheric Multiscale (MMS) spacecraft observed an Alfvén Wing formation along the dawn‐flank of Earth's magnetosphereMMS's observations represent the first in situ measurements of Alfvén Wing current structuresThe current structures are found to be primarily anti‐field‐aligned, electron‐driven, and filamentary [ABSTRACT FROM AUTHOR]

Volcanic rocks, preserving paleorecords of Earth's magnetic field, are essential to constrain the working of the geodynamo, provided their primary signal was not biased. Using a thermomagnetometer, we simulate a situation where a sample's primary record, carried by a thermoremanence (TRM, acquired by cooling in air from 600°C to room temperature), is partly overprinted by a chemical remanence (CRM, acquired by 200 hr of isothermal exposure at 400°C). This situation leads to two directional and intensity components (in the form of linear segments) in the Zijderveld and Arai‐Nagata diagrams. In the case of unstable titanomagnetite grains prior to CRM acquisition, we show that both components can be strongly biased by up to ∼50° for paleodirections and ∼50% for paleointensities. In such a worst‐case scenario, the secondary CRM strongly overprints the primary TRM, rendering the common interpretation of Zijderveld and Arai‐Nagata diagrams in terms of characteristic components invalid. Plain Language Summary: Volcanic rocks, the magnetic minerals of which can acquire a thermoremanent magnetization (TRM) from Earth's magnetic field during their initial cooling, are essential to constrain the working of the geodynamo through Earth's history. However, if the rock is subsequently reheated at moderate temperature in another ambient field, the initial record can be partly overprinted by a chemical remanence (CRM). Starting from a TRM applied on materials of various thermostability, we reproduced in laboratory conditions the acquisition of a CRM by annealing the rock in a controlled ambient field for 200 hr at 400°C. Rock‐magnetic and structural analyses at regular intervals, supplemented by continuous measurements of the remanent magnetization, indicate the creation of new phases as a result of oxidation processes. The paleomagnetic analysis of the final products reveals the existence of two distinct components that can be associated with the initial TRM and the secondary CRM. Whereas the paleomagnetic record of the initial TRM is trustworthy for the most stabilized initial products, directional (up to 50°) and intensity (up to 50%) biases are observed for the least stabilized initial products, illustrating a critical example where the interpretation of characteristic components (linear segments in the interpretation diagrams) is invalid. Key Points: We overprint a thermoremanent magnetization (TRM) by cooling from 600°C with a chemical remanent magnetization by 200 hr exposure at 400°CChemical remanent magnetization was produced by the creation of cation‐deficient titanomagnetite phases, then stabilized by oxyexsolutionPaleodirections and intensities are strongly biased in the worst scenario, making the interpretation of characteristic components invalid [ABSTRACT FROM AUTHOR]

We present the observations of field‐aligned currents and the equatorial electrojet during the 23 March 2023 magnetic storm, focusing on the effect of the drastic decrease of the solar wind dynamic pressure occurred during the main phase. Our observations show that the negative pressure pulse had significant impact to the magnetosphere‐ionosphere system. It weakened large‐scale field‐aligned currents and paused the progression of the storm main phase for ∼3 hr. Due to the sudden decrease of the plasma convection after the negative pressure pulse, the low‐latitude ionosphere was over‐shielded and experienced a brief period of westward penetration electric field, which reversed the direction of the equatorial electrojet. The counter electrojet was observed both in space and on the ground. A transient, localized enhancement of downward field‐aligned current was observed near dawn, consistent with the mechanism for transmitting MHD disturbances from magnetosphere to the ionosphere after the negative pressure pulse. Plain Language Summary: The solar wind is a continuous stream of charged particles blowing from the Sun. The Earth's magnetic field forms a protective shield around our planet, called the magnetosphere, which deflects most of the solar wind particles away from the Earth. Disturbances in the solar wind can interact with the magnetosphere and impact the Earth's upper atmosphere (ionosphere). The interaction creates electric fields forcing charged particles to move in the magnetosphere, which creates electric currents flowing along the magnetic field lines connecting to the high‐latitude ionosphere and drives the movement of charged particles there. The low‐latitude ionosphere is generally shielded from these electric fields. Sudden changes in the solar wind can break such balance, leading to the electric field penetration to low latitudes. We examined how the magnetosphere and ionosphere interacted during the 23 March 2023 geomagnetic storm, focusing on what happened when the solar wind dynamic pressure suddenly decreased. We found the pressure drop caused a sudden decrease of the high‐latitude electric field, resulting in a brief period of overshielding and the electric field in the equatorial ionosphere reversed its direction. This changed the direction of the equatorial electrojet, a major electric current in the ionosphere at the magnetic equator. Key Points: Direct evidence of prompt penetration of electric field in the equatorial ionosphere caused by negative solar wind pressure pulseTransient counter electrojet caused by westward penetration electric field after the arrival of negative pressure pulseSignificant decrease of global large‐scale field‐aligned currents (FACs) and transient enhancement of localized FAC in response to negative pressure pulse [ABSTRACT FROM AUTHOR]

High‐speed jets (HSJs) are commonly observed in the Earth's magnetosheath. The HSJs can drive shock‐like bow waves when compressing the ambient plasma, which are important for the HSJ's evolution and the energization of charged particles. Here we present the first two‐dimensional hybrid simulation of the formation and evolution of jet‐driven bow waves. The simulated bow waves exhibit localized enhanced magnetic field and ion density, with their peaks separated by the order of ion inertial length. The bow waves are formed when a super‐magnetosonic HSJ encounters a magnetic structure with the magnetic field nearly perpendicular to the HSJ's velocity. The magnetic field structure acts as an obstacle to deflect and decelerate the jet, causing the pile up of ions on the jet side and the compression of the magnetic structure on the downstream side. Our study explains the observed properties of bow waves, and helps to better understand the evolution of HSJs. Plain Language Summary: When the solar wind interacts with the Earth's magnetic field, it creates a bow shock in front of the magnetosphere. The bow shock slows down and heats up the solar wind plasma, creating a turbulent area known as the magnetosheath. Sometimes, high‐speed jets (HSJ), observed as high velocity and density pulses, are detected in the magnetosheath. The HSJs can transfer the solar wind plasma through the magnetosheath and potentially affect the near‐Earth space where satellites orbit. Recent studies have found that the HSJs can drive shock‐like bow waves at their front. The bow waves can accelerate and heat up plasma, similar to the primary shock wave, and they play an important role in the evolution of the HSJs. In this study, we present the first simulation result concerning the formation and evolution of the bow waves, which will help us to better understand the HSJs and the magnetosheath dynamics. Key Points: We study the formation and evolution of jet‐driven bow waves with the two‐dimensional hybrid model for the first timePlasma piles up at the ramp of compressed magnetic field, causing separated peaks of density and magnetic field at the bow waveThe magnetic field structure acts as an obstacle to decelerate jet, causing the pile up of jet ions at the leading edge [ABSTRACT FROM AUTHOR]

We obtain current densities from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE), alongside By and Bz from the Interplanetary Magnetic Field (IMF) for March 2010. For each AMPERE spatial coordinate, we cross‐correlate current density with By and Bz, finding the maximum correlation for lags up to 360 min. The patterns of maximum correlation contain large‐scale structures consistent with the literature. For the correlation with By, the lags on the dayside are 10 min at high latitudes but up to 240 min at lower latitudes. Lags on the nightside are 90–150 min. For Bz, the shortest lags on the dayside are 10–20 min; on the equatorward edge of the current oval, 60–90 min; and on the nightside, predominantly 90–150 min. This novel approach enables us to see statistically the timescales on which information is electrodynamically communicated to the ionosphere after magnetic field lines reconnect on the dayside and nightside. Plain Language Summary: We take the minute‐by‐minute behavior of the interplanetary magnetic field (which is in the solar wind) and electric currents flowing along Earth's magnetic field lines in the Northern Hemisphere. We move them with respect to one another to find the time lag required to make them agree best, and then note the best agreement and the time lag that we found. We plot both of these quantities on maps of the Earth's Northern Hemisphere, and then analyze these maps to uncover new information about Earth's reaction to the solar wind. Key Points: We cross‐correlate Birkeland current density with IMF By and Bz and find morphology of R1/R2 and other current systemsBz direct driving timescales are 10‐20 min; expanded polar cap timescales, 60‐90 min; and nightside timescales, 120‐150 minBy shows 10‐min timescales on the dayside at high latitudes, but lags up to 240 min elsewhere [ABSTRACT FROM AUTHOR]

Flux transfer events (FTEs) are transient magnetic flux ropes at Earth's dayside magnetopause formed due to magnetic reconnection. As they move across the magnetopause surface, they can generate disturbances in the ultralow frequency (ULF) range, which then propagate into the magnetosphere. This study provides evidence of ULF waves in the Pc2 wave frequency range (>0.1 Hz) caused by FTEs during dayside reconnection using a global 3D hybrid‐Vlasov simulation (Vlasiator). These waves resulted from FTE formation and propagation at the magnetopause are particularly associated with large, rapidly moving FTEs. The wave power is stronger in the morning than afternoon, showing local time asymmetry. In the pre and postnoon equatorial regions, significant poloidal and toroidal components are present alongside the compressional component. The noon sector, with fewer FTEs, has lower wave power and limited magnetospheric propagation. Plain Language Summary: The Earth's magnetosphere is a dynamic region shaped by the interplay between the solar wind and Earth's magnetic field. This interaction occurs at the boundary of the magnetosphere (magnetopause) through a process known as magnetic reconnection, giving rise to Flux Transfer Events (FTEs), which are magnetic structures that carry flux and energy into the magnetosphere. These FTEs form either in sudden bursts, patchy patterns or in a continuous, and relatively stable way making the magnetopause surface dynamic. As the FTEs move along the boundary of the magnetosphere, they create compressed regions and lead to wave generation that can extend into the magnetosphere. The study uses an advanced 3D hybrid‐Vlasov simulation model to analyze waves originated from FTE formation and propagation at the magnetopause. We find that rapidly moving and large FTEs have a significant impact on the magnetopause, leading to the generation of ULF waves with frequency above 0.1 Hz. This shows first direct evidence supporting previous theoretical speculations regarding the ability of FTEs to generate waves near the magnetopause. Key Points: Dayside Pc2 waves (>0.1 Hz) have been detected in a 3D hybrid‐Vlasov simulationThese waves exhibit lower intensity within the magnetosphere at noon, compared to the prenoon and postnoon sectorsPc2 waves observed in the simulation are associated with largest and fast moving flux transfer events initiated by subsolar reconnection [ABSTRACT FROM AUTHOR]

In this study we use micromagnetic modeling to show that the magnetizations of magnetically single‐vortex particles rotate toward the stress axis on the application of a differential compression stress. This is the exact opposite response to magnetically single‐domain particles, which previously provided the theoretical underpinning of the effect of stress on the magnetic signals of rocks. We show that the magnetization directions of single‐vortex and equant single‐domain particles are altered by much lower stresses than previously predicted, c.f., 100 versus 1,000 MPa; where a change in magnetization is defined as a rotation of >3° after the removal of stress. The magnetization intensity of assemblages also drops by ∼20%–30% on the application and removal of stress of ∼100 MPa. Given that single‐vortex particles are now thought to dominate the magnetization of most rocks, future studies should account for paleomagnetic directional uncertainties and potential underestimation of the ancient magnetic field intensity. Plain Language Summary: Magnetic minerals in rocks can record and retain magnetic information for millions of years. This magnetic information helps us to understand processes like plate tectonics and geomagnetic field behavior in the past. When rocks experience compressional forces, these affect the magnetization of the magnetic minerals within them, potentially altering this magnetic information. Previous theoretical studies of the affect of stress on magnetization used analytical theory, and predicted that stresses of >1,000 MPa are required to alter a rock's otherwise stable magnetization. We show that much lower stresses of only ∼100 MPa are required to compromise magnetic recordings in rocks. The reason for this is a change in our understanding of rock magnetic systems. Previous analytical models were for elongated particles <100 nm in size, as these were thought to be the primary carriers of stable magnetization in minerals. We now know that larger particles can retain stable magnetic recordings to high temperatures over geological time. This study shows that the magnetization of these particles are less stable than smaller particles when stresses are applied. This suggests that common geological processes such earthquakes might provide sufficient near‐surface stress to remagnetize rocks close to the fault zone, leading to a paleo‐stress recording. Key Points: Single‐vortex particles' net magnetization rotates toward the compression axis when subjected to differential stressSingle‐vortex particles display the opposite response to compressional stress compared to single‐domain particlesStress fields of only ∼100 MPa are required to alter paleomagnetic signals, lower than previously predicted [ABSTRACT FROM AUTHOR]

Discrete aurora are sporadic emissions of light originating in Mars upper atmosphere. We report nadir imaging observations from MAVEN's Imaging UltraViolet Spectrograph which identify the conditions which trigger electron precipitation causing these events. Prior studies have shown that discrete aurora events in the strong crustal magnetic field region in the southern hemisphere are the brightest and most repeatable compared to events occurring outside the region. Our new data set offers a more complete and accurate characterization of aurora in this area. The region of strongest crustal fields is composed of two distinct magnetic regions, with magnetic fields in opposite directions; discrete aurora events trigger in one region after dusk and in the other before dawn. Magnetic reconnection in these two adjacent regions with the draped interplanetary field may open the crustal fields in these regions during opposing local times. Particle precipitation can then cause discrete aurora at the observed times and locations. Plain Language Summary: Mars has a surprising variety of types of aurora, all different from Earth's "northern lights." Mars lacks the familiar high‐latitude aurora because it no longer has the same kind of global magnetic field Earth does. This study examines "discrete aurora" events that occur in region in Mars southern hemisphere that has retained some of the ancient magnetic field. It takes the form of long arcades of magnetic loops that resemble a set of arches. As Mars rotates, these arcades are carried around the planet as the solar wind and its imbedded magnetic field are carried past the planet. We show that when conditions are favorable, the magnetic field locked in the solar wind can interact and "reconnect" with Mars magnetic loops, allowing energetic particles to spiral down the field lines into the atmosphere to cause discrete aurora. Key Points: New data confirm that Mars discrete aurora events occur most frequently near strong crustal fields and vary with local timeAuroral events in adjacent regions with opposite magnetic polarity occur before or after midnight depending on local magnetic field directionMagnetic reconnection between crustal fields and the draped interplanetary field appears to control regional and local time behavior [ABSTRACT FROM AUTHOR]

We analyzed the contribution of electromagnetic ion cyclotron (EMIC) wave driven electron loss to a flux dropout event in September 2017. The evolution of electron phase space density (PSD) through the dropout showed the formation of a radially peaked PSD profile as electrons were lost at high L*, resembling distributions created by magnetopause shadowing. By comparing 2D Fokker Planck simulations of pitch angle diffusion to the observed change in PSD, we found that the μ and K of electron loss aligned with maximum scattering rates at dropout onset. We conclude that, during this dropout event, EMIC waves produced substantial electron loss. Because pitch angle diffusion occurred on closed drift paths near the last closed drift shell, no radial PSD minimum was observed. Therefore, the radial PSD gradients resembled solely magnetopause shadowing loss, even though the local pitch angle scattering produced electron losses of several orders of magnitude of the PSD. Plain Language Summary: Extremely energetic charged particles become trapped by Earth's geomagnetic field, forming the Van Allen radiation belts. The total amount of radiation trapped within these belts varies depending on the solar wind conditions, which can disturb the geomagnetic field to produce geomagnetic storms. At the beginning of a geomagnetic storm, there is a relative calm in the radiation belt, produced by the rapid drainage of electrons from the geomagnetic field. It is not fully understood if these electrons are primarily lost into the solar wind, or if they are lost into Earth's atmosphere. In this study, we analyze the remaining trapped electrons to reconstruct the mechanisms of electron escape at the beginning of a geomagnetic storm in September 2017. While previous work found that electrons were primarily lost into the solar wind, we found that loss into the atmosphere also played an important role. Furthermore, we showed that drainage of electrons into the atmosphere can be mistaken for loss into the solar wind if the energy and trajectory of lost electrons are not carefully considered. Key Points: Characterizing electron loss through peaks and minima in radial phase space density can misrepresent simultaneous loss mechanismsAnalysis of electron loss across all adiabatic invariants μ, K, and L*, is necessary to correctly identify loss mechanismsObservational analysis of phase space density data alone cannot be used to quantify individual contributions of simultaneous loss processes [ABSTRACT FROM AUTHOR]

A new observational phenomenon, named Simultaneous Global Ionospheric Density Disturbance (SGD), is identified in GNSS total electron content (TEC) data during periods of three typical geospace disturbances: a Coronal Mass Ejection‐driven severe disturbance event, a high‐speed stream event, and a minor disturbance day with a maximum Kp of 4. SGDs occur frequently on dayside and dawn sectors, with a ∼1% TEC increase. Notably, SGDs can occur under minor solar‐geomagnetic disturbances. SGDs are likely caused by penetration electric fields (PEFs) of solar‐geomagnetic origin, as they are associated with Bz southward, increased auroral AL/AU, and solar wind pressure enhancements. These findings offer new insights into the nature of PEFs and their ionospheric impact while confirming some key earlier results obtained through alternative methods. Importantly, the accessibility of extensive GNSS networks, with at least 6,000 globally distributed receivers for ionospheric research, means that rich PEF information can be acquired, offering researchers numerous opportunities to investigate geospace electrodynamics. Plain Language Summary: Electric fields of solar wind and geomagnetic disturbance origin can penetrate into the low latitude upper atmosphere, influencing the ionospheric dynamics and electron density variations. This study employs a new method that utilizes global and continuous GNSS total electron content (TEC) observations to investigate the electric field effects. The analysis focuses on three geospace disturbance events of different intensities and solar‐terrestrial conditions. The study identifies a novel phenomenon named Simultaneous Global Ionospheric Density Disturbance (SGD), primarily occurring on the sunlit portion of the Earth's ionosphere and also near dawn hours with 1% or larger amplitudes of the background TEC, or a few tenths of a TEC unit (1016 m3). The remarkable global extent of ionospheric responses to minor solar‐geomagnetic conditions is noteworthy. The solar wind magnetic field directed southward is highly correlated with most SGDs, lasting for up to 30 min. The findings present an effective approach for continuously monitoring electric field penetration and ionospheric impacts, leading to an improved understanding of space weather and its technological implications. Key Points: Simultaneous global ionospheric disturbances (SGDs) are often observed even during minor solar and geomagnetic disturbancesSGDs occur predominately on dayside and are related to penetration electric fields (PEFs) of solar wind and geomagnetic disturbance originGlobal GNSS networks offer a novel and effective technique for continuous PEF monitoring, providing rich data sets for further study [ABSTRACT FROM AUTHOR]

Since the discovery of the large‐scale field‐aligned currents it is widely acknowledged that gaps exist between the Region 1 (R1) and Region 2 (R2) currents in which the current values are relatively small as compared to neighboring regions. Assuming that the field‐aligned currents are generated by plasma pressure gradients, we analyzed data collected by the THEMIS satellites between 2007 and 2011 to identify regions with very low plasma pressure gradients (pressure plateaus), which could be responsible for the appearance of these gaps. It was found that the pressure profiles with low radial gradients are typically located between 8 and 10 Radii around the Earth. Projections of pressure plateau regions onto ionospheric altitudes, for both individual events and on a statistical basis, coincide with the locations of gaps between Iijima and Potemra field‐aligned currents. The role played by identified pressure plateaus in shaping the pattern of large‐scale field‐aligned currents is discussed. Plain Language Summary: Field‐aligned currents that flow between the magnetosphere and ionosphere along magnetic field lines play a crucial role in magnetosphere‐ionosphere interactions. Despite being discovered at the start of the space era, their origins are still subject to debate. In our current study, we have established that the observed gaps between the upward and downward currents correspond to the plateau regions of constant plasma pressure, as obtained using data from the THEMIS mission. This finding provides strong evidence in favor of the generation of field‐aligned currents by plasma pressure gradients in the transition region between the dipole and tail‐like geomagnetic field. This is a critical piece of information for understanding the global dynamics of Earth's magnetosphere. Key Points: During quiet geomagnetic conditions, the regions of plasma pressure plateau are observed in the inner magnetosphereThese regions are located at geocentric distances of 8–10 RE forming a ring surrounding the EarthThe localization of the plasma pressure plateau region and gaps between the Region 1 and 2 currents should be located at the same distance [ABSTRACT FROM AUTHOR]

Ultra‐low frequency (ULF) wave observations across the heliosphere often rely on the sign of correlations between plasma (density/pressure) and magnetic field perturbations to distinguish between fast and slow magnetosonic modes. However, the assumptions behind this magnetohydrodynamic result are not always valid, particularly within the magnetosphere which is inhomogeneous and supports standing waves along the geomagnetic field. Through theory and a global simulation, we find both effects can result in anomalous plasma–magnetic field correlations. The interference pattern in standing waves can lead both body and surface magnetosonic waves to have different cross‐phases than their constituent propagating waves. Furthermore, if the scale of gradients in the background are shorter than the wavelength or the waves are near‐incompressible, then advection by the wave of inhomogeneities can overcome the wave's inherent sense of compression. These effects need to be allowed for and taken into account when applying the typical diagnostic to observations. Plain Language Summary: Fluid plasma wave theory provides a key distinguishing feature between the two compressional wave modes: fast magnetosonic waves should have correlated fluctuations in the magnetic field strength and the plasma (density or pressure) whereas anticorrelation relates to the slow magnetosonic mode. This classic result is often used as a diagnostic for waves observed by spacecraft throughout the heliosphere. However, it is important to recognize that this result is derived under the assumption of a single traveling wave in a uniform background plasma. Planetary magnetospheres, in contrast, have spatially‐varying plasma conditions and can reflect waves off of boundaries to form standing modes. We investigate what influence these effects have on both freely propagating waves and those tied to a surface through analytic theory and a global magnetospheric simulation. We find it is possible for the wave's correlation between plasma and magnetic field to be fundamentally altered. For standing waves we show the interference pattern present can lead to this change, whereas for non‐uniform plasmas changes are due to the wave moving plasma with a significantly different background value. These effects need to be allowed for and taken into account when applying the typical plasma–magnetic field test to observed waves. Key Points: We test if the standard plasma–magnetic field relations for magnetosonic waves are altered by standing structure or inhomogeneous plasmaThe interference pattern present in standing (body or surface) waves can lead to anomalous results under certain plasma conditionsAdvection by the wave of plasma inhomogeneities can also result in reversed correlations if gradients are sufficiently large [ABSTRACT FROM AUTHOR]

The off‐equatorial minimum‐B‐pocket (OEMBP) at high latitudes in the dayside outer magnetosphere is expected as another source region of the electrostatic electron cyclotron harmonic (ECH) waves, in addition to the magnetic equator. However, its crucial role in the excitation of the dayside ECH waves has never been directly investigated due to its spatiotemporal‐varying and difficulty to be located. Based on the observations from Magnetospheric Multiscale satellites, we statistically analyze the properties of dayside ECH waves by organizing them according to the latitude distance from the OEMBP located by the spatiotemporal‐varying geomagnetic field model. Such mapping confirms larger occurrence rates and electric field amplitudes of dayside ECH waves in the vicinity of the OEMBP, which gradually fall down nearby with the latitude distance increasing. Our results provide statistical evidence for the OEMBP as another source region of ECH waves besides the magnetic equator. Plain Language Summary: The off‐equatorial minimum‐B‐pocket is the local minimum of magnetic field strength at high latitude in the dayside outer magnetosphere, which is expected as another source region of the electrostatic electron cyclotron harmonic (ECH) waves besides the magnetic equator. However, its importance for the dayside ECH waves at high latitudes has never been directly investigated since the off‐equatorial minimum‐B‐pocket is spatiotemporal‐varying and is difficult to be located. We use the solar wind‐dependent geomagnetic field model to locate the off‐equatorial minimum‐B‐pocket along the field line threading the observational point, and subsequently introduce the latitude distance to quantify the distance from the off‐equatorial minimum‐B‐pocket. Our results prove that dayside ECH waves in the outer magnetosphere are more frequent and stronger in the vicinity of the off‐equatorial minimum‐B‐pocket, demonstrating that it is indeed a source region of ECH waves. Key Points: Dayside electron cyclotron harmonic (ECH) waves on bifurcated magnetic field lines are statistically analyzed according to their latitude distance from minimum‐B‐pocketThese dayside ECH waves have larger occurrence rates and amplitude in the vicinity of the off‐equatorial minimum‐B‐pocketThe occurrence and amplitude of these dayside ECH waves are asymmetric relative to the off‐equatorial minimum‐B‐pocket [ABSTRACT FROM AUTHOR]

In this study employing data from the Swarm satellites at an altitude of ≈500 km, we present evidence of field‐line resonances (FLRs) in the Pc2‐3 (0.02–0.2 Hz) frequency band in the undisturbed post‐sunset region of the equatorial ionosphere. We identify 26 events extending to magnetic latitudes as low as 2°, which was previously thought to be too low for FLRs to exist due to ionospheric damping. Identification of FLRs is supported by a narrow‐band fundamental and first harmonics, an oscillating field‐aligned Poynting vector, a relative phase shift between E and δB of ≈90°, and in one case observation of one wave in conjugate hemispheres by two satellites separated by 4 min. We show that, contrary to the non‐uniform trend of ground‐based observations, the fundamental frequencies of these occurrences decline monotonically with increasing field‐line length. Their frequencies differ from those of ground‐based magnetometer observations reported in the literature. Plain Language Summary: Ultra‐low‐frequency (ULF) waves play a crucial role in the coupling of the magnetosphere‐ionosphere system. These waves have been reported abundantly in the high latitude ionosphere, where they are driven directly by processes in the magnetosphere and solar wind. ULF waves have also been reported at the equatorial ionosphere, but less frequently. In some cases, low‐latitude waves are thought to be driven by high‐latitude waves ducting through the ionosphere or by compressional waves, which can propagate across geomagnetic field lines from high altitudes in the equatorial region. These waves can then excite interhemispheric field‐line resonances (FLRs), in which Earth's magnetic field lines vibrate like a guitar string, with longer field lines oscillating at lower frequencies. In this work, we use the Swarm satellites to study low‐latitude FLRs at an altitude of ≈500 km in the quiet ionosphere at geomagnetic latitudes below 3°. We demonstrate the existence of FLRs with periods between 5 and 50 s which increase with magnetic latitude, as expected when field line lengths increase. Key Points: Field line resonances (FLRs) are identified at the lowest magnetic latitude ever reportedThe fundamental frequency of our events decreases with increasing magnetic latitude in a uniform trendFLRs detected at conjugate latitudes by two spacecraft at the equatorial ionosphere [ABSTRACT FROM AUTHOR]

By using the magnetic field data from Van Allen Probes, we analyzed the distribution characteristics of the electromagnetic environment in the inner magnetosphere on different Dst* index and magnetic local time. Our results show that for the response of different current systems, the dawn‐dusk and noon‐midnight asymmetry distribution of the residual magnetic field δB increases with Dst* index. When Dst* < −60 nT, a "banana"‐shaped geomagnetic field negative disturbance peak region appears in the sector from midnight to dusk. Then, we obtained the azimuthal current density and found the asymmetric internal eastward and external westward ring current. Through the vector analysis of three‐dimensional current density, the current density vector distribution in the magnetic equatorial plane is completely displayed for the first time, which directly proves the existence of banana current near r = 3.0–4.0 RE during strong geomagnetic storms. Plain Language Summary: By using the magnetic field data from Van Allen Probes, we find that the electromagnetic environment of the Earth's inner magnetosphere exhibits different distributions on different geomagnetic disturbance strengths. During the occurrence of strong magnetic disturbances, the residual magnetic field δB will exhibit distinct dawn‐dusk and noon‐midnight asymmetric distributions. And a "banana"‐shaped peak region of negative geomagnetic field perturbation will appear in the midnight to dusk sector, while both the eastward and westward ring currents exhibit asymmetry. The presence of the banana current near r = 3.0–4.0 RE during strong geomagnetic storms is then directly demonstrated by vector analysis of the 3D current density. Key Points: The negative perturbation response of space currents to the geomagnetic field in the inner magnetosphere is analyzedBoth the eastward ring current and westward ring current is asymmetricThe vector distribution diagram of banana current density is obtained [ABSTRACT FROM AUTHOR]

Deep penetration of outer radiation belt electrons to low L (<3.5) has long been recognized as an energy‐dependent phenomenon but with limited understanding. The Van Allen Probes measurements have clearly shown energy‐dependent electron penetration during geomagnetically active times, with lower energy electrons penetrating to lower L. This study aims to improve our ability to model this phenomenon by quantitatively considering radial transport due to large‐scale azimuthal electric fields (E‐fields) as an energy‐dependent convection term added to a radial diffusion Fokker‐Planck equation. We use a modified Volland‐Stern model to represent the enhanced convection field at lower L to match the observations of storm time values of E‐field. We model 10–400 MeV/G electron phase space density with an energy‐dependent radial diffusion coefficient and this convection term and show that the model reproduces the observed deep penetrations well, suggesting that time‐variant azimuthal E‐fields contribute preferentially to the deep penetration of lower‐energy electrons. Plain Language Summary: Electrons trapped by the Earth's magnetic field gather in two regions known as the Van Allen radiation belts. It is well reported that electrons can be transported radially inward from the outer radiation belt during geomagnetically active times. More specifically, low energy (100 s of keV) electrons can be moved radially deeper than higher energy (∼1 MeV) electrons. Previous studies suggested that enhanced convection electric fields could contribute to the earthward transport of low energy (<200 keV) electrons. However, the mechanism which leads to different efficiencies of electron transport at different energies has not been quantified. This study expands the traditional radial diffusion model with an empirically determined convection term and shows that the net convection velocity increases for lower energy electrons. For the first time, we quantitatively modeled the energy‐dependent penetration of radiation belt electrons in a wide energy range (10 s of keV to 2 MeV) in the presence of enhanced large‐scale electric fields, during two geomagnetic storm events observed by the Van Allen Probes mission. Key Points: Convective radial transport of storm‐time enhanced large‐scale E‐fields is an efficient inward transport mechanism of 10–100 s keV electronsThe energy‐dependent electron penetration can be explained by the relation between the timescales of electron drift and large‐scale E‐fieldsA radial diffusion‐convection model is developed to reproduce the storm‐time penetration of lower energy electrons to lower L [ABSTRACT FROM AUTHOR]