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Plasma in the inner magnetosphere is affected by various processes, such as substorms. In this study, we have statistically investigated the ring current properties based on the observations of Van Allen Probes from 2012 to 2019 to examine the substorm effects on the plasma pressure and current system in the inner magnetosphere. The results show that the plasma pressure increases significantly, leading to a ∼3 nT/hr geomagnetic depression during intense substorms. The contribution of <100 keV H+ ions to the plasma pressure increases during substorms, which is more significant at lower L‐shells. Opposite to the H+ ions, the proportion of >10 keV O+ ions contributing to the plasma pressure increases as substorm intensity increases. In addition, the rise of plasma pressure is mainly distributed from dusk to midnight, resulting in the enhancement of asymmetric ring current and the region II field‐aligned currents connecting to the ionosphere. Our results provide a comprehensive view of the variation of plasma pressure and current system in the inner magnetosphere during quiet periods and intense substorms. Key Points: The pressure increase caused by intense substorms can lead to 3 nT/hr magnetic field depressionThe pressure contribution of both <100 keV H+ and >10 keV O+ increase during intense substormsThe ring current densities from dusk to midnight and region II FACs increase dramatically during intense substorms [ABSTRACT FROM AUTHOR]

The D‐shaped ion velocity distribution generated by the time‐of‐flight (ToF) effect can be considered as an important characteristic of reconnection exhausts in the magnetopause and magnetotail reconnection processes. In this study, we reported the D‐shaped velocity distribution of O+ ions produced by the ToF effect in the magnetotail reconnection exhaust based on the observations from the Cluster spacecraft. The observed cutoff velocity of O+ ions is smaller than that of H+ ions at the same time, which is different from the previous theoretical prediction. We suggested that the difference between the two cutoff velocities is probably due to O+ ions having a larger reconnection diffusion region than H+ ions. We also demonstrated a remote‐sensing method to estimate the spatial scale ratio between the O+ and H+ diffusion regions and the distance from the observation site to the center of the magnetotail reconnection region. Plain Language Summary: The reconnection processes at the Earth's magnetopause and magnetotail can convert the magnetic energy into the kinetic energy of the plasma, thus producing high‐speed reconnection exhausts. The exhausting ions usually show a D‐shaped velocity distribution caused by the time‐of‐flight effect. Previous works usually focused only on exhausting protons from the reconnection site, while O+ ions could also be abundant during geomagnetic active times. In this study, we analyzed a reconnection event to investigate the behaviors of O+ and H+ ions in the magnetotail reconnection exhaust region using the observations from the Cluster spacecraft. It is suggested that different behaviors of H+ and O+ ions are probably due to O+ ions having a larger reconnection diffusion region than H+ ions. In addition, we proposed a remote‐sensing method to estimate the spatial scale ratio between the O+ and H+ diffusion regions and the distance from the observation site to the center of the magnetotail reconnection region. Key Points: The time‐of‐flight effect of O+ ions in magnetotail reconnection exhaust was observed by the Cluster spacecraftThe observed cutoff velocity of O+ is smaller than that of H+, indicating that O+ may have a larger diffusion region than H+A remote‐sensing method to estimate the spatial scale ratio between O+ and H+ diffusion regions is introduced [ABSTRACT FROM AUTHOR]

Solar wind parameters play a dominant role in reconnection rate, which controls the solar wind‐magnetosphere coupling efficiency at Earth's magnetopause. Besides, low‐energy ions from the ionosphere, frequently detected on the magnetospheric side of the magnetopause, also affect magnetic reconnection. However, the specific role of low‐energy ions in reconnection is still an open question under active discussion. In the present work, we report in situ observations of a multiscale, multi‐type magnetopause reconnection in the presence of low‐energy ions using NASA's Magnetospheric Multiscale data on September 11, 2015. This study divides ions into cold (10–500 eV) and hot (500–30,000 eV) populations. The observations can be interpreted as a secondary reconnection dominated by electrons and cold ions (mainly in XYGSE plane) located at the edge of an ion‐scale reconnection (mainly in XZGSE plane). This analysis demonstrates a dominant role of cold ions in the secondary reconnection without hot ions' response. Cold ions and electrons are accelerated and heated by the secondary process. The case study provides observational evidence for the simultaneous operation of antiparallel and component reconnection. Our results imply that the pre‐accelerated and heated cold ions and electrons in the secondary reconnection may participate in the primary ion‐scale reconnection affecting the solar wind‐magnetopause coupling and the complicated magnetic field topology could affect the reconnection rate. Key Points: We report Magnetospheric Multiscale observations of a multiscale, multi‐type magnetopause reconnection in the presence of low‐energy ionsAn electron‐cold ion reconnection, dominated by low‐energy ions and electrons, is located at the edge of an ion‐scale reconnectionThe multiscale reconnection provides observational evidence for the simultaneous operation of antiparallel and component reconnection [ABSTRACT FROM AUTHOR]

Six years of Van Allen Probes data are used to investigate cold plasmaspheric electrons affected by ultralow‐frequency (ULF) waves in the inner magnetosphere (L<7) including spatial distributions, occurrence conditions, and resonant energy range. Events exhibit a global distribution within L= 4–7 but preferentially occur at L∼5.5–7 in the dayside, while there is higher occurrence rate in the duskside than dawnside. They can occur under different geomagnetic activities and solar wind velocities (VS), but the occurrence rates are increasing with larger AE, |SYMH|, and VS. These features are closely associated with the generation and propagation of ULF waves in Pc4 (45–150 s) and Pc5 (150–600 s) bands. Combined with electron observations from HOPE instrument, the resonant energies inferred from wave power indicate that cold electrons at ones to hundreds of electron volts can be affected by ULF waves. This study may shed new light on further investigations on the acceleration and transportation of cold plasmaspheric particles that would affect plasmaspheric material release to the Earth's magnetosphere and instabilities for exciting various waves. Key Points: Interactions between ULF waves and cold electrons occur at L = 4–7 with dominant distributions in the dayside and a dawn‐dusk asymmetryThe event occurrence rates are increasing with larger AE, |SYMH|, and solar wind velocityThe resonant energy inferred from wave power and electron observations indicates that electrons at ones to hundreds of electron volts are affected by ULF waves [ABSTRACT FROM AUTHOR]

In this study we focus on the radiation belt dynamics driven by the geomagnetic storms during September 2017. Besides the long-lasting three-belt structures of ultrarelativistic electrons (>2 MeV, existing for tens of days), which has been studied intensively during the Van Allen Probe era, it is found that magnetospheric electrons of hundreds of keVs can also have three-belt structures at similar L extent during storm time. Measurements of 500-800 keV electrons from MagEIS instrument onboard Van Allen Probes show double-peaked (L = 3.5 and 4.5, respectively) flux-versus-L-shell profile in the outer belt, which lasted for 2-3 days. During the time interval of such transient three-belt structure, the energy-versus-L spectrogram shows novel distributions differing from both "S-shaped" and "V-shaped" spectrograms reported previously. Such peculiar distribution also illustrates the energy-dependent occurrence of the three-belt profile. The gradual formation of "reversed energy spectrum" at L~3.5 also indicates that hiss scattering inside the plasmapause contributed to the fast decay of sub-MeV remnant belt. [ABSTRACT FROM AUTHOR]

Drift‐bounce resonance between ultralow frequency (ULF) waves and ions is essential for ion energization in the magnetosphere. Here, we present the first comprehensive study of drift‐bounce resonance in the dayside outer magnetosphere, where off‐equatorial magnetic field minima would strongly distort ions' bounce and drift motion. A generalized theory is proposed, in which the effects of off‐equatorial minima, time‐evolving fields and ion bounce motion are taken into account. In consequence of these effects, ion pitch angle distributions undergo dramatic changes. In the presence of off‐equatorial minima, the time‐of‐flight effect of ion bounce motion forms latitude‐dependent dispersions besides "paw‐track shaped" structures, while evolving wave fields cause time‐dependent phase shifts in "paw‐tracks." All the predicted signatures have been confirmed by 5 years of Magnetospheric Multiscale spacecraft data and numerical simulations. These results allow us to better understand the interactions between ULF waves and thermal ion species in global magnetospheric dynamics. Plain Language Summary: Ultralow frequency (ULF) waves are widely distributed magnetospheric waves that can efficiently accelerate particles and play important roles in geomagnetic activities. Various patterns of ULF wave‐particle interactions have been investigated in detail near the Earth. However, near the dayside boundary of the magnetosphere, the geomagnetic field changes slightly due to solar wind compression. Consequently, the interaction between ULF waves and particles is modified in this region. This study analyzes how the pattern of ULF wave‐particle interactions varies near the dayside boundary of the magnetosphere. We find the compression of the magnetic field, particle motion and ULF wave evolution change the observational signatures of ULF wave‐particle interactions in the region of interest. The results help us better understand the energy transport from the solar wind to the magnetosphere and the role of ULF waves in magnetospheric dynamics. Key Points: Ultralow frequency wave‐ion drift‐bounce resonance in the dayside outer magnetosphere is investigated with 5 years of Magnetospheric Multiscale observationsA generalized theory is proposed to explain the observed ion pitch angle evolutionThe effects of off‐equatorial minima and time‐evolving fields are taken into account successfully to depict the observations [ABSTRACT FROM AUTHOR]

The ultra‐low frequency wave‐particle drift‐bounce resonance in the inner magnetosphere has been studied in detail, due to its important role in particle energization. However, it remains an open question how drift‐bounce resonance manifests in the dayside outer magnetosphere, where particles' orbits show bifurcations because of off‐equatorial magnetic field minima. In this study, we investigate this question, by analyzing Magnetospheric Multiscale observations of the January 20, 2017 event. A test‐particle simulation is conducted to help us understand the observations. The observed pitch angle‐time spectrograms show "pawtrack‐like" structures. We find there are more than two resonant pitch angles at fixed energy, since off‐equatorial minima change the relationship between the bounce (drift) frequency and pitch angle from unimodal function to trimodal function. These results reveal a new drift‐bounce acceleration mechanism in the dayside outer magnetosphere, which potentially affects the efficiency of particle energization during geomagnetic activities like geomagnetic storms. Plain Language Summary: The sun continuously ejects ionized gases called solar winds toward the Earth, affecting Earth's geomagnetic field. When the solar activity is strong, the geomagnetic field is severely disturbed, influencing the safety of power transmissions and space activities. This phenomenon is connected with particle acceleration in the geomagnetic field. The low frequency oscillation of geomagnetic field lines could efficiently transfer energy to charged particles in space, which plays an important role in particle acceleration. This study analyzes special observations which are different from the conventional pattern. A simulation is conducted to help interpret the observations. We find this low frequency magnetic field oscillation interacts with particles differently near the dayside boundary of the geomagnetic field, as the compression of the geomagnetic field by solar winds affects the motion of charged particles nearby. These results help us better understand the formation of geomagnetic activities. Key Points: The effects of off‐equatorial magnetic field minima on ultra‐low frequency wave‐ion interaction have been studied in this letterOff‐equatorial minima lead to more than two resonant pitch angles in drift‐bounce resonanceThe consequent pitch angle distribution shows "pawtrack‐like" structures [ABSTRACT FROM AUTHOR]

Recent work revealed the existence of plasmapause surface waves (PSWs) with Van Allen Probes observations, which are characterized by the periodic modulations of both magnetic/electric fields and plasma densities, and suggested to be the driver of giant undulations (GUs). In this study, six years data from Van Allen Probes were used to investigate the spatial distributions and occurrence conditions of PSWs in the period of 5–30 min. PSWs are found to be mainly distributed at L = 3.5–7 and their occurrence rate is increasing with larger L shells. In the azimuth direction, the spatial distribution of PSWs exhibits an obvious dawn‐dusk asymmetry with highest occurrence rates at MLT = 15–21, which is consistent with the spatial distribution of GUs revealed in the previous study. This further demonstrates that PSWs and GUs are connected, and indicates that PSWs differ from Ps6 waves which have a similar wave period and irregular waveform but mainly concentrate in the dawnside and connect with Ω bands. PSWs preferentially occur under the condition of high solar wind velocities (VSW > 500 km/s) and high dynamic pressures (Pdyn > 5 nPa), and their occurrence rate has a negative correlation with IMF Bz, SYMH and AL. Statistical results also reveal that PSWs preferentially occur around the peak of both magnetic storms and substorms. These findings may shed new lights on the further understanding of PSWs' generation and propagation. Key Points: Six years data from Van Allen Probes were used to investigate the plasmapause surface waves (PSWs) in the period of 5–30 minPSWs have a preferential location in the duskside (MLT = 15–21), which coincides with the statistical result of giant undulationsPSWs' occurrence rate has a positive correlation with geomagnetic activities and they mostly occur around the peak of storms and substorms [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]

This paper investigates the impact characteristics of the 4 November 2021 magnetic storm across different frequency bands based on the electric field data (EFD) from the China Seismo-Electromagnetic Satellite (CSES), categorized into four frequency bands: ULF (Ultra-Low-Frequency, DC to 16 Hz), ELF (Extremely Low-Frequency, 6 Hz to 2.2 kHz), VLF (Very Low-Frequency, 1.8 to 20 kHz), and HF (High-Frequency, 18 kHz to 3.5 MHz). The study reveals that in the ULF band, magnetic storm-induced electric field disturbances are primarily in the range of 0 to 5 Hz, with a significant disturbance frequency at 3.9 ± 1.0 Hz. Magnetic storms also enhance Schumann waves in the ULF band, with 8 Hz Schumann waves dominating in the southern hemisphere and 13 Hz Schumann waves dominating in the northern hemisphere. In the ELF band, the more pronounced anomalies occur at 300 Hz–900 Hz and above 1.8 kHz, with the 300 Hz–900 Hz band anomalies around 780 Hz being the most significant. In the VLF band, the electric field anomalies are mainly concentrated in the 3–15 kHz range. The ELF and VLF bands exhibit lower absolute and relative disturbance increments compared to the ULF band, with the relative perturbation growth rate in the ULF band being approximately 10% higher than in the ELF and VLF bands. Magnetic storm-induced electric field disturbances predominantly occur in the ULF, ELF, and VLF bands, with the most significant disturbances in the ULF band. The electric field perturbations in these three frequency bands exhibit hemispheric asymmetry, with strong perturbations in the northern hemisphere occurring earlier than in the southern hemisphere, corresponding to different Dst minima. No electric field disturbances were observed in the HF band (above 18 kHz). The conclusions of this paper are highly significant for future anti-jamming designs in spacecraft and communication equipment, as well as for the further study of magnetic storms. [ABSTRACT FROM AUTHOR]

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]

In recent years, it has become increasingly clear that space weather disturbances can be triggered by transient upstream mesoscale structures (TUMS), independently of the occurrence of large-scale solar wind (SW) structures, such as interplanetary coronal mass ejections and stream interaction regions. Different types of magnetospheric pulsations, transient perturbations of the geomagnetic field and auroral structures are often observed during times when SW monitors indicate quiet conditions, and have been found to be associated to TUMS. In this mini-review we describe the space weather phenomena that have been associated with four of the largest-scale and the most energetic TUMS, namely, hot flow anomalies, foreshock bubbles, travelling foreshocks and foreshock compressional boundaries. The space weather phenomena associated with TUMS tend to be more localized and less intense compared to geomagnetic storms. However, the quiet time space weather may occur more often since, especially during solar minima, quiet SW periods prevail over the perturbed times. [ABSTRACT FROM AUTHOR]

Interplanetary (IP) shocks are perturbations observed in the solar wind. IP shocks correlate well with solar activity, being more numerous during times of high sunspot numbers. Earth‐bound IP shocks cause many space weather effects that are promptly observed in geospace and on the ground. Such effects can pose considerable threats to human assets in space and on the ground, including satellites in the upper atmosphere and power infrastructure. Thus, it is of great interest to the space weather community to (a) keep an accurate catalog of shocks observed near Earth, and (b) be able to forecast shock occurrence as a function of the solar cycle (SC). In this work, we use a supervised machine learning regression model to predict the number of shocks expected in SC25 using three previously published sunspot predictions for the same cycle. We predict shock counts to be around 275 ± 10, which is ∼47% higher than the shock occurrence in SC24 (187 ± 8), but still smaller than the shock occurrence in SC23 (343 ± 12). With the perspective of having more IP shocks on the horizon for SC25, we briefly discuss many opportunities in space weather research for the remainder years of SC25. The next decade or so will bring unprecedented opportunities for research and forecasting effects in the solar wind, magnetosphere, ionosphere, and on the ground. As a result, we predict SC25 will offer excellent opportunities for shock occurrences and data availability for conducting space weather research and forecasting. Plain Language Summary: Solar activity is quite correlated with sunspot numbers. Alternating periods between solar minima and minima, termed solar cycle, usually occur every ∼11 years. As a result, researchers often attempt to predict sunspot occurrences for the following solar cycle. Solar perturbations occur more frequently during periods of high solar activity, and Earth‐bound perturbations can disturb the Earth's magnetic field in geospace and on the ground, affecting satellites and power infrastructure. In this work, we use an artificial intelligence supervised model to predict the number of shock occurrences in the ongoing solar cycle (beginning December 2019) by training the model with observations of sunspots and solar perturbations in the previous two solar cycles (August 1996–December 2019). Then, sunspot number predictions for the ongoing solar cycle are applied to the model, and predictions for the solar perturbations are obtained. We find that the number of predicted solar perturbations is ∼50% higher than their occurrence number in the previous solar cycle (December 2008–December 2019). Finally, we discuss how this relatively higher number of predicted solar perturbations can impact space weather research given the unprecedented number of data sets available in geospace and on the ground in the upcoming years. Key Points: SSN and shock count data in SC23‐24 are used with SSN predictions for SC25 in a supervised regression model to estimate shock counts in SC25We predict SC25 (275) will have ∼48% more shocks in comparison to SC24 (187), but it will have fewer shocks in comparison to SC23 (343)SC25 will offer unprecedented opportunities for space weather research given the availability of many data sets from space to the ground [ABSTRACT FROM AUTHOR]

The impact of interplanetary (IP) shocks on the Earth's magnetosphere can greatly disturb the geomagnetic field and electric currents in the magnetosphere-ionosphere system. At high latitudes, the current systems most affected by the shocks are the auroral electrojet currents. These currents then generate ground geomagnetically induced currents that couple with and are highly detrimental to ground artificial conductors including power transmission lines, oil/gas pipelines, railways, and submarine cables. Recent research has shown that the shock impact angle, the angle the shock normal vector performs with the Sun-Earth line, plays a major role in controlling the subsequent geomagnetic activity. More specifically, due to more symmetric magnetospheric compressions, nearly frontal shocks are usually more geoeffective than highly inclined shocks. In this study, we utilize a subset (332 events) of a shock list with more than 600 events to investigate, for the first time, shock impact angle effects on the subsequent GICs right after shock impact (compression effects) and several minutes after shock impact (substorm-like effects). We use GIC recordings from the Finnish natural gas pipeline performed near the Mäntsälä compression station in southern Finland. We find that GIC peaks ( > 5 A) occurring after shock impacts are mostly caused by nearly frontal shocks and occur in the post-noon/dusk magnetic local time sector. These GIC peaks are presumably triggered by partial ring current intensifications in the dusk sector. On the other hand, more intense GIC peaks ( > 20 A) generally occur several minutes after shock impacts and are located around the magnetic midnight terminator. These GIC peaks are most likely caused by intense energetic particle injections from the magnetotail which frequently occur during substorms. The results of this work are relevant to studies aiming at predicting GICs following solar wind driving under different levels of asymmetric solar wind forcing. [ABSTRACT FROM AUTHOR]

The magnetopause is the boundary between the interplanetary magnetic field and the terrestrial magnetic field. It is influenced by different solar-wind conditions, which lead to a change in the shape and location of the magnetopause. The interaction between the solar wind and the magnetosphere can be studied from in situ spacecraft observations. Many studies focus on the equatorial plane as this is where recent spacecraft constellations such as THEMIS or MMS operate. However, to fully capture the interaction, it is important to study the high-latitude regions as well. Since the Cluster spacecraft operate in a highly elliptical polar orbit, the spacecraft often pass through the magnetopause at high latitudes. This allows us to collect a dataset of high-latitude magnetopause crossings and to study magnetopause motion in this region, as well as deviations from established magnetopause models. We use multi-spacecraft analysis tools to investigate the direction of the magnetopause motion in the high latitudes and to compare the occurrence of crossings at different locations with the result in the equatorial plane. We find that the high-latitude magnetopause motion is generally consistent with previously reported values and seems to be more often associated with a closed magnetopause boundary. We show that, on average, the magnetopause moves faster inwards than outwards. Furthermore, the occurrence of magnetopause positions beyond those predicted by the model at high latitudes is found to be caused by the solar-wind parameters that are similar to those in the equatorial plane. Finally, we highlight the importance of the dipole tilt angle at high latitudes. Our results may be useful for the interpretation of plasma measurements from the upcoming SMILE mission as this spacecraft will also fly frequently through the high-latitude magnetopause. [ABSTRACT FROM AUTHOR]

Starting from 29 January 2022, a series of solar eruptions triggered a moderate geomagnetic storm on 3 February 2022, followed subsequently by another. Despite the typically minimal impact of unintense storms on space technology, 38 out of the 49 Starlink satellites underwent orbital decay, re‐entering Earth's atmosphere. These satellite losses were attributed to enhanced atmospheric drag conditions. This study employs numerical simulations, utilizing our test particle simulation code, to investigate the dynamics of the inner radiation belt during the two magnetic storms. Our analysis reveals an increase in proton density and fluxes during the transition from the recovery phase of the first storm to the initial phase of the second, primarily driven by intense solar wind dynamic pressure. Additionally, we assess Single Event Upset (SEU) rates, which exhibit a 50% increase in comparison to initial quiet conditions. Plain Language Summary: Starting from 29 January 2022, a series of solar explosions disrupted Earth's magnetic field, resulting in two geomagnetic storms. While such storms typically don't impact space technology significantly, 38 out of the 49 launched Starlink satellites encountered problems, ultimately falling back to Earth and burning up upon reentry due to increased atmospheric drag. In our study, we examined the inner radiation belt during these two consecutive magnetic storms. Using a simulation code, we conducted a thorough analysis of the inner proton belt's behavior in February 2022. Notably, we observed a rise in proton density within the inner belt from the end of the first magnetic storm to the beginning of the subsequent storm. Additionally, we assessed the resulting Single Event Upset rates, which increased by 50% compared to the initial quiet conditions. Key Points: We simulated the inner proton belt's activity during the two successive geomagnetic storms of February 2022The proton flux in the South Atlantic Anomaly increased between the first storm's recovery phase and the second storm's initial phaseWe assessed the resulting Single Event Upset rates, which rose by 50% compared to the initial quiet conditions [ABSTRACT FROM AUTHOR]

The ring current is an important component of the Earth's near‐space environment, as its variations are the direct driver of geomagnetic storms that can disrupt power grids, satellite communications, and navigation systems, thereby impacting a wide range of technological and human activities. Oxygen ions (O+) are one of the major components of the ring current and play a significant role in both the enhancement and depletion of the ring current during geomagnetic storms. Although a standard statistical study can provide average global distributions of ring current ions, it can't offer insight into the short‐term dynamic variations of the global distribution. Therefore, we employed the Artificial Neural Network technique to construct a global ring current O+ ion model based on the Van Allen Probes observations. Through optimization of the combination of input geomagnetic indices and their respective time history lengths, the model can well reproduce the spatiotemporal variation of the oxygen ion flux distributions and demonstrates remarkable accuracy and minimal errors. Additionally, the model effectively reconstructs the temporal variation of ring current O+ ions for non‐training set data. Furthermore, the model provides a comprehensive and dynamic representation of global ring current O+ ion distribution. It accurately captures the dynamics of O+ ions during a geomagnetic storm with the oxygen ion fluxes enhancement and decay, and reveals distinct characteristics for different energy levels, such as injection from the plasma sheet, outflow from the ionosphere, and magnetic local time asymmetry. Plain Language Summary: The ring current, a significant part of Earth's space environment, is closely linked to geomagnetic disturbances that can disrupt power grids, satellite communications, and navigation systems, affecting our daily lives. Oxygen ions (O+) are a key component of the ring current during geomagnetic active time. In our study, we use a powerful tool called Artificial Neural Networks to create a model for ring current O+ behavior. This model only requires F10.7 and geomagnetic indices as inputs. By carefully selecting the best combination of the geomagnetic indices and their time history, we built a model that can accurately mimic the behavior of oxygen ions observed by satellites. Our model demonstrates that the levels of O+ ion fluxes rise and fall at various locations during geomagnetic disturbance, which is consistent with previous observational trend. The model also helps us to understand the global distribution of O+ ions in real time. During a test case of a geomagnetic storm, the model revealed details about energy‐dependent O+ ion flux enhancement and decay. Key Points: An artificial neural network model was built to reconstruct ring current oxygen ions using geomagnetic indices as inputThe model shows high accuracy in predicting non‐training set data and successfully captures the enhancement and decay of oxygen ion fluxesThe model successfully reproduces the variation of global distribution for oxygen ions of different energies during geomagnetic storms [ABSTRACT FROM AUTHOR]

If the interplanetary magnetic field (IMF) has a northward component, a portion of the solar wind ions flowing in the high-latitude magnetosheath drastically alters the flow direction owing to lobe reconnection and follows the Earth's magnetic field lines toward the low-altitude cusp. Partly because of this folded path, the relationship between the speed of ion flow in the magnetosheath and the energy of the precipitating ions in the low-altitude cusp remains unclarified. Herein, we examined particle data from 11 years of observations by Defense Meteorological Satellite Program satellites to obtain 1990 cases through an automated event identification of the cusp. This corresponds to the largest number of cusp ion events reported to date for a stable northward IMF. The results revealed a positive correlation between the energy of the precipitating ions and the estimated flow speed in the magnetosheath, which answers the question that has remained unexplained for decades: why is the brightness of the cusp proton aurora more strongly correlated with the solar wind dynamic pressure than with the solar wind number density? We suggest additional energizations of the exhausted ions from lobe reconnection in the outflow region because of the turbulent electric field formed by the opposite flow of two ion populations—one in the tailward-directed magnetosheath flow and the other in the earthward-directed outflow jet. [ABSTRACT FROM AUTHOR]

We present the first high resolution global MHD with coupled inner magnetosphere simulation results of an observed theta aurora event. We use the Space Weather Modeling Framework in the Geospace configuration, which produces accurate field aligned current closure in the ionosphere that is integral to theta aurora formation. At the location of the observed theta aurora, the simulation produces a narrow channel of Joule heating along both open and closed field lines, and between a pair of oppositely directed field‐aligned current sheets in the ionosphere. We demonstrate that this Joule heating pattern that we identify as theta aurora maps to a reconnection region at the magnetotail flanks as well as in the distant magnetotail. The theta aurora maps to a cross‐tail current disruption and field‐aligned current source region in a highly twisted magnetotail. Plain Language Summary: The light of the aurora observed in the Earth's atmosphere is a signature of magnetic processes in outer space. The Sun produces hot magnetized plasma that blows through the solar system like a wind. The Earth's magnetic field shields us from the harmful impacts of the Sun's magnetized plasma wind. The interaction between the Earth's magnetic field and the Sun's plasma wind causes the ring of auroral light we see in the polar regions. Sometimes the ring of auroral light grows a bar of auroral light across its center (over the Earth's magnetic poles), transforming the ring into a structure like the Greek letter theta. We call this the theta aurora. It is unknown what physical processes in the Earth's magnetic field create the theta aurora. In this study we show the first realistic simulation that uses solar wind driver conditions of an observed theta aurora event, and demonstrate that we can successfully produce the theta aurora structure in the simulation. We find that the theta aurora's center bar of auroral light comes from multiple regions in the vast Earth's magnetotail. This new result helps us better understand the dynamics of the Earth's space environment and better protect ourselves against the Sun's hazardous plasma wind effects. Key Points: We show first global MHD simulation event study that successfully produces a Joule heating signature that we identify as theta auroraSimulation associates transpolar arc signatures with reconnection at the magnetospheric flank and in the distant magnetotailThe transpolar arc maps to a cross‐tail current disruption and field‐aligned current source region in a highly twisted magnetotail [ABSTRACT FROM AUTHOR]

This study presents observations of magnetopause reconnection and erosion at geosynchronous orbit, utilizing in situ satellite measurements and remote sensing ground‐based instruments. During the main phase of a geomagnetic storm, Geostationary Operational Environmental Satellites (GOES) 15 was on the dawnside of the dayside magnetopause (10.6 MLT) and observed significant magnetopause erosion, while GOES 13, observing duskside (14.6 MLT), remained within the magnetosphere. Combined observations from the THEMIS satellites and Super Dual Auroral Radar Network radars verified that magnetopause erosion was primarily caused by reconnection. While various factors may contribute to asymmetric erosion, the observations suggest that the weak reconnection rate on the duskside can play a role in the formation of asymmetric magnetopause shape. This discrepancy in reconnection rate is associated with the presence of cold dense plasma on the duskside of the magnetosphere, which limits the reconnection rate by mass loading, resulting in more efficient magnetopause erosion on the dawnside. Plain Language Summary: The boundary of Earth's magnetosphere, knows as the magnetopause, moves back and forth in response to the upstream solar wind conditions, mainly solar wind dynamic pressure and the north‐south component of the interplanetary magnetic field (IMF). It is well known that the magnetopause moves closer to the Earth during periods of southward IMF compared to northward IMF. This phenomenon occurs because reconnection between the IMF and Earth's magnetic field removes magnetic flux at the boundary layer, resulting in magnetopause erosion. Occasionally, the magnetopause approaches geosynchronous orbit when the magnetosphere is highly compressed and/or eroded, mainly during geomagnetic storms. Statistical studies have shown the magnetopause erosion often shows a dawn‐dusk asymmetry at geosynchronous orbit. This study presents concurrent observations of magnetopause erosion and related dayside reconnection at geosynchronous orbit by utilizing data from THEMIS, Geostationary Operational Environmental Satellites, and Super Dual Auroral Radar Network radar. The observations suggest that the presence of cold dense plasma in the duskside magnetosphere weakens reconnection rates and, consequently, it contributed to the dawn‐dusk asymmetry of magnetopause erosion. Key Points: We present observations of magnetopause erosion at geosynchronous orbit caused by reconnectionMagnetopause erosion showed dawn‐dusk asymmetryAsymmetric erosion can plausibly be attributed to a reduced reconnection rate at the duskside magnetopause due to cold dense plasma [ABSTRACT FROM AUTHOR]

This document is an editorial for a special issue of the journal Remote Sensing, which focuses on using satellite data and new methodologies to understand the preparatory phase of medium-large earthquakes. The issue includes 15 papers from authors in various countries, covering topics such as seismo-electromagnetic processes, lithospheric structure, atmospheric anomalies, ionospheric disturbances, and interactions between the lithosphere, atmosphere, and ionosphere. The editorial emphasizes the need for further research to explain the different patterns observed in earthquakes and the potential role of tectonic settings and water in these phenomena. Additionally, there is an acknowledgment section from a research paper published in the journal, expressing gratitude to the academic editors who helped evaluate the papers in the special issue. [Extracted from the article]

In this study, we investigate the effects caused by interplanetary (IP) shock impact angles on the subsequent ground dB/dt variations during substorms. IP shock impact angles have been revealed as a major factor controlling the subsequent geomagnetic activity, meaning that shocks with small inclinations with the Sun‐Earth line are more likely to trigger higher geomagnetic activity resulting from nearly symmetric magnetospheric compressions. Such field variations are linked to the generation of geomagnetically induced currents (GICs), which couple to artificial conductors on the ground leading to deleterious consequences. We use a sub‐set of a shock data base with 237 events observed in the solar wind at L1 upstream of the Earth, and large arrays of ground magnetometers at stations located in North America and Greenland. The spherical elementary current system methodology is applied to the geomagnetic field data, and field‐aligned‐like currents in the ionosphere are derived. Then, such currents are inverted back to the ground and dB/dt variations are computed. Geographic maps are built with these field variations as a function of shock impact angles. The main findings of this investigation are: (a) typical dB/dt variations (5–10 nT/s) are caused by shocks with moderate inclinations; (b) the more frontal the shock impact, the more intense and the more spatially defined the ionospheric current amplitudes; and (c) nearly frontal shocks trigger more intense dB/dt variations with larger equatorward latitudinal expansions. Therefore, the findings of this work provide new insights for GIC forecasting focusing on nearly frontal shock impacts on the magnetosphere. Plain Language Summary: Space weather effects caused by solar perturbations can increase the intensity of electric currents flowing in the Earth's upper atmosphere. These currents in turn induce geoelectric currents on the ground that couple with ground infrastructure, such as power transmission lines, oil/gas pipelines, and railways, which further cause deleterious consequences. Recent studies have shown that the angle with which solar perturbations impact the Earth closely controls intensifications of electric currents in the upper atmosphere and the subsequent ground magnetic field perturbations causing geoelectric currents. When these space weather drivers impact Earth more frontally, the resulting atmospheric current enhancements and the subsequent ground magnetic field perturbations are generally more intense. In this study, we use a large amount of magnetic field data recorded in North America and Greenland. We find that the most intense geomagnetic field disturbances are caused by the impacts of nearly frontal solar perturbations on the Earth and cover large geographic areas reaching southern Canada and northern United States. The results of this work can be used by space weather forecasters when predicting events with significant magnetic field variations leading to intense geoelectric currents, particularly when tracking space weather drivers that are forecasted to impact Earth nearly frontally. Key Points: Typical substorm‐time ground dB/dt variations (5–10 nT/s) are caused by interplanetary shocks with moderate inclinations (140°–160°)The more frontal the shock, the more intense and the broader the ionospheric current amplitudesThe more frontal the shock, the more intense, larger the area coverage, and the lower the equatorward latitudinal extent of dB/dt variations [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]

We examined rapid variations in the electron zebra stripe patterns, specifically at L = 1.5, over a three‐month duration, using twin Van Allen Probes within Earth's inner magnetosphere. During geomagnetically quiet intervals, these stripes exhibit a peak‐to‐valley ratio (Δj) ∼1.25 in detrended electron fluxes. However, during geomagnetic storms, they became highly prominent, with Δj > 2.5. The correlation between Δj and net field‐aligned currents (FACs) is observed to be high (0.70). Global magnetohydrodynamic (MHD) simulation results indicate that the westward electric field at midnight at low latitudes in the deep inner magnetosphere correlates well with net FACs. An increase in net FACs could amplify the dawn‐to‐dusk electric field in the deep inner magnetosphere, thereby causing the inward transport of electrons. Given that FACs are linked to the interaction between solar wind and the magnetosphere, our findings emphasize the importance of solar wind‐magnetosphere coupling in the deeper regions of the inner magnetosphere. Plain Language Summary: The intensity of hundreds of keV electron fluxes displays a distinctive pattern in the energy versus L‐value spectrogram, characterized by periodic valleys and peaks, commonly referred to as zebra stripes. These patterns have been observed in the magnetospheres of multiple planets, including Earth, Jupiter, and Saturn. Our study reveals that during geomagnetically quiet intervals, Earth's inner magnetospheric zebra stripes exhibit well‐defined banded features. However, these bands become highly pronounced during geomagnetic storms. The peak‐to‐valley ratio (Δj) of detrended electron fluxes shows a correlation with net field‐aligned currents (FACs), and these FACs, in turn, align with the westward component of the electric field at midnight. Consequently, FACs play a significant role in controlling electron flux dynamics deep within the inner magnetosphere. This research illuminates the solar wind‐magnetosphere‐ionosphere couplings. Key Points: Electron zebra stripes are a persistent feature in Earth's inner magnetosphere, although they become intensified during geomagnetic stormsPeak‐valley ratio (Δj) in detrended electron flux within the zebra stripes enhances by ≥1 factor at L = 1.5 during three geomagnetic stormsΔj is well correlated with the net field‐aligned current (FAC) in polar region, suggesting the dominant role of convection driven by FAC [ABSTRACT FROM AUTHOR]

Geomagnetic field measurements indicate that at present we may be on the brink of the Earth's magnetic field reversal, potentially resulting in all the accompanying negative consequences for the mankind. Mathematical modelling is necessary in order to find precursors for reversals and excursions of the magnetic field. With this purpose in mind, following the Podvigina scenario for the emergence of the reversals, we have studied convective flows not far (in the parameter space) from their onset and the onset of magnetic field generation, and found a flow demonstrating reversals of polarity of some harmonics comprising the magnetic field. We discuss a simulated regime featuring patterns of behaviour that apparently indicate future reversals of certain harmonics of the magnetic field. It remains to be seen whether reversal precursors similar to the observed ones exist and might be applicable for the much more complex geomagnetic dynamo. [ABSTRACT FROM AUTHOR]

Changes in the Earth's geomagnetic field induce geoelectric fields in the solid Earth. These electric fields drive Geomagnetically Induced Currents (GICs) in grounded, conducting infrastructure. These GICs can damage or degrade equipment if they are sufficiently intense—understanding and forecasting them is of critical importance. One of the key magnetospheric phenomena are Sudden Commencements (SCs). To examine the potential impact of SCs we evaluate the correlation between the measured maximum GICs and rate of change of the magnetic field (H′) in 75 power grid transformers across New Zealand between 2001 and 2020. The maximum observed H′ and GIC correlate well, with correlation coefficients (r2) around 0.7. We investigate the gradient of the relationship between H′ and GIC, finding a hot spot close to Dunedin: where a given H′ will drive the largest relative current (0.5 A nT−1 min). We observe strong intralocation variability, with the gradients varying by a factor of two or more at adjacent transformers. We find that GICs are (on average) greater if they are related to: (a) Storm Sudden Commencements (SSCs; 27% larger than Sudden Impulses, SIs); (b) SCs while New Zealand is on the dayside of the Earth (27% larger than the nightside); and (c) SCs with a predominantly East‐West magnetic field change (14% larger than North‐South equivalents). These results are attributed to the geology of New Zealand and the geometry of the power network. We extrapolate to find that transformers near Dunedin would see 2000 A or more during a theoretical extreme SC (H′ = 4000 nT min−1). Plain Language Summary: A changing magnetic field at the surface of the Earth will induce anomalous currents in conducting infrastructure, such as a power network. There are many processes that can cause the Earth's magnetic field to change, but we investigate one of the simplest: Sudden Commencements (SCs). SCs are caused by rapid increases in the density or velocity of the solar wind, and can be measured as a fast, mostly Northward change of the magnetic field on the ground. We compare the changes in the magnetic field with the currents observed at 75 locations across the New Zealand power network. We find a link between the changes in the magnetic field and the currents, but several locations appear to be more susceptible to large currents. We also find that some types of SC appear to cause larger currents and that the effect of SCs is greater on the sunlit side of the Earth. Finally, we use the relationships we have seen over the last 20 years to see what would happen if a much larger event were to occur in the future. Key Points: The maximum H′ and Geomagnetically Induced Current (GIC) observed during Sudden Commencements (SCs) correlates well (r2 ∼ 0.7) across New ZealandSCs that occur when New Zealand is on the dayside of the Earth are associated with 27% greater GICs for the same H′ on averageExtrapolation suggests that a hypothetical extreme SC (4000 nT min−1) would be related to GICs over 2000 A near Dunedin [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]

During a few solar energetic particle (SEP) events, solar protons were trapped within the geomagnetic field and reached the outer edge of the inner radiation belt. We reproduced this phenomenon by modeling the proton flux distribution at the Low‐Earth Orbit (LEO) for different geomagnetic conditions during solar particle events. We developed a three‐dimensional relativistic test particle simulation code to compute the 70–180 MeV solar proton Lorentz trajectories in low L‐shell range from 1 to 3. The Tsyganenko model (T01) generated the background static magnetic field with the IGRF (v12) model. We have selected three Dst index values: −7, −150, and −210 nT, to define quiet time, strong, and severe geomagnetic storms and to generate the corresponding inner magnetic field configurations. Our results showed that the simulated solar proton flux was more enhanced in the high‐latitude regions and more expanded toward the lower latitude range as long as the geomagnetic storm was intensified. Satellite observations and geomagnetic cutoff rigidities confirmed the numerical results. Furthermore, the LEO proton flux distribution was deformed, so the structure of the proton flux inside the South Atlantic Anomaly (SAA) became longitudinally extended as the Dst index decreased. Moreover, we have assessed the corresponding radiation environment of the LEO mission. We realized that, for a higher inclined LEO mission during an intense geomagnetic storm (Dst = −210 nT), the probability of the occurrence of the Single Event Upset (SEU) rates increased by 19% and the estimated accumulated absorbed radiation doses increased by 17% in comparison with quiet conditions. Plain Language Summary: Solar energetic particles are high‐energy particles emitted from the Sun during intense activity. When they reach the Earth, they become trapped in its magnetic field. In some cases, these trapped solar particles can penetrate deeper toward the Earth's surface when some fluctuations in the Earth's magnetic field occur. The impact of solar particles on satellite technologies and astronauts is significant and dangerous. In this work, we reproduced this phenomenon by developing a numerical code to calculate the solar proton trajectories inside the Earth's magnetic field according to several geomagnetic conditions. Our new results successfully produced the same physical process and did agree with other methodologies, such as satellite observations. Afterward, we estimated the resulting radiation environment of a Low‐Earth Orbit mission. We found that a satellite could suffer more radiation risks of around 20% if a Solar Energetic Particle event occurred during an intense geomagnetic storm. Key Points: We have numerically modeled the Low‐Earth Orbit proton flux due to precipitated solar protons for different geomagnetic storm conditionsOur test particle simulations reproduced the proton flux enhancement at high latitudes, agreeing with observations and cutoff rigiditiesDuring a stronger geomagnetic storm and solar energetic particle event, a spacecraft that enters the polar cap region would be subject to more radiation risks [ABSTRACT FROM AUTHOR]

The Earth's magnetosphere supports a variety of Magnetohydrodynamic (MHD) normal modes with Ultra Low Frequencies (ULF) including standing Alfvén waves and cavity/waveguide modes. Their amplitudes and frequencies depend in part on the properties of the magnetosphere (size of cavity, wave speed distribution). In this work, we use ∼13 years of Time History of Events and Macroscale Interactions during Substorms satellite magnetic field observations, combined with linearized MHD numerical simulations, to examine the properties of MHD normal modes in the region L > 5 and for frequencies <80 mHz. We identify persistent normal mode structure in observed dawn sector power spectra with frequency‐dependent wave power peaks like those obtained from simulation ensemble averages, where the simulations assume different radial Alfvén speed profiles and magnetopause locations. We further show with both observations and simulations how frequency‐dependent wave power peaks at L > 5 depend on both the magnetopause location and the location of peaks in the radial Alfvén speed profile. Finally, we discuss how these results might be used to better model radiation belt electron dynamics related to ULF waves. Plain Language Summary: The solar wind constantly disturbs plasma in the near‐Earth space environment on a broad range of frequencies. However, plasma waves in the Earth's magnetosphere, a region of space where the Earth's magnetic field plays a dominant role in shaping plasma dynamics, often exhibit standing wave structure with a narrow range of frequencies. In other words, the magnetosphere selects standing waves with discrete frequencies from drivers with a broadband frequency spectrum. These standing waves have properties that depend on the size of the magnetosphere and plasma wave speeds. In this study, we use a database of magnetic field measurements from the Time History of Events and Macroscale Interactions during Substorms satellites along with numerical simulations to isolate natural frequencies from noisy and variable driving conditions and extract standing wave spatial structure. We show how the standing wave properties change as the outer boundary of the magnetosphere and internal wave speeds change. We finally discuss how the properties of these standing waves might be used to improve space weather models. Key Points: Magnetohydrodynamic normal modes are identified using ∼13 years of observations and comparisons with numerical simulationsRadial Alfvén speed profile peaks outside 8 Earth radii significantly alter frequencies and spatial structure of normal modesFrequencies and nodal structure of cavity/waveguide modes vary with magnetopause location, with power peaks well inside the magnetopause [ABSTRACT FROM AUTHOR]

Universal Time (UT) variations in many magnetospheric state indicators and indices have recently been reviewed by Lockwood and Milan (2023, https://doi.org/10.3389/fspas.2023.1139295). Key effects are introduced into magnetospheric dynamics by the eccentric nature of Earth's magnetic field, features that cannot be reproduced by a geocentric field model. This paper studies the UT variation in the occurrence of substorm onsets and uses a simple Monte‐Carlo model to show how it can arise for an eccentric field model from the effect of the diurnal motions of Earth's poles on the part of the geomagnetic tail where substorms are initiated. These motions are in any reference frame that has an X axis that points from the center of the Earth to the center of the Sun and are caused by Earth's rotation. The premise behind the model is shown to be valid using a super‐posed epoch study of the conditions leading up to onset. These studies also show the surprising degree of preconditioning ahead of the growth phase that is required, on average, for onset to occur. A key factor is the extent to which pole motions caused by Earth's rotation influence the near‐Earth tail at the relevant X coordinate. Numerical simulations by a global MHD model of the magnetosphere reveal the effect required to generate the observed UT variations and with right order of amplitude, albeit too small by a factor of about one third. Reasons why this discrepancy may have arisen for the simulations used are discussed. Plain Language Summary: Earth's magnetic field is eccentric in that the main magnetic (dipole) axis does not pass through the center of the Earth. This introduces a wobble into many aspect of near‐Earth space (the "magnetosphere") as Earth rotates. Many consequences of this have been noted in previous papers. This paper investigates the effect of the eccentricity on the phenomenon of magnetospheric substorms. It is shown that the explosive releases of energy stored in the tail of the magnetosphere are more likely to start ("onset") at some Universal Times (and therefore geographic longitudes) than others and an explanation of why is provided. Key Points: Universal Time (UT) effects in the magnetosphere are caused by the eccentric nature of Earth's intrinsic magnetic fieldThere is a UT dependence of the open flux (and hence also the integrated magnetopause reconnection voltage) needed to trigger substorm onsetGrowth phases that lead to substorm onset show considerable preconditioning by prior reconnection [ABSTRACT FROM AUTHOR]

Chorus waves play a significant role in both electron precipitation and acceleration in Earth's radiation belts. Their generation includes the initial linear process and subsequent nonlinear process. After the amplitude of whistler‐mode waves reaches a sufficient value in the linear phase, the nonlinear process takes effect and results in the frequency chirping of chorus waves. Here we present the first report on the generation and intensification of rising‐tone chorus driven by two sudden increases of solar wind dynamic pressure observed by Van Allen Probes on 20 December 2015. First, whistler‐mode waves are excited by the anisotropic thermal electrons (∼10s keV), which is quite consistent with the linear theoretical expectation. Then, rising‐tone chorus waves are nonlinearly generated from the existing whistler‐mode waves, triggered by the first increase of solar wind dynamic pressure. Subsequently, the second increase of solar wind pressure further intensifies the rising‐tone chorus. Based on the nonlinear theoretical model and Ts04 magnetic field model, we demonstrate that the decreasing inhomogeneity of the background magnetic field due to the increasing solar wind dynamic pressure is the major cause to trigger rising tones, which essentially reduces the wave amplitude threshold of the nonlinear process. Our study emphasizes the significance of solar wind dynamic pressure in the Earth's radiation belts from a micro perspective. Key Points: We report the first direct observation of the rising‐tone chorus triggered by the enhanced solar wind pressure with Van Allen ProbesRising tones are triggered from the preexisting whistlers by the first increase of solar wind pressure, and then intensified by the second riseThe reduced inhomogeneity of geomagnetic fields caused by the enhanced solar wind pressure is the major factor to control the nonlinear evolution [ABSTRACT FROM AUTHOR]

Magnetic storms and substorms cause global disturbances in the Earth's magnetosphere. Plasma clouds injected from the magnetotail during storm or substorm drift around the Earth and generate ultra‐low frequency (ULF) waves via various mechanisms. At the same time, the inner part of the magnetosphere called plasmasphere is filled with cold particles and its characteristics are sensitive to the geomagnetic activity level. Previous theoretical and some observational studies suggested plasmasphere and its boundary, plasmapause, are special regions for ULF waves to interact with charged particles. We present a statistical analysis of ULF waves during different geomagnetic conditions. We utilized Arase satellite magnetic field and electron density measurements from March 2017 to December 2020 to investigate spatial distribution of ULF waves and its dependence on the plasmapause location. A 1–2 RE gap between the plasmapause and a region of high transverse waves occurrence rate was found. This gap keeps during quiet geomagnetic conditions when plasmasphere expands, and we concluded that the plasmapause controls the ULF wave distribution in the magnetosphere. ULF wave occurrence rate significantly decreases at quiet time, but dayside and dawnside maxima still occur for poloidal and compressional, and toroidal waves, respectively. Thus, we can distinguish internally and externally excited waves. Average wave frequency distribution revealed field‐line resonance character of toroidal waves as frequency increases toward the Earth. Poloidal and compressional waves distributions clearly distinguish low frequency externally excited waves and high frequency storm‐time pulsations. Plain Language Summary: Plasma environment and magnetic field inside the magnetosphere are very dynamic and strongly depend on geomagnetic activity level. Magnetic storms and substorms affect all the processes and systems, including the plasmasphere formation and ultra‐low frequency (ULF) wave generation. The plasmasphere is a dense inner part of the magnetosphere filled with cold protons and electrons, while ULF waves are responsible for energy transfer on a large distance and intensively interact with charged particles. We used 46 months statistics of Arase satellite measurements and found a clear dependence between ULF waves observations and the plasmasphere boundary location. ULF waves are mostly observed in 1–2 Earth radii from the plasmasphere boundary even when it moves outward during long period of quiet geomagnetic conditions. Although ULF waves rare occur at quiet geomagnetic conditions, they still appear at the dayside and dawnside magnetosphere. We think, these waves are excited by external sources from the solar wind or magnetosphere boundary—magnetopause. This conclusion is supported by wave frequency analysis showing a clear difference between lower frequency dayside waves and higher frequency nightside waves, which are associated with external and internal sources, respectively. Thus, we found distinct signatures of plasmasphere control of ULF wave spatial distribution. Key Points: Poloidal and toroidal waves are detected mostly outside the plasmasphere with 1–2 Earth radii from itPlasmasphere expansion and shrinkage impact the ultra‐low frequency wave distribution at the dusk and night sectors of the magnetosphereExternally excited dayside waves usually have lower frequencies than nightside waves excited by internal instabilities [ABSTRACT FROM AUTHOR]

Global ultra‐low frequency (ULF) oscillations are believed to play a significant role in the mass, energy, and momentum transport within the Earth's magnetosphere. In this letter, we observe a ∼1.2 mHz radial standing wave in the dusk‐sector magnetosphere accompanied by the field line resonance (FLR) on 16 July 2017. The frequency estimation from the simple box model also confirms the radial standing wave. The essential characteristics of FLR are concurrently identified at the dusk‐sector magnetosphere and the conjugated ground location. Further, the radial standing wave dissipates energy into upper atmosphere to enhance the local aurora by coupling itself to the FLR. The magnetospheric dominant 1.2/1.1 mHz ULF waves plausibly correspond well with the discrete ∼1 mHz magnetosheath ion dynamic pressure/velocity oscillation, suggesting this radial standing wave and FLR in the flank magnetosphere may be triggered by the solar‐wind and/or magnetosheath dynamic pressure/velocity fluctuations. Plain Language Summary: Just like thumping the strings, the Earth's magnetic field line can also be disturbed by the external impulses or internal instability, generating the rich ultra‐low frequency (ULF) oscillations with period of 1–1,000 s. They are believed to play a significant role in the mass, energy, and momentum transport within the Earth's magnetosphere. But the question of how an external driving mechanism can produce field line disturbances deep inside the magnetosphere is a topic of much debate. One classical global mode resonance model suggests that the magnetospheric space sometimes acts as a huge closed or semi‐closed cavity, where the ULF waves form the radial standing wave structure. So far, the observational evidence of radial standing waves with period of above 8 min accompanied by the field line disturbance in the flank magnetosphere is sparse. In this letter, we report a radial standing wave with period of ∼14 min within dusk‐sector magnetosphere accompanied by the field line disturbance phenomenon by multiple‐satellite observations. The radial standing wave energy sinks down to upper atmosphere to light the local aurora by coupling itself to the field line resonance. We conclude the radial standing wave may be caused by the solar‐wind and/or magnetosheath dynamic pressure/velocity fluctuations. Key Points: A radial standing Pc5‐6 ultra‐low frequency wave is identified within the dusk‐sector magnetosphereRadial standing wave dissipates energy into upper atmosphere by coupling itself to the field line resonanceSolar‐wind and/or magnetosheath dynamic pressure/velocity fluctuations possibly trigger this radial standing wave [ABSTRACT FROM AUTHOR]

We investigate the differences between the effects of geomagnetic storms due to Interplanetary Coronal Mass Ejections (ICME) and due to Stream Interaction Regions or Corotating Interaction Regions (SIR/CIR) on the ionospheric F2-layer during the maximum of solar cycle 24. We have created a unique list of the ICME- and SIR/CIR-driven geomagnetic storm events for the time interval between November 2012 and October 2014. Finally, 42 clear ICME and 34 clear SIR/CIR events were selected for this analysis. The individual geomagnetic storm periods were grouped by seasons, time of day, and local time of Dstmin and were analyzed using three different methods: linear correlation analysis using 4-h averages of foF2 parameters and the geomagnetic indices (1st), daily variation of deltafoF2 (2nd), and 3D plotting: geomagnetic indices vs. time vs. deltafoF2 (3rd). The main phase day of the ICME- and SIR/CIR-induced geomagnetic storms was our main focus. We used manually evaluated ionospheric foF2 parameters measured at the Sopron ionosonde station and the geomagnetic indices (Kp, Dst, and AE) for this analysis. We have found that in most cases, the variation of the Dst index is the best indicator of the impact caused in the F2 layer. We conclude as well that the representation of the data by the third method gives a better description of the ICME and SIR/CIR-triggered storm behavior. In addition, our investigation shows that the SIR/CIR-related perturbations can be predicted with greater accuracy with the second method. [ABSTRACT FROM AUTHOR]

The density in the D region is hard to observe on a large scale, because the density in this region is too low. In addition to solar radiation changes with a sinusoidal period of 24 hr, the energetic electron precipitation (EEP) is the major source for the ionospheric D region. In this study, we utilized observations from NOAA/MetOp satellites to derive the electron density profile from 60 to 150 km under the influence of EEP. It seems that the derived electron profile is roughly consistent with the observed one. Additionally, using above derived method of low ionospheric (D and E regions) electron profile based on the observations of NOAA/MetOp satellites, the actions of EEP on global ionosphere under different level of geomagnetic activity are analyzed. We find that EEP has a significant impact on the electron density in the altitude range from 60 to 90 km in the L ∼ 4.5–8 and MLT ∼ 9–14 regions, particularly during strong geomagnetic activity. A peak in Total Electron Content (TEC) was observed at MLT ∼ 11–12 and L ∼ 5.5–6, exhibiting an enhancement approximately 19.96 times higher than that observed during quiet periods. However, the effects of EEP decrease as altitude increases. The TEC enhancement at altitudes from 90 to 120 km during strong geomagnetic activity is like that at 60–90 km, but only about 2.08 times higher than that during quiet periods. In addition, only a weak increase is detected around midnight at altitudes from 120 to 150 km. Key Points: The electron density profile from 60 to 150 km under the influence of energetic electron precipitation (EEP) is derivedEEP has a significant impact on the electron density for the ranges from 60 to 90 km, particularly in L ∼ 4.5–8 and MLT ∼ 9‐14The effect of EEP is very weak on the electron density while the altitude is more than 120 km [ABSTRACT FROM AUTHOR]

Poleward‐moving auroral arcs (PMAAs) with recurrence periods on the order of minutes have been established as a distinct form of auroral activities. Their origin and characteristics, especially those on the dayside, are less understood due to the paucity of observations. Here, we investigate a periodic PMAA event at the postdawn sector based on optical measurements from the high‐latitude Yellow River Station and magnetic measurements from multiple ground‐based stations. The optical observations demonstrate a long duration (∼2.5 hr) of the periodic auroral arcs at not only the well‐established red‐line but also the green‐ and the blue‐lines. This multi‐band feature, together with images at far‐ultraviolet wavelengths, demonstrates a wide energy range of precipitating electrons from hundreds of eV to several keV, with a characteristic energy of ∼2 keV. The auroral luminosity variations at different wavebands show a consistent periodicity within 1.67–3 mHz, which is also shown in the ground‐based magnetic pulsations. The auroral arc and magnetic field observations display signatures of field line resonance, which are likely driven by perturbations at the magnetopause boundary layer. These results complement our knowledge of the dayside periodic PMAAs and improve our understanding of their relationship with ultra‐low frequency waves. Plain Language Summary: Aurora is one of the most important phenomena associated with magnetosphere‐ionosphere coupling. Auroral arcs, one of the most prominent auroral forms, are characterized by a narrow width in the north‐south direction but an elongated structure in the east‐west direction. The advanced ground‐based optical instrumentation has demonstrated a distinct type of auroral arcs, the poleward‐moving auroral arcs (PMAAs) with a recurrence period on the order of minutes. However, since stations at relatively‐low latitudes cannot provide optical observations at the dayside due to the sunlit interference, our understanding of periodic PMAAs on the dayside and their associated electron precipitation is very limited. Here, we utilize observations from the high‐latitude Yellow River Station to investigate a periodic PMAA event at the postdawn sector. The optical observations demonstrate the long‐duration characteristics and the multi‐wavelength feature of the periodic arcs. The variations of the auroral luminosity and the ground‐based magnetic field pulsations show consistent periodicity within the ultra‐low frequency (ULF) range, with the spatiotemporal characteristics displaying signatures of field line resonance. These observations provide a valuable opportunity for us to study the dayside periodic PMAAs and their relationship with ULF waves, indicating that the flank region could serve as an active source for periodic PMAAs. Key Points: Multi‐band auroral observations at postdawn sector indicate the successive appearance of poleward‐moving auroral arcs, lasting ∼2.5 hrAuroral intensity and geomagnetic field variations, with similar periodicity within 1.67–3 mHz, show characteristics of field line resonanceThe auroral intensity is strongly enhanced within a 20‐min interval, which is likely driven by localized perturbations in the magnetopause [ABSTRACT FROM AUTHOR]

In this paper, we statistically analyzed substorm activity at auroral latitudes for 2007–2020 and its relationship with magnetic disturbances at middle latitudes using the INTERMAGNET, SuperMAG, and IMAGE magnetometer data. The appearance and development of magnetic disturbances at auroral latitudes was monitored by the IL index (similar to the AL index, but calculated according to IMAGE data). For the 2007–2020 period, events that were observed near the meridian of the IMAGE network, in the night sector (2103 MLT), were selected. Two samples of events were used: (1) IL < –200 nT for at least 10 min, with an additional criterion for the presence or absence of positive bays at the Panagyurishte station in Bulgaria, and (2) isolated substorms observed on the IMAGE meridian according to the list of Ohtani and Gjerloev (2020). The distributions of the IL index, as well as the empirical and theoretical cumulative distribution functions, are obtained, and the of the occurrence of extreme events are also estimated. It is shown that, in general, the IL distributions are described well by exponential functions, and out of all events, events accompanied by mid-latitude positive bays were observed in ~65% of cases while their fraction increased with increasing disturbance intensity. Events with positive bays at midlatitudes of MPB and isolated substorms were better described by the Weibull distribution for extreme events. From both distributions, annual and semi-annual variations were identified: annual variations have a summer minimum and a winter maximum, and semi-annual variations have maxima near the equinoxes, which is most likely due to the Russell-McPherron effect. The semi-annual variation is also shown to be more pronounced for events with accompanying mid-latitudinal positive bays. [ABSTRACT FROM AUTHOR]

Magnetic dip plays a potentially important role in the ring‐current‐radiation‐belt‐coupling. Here, we report a proton‐driven magnetic dip event accompanied by the expected generation of electromagnetic ion cyclotron (EMIC) wave and the unexpected disruption of fast magnetosonic (MS) wave observed by the Van Allen Probe B on 03 November 2015. The effect of magnetic dip on the EMIC wave generation and MS wave propagation is analyzed. The analysis of EMIC wave instability demonstrates that EMIC waves are more easily excited in the magnetic dip. Moreover, the cold plasma density shows a highly positive correlation with the magnetic field fluctuation, and they are closely related to the disappearance of MS wave during the magnetic dip structure, which is confirmed by theoretical estimation. Our results indicate that magnetic dip may play different roles in the activities of EMIC and MS waves, which has rarely been discussed before. Plain Language Summary: Magnetic dip, a localized minimum of magnetic field intensity, is a frequently observed magnetic field structure in the terrestrial magnetosphere. The formation of magnetic dips is due to the increasement of ∼10 keV proton and electron fluxes in terms of pressure balance. In turn, the magnetic dips can change the distributions of protons and electrons and further the generation and propagation of electromagnetic waves. Previous studies have found that magnetic dips favor the generation of electromagnetic ion cyclotron waves and magnetosonic waves. However, in this study, we find magnetic dips can modulate the cold electron density and indirectly inhibit the propagation of MS waves, which is not reported yet. These findings will advance our understanding of the role of magnetic dip in the inner magnetosphere dynamics. Key Points: The expected generation of electromagnetic ion cyclotron (EMIC) waves and the unexpected disruption of magnetosonic (MS) waves are simultaneously observed in a magnetic dipThe magnetic dip plays different roles in the activities of EMIC and MS wavesThe cold plasma density correlated with the magnetic field fluctuation may account for the disruption of MS waves [ABSTRACT FROM AUTHOR]

Interplanetary (IP) shocks can trigger substorms when they interact the magnetosphere. In study of Hajra and Tsurutani (2018, https://doi.org/10.1016/0032-0633(77)90001-0), they reported a shock‐induced substorm event where the energy dissipation exceeded the energy input (ε $\varepsilon $ parameter). In this study, we examined 198 IP shock‐induced substorms from 1995 to 2021 and found 32 underpowered events where the energy dissipation exceeded the energy input calculated from IP shock sheath parameters. We also found underpowered events using other newly developed energy functions. To resolve this dilemma, we introduce the concept of dual compressions of the interplanetary magnetic field by both the IP shock and the bow shock, respectively. Based on in situ observations in the magnetosheath, we obtained a set of new parameters from the dual compressions. The stronger magnetic field resulting from the double compressions reconnects the geomagnetic field, leading to an increased input of energy into the magnetosphere. This new energy input is generally sufficient to balance the energy dissipation. Plain Language Summary: The magnetosphere is the region controlled by the geomagnetic field. The ε $\varepsilon $ parameter estimates the magnetosphere energy input by considering the Poynting flux in the solar wind. In some cases, however, the energy dissipation exceeds the energy input calculated from solar wind parameters. Considering that the solar wind slows down to form a bow shock when it interacts the Earth's magnetosphere with the solar wind kinetic energy converted into magnetic energy, it is more appropriate to consider the Poynting flux in the magnetosheath. Therefore, we suggest that the ε $\varepsilon $ parameter should be calculated in the magnetosheath. By taking the values of the magnetic field and velocity in the magnetosheath after dual compressions by IP shock and Bow shock for IP shock‐induced substorm events, we obtain larger energy input compared with the ε $\varepsilon $ parameter. The new energy input is generally sufficient to balance the energy dissipation. Key Points: The energy dissipation during some IP shock‐induced substorms exceeded the energy input by using various energy coupling functionsThe dilemma can be solved by the dual compressions of interplanetary magnetic field by IP shock and bow shock, which result in more energy inputA set of new parameters based on the dual compressions are obtained, the new energy input is large enough to provide sufficient energy input [ABSTRACT FROM AUTHOR]

Waves which couple to energetic electrons are particularly important in space weather, as they drive rapid changes in the topology and intensity of Earth's outer radiation belt during geomagnetic storms. This includes Ultra Low Frequency (ULF) waves that interact with electrons via radial diffusion which can lead to electron dropouts via outward transport and rapid electron acceleration via inward transport. In radiation belt simulations, the strength of this interaction is specified by ULF wave radial diffusion coefficients. In this paper we detail the development of new models of electric and magnetic radial diffusion coefficients derived from in‐situ observations of the azimuthal electric field and compressional magnetic field. The new models use L∗ ${L}^{\ast }$ as it accounts for adiabatic changes due to the dynamic magnetic field coupled with an optimized set of four components of solar wind and geomagnetic activity, Bz ${B}_{z}$, V $V$, Pdyn ${P}_{\mathrm{d}\mathrm{y}\mathrm{n}}$, and Sym−H $\mathrm{S}\mathrm{y}\mathrm{m}-H$, as independent variables (inputs). These independent variables are known drivers of ULF waves and offer the ability to calculate diffusion coefficients at a higher cadence then existing models based on Kp. We investigate the performance of the new models by characterizing the model residuals as a function of each independent variable and by comparing to existing radial diffusion models during a quiet geomagnetic period and through a geomagnetic storm. We find that the models developed here perform well under varying levels of activity and have a larger slope or steeper gradient as a function of L∗ ${L}^{\ast }$ as compared to existing models (higher diffusion at higher L∗ ${L}^{\ast }$ values). Plain Language Summary: The outer radiation belt is a region of space comprising highly energetic electrons. During periods of extreme space weather, the number and energy of these electrons can rapidly vary. During these periods as the electron energies and numbers become enhanced, they can pose a threat to satellite and space infrastructure. While we have an excellent understanding of the physical processes which drive radiation belt electron dynamics, we still have a limited ability to model and forecast radiation belt dynamics; this is a result of the complexity of Earth's radiation belt system. One of the key processes controlling radiation belt dynamics is Ultra Low Frequency (ULF) wave radial diffusion. In this work we detail the development a new model quantifying the strength of ULF wave radial diffusion in the outer radiation belt utilizing space base observations of the electric and magnetic fields in Earth's magnetosphere. Accurately quantifying ULF wave radial diffusion is fundamental to understanding radiation belt dynamics and any improvement or refinements in radial diffusion models can help to provide a better understanding of the complex radiation belt system and importantly improve hindcasts, nowcasts, and forecasts. Key Points: A new four‐component L*‐dependent radial diffusion modelThe new model uses L* accounting for adiabatic changes under varying levels of solar and geomagnetic activityA detailed parametric study identifies an optimized set of independent variables for the new radial diffusion models [ABSTRACT FROM AUTHOR]

The IAGA‐endorsed Polar Cap Indices for the northern and southern hemispheres, PCN and PCS, are compared for 1998–2018, inclusive. Potential effects of the slightly different, and changing, magnetic coordinates of the two magnetic stations employed, Thule (Qaanaaq) in Greenland and Vostok in Antarctica, are investigated. It is shown that the agreement in overall behavior of the two indices is very close indeed but that PCS consistently correlates slightly better with solar wind parameters than PCN. Optimum lags for these correlations are 19 min for 1‐min data and 37 min for hourly averages. The correlations are significantly highest for the predicted magnetopause reconnection voltage, which is a linear predictor of PCN and PCS for all 1‐hr data and for all but the largest 0.1% of 1‐min values. The indices show lower correlation and marked non‐linearity (tending to saturation) at all levels with the estimated magnetopause reconnection electric field or the estimated power input into the magnetosphere. The PCN index is shown to correlate closely with the transpolar voltage measured by the northern‐hemisphere SuperDARN radar network and both PCN and PCS clearly show the Russell‐McPherron effect of dipole tilt and the Y‐component of the interplanetary magnetic field. However the patterns in time‐of‐year and Universal Time (UT) are complicated by lobe reconnection during northward‐IMF, the effect of which on the indices is shown to be predominantly a summer hemisphere phenomenon and gives a UT dependence on the IMF Y‐component that is predicted theoretically. Plain Language Summary: The Polar Cap Indices are generated from geomagnetic recordings made at Thule (Qaanaaq) in Greenland and Vostok in Antarctica and are used as monitors of the coupling of solar wind energy and momentum into Earth's magnetosphere. These stations are at similar locations relative to the nearby magnetic poles of the Earth but there are small differences, the effect of which is investigated. There has been debate about the processing of the data to generate the indices and the extent to which the results from the two hemisphere do ‐ and should ‐ agree with each other. It is shown that the overall agreement of the northern and southern hemisphere indices is very good indeed. It has been proposed that these indices reflect a number of aspects of coupling of the solar wind and Earth's magnetosphere, but it is shown here that they are optimum indicators of the voltage generated by magnetic reconnection between the geomagnetic field and the interplanetary magnetic field. This is shown to be consistent with the variations of the indices with time‐of‐year and time‐of‐day. Key Points: Simultaneous 1‐min values can differ, but the distributions of the north and south polar cap indices over 1998–2018 are very similarBoth indices give significantly higher correlations with the predicted voltage of open flux generation and with observed transpolar voltageBoth indices show a Russell‐McPherron effect plus a northward‐IMF lobe reconnection effect that is predominantly in the summer hemisphere [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]

Dipolarization fronts (DFs), ion‐scale magnetic transients characterized by dramatic enhancement of northward magnetic field, have been documented as crucial energy transfer regions in the magnetosphere. DF‐driven energy transfer has hitherto been studied mainly in the laminar regime. Energy transfer driven by turbulent processes, however, remains unclear. Here we perform a comprehensive investigation of turbulent energy transfer (TET) developed at DFs, via using high‐cadence data from Magnetospheric Multiscale mission. We find that: (a) TET is equally governed by energy loads and generators, different from laminar energy transfer which is typically dominated by energy loads; (b) ion and electron currents play comparable roles in driving TET; (c) TET is positively correlated with local magnetic field strength and ion speed; (d) TET shows asymmetric global distributions along the dawn‐dusk direction. These features implicate that TET is primarily related to electromagnetic turbulence at electron‐ion hybrid scales. These new results, uncovering unique characteristics of DF‐driven TET, can deeply advance our understanding of energy budgets in the magnetosphere. Plain Language Summary: The sun constantly emits outward‐propagating plasma flows called solar wind. When arriving close to Earth, the solar wind interacts with Earth's internal, dipolar magnetic field, leading to formation of long, stretched magnetic field lines in the Earth's nightside, dubbed as magnetotail. The magnetotail hosts a variety of plasma structures which contribute to energy transport therein. Dipolarization fronts (DFs), characterized by sharp enhancement of northward component of magnetic field at ion scale, have been suggested as the leading boundaries of plasma jets (bursty bulk flows) in the magnetotail. They usually host the interaction between the jets and the ambient plasma. Hence, DFs have been suggested as the crucial regions in which intense energy transfer occurs. So far, it has been well documented that the DF‐driven energy transfer is closely related to large‐scale electric fields and ion currents, which are essentially laminar (i.e., direct‐current). Contributions from turbulent (i.e., alternating ‐current) fields and currents remain not well understood. In this study, we use high‐cadence data from NASA's Magnetospheric Multiscale mission to perform a comprehensive investigation of turbulent energy transfer (TET) developed at DFs, uncovering some important features. These results help better understand energy transport in the terrestrial magnetotail. Key Points: Turbulent energy transfer (TET) is equally governed by energy loads and generators, with comparable contributions from electron and ion currentsTET is positively correlated with local magnetic field strength and ion speedTET may be related to electromagnetic turbulence at electron‐ion hybrid scales [ABSTRACT FROM AUTHOR]

Solar wind and magnetospheric processes play important roles in relativistic electron acceleration in the Earth's outer radiation belt. However, how geomagnetic activity affects the acceleration region of relativistic electrons is still not clear. Here we first report the local acceleration regions during the strong storm of 1 June 2013. The first local acceleration region occurs at the lower L shell (L* ∼ 4.0) during the early recovery phase, which is associated with non‐continuous substorm activities. The second local acceleration region occurs at the higher L shell (L* ∼ 5.0) during the late recovery phase, which is associated with continuous substorm activities. To provide a more comprehensive picture, we additionally analyze two storm events with different geomagnetic activities. These events suggest that (a) the strong storm makes it possible for the local acceleration regions to occur at different L shells; (b) the timing, intensity, and duration of substorms during the recovery phase are crucial to the location and dynamics of the local acceleration region. These results can help us further understand the dynamics of relativistic electrons in the Earth's outer radiation belt. Plain Language Summary: Previous studies have shown that the local acceleration by whistler‐mode chorus waves is the dominant process for the seed populations (100s keV) up to MeV energies in the Earth's outer radiation belt. Where and when the local acceleration occurs are very important for the dynamics of relativistic electrons in the outer radiation belt. In this study, the region associated with the local acceleration process of relativistic electrons is referred to as the acceleration region. Using the data from the Van Allen Probes, two different acceleration regions during the strong storm of 1 June 2013 are found. To provide a more comprehensive picture, we additionally analyze three storm events with different geomagnetic activities. These storm events suggest that geomagnetic activities are very important for the dynamics of the acceleration region in the outer radiation belt. This can help us further understand the dynamics of relativistic electrons in the Earth's outer radiation belt. Key Points: The two local acceleration regions of relativistic electrons are observed at L* ∼ 4.0 and 5.0 during the 1 June 2013 stormThe local acceleration regions can occur at different L shells during strong geomagnetic stormsSubstorms during the recovery phase are important for the location and dynamics of the acceleration region [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]

Terrestrial ring current dynamics are a critical part of the near‐space environment, in that they directly drive geomagnetic field variations that control particle drifts, and define geomagnetic storms. The present study aims to specify a global and time‐varying distribution of ring current proton using geomagnetic indices and solar wind parameters with their history as input. We train an artificial neural network (ANN) model to reproduce proton fluxes measured by the Radiation Belt Storm Probes Ion Composition Experiment instrument onboard Van Allen Probes. By choosing optimal feature parameters and their history length, the model results show a high correlation and a small error between model specifications and satellite measurements. The modeled results well capture energy‐dependent proton dynamics in association with geomagnetic storms, including inward radial diffusion, acceleration and decay. Our ANN model produces proton fluxes with their corresponding 3D spatiotemporal variations, capturing the latitudinal distribution and local time asymmetry that are consistent with observations and that can further inform theory. Plain Language Summary: The Earth's ring current is an electric current encircling the Earth at a geocentric distance of three to eight Earth radii near the magnetic equatorial plane. The enhancement of ring current causes global magnetic field variations, including decreases at the Earth's surface known as geomagnetic storms. The present study establishes a global, time‐varying model of ring current proton flux using machine learning. This model was trained using geomagnetic indices and their history as input, and proton fluxes as output. This machine learned model shows a good performance on the out‐of‐sample test set, yielding a high correlation and a low error between model results and satellite measurements. The modeled results show that the variation of proton distribution is in association with geomagnetic storms, including their inward radial diffusion, enhancement and decay. This model produces a global distribution of proton fluxes with spatiotemporal variations, capturing the latitudinal distribution and local time variations that are consistent with observations and useful for informing theory. Key Points: An artificial neural network model (RCPANN) was established to reconstruct ring current proton distributions using geomagnetic indicesThe RCPANN model successfully captures the energy‐dependent proton flux enhancement and decayThe RCPANN model captures the latitudinal variation and local time asymmetry of ring current proton distribution [ABSTRACT FROM AUTHOR]

A study was made of simultaneous observations of ultra-low-frequency oscillations in the 0.1–5.0 Hz frequency range of the serpentine emissions type (Serpentine Emissions, SE) observed at the polar cap region and disturbances in the auroral zone. The unique analog magnetic records of the Vostok Antarctic Observatory (corrected geomagnetic coordinates Φ' = –85.41°, Λ' = 69.01°) have been digitized with a high resolution (20 Hz) and are freely available on the website of the World Data Center for Solar-Terrestrial Physics, Moscow. For 1966 (November and December), 1968 (March–July), and 1970–1972, 1973 (January–March) the behavior of "serpentine emissions" was analyzed during the development of 180 isolated substorms identified by variation in AL index. An interruption in the mode of generation of serpentine emissions in the region of the polar cap was found during the active phase of intense isolated substorms (with a maximum magnitude of the AE index of ∼500–600 nT). During the expansion phase of the substorms, broadband noise electromagnetic emission appears with a sharp leading edge in the Pc1–2 range and is also recorded in the polar cap. Noise emission has a sharp leading edge and occurs approximately 2 hours after reorientation of the Bz-components of the interplanetary magnetic field (IMF) from north to south. The time of the interruption of SE coincides with the beginning this emission and moment of achievement Bz-component of maximum negative IMF values. Interruption of the SE effect is observed against the background of relatively stable other geoeffective parameters of the solar wind and IMF. The average duration of the interruption of emissions is ∼3 hours. Indirect confirmation of the impact of substorm activity on the SE generation regime is found in the coincidence of the patterns of daily and seasonal variation of SE interruption intervals and the probability of observing substorms. Due to the fact that noise emission occurs during the active phase of isolated substorms and ∼2 hours after reorientation of the Bz-components of the IMF in the solar wind, there is reason to believe that it is associated with plasma flows directed towards the Earth from the magnetotail. Apparently, the energy of plasma flows during the active phase of a substorm stimulates the appearance of noise emission, thus interrupting SE. [ABSTRACT FROM AUTHOR]

Four supersubstorms of the solar cycle 24 are analyzed to investigate the global variations in the H‐component and geomagnetically induced currents (GICs). The response to the storm sudden commencement (SSC) of the 2012 and 2017 events is observed differently over different latitude bands. Initially, the latitude band of 0°–45° shows a step‐like preliminary positive impulse (PPI), 45°–65° shows a Gaussian‐kind of PPI and 65°–90° shows a preliminary reverse impulse, followed by a main impulse all over. The H‐component variations during the supersubstorm periods show a strong north‐south asymmetry over the high latitudes which is attributed to the seasonal dependence of the growth and decay of ionospheric currents respectively in the summer and winter hemispheres. The co‐latitude band of ∼55°–65° shows a complete reversal of phase (i.e., global positive peak) of the H‐component compared to the maximum depression observed from the close‐by latitude bands and peak depressions in the SYM‐H, SML, and AE indices. The low latitude variations exhibit a dominant local time‐dependent control of the perturbation electric fields over the short and long terms during the geomagnetic events. The D‐component variations reflect complex ionospheric contributions during the SSC and the main phase with more asymmetries and variability. The GIC threat represented by the dB/dt peaks during the supersubstorms shows the highest magnitude (∼900 nT/min) in the latitude band of 60°–75° with a secondary peak over the dip equatorial regions. Further, some scattered and prominent peaks over the mid and high latitudes outside the supersubstorm periods are also observed. Plain Language Summary: Supersubstorms are the extreme class of geomagnetic substorms associated with an intense short‐duration solar energy input in the magnetosphere‐ionosphere system. Such supersubstorms are known to cause severe threats to the technical and electrical systems in polar region and near‐Earth space. Recently completed solar cycle exhibited only four 4 supersubstorms during May 2011, March 2012 and September 2017 which have been also associated with geomagnetic storm. This study utilizes geomagnetic signatures observed from magnetometer stations spread over the globe. Some interesting patterns are found which include a latitude‐specific response during the first shock and later during the main phase. A reversal in polarity of the geomagnetic perturbation during the supersubstorms is highly striking observation, which is commonly observed from a mid‐to‐high latitude band during these events. Further, each latitude band is found to show a dependency on the local time. An effort is made to classify the contribution from disturbed ionospheric current from a sum of the total disturbance current. This study also provides intriguing results on the geomagnetically induced currentss during these extreme events, which is necessary with respect to understand the cause‐effect relationship of the space weather disturbances and the associated hazards that the earth and the human civilization may face. Key Points: Three distinct latitude‐specific signatures are established in response to the sudden impulseIntriguing mid‐to‐high latitude positive reversals during minimum depressions in the SML index and H‐component variations elsewhereLarge amplification of dB/dt depicting unusually large geomagnetically induced currents during and outside the supersubstorm occurrences [ABSTRACT FROM AUTHOR]

Electron heating at collisionless shocks in space is a combination of adiabatic heating due to large‐scale electric and magnetic fields and non‐adiabatic scattering by high‐frequency fluctuations. The scales at which heating happens hints to what physical processes are taking place. In this letter, we study electron heating scales with data from the Magnetospheric Multiscale (MMS) spacecraft at Earth's quasi‐perpendicular bow shock. We utilize the tight tetrahedron formation and high‐resolution plasma measurements of MMS to directly measure the electron temperature gradient. From this, we reconstruct the electron temperature profile inside the shock ramp and find that the electron temperature increase takes place on ion or sub‐ion scales. Further, we use Liouville mapping to investigate the electron distributions through the ramp to estimate the deHoffmann‐Teller potential and electric field. We find that electron heating is highly non‐adiabatic at the high‐Mach number shocks studied here. Plain Language Summary: Shock waves appear whenever a supersonic medium, such as a plasma, encounters an obstacle. The plasma, which consists of charged ions and free electrons, is heated by the shock wave through interactions with the electromagnetic fields. In this work, we investigate how electrons are heated at plasma shocks. A key parameter to electron heating is the thickness of the layer where the heating takes place. Here, we use observations from the four Magnetospheric Multiscale spacecraft that regularly cross the standing bow shock that forms when the supersonic plasma, known as the solar wind, encounters Earth's magnetic field. We find that the thickness of the shock is larger than previously reported and is on the scales where ion physics dominate. We also find that the electron heating deviates significantly from simple adiabatic heating. Key Points: Using multipoint data from Magnetospheric Multiscale, we find that electron heating takes place on ion scales in the quasi‐perpendicular shock rampWe show that the time series of the temperature does not represent the spatial profile due to varying shock ramp speedElectron distributions in the ramp and downstream of the shock show that electrons are heated non‐adiabatically [ABSTRACT FROM AUTHOR]

We study Universal Time (UT) variations in the magnetospheric response to Coronal Mass Ejection (CME) impacts, using the example of the two CMEs that led to the destruction of 38 out of 49 Starlink satellites in early February 2022. We employ the Expanding‐Contracting Polar Cap model to analyze the variation in the size of the ionospheric polar caps and an eccentric dipole model of the geomagnetic field and thereby quantify the UT variations caused by the inductive effect of the diurnal motions of the geomagnetic poles in a "geocentric‐solar" frame of reference (i.e., a frame with an X axis that points from the center of the Earth to the center of the Sun). The results show that use of a quasi‐steady convection model predicts a similar global power deposition into the thermosphere as that inferred here, but does not give the same division of that power between the northern and southern hemispheres. We demonstrate that, through the combined effects of the Russell‐McPherron dipole‐tilt mechanism on solar‐wind magnetosphere coupling and of the diurnal polar cap motions in a geocentric‐solar frame, the power deposited varies significantly with the arrival UT of the CMEs at Earth. We also show that in the events of early February 2022, both CMEs arrived at almost the optimum UT to cause maximum thermospheric heating. Plain Language Summary: We use a recent well‐publicized space‐weather event as an example of a previously‐overlooked aspect of the behavior of near‐Earth space. The event took place in early February 2022, when 38 out of 49 Starlink satellites burned up in Earth's atmosphere because two Coronal Mass Ejections (CMEs) emitted from the Sun hit the Earth and had a larger heating effect on the upper atmosphere than expected. The new element that we introduce is the effect of the eccentricity of Earth's magnetic field which is reflected in the offset of the magnetic pole from the geographic pole being considerably greater in the southern hemisphere than in the northern hemisphere. This introduces a daily variation into the response of Earth's magnetosphere to a given solar wind disturbance and we show that the effect would have been less severe during the February 2022 event had the CMEs arrived either earlier or later than they did. Key Points: Analysis of the CME effects causing the loss of 38 Starlink satellites shows that the terrestrial response to a CME depends on its impact UTUT effects are caused by diurnal motions of the poles and the eccentric nature of the geomagnetic fieldJoule heating dominated in the southern polar cap during the first CME and initially during the second but later was dominant in the north [ABSTRACT FROM AUTHOR]