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Flux Transfer Events (FTEs) are transient magnetic flux ropes typically found at the Earth's magnetopause on the dayside. While it is known that FTEs are generated by magnetic reconnection, it remains unclear how the details of magnetic reconnection controls their properties. A recent study showed that the helicity sign of FTEs positively correlates with the east‐west (By) component of the Interplanetary Magnetic Field (IMF). With data from the Cluster and Magnetospheric Multiscale missions, we performed a statistical study of 166 quasi force‐free FTEs. We focus on their helicity sign and possible association with upstream solar wind conditions and local magnetic reconnection properties. Using both in situ data and magnetic shear modeling, we find that FTEs whose helicity sign corresponds to the IMF By are associated with moderate magnetic shears while those that do not correspond to the IMF By are associated with higher magnetic shears. While uncertainty in IMF propagation to the magnetopause may lead to randomness in the determination of the flux rope core field and helicity, we rather propose that for small IMF By, which corresponds to high shear and low guide field, the Hall pattern of magnetic reconnection determines the FTE core field and helicity sign. In that context we explain how the temporal sequence of multiple X‐line formation and the reconnection rate are important in determining the flux rope helicity sign. This work highlights a fundamental connection between kinetic processes at work in magnetic reconnection and the macroscale structure of FTEs. Plain Language Summary: In the vicinity of the Earth's magnetosphere outer boundary, the magnetopause, twisted magnetic field structures known as "Flux Transfer Events" (FTEs) are often detected by spacecraft in‐situ. They temporarily connect the solar wind to the Earth's ionosphere, allowing the transfer of solar wind flux into the magnetosphere. It is known that FTEs are produced as a consequence of magnetic reconnection, a process that rearranges the topology of sheared magnetic fields, between the shocked solar wind and the geomagnetic field. However, our understanding of how the microphysics of magnetic reconnection can lead to the macroscopic structures of FTEs is still limited. We revisit the in‐situ observations of FTEs made by the Cluster and Magnetospheric Multiscale missions. We focus on the twist feature of FTEs as characterized by their helicity and investigate its relationship to solar wind conditions and possible link to magnetic reconnection properties. By investigating local magnetic shear conditions around FTE locations, we found that the FTE helicity is determined by a kinetic feature of magnetic reconnection known as the "Hall magnetic field". Our study highlights a close connection between a kinetic process of magnetic reconnection and the global structure of FTEs, constituting a cross‐scale coupling effect in solar‐terrestrial interaction. Key Points: We study the helicity sign of Flux Transfer Events and investigate upstream solar wind conditions and local magnetic shear around themThe helicity sign is found to be unassociated to the Interplanetary Magnetic Field (By) component when the local magnetic shear is highThe FTEs' helicity sign in such cases may relate to the Hall field of magnetic reconnection in the absence of a guide field [ABSTRACT FROM AUTHOR]

Embedded Region 1 and 2 field‐aligned currents (FACs), intense FAC layers of mesoscale latitudinal width near the interface between large‐scale Region 1 and Region 2 FACs, are related to dramatic phenomena in the ionosphere such as discrete arcs, inverted‐V precipitation, and dawnside auroral polarization streams. These relationships suggest that the embedded FACs are potentially important for understanding ionospheric heating and magnetosphere‐ionosphere (M‐I) coupling and instabilities. Previous case studies of embedded FACs have led to the speculation that they may result from enhanced M‐I convection during active times. To explore this idea further, we investigate statistically their occurrence rates under a variety of geomagnetic conditions with a large event list constructed from 17 years of Defense Meteorological Satellite Program observations. The identification procedure is fully automated and explicit. The statistical results indicate that embedded Region 1 and 2 FACs are common, and that they have a higher chance to occur when the level of geomagnetic activity is higher (given by various indices), supporting the idea that they result from enhanced M‐I convection. Plain Language Summary: Earth's magnetosphere is coupled to its ionosphere by electric currents of different scales flowing along magnetic fields. Sometimes a special type of medium‐scale currents may significantly impact the ionosphere. From this statistical study, we found that these medium‐scale currents appear frequently and more often when the magnetosphere and ionosphere are characterized by a higher level of activity. These findings provide important information on how these medium‐scale systems are generated. Key Points: We designed an automated, explicit selection algorithm to build a large database of embedded Region 1 and 2 currentsEmbedded Region 1 and 2 (ER1 and ER2) currents appear frequently and preferentially under geomagnetically active conditionsThe latitudinal widths of ER1 and ER2 currents increase with geomagnetic activity but not as much as those of R1 and R2 currents do [ABSTRACT FROM AUTHOR]

Auroral brightening is one of the most common phenomena that occur during substorm onset and is usually recognized as a projection of the substorm‐associated magnetospheric plasma dynamics to the ionosphere. However, electromagnetic fields and plasma features associated with the substorm brightening arc have not been well understood. In this study, we present a comprehensive observation of the source plasma and field variations of a substorm brightening aurora in the inner magnetosphere. We performed a unique conjugate observation of a substorm brightening auroral arc observed by a ground‐based camera and by the Arase satellite in the magnetospheric source region at L ∼ 6. The event was observed at Tromsø (69.6°N, 19.2°E), Norway, on 12 October 2017. The brightening arc indicates east‐west structures with longitudinal scales of ∼0.5°–2.0°. Field‐aligned bi‐directional electrons with an energy range between 66 and 1,800 eV were detected by the satellite, simultaneously with the appearance of the brightening arc in the camera. These electrons were probably supplied from the auroral brightening region in the ionosphere, indicating that the satellite was on the same field line of the brightening aurora. The magnetic and electric field data show characteristic fluctuations and earthward Poynting flux around the time that the satellite crossed the aurora. Anti‐phase oscillations between the thermal pressure and the magnetic pressure are also reported. Based on these observations, we suggest the possibility that a ballooning instability occurred in the source region of the substorm brightening arc in the inner magnetosphere at L ∼ 6. Plain Language Summary: A frequently occurring source of variations in the magnetosphere is the substorm, a process that causes energy dissipation into the atmosphere. Substorm is presented as the development of aurorae at high latitudes in the ionosphere. The study of substorm processes helps in understanding the near‐Earth space environment and the space weather. Along Earth's magnetic field lines, the aurora at a latitude of ∼65°N can be traced to ∼4–7 Earth radii away from the Earth at the equatorial plane in space. Using a ground‐based auroral camera, we can construct the correspondence between auroral motion and field and plasma variation at the satellite. This study reports such a unique event of substorm brightening arc observed at Tromsø, Norway, on 12 October 2017. Satellite observed bi‐directional electrons prove the connection between aurora break‐up at ∼100 km altitude and its source region in the magnetosphere at ∼30,000 km away from Earth. Based on the magnetic wave spectrograms, auroral bead‐like structures and other observational results, we suggest the possibility that a ballooning plasma instability occurred in the source region of the substorm brightening arc in the inner magnetosphere. Key Points: Observation of plasma and field features in the source region of a sudden brightening auroral arc during a minor substorm onset at L ∼ 6Energization of particles, field‐aligned electrons, and electromagnetic field fluctuations were observed during the arc crossing by AraseSeveral observational facts indicate the possibility of ballooning instability occurring at this substorm onset [ABSTRACT FROM AUTHOR]

Nearly all studies of impulsive geomagnetic disturbances (GMDs, also known as magnetic perturbation events MPEs) that can produce dangerous geomagnetically induced currents (GICs) have used data from the northern hemisphere. In this study, we investigated GMD occurrences during the first 6 months of 2016 at four magnetically conjugate high latitude station pairs using data from the Greenland West Coast magnetometer chain and from Antarctic stations in the conjugate AAL-PIP magnetometer chain. Events for statistical analysis and four case studies were selected from Greenland/AAL-PIP data by detecting the presence of >6 nT/s derivatives of any component of the magnetic field at any of the station pairs. For case studies, these chains were supplemented by data from the BAS-LPM chain in Antarctica as well as Pangnirtung and South Pole in order to extend longitudinal coverage to the west. Amplitude comparisons between hemispheres showed (a) a seasonal dependence (larger in the winter hemisphere), and (b) a dependence on the sign of the By component of the interplanetary magnetic field (IMF): GMDs were larger in the north (south) when IMF By was >0 (<0). A majority of events occurred nearly simultaneously (to within ±3 min) independent of the sign of By as long as |By| ≤ 2 |Bz|. As has been found in earlier studies, IMF Bz was <0 prior to most events. When IMF data from Geotail, Themis B, and/or Themis C in the near-Earth solar wind were used to supplement the time-shifted OMNI IMF data, the consistency of these IMF orientations was improved. [ABSTRACT FROM AUTHOR]

The intensity of the equatorial electrojet (EEJ) shows temporal and spatial variability that is not yet fully understood nor accurately modeled. Atmospheric solar tides are among the main drivers of this variability but determining different tidal components and their respective time series is challenging. It requires good temporal and spatial coverage with observations, which, previously could only be achieved by accumulating data over many years. Here, we propose a new technique for modeling the EEJ based on principal component analysis (PCA) of a hybrid ground‐satellite geomagnetic data set. The proposed PCA‐based model (PCEEJ) represents the observed EEJ better than the climatological EEJM‐2 model, especially when there is good local time separation among the satellites involved. The amplitudes of various solar tidal modes are determined from PCEEJ based tidal equation fitting. This allows to evaluate interannual and intraannual changes of solar tidal signatures in the EEJ. On average, the obtained time series of migrating and nonmigrating tides agree with the average climatology available from earlier work. A comparison of tidal signatures in the EEJ with tides derived from neutral atmosphere temperature observations show a remarkable correlation for nonmigrating tides such as DE3, DE2, DE4, and SW4. The results indicate that it is possible to obtain a meaningful EEJ spectrum related to solar tides for a relatively short time interval of 70 days. Key Points: A novel technique to model the equatorial electrojet (EEJ) based on the principal component analysis of a hybrid ground‐satellite data setThe new modeling matches observations better than the EEJM‐2 model, especially when the Swarm satellites have optimum local time coverageTime series of migrating and nonmigrating tides amplitude in the EEJ are derived from 70‐day window [ABSTRACT FROM AUTHOR]

The banded structure of electromagnetic ion cyclotron (EMIC) wave spectra and their resonant interactions with radiation belt electrons depend on the cold ion composition. However, there is a great deal of uncertainty in the composition in the inner magnetosphere due to difficulties in direct flux measurements. Here, we show that the hydrogen and helium band wave spectra are most consistent with a helium and oxygen composition of a few percent. Less than 10% of hydrogen band wave intensity is consistent with a high helium fraction of ∼20%. Similarly, only ∼20% helium band wave intensity is consistent with an oxygen torus ion composition. Furthermore, we find that the decay of the ultra‐relativistic electrons in the radiation belts by EMIC waves depends on the ion composition. The decay is most sensitive to the helium fraction, and the strongest agreement with Van Allen Probes data is found when the helium fraction is a few percent. We suggest that more observations of the cold ion composition would significantly help understand and set constraints on the decay of ultrarelativistic electrons in the radiation belts. Plain Language Summary: The radiation belts are rings of charged particles around the Earth that are largely confined by the Earth's magnetic field. Electromagnetic waves can interact with these particles, leading to acceleration, transport, and loss. In the outer belt, electrons can be rapidly accelerated during geomagnetic storms leading to a large population of ultrarelativistic electrons. Electromagnetic ion cyclotron waves are particularly important for the decay of this ultrarelativistic population, as they are effective at scattering these electrons into the atmosphere, removing them from the belts. In this work, we show that the cold ion composition is important in determining the rate of decay of these electrons and find that the strongest agreement between models and observations when the He+ fraction is only a few percent. Key Points: Electromagnetic ion cyclotron wave spectra are most consistent with low helium and oxygen fractionsRadiation belt simulations are most sensitive to the cold ion helium fractionStronger agreement with observations is found using ion compositions with low helium fractions [ABSTRACT FROM AUTHOR]

Energetic Electron Precipitation (EEP) from the plasma sheet and the radiation belts ionizes the polar lower thermosphere and mesosphere. EEP increases the production of NOx and HOx, which will catalytically destroy ozone, an important element of atmospheric dynamics. Therefore, measurement of the latitudinal extent of the precipitation boundaries is important in quantifying the atmospheric effects of the Sun‐Earth interaction. This study uses measurements by the Medium Energy Proton Electron Detector (MEPED) of six NOAA/POES and EUMETSAT/METOP satellites from 2004 to 2014 to determine the latitudinal boundaries of EEP and their variability with geomagnetic activity and solar wind drivers. Variation of the boundaries for different electron energies and Magnetic Local Time (MLT) is studied. Regression analyses are applied to determine the best predictor variable based on solar wind parameters and geomagnetic indices. The highest correlation was found for the pressure‐corrected Dst index when applying a linear regression model. A model of the equatorward EEP boundary is developed separately for three different energy channels, >43, >114, and >292 keV, and for 3 hour MLT sectors. For >43 keV EEP, 80% of the equatorward boundaries predicted by the model are within ±2.2° cgmlat. The model exhibits a solar cycle bias where it systematically exaggerates the equatorward movement of the EEP region during solar minimum. The highest accuracy of the model is found in periods dominated by corotating interaction regions/high speed solar wind streams. The result will be a key element for constructing a model of EEP variability to be applied in atmosphere climate models. Plain Language Summary: Charged particles trapped in the Earth's magnetic field get accelerated to high energies through various magnetospheric processes. They can eventually precipitate into the Earth's atmosphere in a process known as Energetic Electron Precipitation or EEP. EEP deposits energy in the mesosphere and lower thermosphere which increase the production of ozone‐depleting substances. Vertical transport of these, in particular during winter, can lead to indirect destruction of stratospheric ozone, a crucial element of atmospheric dynamics. Therefore, measurement of the latitudinal extent of EEP is important in quantifying the atmospheric effects of the Sun‐Earth interaction. We use measurements from six NOAA/POES and EUMETSAT/METOP satellites from 2004 to 2014 to determine the equatorward latitudinal boundaries of EEP. We investigate how they correlate with solar wind parameters and geomagnetic indices to identify the best predictor for EEP boundaries. The result will be a key element for constructing a model of EEP variability to be applied in atmosphere climate models. Key Points: A model predicting equatorward extent of >43 keV electron precipitation is developed based on pressure‐corrected DstThe model has an error estimate of ±2.2° cgmlat over a full solar cycle (2004–2014)The model has the highest accuracy during periods dominated by high‐speed solar wind streams [ABSTRACT FROM AUTHOR]

Geomagnetic activity in Earth's outer magnetosphere stimulates intense and sometimes explosive flows of electromagnetic power propagating earthward as Alfvén waves. Observations show that among its various magnetospheric sources, Alfvénic Poynting flux from the magnetotail reaches the ionosphere with the greatest intensity. Dayside fluxes are statistically weaker though more persistent. Flankside fluxes are weakest. We show using global magnetohydrodynamic simulations that these distributions can be attributed to the formation (or not) of meridionally narrow, relatively uniform, low Alfvén conductance channels that efficiently transmit Alfvénic power to the near‐Earth space. These Alfvén ducts form naturally at the heads of flow bursts in the magnetotail but not in flankside flux tubes, where the transmission of Alfvénic power is strongly attenuated by reflections at large conductance gradients along the propagation path. The results elucidate the underlying physics that control efficient transmission of electromagnetic power from the magnetotail to low altitude to power Alfvénic aurora. Plain Language Summary: Geomagnetic activities usually produce Alfvén waves in Earth's outer magnetosphere. In simplest form, an Alfvén wave behaves as a wave on a transmission line with magnetic field lines serving as the lines. Injections of Alfvénic power into Earth's ionosphere cause spectacular dynamic aurora. In this study, we present the discovery of a new phenomenon revealed in global geospace simulations—formation of Alfvén wave ducts. Their occurrence explains the efficient transmission of electromagnetic power from Earth's magnetotail to its ionosphere and atmosphere to produce dynamic aurora as observed in the past. More importantly, this study serves as a new foundation of powering terrestrial (and outer planetary) aurora with global magnetic field, from the view of large‐scale electromagnetic wave energy transmission. Key Points: We discovered a new phenomenon revealed in global geospace simulations—formation of Alfvén wave ductsThe suggested Alfvén ducts promote low‐loss transmission of equatorial Alfvénic power to the near‐Earth spaceAlfvén ducts elegantly explain the different propagation efficiencies of Alfvénic power in the magnetosphere [ABSTRACT FROM AUTHOR]

Earth's magnetotail plays a critical role in the dynamics of the magnetosphere, particularly during intervals of geomagnetic activity. To improve our understanding of the ion dynamics in this region, energetic neutral atom (ENA) imaging can provide global measurements to place in situ measurements in context and validate simulations. The NASA Two Wide‐angle Imaging Neutral‐atom Spectrometers mission provided near‐continuous observations using ENA imagers. ENA data can be used to calculate maps of equatorial ion temperatures that often show observations of regions of enhanced temperatures associated with phenomena in the magnetotail such as magnetic reconnection and narrow flow channels. We present an algorithm that can be used to search through a collection of these maps to identify intervals with such enhancements for further study. The algorithm results are validated against two sets of related phenomena: (a) a database of dipolarizing flux bundle (DFB) measurements from THEMIS and (b) a list of substorm onsets from SuperMAG. We demonstrate that the algorithm is very good at identifying intervals when there are DFB measurements or substorm onsets as long as there sufficient ENA data. We discuss some potential scientific studies that can result from use of the algorithm. We also show a preliminary application of the algorithm to simulation output to demonstrate the usefulness for other datasets, facilitate comparative studies, and introduce a new method for model validation. Key Points: An algorithm has been developed to identify regions of increased ion temperatures in maps calculated from Two Wide‐angle Imaging Neutral‐atom Spectrometers energetic neutral atom dataThe algorithm is validated against a list of dipolarizing flux bundle measurements and a list of substorm onsetsThe algorithm will be useful for future case and statistical studies of mesoscale phenomena as well as model validation [ABSTRACT FROM AUTHOR]

We present the first comparative statistical study of subauroral arc detachment from the main auroral oval at Athabasca (magnetic latitude = 61.5°N), Canada, for three different types of subauroral arcs: pure red arc, red arc with simultaneous emission in green‐line (red + green arc), and STEVE (strong thermal emission velocity enhancement). Based on 15‐years (2006–2020) of all‐sky imaging observations, this study not only uncovers the occurrence characteristics of different arcs but also provides important insights into the specific geomagnetic conditions under which STEVE develops. Red arc was the most common subauroral arc (139 events), followed by red + green arc (42 events), and STEVE (26 events) was a rare phenomenon. The detachment rate of red and red + green arcs exhibits dependence on both the solar flux and geomagnetic activity. The detachment rate of STEVE was higher during premidnight, whereas that of red and red + green arcs was higher around the midnight sector. The geomagnetic activity was relatively higher for STEVE, the decrease in the AL index and local X‐component magnetic variations being ∼2–3 times higher for STEVE as compared to other arcs. STEVE shows a strong association with asymmetric ring current in terms of prominent bay‐like enhancement in ASY‐H index prior to its detachment. Such bay‐like enhancement was ∼4 times higher for STEVE as compared to other arcs. STEVE events were accompanied by dispersionless injection for both electron and proton flux at geosynchronous orbit. These results unambiguously suggest that STEVE develops after substorm associated energy injection and significant intensification of the asymmetric ring current. Key Points: The first comparative statistical study of arc detachment from the main auroral oval at subauroral latitudes for three types of arcsThe substorm intensities estimated by the magnetic field variations were ∼2–3 times higher during STEVE as compared to other arcsThe ASY‐H index shows prominent bay‐like structure (∼4 times higher as compared to other arcs) just prior to the STEVE detachment [ABSTRACT FROM AUTHOR]

Cross‐satellite calibration of energetic particle fluxes is essential to understanding Earth's radiation belt dynamics and modeling the space radiation environment. Using the method of comparing phase space density (PSD) conjunctions in the same set of phase space coordinates, we perform a cross‐satellite calibration of energetic electron fluxes measured by the Low Earth Orbit Chinese FengYun‐3B satellite (FY‐3B) and the near‐equatorial Highly Eccentric Orbit Van Allen Probe‐A (VAP‐A), respectively. VAP‐A provides pitch angle resolved electron (directional) differential fluxes while FY‐3B provides omnidirectional electron differential fluxes. Before calculating the PSDs from FY‐3B high energy electron detector (HEED), a method is introduced to estimate the local pitch angle of FY‐3B based on the installation direction information of HEED and the T89c magnetic field model. We calculate the calibration factors by using the PSD conjunctions inferred from the two satellites for three fixed sets of (μ, K, L*) during the geomagnetic storm times. The calibration factors range from 0.76 to 0.86 varying with fixed sets of (μ, K, L*), which indicates that the electron differential fluxes from FY‐3B are higher than that from VAP‐A. During geomagnetic quiet times, we perform calibration by using independent Gaussian fitting analysis of the number of points with the unaveraged PSDs of FY‐3B and VAP‐A for three fixed sets of (μ, K, L*), with the corresponding calibration factors of 0.78, 0.72, and 0.46. The calibrated electron differential fluxes of FY‐3B measured at the high magnetic latitudes, can be used in the future modeling of the radiation belt electron dynamics. Key Points: Cross‐satellite calibrations of electron fluxes measured by FengYun‐3B satellite and Van Allen Probe‐A are conducted using electron phase space densities (PSDs)We compare PSD conjunctions and perform independent Gaussian fitting analysis in the fixed sets of (μ, K, L*)The cross‐satellite calibration factors are 0.76–0.86 and 0.46–0.78 during geomagnetic storm times and quiet times, respectively [ABSTRACT FROM AUTHOR]

We perform a geomagnetic event simulation using a newly developed magnetohydrodynamic with adaptively embedded particle‐in‐cell (MHD‐AEPIC) model. We have developed effective criteria to identify reconnection sites in the magnetotail and cover them with the PIC model. The MHD‐AEPIC simulation results are compared with Hall MHD and ideal MHD simulations to study the impacts of kinetic reconnection at multiple physical scales. At the global scale, the three models produce very similar SYM‐H and SuperMag Electrojet indexes, which indicates that the global magnetic field configurations from the three models are very close to each other. We also compare the ionospheric solver results and all three models generate similar polar cap potentials and field‐aligned currents. At the mesoscale, we compare the simulations with in situ Geotail observations in the tail. All three models produce reasonable agreement with the Geotail observations. At the kinetic scales, the MHD‐AEPIC simulation can produce a crescent shape distribution of the electron velocity space at the electron diffusion region, which agrees very well with MMS observations near a tail reconnection site. These electron scale kinetic features are not available in either the Hall MHD or ideal MHD models. Overall, the MHD‐AEPIC model compares well with observations at all scales, it works robustly, and the computational cost is acceptable due to the adaptive adjustment of the PIC domain. It remains to be determined whether kinetic physics can play a more significant role in other types of events, including but not limited to substorms. Key Points: We perform a global simulation of a geomagnetic storm event with kinetic modeling of the magnetotail reconnectionThe kinetic region is adaptively embedded to the magnetohydrodynamic model and the reconnection sites are identified by physical criteria during the runtimeThe global scale, mesoscale, and electron scale features are observed simultaneously in one simulation [ABSTRACT FROM AUTHOR]

The modulation of electromagnetic ion cyclotron (EMIC) waves by different geomagnetic pulsations is known to us from both ground and satellite observations. However, their dependence on the EMIC wave characteristics is not well explored. We report a statistical analysis of modulation of EMIC waves by short and long periodicities at the Indian Antarctic station, Maitri (L ≈ 5). We have analyzed the induction coil magnetometer data for the period of 2011–2017 and identified 6,845 EMIC wave events, out of which 5,502 events (80%) clearly showed the presence of short period modulation. These short period modulations are associated with repetitive rising tone EMIC wave emissions. Among these 5,502 EMIC wave events only 2,413 events showed presence of long period modulation, in addition to the short period modulation. Next we have examined the characteristics like start time, end time, peak frequency, frequency extent, maximum power, and dominant short and long periodicities present in each EMIC wave event. Based on the statistical analysis, we found that the dominant short and long periodicities in the EMIC waves are in the range of 1.5–3 min and 10–60 min, respectively. These short period decreases with an increase in the peak frequency of the EMIC wave. It is attributed to the decrease in magnetic field line oscillation period at lower L‐shells. Additionally, we noticed that the stronger EMIC wave events are likely to have a higher peak frequency. All these observed tendencies are examined in light of nonlinear theory, and they are found to be in good agreement. Plain Language Summary: The electromagnetic ion cyclotron (EMIC) waves play an important role in the precipitation of relativistic electrons from the outer radiation belt to the upper atmosphere. Thus, it forms an important component of the magnetospheric research. The modulation of the EMIC waves by different geomagnetic pulsations is known to us from both ground and satellite observations. It is believed that the modulation of the EMIC wave is governed through the modulation of the source region ambient plasma parameters by the ULF geomagnetic pulsations. Therefore it is possible that the modulation periodicities are linked to the EMIC wave characteristics. We examined this aspect using long‐term EMIC wave observations from Indian Antarctic ground station and validated the observed trends using nonlinear theory. Such study is important to improve our understanding about the EMIC wave modulation. Key Points: Short and long periodicities in the electromagnetic ion cyclotron (EMIC) waves are in the range of 1.5–3 min and 10–60 min, respectivelyShort period modulation is common and dependent on EMIC wave frequency such that it has larger values at higher L‐shellsStronger EMIC waves likely to have higher frequency, which is in agreement with nonlinear theory [ABSTRACT FROM AUTHOR]

We report on the solar and interplanetary (IP) causes of the third largest geomagnetic storm (26 August 2018) in solar cycle 24. The underlying coronal mass ejection (CME) originating from a quiescent filament region becomes a 440 km/s magnetic cloud (MC) at 1 au after ∼5 days. The prolonged CME acceleration (for ∼24 hr) coincides with the time profiles of the post‐eruption arcade intensity and reconnected flux. Chen et al. (2019, https://doi.org/10.3847/1538-4357/ab3f36) obtain a lower speed since they assumed that the CME does not accelerate after ∼12 hr. The presence of multiple coronal holes near the filament channel and the high‐speed wind from them seem to have the combined effect of producing complex rotation in the corona and IP medium resulting in a high‐inclination MC. The Dst time profile in the main phase steepens significantly (rapid increase in storm intensity) coincident with the density increase (prominence material) in the second half of the MC. Simulations using the Comprehensive Inner Magnetosphere‐Ionosphere model show that a higher ring current energy results from larger dynamic pressure (density) in MCs. Furthermore, the Dst index is highly correlated with the main‐phase time integral of the ring current injection that includes density, consistent with the simulations. A complex temporal structure develops in the storm main phase if the underlying MC has a complex density structure during intervals of southward IP magnetic field. We conclude that the high intensity of the storm results from the prolonged CME acceleration, complex rotation of the CME flux rope, and the high density in the 1‐au MC. Plain Language Summary: Powerful coronal mass ejections (CMEs) are responsible for very intense geomagnetic storms due to the out of the ecliptic component of the magnetic field in the CME flux rope or in the sheath if shock‐driving (Gosling, 1993, https://doi.org/10.1029/93ja01896). The 26 August 2018 storm was very intense, but the CME was inconspicuous and weak near the Sun. However, over an extended period of time the CME accelerated slowly and picked up adequate speed to cause an intense storm. Due to the presence of coronal holes near the eruption region, the CME rotated in such a way that the CME magnetic field and Earth's magnetic field can efficiently couple to transfer energy into the magnetosphere to cause the geomagnetic storm. The energy transfer is expedited by the presence of dense material deep inside the CME. Key Points: A coronal mass ejection characterized by prolonged acceleration, rotation, and high‐density content results in the intense geomagnetic stormSimulations with a kinetic code confirm that the ring current injection involves both solar wind dynamic pressure and electric fieldEmpirical formulas for predicting Dst based on solar wind electric field work when the magnetic cloud has no high‐density enhancement [ABSTRACT FROM AUTHOR]

We use the framework of Physics‐Informed Neural Network (PINN) to solve the inverse problem associated with the Fokker‐Planck equation for radiation belts' electron transport, using 4 years of Van Allen Probes data. Traditionally, reduced models have employed a diffusion equation based on the quasilinear approximation. We show that the dynamics of "killer electrons" is described more accurately by a drift‐diffusion equation, and that drift is as important as diffusion for nearly‐equatorially trapped ∼1 MeV electrons in the inner part of the belt. Moreover, we present a recipe for gleaning physical insight from solving the ill‐posed inverse problem of inferring model coefficients from data using PINNs. Furthermore, we derive a parameterization for the diffusion and drift coefficients as a function of L only, which is both simpler and more accurate than earlier models. Finally, we use the PINN technique to develop an automatic event identification method that allows identifying times at which the radial transport assumption is inadequate to describe all the physics of interest. Key Points: We analyze the relative importance between drift and diffusion, with varying L, geomagnetic activity, and phase space density valuesWe derive a simple and interpretable parameterization of drift and diffusion coefficients as functions of L onlyWe use the PINN framework to automatically identify events for which the one‐dimensional radial approximation does not hold [ABSTRACT FROM AUTHOR]

Modeling studies have predicted that the acoustic resonance of the atmosphere during geophysical events such as earthquakes and volcanos can lead to an oscillation of the geomagnetic field with a frequency of about 4 mHz. However, observational evidence is still limited due to scarcity of suitable events. On 15 January 2022, the submarine volcano Hunga Tonga‐Hunga Ha'apai (20.5°S, 175.4°W, Tonga) erupted in the Pacific Ocean and caused severe atmospheric disturbance, providing an opportunity to investigate geomagnetic effects associated with acoustic resonance. Following the eruption, geomagnetic oscillation is observed at Apia, approximately 835 km from Hunga Tonga, mainly in the Pc 5 band (150–600 s, or 1.7–6.7 mHz) lasting for about 2 hr. The dominant frequency of the oscillation is 3.8 mHz, which is consistent with the frequency of the atmospheric oscillation due to acoustic resonance. The oscillation is most prominent in the eastward (Y) component, with an amplitude of ∼3 nT, which is much larger than those previously reported for other events (<1 nT). Comparably large oscillation is not found at other stations located further away (>2700 km). However, geomagnetic oscillation with a much smaller amplitude (∼0.3 nT) is observed at Honolulu, which is located near the magnetic conjugate point of Hunga Tonga, in a similar wave form as at Apia, indicating interhemispheric coupling. This is the first time that geomagnetic oscillations due to the atmospheric acoustic resonance are simultaneously detected at magnetic conjugate points. Key Points: The effect of the January 2022 Hunga Tonga‐Hunga Ha'apai volcano eruption on the geomagnetic field is examinedGeomagnetic oscillation with a frequency of ∼3.8 mHz is observed simultaneously near the volcano and its magnetic conjugate pointThe oscillation is attributed to the acoustic resonance of the atmosphere [ABSTRACT FROM AUTHOR]

The turbulent energy cascade that is characteristic of bursty bulk flow (BBF) braking regions in the Earth's magnetotail has been shown to be the energy source of large‐amplitude electric fields (>50 mV/m) which can, in turn, result in local energetic electron acceleration. These pre‐energized electrons can move inward to stronger magnetic fields being further accelerated and can eventually supply an energetic tail to electron distributions in the outer radiation belt. Using wave and plasma measurements from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites during four tail seasons from 2015 to 2019, we study the process of BBF magnetic and kinetic energy being transferred to electrons by turbulent electric fields from a statistical perspective. We identify turbulent BBF regions by the presence of high‐amplitude electric fields. We show that the high‐amplitude electric fields are associated with an increase in electron temperature by three times compared to quiet times and with a ten‐fold increase in temperature fluctuations. They are also associated with strong variations of energetic electron fluxes, indicative of local acceleration. We further discuss the implications of these findings and the role of this pre‐energized electron population in supplying the outer radiation belt. Plain Language Summary: Bursty bulk flows are high‐speed plasma flows, observed in Earth's nightside magnetosphere. They move toward Earth from distances 20–30 Earth radii in the magnetotail where they are generated by magnetic reconnection. Closer to Earth (6–12 Re tailward), they slow down and deflect, as their energy dissipates in a turbulent cascade. Using data from NASA THEMIS satellites, we show that high‐amplitude electric fields generated in this cascade, strongly energize electrons. If further pushed inward, these pre‐energized electrons will be accelerated by Earth's magnetic field, and can eventually supply the outer radiation belt. Key Points: Large‐amplitude electric fields can lead to ten‐fold electron temperature fluctuations and three‐fold energetic electron flux variationsTemperature and flux variations rather than absolute temperature and flux values are indicative of local accelerationThe accelerated electrons may serve as a seed population for the high‐energy tail of the outer radiation belt [ABSTRACT FROM AUTHOR]

In this study, we examined the response of geomagnetic daily variation and ionospheric currents to the solar annular eclipse that passed through China at 10:00–17:30 LT on 21 June 2020. We utilized geomagnetic field data obtained from the Geomagnetic Network Center of China, Institute of Geophysics, China Earthquake Administration and International Real‐time Magnetic Observatory Network, which extend from middle latitudes to equatorial regions in China and surrounding areas. It is found that the eclipse obscuration reduces the intensity of geomagnetic daily variation at most stations as compared with the 5‐quiet‐day average. The response of geomagnetic daily variation to the solar eclipse depends on local time and latitude. In most shadowed areas of the eclipse, there is a reduction of northward (southward) component of the meridional magnetic daily field (ΔX) below (above) ∼30° magnetic latitudes. The solar eclipse can also induce a decrease of the zonal magnetic field variation (ΔY). Meanwhile, we observed that the solar eclipse reduces the intensity of vertical geomagnetic component (ΔZ) at the stations around the early afternoon aside the eclipse totality. In general, these geomagnetic responses imply that the solar eclipse can reduce the ionospheric solar quiet (Sq) current system. These ionospheric currents and ground geomagnetic field disturbances during the solar eclipse are also verified by a global physical model. The simulation demonstrates that the ionospheric currents in the southern hemisphere, where the solar eclipse does not cover, also have a weak response. Overall, the ionospheric current disturbance induced by the solar eclipse shows a counter‐Sq pattern. Key Points: The solar eclipse effects on global ionospheric electrodynamics are studied by extensive geomagnetic data in China and a numerical modelIonospheric electrodynamic response to the annular solar eclipse shows a local time and latitudinal dependenceThe ionospheric current system caused by the solar eclipse has a counter‐Sq pattern [ABSTRACT FROM AUTHOR]

Few remote sensing or in‐situ techniques can measure winds in Earth's thermosphere between altitudes of 120 and 200 km. One possible approach within this region uses Doppler spectroscopy of the optical emission from atomic oxygen at 558 nm, although historical approaches have been hindered in the auroral zone because the emission altitude varies dramatically, both across the sky and over time, as a result of changing characteristic energy of auroral precipitation. Thus, a new approach is presented that instead uses this variation as an advantage, to resolve height profiles of the horizontal wind. Emission heights are estimated using the Doppler temperature derived from the 558 nm emission. During periods when the resulting estimates span a wide enough height interval, it is possible to use low order polynomial functions of altitude to model the Doppler shifts observed across the sky and over time, and thus reconstruct height profiles of the horizontal wind components. The technique introduced here is shown to work well provided there are no strong horizontal gradients in the wind field. Conditions satisfying these caveats do occur frequently and the resulting wind profiles validate well when compared to absolute in‐situ wind measurements from a rocket‐borne chemical release. While both the optical and chemical tracer techniques agreed with each other, they did not agree with the HWM‐14 horizontal wind model. Applying this technique to wind measurements near the geomagnetic cusp footprint indicated that cusp‐region forcing did not penetrate to atmospheric heights of 240 km or lower. Plain Language Summary: We present a fundamentally new capability for measuring height variations in horizontal winds for altitudes between 100 and 150 km. It is difficult to measure these winds because space‐based platforms are unable to maintain such low orbits, whereas ground‐based remote sensing depends on an optical emission that behaves in ways that are difficult to account for. In particular, there are times when the emission height varies dramatically which, historically, made derivation of horizontal winds intractable. Here, we instead use this variation as a means to resolve how the wind changes with height. Not only does this allow height profiling, but we can now also make use of previously rejected data periods. Our results were validated by comparing to absolute wind measurements from a chemical tracer released by a sounding rocket. Comparing these profiles to the most‐commonly used empirical wind model (HWM14) showed that the model does not agree with the measured wind behavior. Finally, we have applied this technique to one of Earth's least understood atmospheric regions: the terrestrial footprints of the polar cusps. Results of this study placed a lower bound on the altitudes influenced by the cusps; its effects were not observed below 240 km. Key Points: We present height profiles of horizontal winds in the space‐atmosphere interaction region where measurements have previously been difficultHeight profiles from both this technique and chemical tracers are not consistent with the HWM‐14 modelApplying this approach to 2 years of data from two sites showed no indication of cusp‐region forcing penetrating down to E‐region heights [ABSTRACT FROM AUTHOR]

Polar Cap (PC) indices, PCN North based on magnetic observations from Qaanaaq (THL) in Greenland and PCS South based on magnetic data from Vostok in Antarctica, are very useful indices for studies of solar wind‐magnetosphere interactions and for space weather monitoring and forecasts. PCN indices are issued from the Danish Space Research Institute (DTU Space) while PCS indices are issued from the Arctic and Antarctic Research Institute (AARI) in St Petersburg. Unfortunately, series of invalid PCS values have been provided from AARI and used in a number of publications issued since the PC index concept was endorsed by the International Association of Geomagnetism and Aeronomy (IAGA) in 2013. In spite of the disclosure of the failure in the PCS indices in 2018, publishing at the IAGA‐supported International Service of Geomagnetic Indices of the invalid indices has continued up to now (2022). The present contribution defines the invalidating features of the PCS indices in question, conveys examples, and discusses the applications in peer‐reviewed publications. Plain Language Summary: The polar cap south (PCS) index series submitted from the Arctic and Antarctic Research Institute (AARI) in St Petersburg since 2014 holds extended sections of invalid index values with errors up to magnetic storm level as documented in the present contribution. Indices of this PCS index series, still published, have been used in peer‐reviewed publications. Thus, it is suggested that the authors examine these works, of which a number (9) is listed here, to look for devaluating effects from using the invalid PCS indices. The submission from AARI in December 2021 of a new PCS index series has confirmed the failure of the former PCS series. Key Points: The present work has analyzed published Polar Cap South (PCS) index series and disclosed comprehensive occurrences of invalid index valuesThe investigations have provided a survey of peer‐reviewed publications being devaluated by having used invalid PCS index valuesThe reported analyses have shown that unclear resolutions issued by the International Association of Geomagnetism and Aeronomy (IAGA) have supported the use of PCS index series without questioning their validity [ABSTRACT FROM AUTHOR]

Since the launch in 2013, the three satellites of the Swarm constellation have been conducting multipoint observations of ionospheric plasma density. The variety of their flight formations is advantageous for investigating (a) coherence scale and (b) directivity of nighttime Equatorial Plasma Irregularities (EPIs). In this study, we address the two topics statistically using in situ plasma density measured at 2 Hz rates by the Swarm constellation from 2013 to 2021. Maximum cross‐correlation coefficients between two Swarm density profiles decrease as longitude differences between the observation pair increase. The coefficient is larger than 0.6 only when two satellites are within about 0.1° in geographic longitude (GLON), which approximately corresponds to 10 km. When two density profiles are considerably correlated, we can determine preferred bearings of the EPI structure. A majority of EPIs conform to the backward‐C shapes astride the dip equator. The preference for backward‐C is more conspicuous later at night than in the early evening. Different GLON sectors exhibit slightly different directions of EPI structures, but the behavior is not well organized with the geomagnetic declinations of the respective sectors. EPI directions do not display monotonic dependence on Ap or F10.7, but further studies during the coming solar maximum are necessary to better represent high solar/geomagnetic activity. Plain Language Summary: Equatorial Plasma Irregularities (EPIs) in the nightside ionosphere have profound effects on the contemporary radio communications and navigation systems. In spite of the long history of study since the 1930s, the coherence scale and directivity of EPIs have not yet been investigated with global‐scale multiyear data sets. In this study we address the two EPI characteristics by cross‐correlating in situ plasma density profiles from three satellites under formation flight in polar orbits: the Swarm constellation. As East‐West separation between satellites increases, the pair of plasma density profiles lose similarity. The cross‐correlation coefficients go down below 0.6 when the East‐West separation is larger than ∼0.1° in geographic longitude (GLON), which amounts to about 10 km. The satellite pairs allow us to further investigate the directional preference of EPI structures. A majority of the EPI structures constitute backward‐C shapes straddling the dip equator. The preference for backward‐C becomes more and more evident as the night progresses. The directions of the EPI structures are not well organized with geomagnetic declination angles of the GLON sector. Neither do the directions exhibit monotonic changes with geomagnetic or solar activity, but further studies during the coming solar maximum will give us more decisive conclusion. Key Points: Latest changes in the flight formation of the Swarm mission are beneficial for interspacecraft correlation analysesAs zonal distance between two Swarm spacecraft increases, the two along‐track profiles of density irregularities become less coherentMost nighttime equatorial ionospheric irregularities, especially late at night, conform to backward‐C shapes straddling the dip equator [ABSTRACT FROM AUTHOR]

In response to a solar wind dynamic pressure enhancement, the compression of the magnetosphere generates strong ionospheric signatures and a sharp variation in the ground magnetic field, termed sudden commencement (SC). While such compressions have also been associated with a contraction of the ionospheric polar cap due to the triggering of reconnection in the magnetotail, the effect of any changes in dayside reconnection is less clear and is a key component in fully understanding the system response. In this study, we explore the time‐dependent nature of dayside coupling during SC by performing global simulations using the Gorgon magnetohydrodynamic code and impact the magnetosphere with a series of interplanetary shocks with different parameters. We identify the location and evolution of the reconnection region in each case as the shock propagates through the magnetosphere, finding strong enhancement in the dayside reconnection rate and prompt expansion of the dayside polar cap prior to the eventual triggering of tail reconnection. This effect pervades for a variety of interplanetary magnetic field orientations, and the reconnection rate is most enhanced for events with higher dynamic pressure. We explain this by repeating the simulations with a large explicit resistivity, showing that compression of the magnetosheath plasma near the propagating shock front allows for reconnection of much greater intensity and at different locations on the dayside magnetopause than during typical solar wind conditions. The results indicate that the dynamic behavior of dayside coupling may render steady models of reconnection inaccurate during the onset of a severe space weather event. Plain Language Summary: The Earth's magnetic field is often impacted by strong impulses of magnetized plasma ejected from the Sun. These compress the boundaries of the region shielded by the magnetic field and generate sudden signatures in the upper atmosphere and on the ground which can have societal effects. During the compression, some mass and energy is able to penetrate this shield more effectively than usual through a process called magnetic reconnection. We perform computer simulations of several such compressions to explore the effect on reconnection in detail, quantifying its enhancement and the dependence on various parameters. We find that reconnection is intensified immediately at the point of compression and undergoes a highly time‐dependent behavior for several minutes after impact. This demonstrates that basic models of reconnection that do not account for this time‐dependence may be inaccurate at the onset of severe space weather events. Key Points: The dayside reconnection line responds dynamically to the compression and distortion of the magnetopause by an interplanetary shockThe reconnection rate is enhanced near the shock as it propagates and appears to be modulated by large‐scale magnetopause motionsThe complex time‐dependence of dayside coupling during sudden commencement deviates significantly from steady models of reconnection [ABSTRACT FROM AUTHOR]

We present statistical results of traveling ionospheric disturbances (TIDs) with origin near the geomagnetic equator during geomagnetic storms that occurred within the period of 2010–2018. Based on storm criteria of Kp > 4 and Dst ≤ −50 nT, we have analyzed total electron content perturbations derived from Global Navigational Satellite Systems observations within a latitude range of 40°S–60°N and longitude ranges of 20°–40°E and 50°–70°W representing the African and American sectors. Although the northern hemispheric part of the African sector has limited data coverage, results show that the launched TIDs do not exceed the latitudinal distance of 20°–25° from their origin during the analyzed period. A statistically similar result is observed over the American sector with launched poleward TIDs constrained largely within ±20°–30° around the geomagnetic equator. Where data are available, majority of these cases are linked to changes in ionospheric electrodynamics, especially the enhancement of equatorial electrojet (EEJ), although there are other observations that are not explained by EEJ variability. This indicates that there may be other physical mechanisms that play a role in launching TIDs at the geomagnetic equator during disturbed conditions. An important result is that large‐scale and medium‐scale TIDs have been found to occur predominantly during the main and recovery phases of geomagnetic storms, respectively, at least over the African sector. Key Points: Storm‐time‐related poleward traveling ionospheric disturbances (TIDs) originating from the geomagnetic equatorial region occur mainly during local daytimeLarge scale poleward TIDs are observed during the main phase while medium‐scale TIDs dominate the recovery phase, at least over the African sectorIncreased vertical E × B drift plays a crucial role in launching poleward TIDs [ABSTRACT FROM AUTHOR]

Flux enhancements of field‐aligned low‐energy O+ ion (FALEO) are simultaneously observed by Arase, Van Allen Probes A and B in the nightside inner magnetosphere during 05–07 UT on September 22, 2018. FALEOs appear after a magnetic dipolarization signature with approximately 6–20 min delay. It has the energy‐dispersion signature from a few keV to ∼100 eV only in the direction parallel to the magnetic field at Arase, while it decreases its energy from a few keV down to 10 eV in both the parallel and antiparallel directions at Probes A and B. We perform a numerical simulation to trace trajectories of test O+ ions in a model magnetosphere, which are launched from above the ionosphere 3–15 min after a substorm. Flying virtual satellites that have the same orbits as the real satellites, we create virtual energy‐time spectrograms of O+ ions to compare with the observed ones. Results show a very good correspondence between them, indicating that FALEOs originate from ionospheric O+ ions that are extracted from the upper ionosphere at substorm onset and flow along the magnetic field toward the geomagnetic equator. It is also revealed that 3–9 hr after their launch, test O+ ions less than 400 eV have a spatial distribution in the inner magnetosphere which is similar to those of the warm plasma cloak and the oxygen torus. We therefore conclude that FALEO is a source of those cold ion populations. Key Points: Flux enhancements of field‐aligned low‐energy O+ ion (FALEO) are simultaneously observed by Arase, Van Allen Probes A and BNumerical simulation well reproduces observed energy‐time spectrograms of FALEO with the energy‐dispersed signature from 1 keV to 10 eVFALEO is a source of the warm plasma cloak and the oxygen torus that appear in the dawn to noon sectors [ABSTRACT FROM AUTHOR]

The nightside magnetic field at the middle and low latitude observatory is usually disturbed, decreasing in its north component (noted as Bx) during the periods of enhanced space weather activity. As indicated by the SYM‐H index, the decreasing field mainly originated from the magnetospheric westward ring current and the cross‐tail currents. In the meridian direction, the maximum of the disturbed field has been observed not always located at the geomagnetic dipole equator. One possible explanation is that the latitudinal position of the magnetotail current systems is affected by the orientation of solar wind to the Earth's geomagnetic dipole axis (noted as tilt angle). For a non‐zero tilt angle, the current deviates from the equatorial position (e.g., Malin & Mete Işikara, 1976; 10.1111/j.1365-246x.1976.tb07096.x). In this study, the mid‐low latitudinal distribution of the nightside disturbed field is systematically investigated every ∼1.5 hr, based on the magnetic surveys of the Swarm satellite constellation at polar orbits of ∼450 km altitude. The meridian distribution of Bx regularly appears like an upward‐opening parabolic shape with its vertex representing the magnetic minimum. In summer, the minimum is found generally located in the southern hemisphere, while in winter the pattern is reversed. The overall distribution demonstrates north‐south asymmetry. When the disturbed level is low, the deviation of the magnetic minimum from the dipole equator usually gets larger. The north‐south asymmetric distribution during storm times is further analyzed. In the initial phase, the magnetic minimum was shifted prominently (about 10°–30°) from the equator, then moved back (with the offset less than 10°) in the main phase and recovery phase in most cases. Moreover, the latitudinal deviation undergoes a diurnal variation, which is conspicuous in spring and autumn. It seems the asymmetric distribution is anti‐correlated to the contribution of the ring current, indicated by the SYM‐H index. The warping structure of the cross‐tail current sheet probably plays a predominant role in generating the north‐south asymmetry of the magnetic field disturbances for non‐zero dipole tilt angles. Monitoring the external magnetic configuration via the low‐Earth‐orbit satellites will provide fundamental evidence to identify the relative position of the ring current and the cross‐tail current, as well as the dynamic processes in the near magnetotail. Key Points: The nightside magnetic disturbed maximum at mid‐low latitudes annually shifts in meridian direction, which is notable when SYM‐H is quietThe disturbed maximum generally stayed close to the dipole equator in the main phase and the recovery phase of the geomagnetic stormThe cross‐tail current largely contributes to the near‐Earth disturbed fields, dominating the magnetic north‐south asymmetric distribution [ABSTRACT FROM AUTHOR]

For over a decade, the Super Dual Auroral Radar Network and the Active Magnetosphere and Planetary Electrodynamics Response Experiment have been measuring ionospheric convection and field‐aligned currents in the high‐latitude regions, respectively. Using both, high‐latitude maps of the magnetosphere‐ionosphere energy transfer rate (the Poynting flux) have been generated with a time resolution of 2 min between 2010 and 2017. These data driven Poynting flux (PF) patterns are used in this study to perform a superposed epoch analysis of the northern hemisphere ionospheric response to transitions of the interplanetary magnetic field Bz component, upwards of 60° geomagnetic latitude. We discuss the difference in the distribution of PF between the magnetosphere‐ionosphere Dungey cycle "switching on" and "switching off" to solar wind driving, revealing that they are not symmetric temporally or spatially. Plain Language Summary: The Earth's high‐latitude upper atmosphere (the ionosphere, upwards of 100 km in altitude) is consistently bombarded with solar energy that takes the form of electric currents aligned with Earth's magnetic field. The magnetosphere has two generalized states, "open" and "closed." Open is when the Earth and interplanetary magnetic fields (IMFs) connect to each other on the dayside, allowing energy into the atmosphere from the solar wind. Closed is when the fields do not connect (or do not connect simply) and thus not as much energy enters the atmosphere. The aforementioned open or closed states depend on the direction of the IMF, which varies constantly, as well as the history of the field as the magnetosphere takes time to reconfigure. In this study, we utilize nearly 8 years of data to generate average patterns of ionospheric energy input at various intervals before and after the IMF flips in direction. We discuss the spatial and temporal timescales upon which the energy varies in response to the relatively symmetric IMF transitions, finding that they do not result in symmetric changes in the ionospheric energy input patterns. Key Points: The Super Dual Auroral Radar Network and the Active Magnetosphere and Planetary Electrodynamics Response Experiment derived Poynting flux (PF) distributions are used to perform a superposed epoch analysis of interplanetary magnetic field (IMF) Bz transitionsChanges in the IMF Bz component have immediate responses, but continue to evolve for several tens of minutesThere is spatial and temporal asymmetry to how the PF morphology responds to opposite Bz transitions [ABSTRACT FROM AUTHOR]

We present a statistical study on the properties of equatorial electron pitch angle distributions (PADs) in the Earth's outer radiation belt, for the first time based on particle measurements from the entire Van Allen Probes mission. A detailed selection criteria is used to identify intervals when flux measurements at energies from 0.2 to 3.4 MeV are available across a wide range of pitch angles close to the geomagnetic equatorial plane. To better characterize the shape of each pitch angle distribution, the flux data is fitted to functions of the equatorial pitch angle based on Legendre polynomials. Using this technique, we show that the shape of the PADs strongly depends on the particle's energy, location and geomagnetic activity. These results are used to identify the dominant physical processes responsible for creating PADs with different shapes. The results presented here mainly focus on the occurrence statistics and properties of butterfly PADs. Significantly, we find clear evidence that butterfly PADs have a peak flux that preferentially occurs at two distinct and discrete equatorial pitch angles, either 35° ± 5° or 65° ± 5°. Energy, L‐shell and magnetic local time variations indicate that the flux peaks at equatorial pitch angles ∼35° appear consistent with magnetopause shadowing, whilst those peaking at ∼65° may be associated with wave‐particle interactions. We also show that the flux at low and high pitch angles, for both butterfly and nonbutterfly PADs, remains remarkably well correlated even as the flux intensity varies by four orders of magnitude. Plain Language Summary: We present a statistical study on the properties and occurrence of equatorial electron pitch angle distributions (PADs) in the Earth's outer radiation belt based on particle measurements from the entire Van Allen Probes mission. The flux data is fitted to functions of the equatorial pitch angle. Using the PAD fits, the characteristics of PADs that have a local flux minimum near equatorial pitch angles of 90°, known as butterfly PADs, are determined. We find clear evidence that these butterfly PADs have a peak flux that preferentially occurs at two distinct and discrete equatorial pitch angles, either 35° ± 5° or 65° ± 5°. Energy, L‐shell and magnetic local time variations indicate that the butterfly PADs with flux peaks at equatorial pitch angles of ∼35° appear consistent with outer radiation belt electron loss through the magnetopause, whilst those peaking at ∼65° may be associated with wave‐particle interactions. We also show that the flux at low and high pitch angles, for both butterfly and nonbutterfly PADs, remains remarkably well correlated even as the flux intensity varies by four orders of magnitude. Key Points: Equatorial butterfly pitch angle distributions have preferred flux peaks at ∼35° or ∼65°, each occurring across different L and MLT rangesThe depth of butterfly distribution dips tends to get smaller as the flux increases, inconsistent with growing off‐equatorial peaksFrom L*∼3 to ∼5, the flux at 90° is highly correlated with the flux at lower pitch angles, even as the flux varies by four orders of magnitude [ABSTRACT FROM AUTHOR]

By using Wuhan VHF radar, we show the morphological features of E‐region field‐aligned irregularity (FAI) occurrence at Wuhan during 2015–2020. Statistical results present that E‐region FAI occurrence reaches a maximum after sunset in summer season. According to Doppler spectrum features, type‐2 irregularity is predominantly observed at Wuhan. In addition, we observed a remarkable correlation between E‐region FAI occurrence and geomagnetic activity, which includes periods of positive correlation and negative correlation depending on different geomagnetic conditions. The strong negative correlation also exists between E‐region FAI occurrence and solar activity. In our observed results, we find that E‐region FAI occurrence shows a strong linkage with local sporadic E (ES) layer. A quantitative analysis of linear theory of plasma instability in the E‐region at midlatitudes is also presented in our study. The calculated results of linear growth rate indicate the importance of plasma density gradient of local ES layer and field‐line‐integrated Pedersen conductivity ΣPE+F ${{\Sigma}}_{P}^{E+F}$ on the generation of E‐region FAI. The geomagnetic and solar variations of E‐region FAI occurrence are also discussed in this study, which show a dependence on the geomagnetic and solar variations of both meteor rate and medium‐scale traveling ionospheric disturbance occurrence. Key Points: Midlatitude E‐region irregularity occurrence shows a strong dependence on local time, season, geomagnetic, and solar activitiesThe strong correlation exists between E‐region field‐aligned irregularity and local ES layer based on observed results and quantitative analysisThe variations of meteor rate and medium‐scale traveling ionospheric disturbance occurrence with geomagnetic and solar activities affect the distributions of E‐region irregularity [ABSTRACT FROM AUTHOR]

Geomagnetic storms on 7–8 September 2017 triggered severe ionospheric disturbances that had a serious effect on satellite navigation and radio communication. Multiple observations derived from Global Navigation Satellite System receivers, Earth's Magnetic Field and Environment Explorers (SWARM) and the Thermosphere‐Ionosphere ‐Electrodynamics General Circulation Model's simulations are utilized to investigate the spatial‐temporal ionospheric behaviors under storm conditions. The results indicate that the electron density in the Asia‐Australia, Europe‐Africa and America sectors suddenly changed with the Bz southward excursion, and the ionosphere over low‐middle latitudes under the sunlit hemisphere is easily affected by the disturbed magnetic field. The SWARM observations verified the remarkable double‐peak structure of plasma enhancements over the equator and middle latitudes. The physical mechanism of low‐middle plasma disturbances can be explained by a combination effect of equatorial electrojets, vertical E × B drifts, meridional wind and thermospheric O/N2 change. Besides, the severe storms triggered strong Polar plasma disturbances on both dayside and nightside hemispheres, and the Polar disturbances had a latitudinal excursion associated with the offset of geomagnetic field. Remarkable plasma enhancements at the altitudes of 100–160 km were also observed in the auroral zone and middle latitudes (>47.5°N/S). The topside polar ionospheric plasma enhancements were dominated by the O+ ions. Furthermore, the TIE‐GCM's simulations indicate that the enhanced vertical E × B drifts, cross polar cap potential and Joule heating play an important role in generating the topside plasma perturbations. Key Points: Positive disturbances over low‐middle latitudes only appear on the dayside, but polar disturbances occur on both dayside and nightsideStorms trigger bottomside plasma enhancements over middle‐high latitudes (>47.5 N/S) that have a latitudinal offset with geomagnetic fieldA combination of equatorial electrojet, E × B drifts, meridional wind and O/N2 change is responsible for low‐middle latitudinal disturbances [ABSTRACT FROM AUTHOR]

We present a statistical study of magnetic field vector residual between Swarm observations and two Earth magnetic field models: the 13th generation International Geomagnetic Reference Field (IGRF) model and the CHAOS‐7 model. Statistics of these residuals are important for estimating potential errors for satellite operations when using Earth magnetic models as a reference, as well as for magnetosphere‐ionosphere‐thermosphere studies examining energy input into the system. Magnetic field residuals are calculated as vector differences between observations and model estimation at Swarm satellite positions from 2014 to 2020. Magnetic field residuals for both models increase as geomagnetic activity level increases, and the largest magnitude of vector difference can be around 1800 nT with relatively small angle differences. The CHAOS‐7 model shows lower magnetic field residuals compared to IGRF‐13. North‐south hemispheric asymmetries are seen in magnetic field residuals for high Kp values larger than 6 with the southern hemisphere (SH) having more frequent occurrence of magnetic field residuals larger than 300 nT, especially during SH summer. Most large residual values appear in the high‐latitude region with SH seeing additional large residuals around the South Atlantic Anomaly region. Midnight and noon sectors show the strongest interhemispheric asymmetries. The northern hemisphere shows more frequent occurrence of large residuals above 75° magnetic latitude throughout all local times compared to the SH. Identifying asymmetries in large magnetic residuals under high geomagnetic activity levels is helpful for studying the difference in response to ionospheric disturbances in the two hemispheres. Key Points: Comparison between Swarm magnetic field vector observations and Earth magnetic field modelsStatistical magnetic field residual maps show largest differences in high latitude region and around the South Atlantic AnomalyMore frequent occurrence of large magnetic field residual under high geomagnetic activity levels in southern hemisphere [ABSTRACT FROM AUTHOR]

The Super Dual Auroral Radar Network (SuperDARN) was built to study ionospheric convection and has in recent years been expanded geographically. Alongside software developments, this has resulted in many different versions of the convection maps data set being available. Using data from 2012 to 2018, we produce five different versions of the widely used convection maps, using limited backscatter ranges, background models and the exclusion/inclusion of data from specific radar groups such as the StormDARN radars. This enables us to simulate how much information was missing from older SuperDARN research. We study changes in the Heppner‐Maynard boundary (HMB), the cross polar cap potential (CPCP), the number of backscatter echoes (n) and the χ2/n statistic which is a measure of the global agreement between the measured and fitted velocities. We find that the CPCP is reduced when the PolarDARN radars are introduced, but then increases again when the StormDARN radars are added. When the background model is changed from the RG96 model, to the most recent TS18 model, the CPCP tends to decrease for lower values, but tends to increase for higher values. When comparing to geomagnetic indices, we find that there is on average a linear relationship between the HMB and the geomagnetic indices, as well as n, which breaks when the HMB is located at latitudes below ∼50° due to the low observational density. Whilst n is important in constraining the maps (maps with n > 400 data points are unlikely to differ), it is insufficient as the sole measure of quality. Plain Language Summary: The ionosphere, where space begins and the atmosphere ends, moves as a result of the Earth's magnetic field coupling with the Sun. The Super Dual Auroral Radar Network (SuperDARN) was built around the Earth's magnetic poles to study this phenomenon, known as ionospheric convection. Combining many line‐of‐sight convection measurements, we are able to build global maps of ionospheric convection from SuperDARN data. This encapsulates dynamics which are central to space weather phenomena. SuperDARN, which has been gathering data for decades, has over time undergone numerous transformations, including the development of new processing software and more radars being added to the network. Using data from the years 2012–2018, we perform a statistical analysis on processed SuperDARN convection maps for the entire data set and assess systematically how the data set has changed over the years. We consider how the addition of more data and differences to the convection mapping procedures can affect scientific studies in the context of this large database. Key Points: We identify changes in derived convection maps when PolarDARN and StormDARN are added, and show the impact of different processingDerived convection parameters are highly susceptible to processing variables and which radars are includedWe show how the number of backscatter echoes per map is critical to the integrity of the maps, and discuss how this impacts map quality [ABSTRACT FROM AUTHOR]

Electromagnetic ion cyclotron (EMIC) waves have long been thought to contribute significantly to the variability of Earth's outer radiation belt because of their ability to scatter relativistic (>0.5 MeV) electrons into the loss cone on short timescales. We investigate statistically such pitch angle scattering signatures ("bite‐outs") and their quantitative agreement with cold plasma theory. Our analysis is based on EMIC wave events and local plasma parameters observed simultaneously with energetic electrons by the Van Allen Probes between February and May 2017. We determine the energy and pitch angle ranges in which relativistic electron scattering is likely and find that, on average, EMIC wave‐driven scattering of core (E ≤ 2 MeV) electrons is possible in 28% of events. By comparing the expected ranges for scattering to electron flux measurements co‐located with the wave activity, we determine that scattering signatures are found in 46% of those. Additionally, we find that the scattering signatures are more likely to occur under increased geomagnetic activity and for waves with greater wave power. These results provide observational evidence that EMIC wave‐driven scattering of core relativistic electrons is common in the duskside outer radiation belt. Key Points: The minimum resonant energy of electrons falls below 2.0 MeV in ∼28% of EMIC wave events surveyedApproximately 46% of events were co‐located with relativistic electron pitch angle scattering signaturesThe presence of scattering signatures is dependent on geomagnetic activity and wave power [ABSTRACT FROM AUTHOR]

Sporadic‐E (Es) layers are investigated over five Digisonde stations located inside (Santa Maria and Cachoeira Paulista), boundary (São Luís), and outside (Millstone Hill and Port Stanley) the South American Magnetic Anomaly (SAMA) in the course of the 25 August 2018, geomagnetic storm. Data from Digisondes and ground‐based magnetometer and a numerical model of the E‐region (MIRE) are analyzed to search the roles of neutral winds and electric fields as physical processes responsible for the generation of Es layers. Moreover, it is observed for the first time over Santa Maria a type of Es layer that resembles the Esa detected in the auroral region. The characterization of such signatures in ionograms and the investigations of its formation mechanism are the main focus of this study. Hiss waves activities detected by the Van Allen Probe‐B data provide evidence that energetic particle precipitations are the most likely process responsible for this type of Es layer inside the SAMA in the night hours during the geomagnetic storm recovery phase. An attempt is also made to explain the presence of Esq over São Luís in terms of the expansion of the Equatorial Electrojet to higher latitudes. Finally, the results bring a new observational and modeling indication of a concomitant action of multiple formation mechanisms in the Es layers development in the American sector during the 25 August 2018, geomagnetic storm. Key Points: Sporadic‐E layer of auroral type (Esa) is reported for the first time over Santa Maria station (29.7°S, 53.8°W), BrazilHiss waves may be responsible for the particle precipitation over SAMA during the generation of the Esa layers during the recovery phaseEsa layers are appearing in stations further west in South America due to the westward motion of the SAMA [ABSTRACT FROM AUTHOR]

Magnetic anomalies on the surface of the Moon interact with the solar wind plasma flow, resulting in both magnetic and electrostatic deflection/reflection of charged particles. Consequently, surface charging in these regions differs from regions without magnetic fields. Using the Colorado Solar Wind Experiment facility, this interaction is investigated with high-energy flowing plasmas (100-800 eV beam ions) that are incident upon a magnetic dipole embedded beneath an insulating surface. The dipole moment is perpendicular to the surface. The plasma potential distribution is measured above the surface using an emissive probe. In the dipole lobe regions the surface is charged to significantly higher positive potentials by the unmagnetized ion beam impinging on the surface while the electrons remain excluded by the magnetic field. At low ion beam energies the results agree with these expectations as the surface potential follows the ion beam energy. However, at high beam energies, the surface potentials in the electron-shielded lobe regions remain significantly lower than the expected magnitude. Surprisingly, electrons are detected in the shielded regions by a Langmuir probe. A test particle simulation indicates that secondary electrons induced by the high energy ion beams impinging on the surface can enter the shielded regions, thus lowering the surface potential. [ABSTRACT FROM AUTHOR]

Dynamical variations of radiation belt trapped electron fluxes are examined to better understand the variability of enhancements linked to substorm clusters. Analysis is undertaken using the Substorm Onsets and Phases from Indices of the Electrojet substorm cluster algorithm for event detection. Observations from low earth orbit are complemented by additional measurements from medium earth orbit to allow a major expansion in the energy range considered, from medium energy energetic electrons up to ultra-relativistic electrons. The number of substorms identified inside a cluster does not depend strongly on solar wind drivers or geomagnetic indices either before, during, or after the cluster start time. Clusters of substorms linked to moderate (100 nT < AE = 300 nT) or strong AE (AE = 300 nT) disturbances are associated with radiation belt flux enhancements, including up to ultra-relativistic energies by the strongest substorms (as measured by strong southward Bz and high AE). These clusters reliably occur during times of high speed solar winds streams with associated increased magnetospheric convection. However, substorm clusters associated with quiet AE disturbances (AE = 100 nT) lead to no significant chorus whistler mode intensity enhancements, or increases in energetic, relativistic, or ultra-relativistic electron flux in the outer radiation belts. In these cases the solar wind speed is low, and the geomagnetic Kp index indicates a lack of magnetospheric convection. Our study clearly indicates that clusters of substorms occurring outside of high speed wind streams are not by themselves sufficient to drive acceleration, which may be due to the lack of pre-cluster convection. [ABSTRACT FROM AUTHOR]

Heliospheric plasma turbulence plays a key role in transferring the energy of large‐scale magnetic field and plasma flow fluctuations to smaller scales where the energy can be dissipated, ultimately leading to plasma heating. High‐quality measurements of electromagnetic fields and electron velocity distributions by the Magnetospheric Multiscale (MMS) mission in Earth's magnetosheath present a unique opportunity to characterize plasma turbulence and to determine the mechanisms responsible for its dissipation. We apply the field‐particle correlation technique to a set of 20 MMS magnetosheath intervals to identify the dissipation mechanism and quantify the dissipation rate. It is found that 95% of the intervals have velocity‐space signatures of electron Landau damping that are quantitatively consistent with linear kinetic theory for the collisionless damping of kinetic Alfvén waves. About 75% of the intervals contain asymmetric signatures, indicating a local imbalance of kinetic Alfvén wave energy flux in one direction along the magnetic field than the other. About one‐third of the intervals have an electron energization rate with the same order‐of‐magnitude as the estimated turbulent cascade rate, suggesting that electron Landau damping plays a significant, and sometimes dominant, role in the dissipation of the turbulent energy in these magnetosheath intervals. Key Points: Electron Landau damping plays a ubiquitous role in dissipating Alfvénic turbulent energy in the magnetosheathElectron Landau damping signatures correspond to the wave energy flux direction, implying the dissipation of locally imbalanced turbulenceElectron Landau damping sometimes plays a dominant role in dissipating turbulent energy in the magnetosheath [ABSTRACT FROM AUTHOR]

During periods of strong geomagnetic activity, intense currents and electric fields originating in the magnetosphere inundate the high‐latitude E‐region ionosphere. These strong electric fields drive plasma instabilities, including the Farley‐Buneman instability (FBI). These instabilities give rise to small‐scale plasma turbulence that modifies the large‐scale ionospheric conductance that, in turn, affects the evolution of the magnetosphere‐ionosphere‐thermosphere system. Also, during geomagnetic storms, high‐energy precipitating electrons, ≳5 keV, frequently penetrate down to the same regions where the intense currents and electric fields exist. This research examines the effects of precipitating electrons on the generation of the FBI and shows that, under many common conditions, it can easily suppress the FBI in a predictable manner. We demonstrate that the plasma pressure of superthermal electrons may significantly exceed the regular plasma pressure of the cold ionospheric plasma. This effect will increase the FBI threshold and suppress the instability in auroral regions. However, our detailed theoretical analysis shows that the effect of the superthermal precipitating electrons on the FBI threshold is much stronger than the pressure effect. The energy dependence of the electron‐N2 collision frequency can greatly enhance the effect of this additional pressure, further suppressing the FBI, even at a moderate precipitation level. Therefore, we expect that precipitation will exert an additional significant feedback on the magnetosphere by preventing the elevated conductivity caused by FBI driven turbulence. Both the turbulence‐enhanced conductivities and this suppression should be taken into account in global modeling of the magnetosphere‐ionosphere coupling. Plain Language Summary: From time to time, elevated solar activity disturbs the near‐Earth environment, creating within the Earth's magnetosphere tangible geomagnetic storms. During these events, strong electric fields often penetrate down to the lower high‐latitude Earth's ionosphere where intense electric currents close. These currents provide strong magnetosphere‐ionosphere coupling. At altitudes between 100 and 120 km, the penetrated electric fields are often intense enough to generate plasma instability which, in turn, causes turbulence, heats electrons, and affects the large‐scale ionospheric conductance. This conductance is an important element of a global electric circle surrounding our planet. During the storm time, it is always possible that highly energetic electrons from the distant magnetospheric regions precipitate into the Earth's atmosphere, leading to the spectacular effect of Aurora Borealis. This theoretical research predicts that if these energetic electrons penetrate down to the lower‐ionosphere regions at the time of strong electric field able to generate plasma instability then these electrons will suppress this instability within the auroral regions, thus affecting the global ionospheric conductance. Proper understanding of these effects is important for accurate predictive modeling of the near‐Earth space plasmas, especially for the timely predictions of potentially devastating effects of solar activity on the human life and technology. Key Points: During geomagnetic storms, strong electric fields and intense electron precipitation frequently overlap in the E‐region ionosphereWithout precipitation, strong electric fields drive E‐region instabilities, leading to plasma turbulence and increased conductanceStrong electron precipitation dramatically raises the instability threshold, largely suppressing the instability inside the auroral regions [ABSTRACT FROM AUTHOR]

The Earth's background free oscillations at ∼3.7 and ∼4.4 mHz resonantly couple with the atmospheric acoustic modes and thus energy cross‐talk between the earth‐atmosphere system is maximum at these frequencies. The present study proposes resonant coupling between the Earth's surface and atmosphere during the 11 April 2012 Sumatra doublet earthquake and offer a possible explanation to this occurrence. Following both these earthquakes, prolonged ionospheric oscillations centered at frequency of ∼4 mHz were observed in GPS (Global Positioning System) derived total electron content (TEC) towards north‐northeast of the epicenters. We scrutinize these oscillations in terms of the manifestations of plausible non‐tectonic and tectonic forcing mechanisms surrounding the epicentral region. Non‐tectonic forcing such as the geomagnetic field coupling factor and observation geometry played a critical role in determining the amplitude anisotropy of resonant ionospheric signatures. Further, the Rayleigh waves of the first earthquake (Mw 8.6) were already characterized by an excess of energy at ∼4 mHz. We propose this could make the Mw 8.6 earthquake particularly efficient to excite the 4 mHz resonance in the atmosphere. The resonant ionospheric signatures after the second earthquake (Mw 8.2) were observed to be closely associated with the Earth's free oscillations caused by R2 Rayleigh wave train of the Mw 8.6 earthquake event. Together, all the above point to a scenario where the resonant ionospheric signatures during the Sumatra doublet event were indeed related to the seismic source. Therefore, resonant co‐seismic ionospheric signatures could provide additional information on the low‐frequency features of seismic ruptures. Key Points: Resonant ionospheric signatures observed after Sumatra 2012 doublet earthquake could be linked to seismic sourcesResonant ionospheric signatures can provide more insight into rupture process of large earthquakesEffects of geomagnetic field coupling factor and satellite geometry on co‐seismic ionospheric signals can be inconclusive [ABSTRACT FROM AUTHOR]

During the solar minimum the F‐region plasma density can become extremely low, and in some exceptional cases the F‐layer can completely disappear. These F‐region density depletions are particularly deep during geomagnetically quiet nights, when slow plasma convection creates polar holes, extending 100–1000 km in size, where the electron density is several orders of magnitude lower than the background values. Polar holes are believed to be formed during the periods of the very slow anti‐sunward convection, when the plasma is trapped just poleward of the auroral oval in the absence of any ionization sources. At the same time, fast convection with rapid vertical plasma transport has also been associated with the polar hole formation. Combining the electron density measurements from the Resolute Bay Incoherent Scatter Radar (RISR) and Poker Flat ISR (PFISR), the state of art variational data assimilation tool Ionospheric Data Assimilation Four‐Dimensional (IDA4D), and the convection data from the Super Dual Auroral Radar Network (SuperDARN) array, the polar hole formation and evolution are experimentally investigated. The F‐region plasma is traced back in time prior to the polar hole formation to analyze the favorable conditions for the formation of the polar holes. The role of plasma convection in the formation of the polar holes is experimentally investigated. The results show that the polar holes can be formed in the twilight condition and without being trapped in the closed convection loops. The formation of the polar holes were equally possible during slow and strong convection flows, predominantly with a standard two‐cell convection configuration. Key Points: The role of plasma convection in the formation of the polar holes is investigatedPolar holes has larger latitudinal extent than previously thoughtPlasma does not have to be trapped in the closed convection loops is full darkness to become depleted [ABSTRACT FROM AUTHOR]

Using 65,133 hourly averages of transpolar voltage (ΦPC) from observations made over 25 yr by the SuperDARN radars, with simultaneous SML and interpolated am geomagnetic indices, we study their optimum interplanetary coupling functions. We find mean lags of 18, 31, and 45 min for ΦPC, am, and SML, respectively, and fit using a general coupling function with three free fit exponents. To converge to a fit, we need to average interplanetary parameters and then apply the exponent which is a widely used approximation: we show how and why this is valid for all interplanetary parameters, except the factor quantifying the effect of the clock angle of the interplanetary magnetic field, sind(θ/2), which must be computed at high time resolution and then averaged. We demonstrate the effect of the exponent d on the distribution, and hence weighting, of samples and show d is best determined from the requirement that the coupling function is a linear predictor, yielding d of 2.50 ± 0.10, 3.00 ± 0.22, and 5.20 ± 0.41 for ΦPC, am, and SML, respectively. To check for overfitting, fits are made to half the available data and tested against the other half. Ensembles of 1,000 fits are used to study the effect of the number of samples on the distribution of errors in individual fits and on systematic biases in the ensemble means. We find only a weak dependence of solar wind density for ΦPC and SML but a significant one for am. The optimum coupling functions are shown to be significantly different for ΦPC, am, and SML. Plain Language Summary: Coupling functions are mathematical combinations of measured variables observed in the solar wind, just before it impacts near‐Earth space. They are used to predict the effect that the solar wind will have (or, for retrospective studies, will have had) on the space‐weather environment of the Earth. There is a very wide variety of proposed optimum forms for coupling functions in the literature, some of which work better than others and we show which performs best depends on which terrestrial disturbance indicator we are trying to predict and on what timescale. We look at the validity of some commonly used assumptions made when compiling a coupling function and, using an unprecedentedly large data set of different types of terrestrial space weather disturbance indicator, we derive the optimum coupling functions and their statistical uncertainties. We show that that the required coupling functions are significantly different in the three cases. The results establish some important principles for the development of these coupling functions and show they need to be tailored to the specific space weather disturbance indicator, timescale, and activity level that they aim to predict. Key Points: Using a very large dataset we analyze the sources and effects of noise in correlation studies used to derive solar wind coupling functionsWe study effects of weighting by the distribution of samples which varies with the choice of interplanetary magnetic field orientation factor and averaging timescaleThe optimum coupling functions for transpolar voltage and a planetary geomagnetic activity index are significantly different [ABSTRACT FROM AUTHOR]

We report one of the first comprehensive ground‐based investigations of energy transfer rates in the E‐region ionosphere compared relative to geomagnetic activity, seasonal effects, and solar activity level using nearly continuously sampled data collected with the Poker Flat Incoherent Scatter Radar (PFISR) between 2010 and 2019. We quantified the integrated electromagnetic (EM) energy transfer rate and the integrated Joule heating rate in the E‐region between 90 and 130 km, which includes the contribution from the neutral winds. We find that (a) the median Joule heating rate and EM energy transfer rate in the evening sector are larger in the winter versus the summer and have similar magnitudes in the spring and fall for the same solar activity and geomagnetic conditions. (b) The seasonal dependence of the energy transfer rates is closely associated with the seasonal variations of the electric fields. Our analysis shows that the larger EM energy transfer and Joule heating rates in disturbed conditions in the winter versus the summer are associated with the combined effects of both the electric field and Pedersen conductance with the electric field playing a dominant role. Given that the Pedersen conductance in the evening sector is closely related to the particle precipitation and field aligned currents in the auroral region, this study provides complementary ionospheric evidence of the winter‐summer asymmetry of the intensity and density of field‐aligned currents (e.g., Ohtani et al., 2009, https://doi.org/10.1029/2009ja014115). (c) The geomagnetic activity level has the most significant impact on the magnitude of the energy transfer rates, followed by seasonal variations, and last the solar activity level. Plain Language Summary: We reported a comprehensive statistical study of energy transfer rates (EM transfer rate and Joule heating rate) in the auroral E‐region in aspects of season, geomagnetic activity, and solar activity. This study is done by collecting ground‐based 10 years of measurements from Poker Flat Incoherent Scatter Radar. We find a consistent seasonal variation of energy transfer rates for different geomagnetic and solar activity levels. That is the EM energy transfer and Joule heating experience larger enhancements in the evening sector in the winter versus the summer and show similar magnitudes in the spring and fall equinoxes. We further find that the seasonal variation of the enhancement is mainly associated with the seasonal variation of the electric field while the seasonal variation of the E‐region conductance could also play a smaller role during disturbed conditions. We also compared the relative importance of season, geomagnetic, and solar activity levels in impacting the energy transfer rates and find that the geomagnetic activity has the largest impact, followed by season and solar activity. Key Points: Geomagnetic activity, seasonal, and solar activity variability of the integrated energy transfer rates are quantified in the E‐region between 90 and 130 km spanning 2010–2019 using PFISR observations for the first timeThe integrated Joule heating and EM energy transfer rates in the evening sector are larger in winter versus summer and have similar magnitudes in spring and fall equinoxesThe larger energy transfer rates in winter relative to summer in disturbed conditions are associated with a combination of electric field and Pedersen conductance [ABSTRACT FROM AUTHOR]

Previous studies show equatorial plasma bubbles (EPBs) mainly occur after sunset and usually drift from west to east. In this work, two EPBs occurred after midnight and lasted to sunrise, and were observed by multi‐instruments (two all‐sky imagers (ASIs), global positioning system, Swarm satellite, and a digisonde) during a geomagnetically quiet night of December 18–19, 2018. Our observations also show that plasma depletion structure (PDS) also occurred at that night. EPBs occurred in the region of PDS and showed a special evolution process during the night. Those PDS gradually disappeared with time, but strip‐like EPBs remained clearly in airglow images. Near sunrise, new PDS appeared to merge with those EPBs to form large PDS. Meanwhile, EPBs showed different zonal drifts within a very narrow longitudinal zone (∼2°). One EPB remained stationary while the other showed a small eastward drift. These results are different from post‐sunset EPBs in previous studies. During the lifetime of those ionospheric plasma irregularities, upward and downward motion of the peak height and virtual height of the ionospheric F region was observed by a digisonde, which indicates that the evolution of EPBs should be related to the vertical drift of ionospheric plasma. Besides, based Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIE‐GCM) simulations, we suggest the different zonal drifts of those EPBs might related to plasma zonal drifts caused by zonal winds. Key Points: The evolution of equatorial plasma bubbles (EPBs) occurring in the region of plasma depletion structure, from post‐midnight to sunrise, were observed during a geomagnetically quiet nightEPBs showed different zonal drifts in the observations: one remained stationary and the other drifted eastwardVertical drifts of ionospheric plasma are the driver of EPB evolution and different zonal drifts may be related to zonal wind variations [ABSTRACT FROM AUTHOR]

Chorus waves outside the plasmapause influence the Earth's radiation belt dynamics by interacting with energetic electrons via cyclotron and Landau resonance. Recent numerical diffusion experiments indicate that the diffusion process is sensitive to the spatial and temporal scale of variability in the wave‐particle interaction, which is reported to be more efficient than that based on the traditional average model. Using Van Allen Probes A and B data from November 2012 to July 2019, the spatial and temporal scale size of chorus waves are calculated by the correlation between the wave amplitudes detected by two satellites with varying spatial separation or time lag. We found that, the chorus wave is incoherent when the spatial extent is greater than 433 km or the time lag lasts ∼10 s, which are significantly smaller than that of plasmaspheric hiss. In addition, the spatial correlations of chorus tend to be higher near noon or with lower geomagnetic activity. The temporal correlations of chorus are always statistically near zero, which are not influenced by the location and geomagnetic activity. Our results can help refine the model of the interactions between energetic particles and chorus waves in the radiation belt. Key Points: Chorus waves tend to be incoherent when the spatial extent is greater than 433 km or the time lag lasts 10 s at <∼6.5The spatial scale size of chorus wave is relatively larger near noon or with lower geomagnetic activityThe temporal scale size of chorus wave is always statistically near zero, not influenced by L‐shell, magnetic local time, or AL* index [ABSTRACT FROM AUTHOR]

A strong westward wind pattern at altitudes of ∼140–250 km at high latitudes (∼65°) in the approximate longitude range of 100°E–200°E in the Southern Hemisphere, and from 200°E to 300°E in the Northern Hemisphere, known as a "wind wall," is investigated. Simulations from the Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM‐X) and the Global Airglow (GLOW) model, along with Wind Imaging Interferometer (WINDII) observations, allowed the description of most of the wind wall features. The results indicate that the wind wall is linked to the well‐known high‐latitude wind convection pattern. This atmospheric circulation is significantly stronger during active geomagnetic conditions, where its dusk anticyclonic circulation cell is also enhanced, which results in the origin of the wind wall. The study also reveals the impacts of the wind wall on the vertical velocity and its consequences for the O(1S) green line emission on the lower thermosphere, as well as its influence on the atomic oxygen depletion at high latitudes, which is mainly a response to the complex high‐latitude wind convection pattern and its spatial and temporal variation. Key Points: Strong thermospheric westward winds at polar regions, known as a wind wall, are associated with the high‐latitude convection patternThe wind wall pattern has its origin in the active geomagnetic conditionsOver the lower thermosphere, the wind wall leads to perturbations on the vertical winds, the O(1S) emission rates, and the atomic oxygen [ABSTRACT FROM AUTHOR]

We present new results using data collected by the Poker Flat Incoherent Scatter Radar (PFISR) of energy transfer rates, which include the effects from neutral winds in the high latitude E‐region ionosphere‐thermosphere (IT) during Fall 2015. The purpose of our investigation is to understand the magnetic local time (MLT) dependence of the peak energy transfer, which occurs asymmetrically in the morning‐evening (dawn‐dusk) MLT sector. The statistical characteristics of both altitude‐resolved and altitude‐integrated energy transfer rates in the auroral E region local to PFISR during different geomagnetic conditions are quantified. Our analysis shows that the geomagnetic activity level has large impacts on the energy transfer rates. In contrast with previous investigations, we find both the altitude integrated electromagnetic (EM) energy transfer rate and Joule heating rate are larger in the evening sector than in the morning sector during all geomagnetic activity conditions. We also observe a non‐negligible negative EM energy transfer rates below 110 km in the morning sector during active conditions, which is associated with neutral winds during this MLT interval. The statistical results show that the neutral winds tend to increase the Joule heating rate in a narrow altitude range in the morning sector and impact a broader region with respect to altitude and time in the evening sector in the E region under moderate and active conditions. We find that during quiet conditions that the neutral winds have a significant contribution to the Joule heating and contribute up to 75% of the Joule heating. However, during active conditions the enhanced electric fields are a dominant driver of Joule heating, while the neutral wind effects can reduce the Joule heating rates by 25% or more relative to the passive heating rates. Key Points: Energy transfer rates are estimated from Poker Flat Incoherent Scatter Radar (PFISR) measurements for the first timeThe electromagnetic energy transfer rate and the Joule heating rate are larger in the evening sector versus the morning sectorNegative electromagnetic energy transfer rates occur below 110 km in the morning sector during disturbed geomagnetic conditions [ABSTRACT FROM AUTHOR]

We examine the drift‐resonant particle dynamics for toroidal ultralow frequency (ULF) waves in a pure dipole background geomagnetic field. We confirm that the resonant condition originally believed to apply only for poloidal ULF waves, mωd=ω, also applies for compressional toroidal waves. The predicted phase relationships have been confirmed from Van Allen Probes observations. Their good agreement provides the first observational evidence for the drift resonance condition controlled by the compressional toroidal ULF wave. Moreover, we extend the drift resonance theory into a nonlinear regime. The resulting particle motion can be described by a modified pendulum equation with solutions depending on the wave number m. For high‐m toroidal waves, the resonant islands become asymmetric to perturb the particle trajectories within each potential well and consequently increase the trapping widths in both energy and L‐shell. We further carry out test‐particle simulations to show the evolution of electron distribution functions when they interact with either toroidal or poloidal waves. These findings demonstrate that toroidal ULF waves, like their poloidal counterparts, play an important role in magnetospheric particle dynamics. Plain Language Summary: In the magnetosphere, ultralow frequency (ULF) electromagnetic waves in the mHz range are usually categorized into poloidal and toroidal modes. It has been widely accepted that poloidal ULF waves, characterized by electric field oscillations in the azimuthal direction, play a key role in accelerating and transporting charged particles in the inner magnetosphere. On the other hand, toroidal waves have long been considered incapable of accelerating particles since their radial electric field is perpendicular to the azimuthal drift velocity of inner magnetospheric particles unless their drift paths are significantly distorted by dayside compression of the magnetosphere. Here we show that even in a pure dipole field, the toroidal waves can still resonate with energetic particles because of the nonzero curl of the wave electric field in association with the compressional magnetic field oscillations, and the resonant condition remains the same as its poloidal counterpart. The predictions of compressional toroidal wave‐particle drift resonance show good agreement with spacecraft observations. Further analysis shows that the particle nonlinear behavior in the toroidal wave field can be described by a modified pendulum equation. Therefore, it is important to consider not only the poloidal but also the toroidal modes in the study of ULF wave‐particle interactions. Key Points: Compressional toroidal ultra low frequency (ULF) waves have the same drift resonance condition as poloidal waves in a pure dipole modelDrift resonance between electrons and compressional‐toroidal ULF waves is identified from Van Allen Probes observationsA modified pendulum equation is derived to describe the nonlinear drift resonance between toroidal waves and charged particles [ABSTRACT FROM AUTHOR]

Electromagnetic waves observed in the inner magnetosphere at frequencies between about 0.5 and 4 kHz sometimes exhibit a quasiperiodic (QP) time modulation of the wave intensity with modulation periods from tens of seconds up to a few minutes. Such waves are typically termed "QP emissions" and their origin is still not fully understood. We use a large set of more than 2,000 of these events identified in the low‐altitude DEMETER spacecraft data to check for energetic electron flux variations matching the individual QP wave elements. Altogether, seven such events are identified and their detailed analysis is performed. Energetic electron fluxes are found to be modulated primarily at energies lower than about 250 keV. While the waves may propagate unducted across L‐shells, the energetic particles follow magnetic field lines from the interaction region down to the observation point. This is used to estimate the locations of anticipated generation regions to L‐shells between about 4 and 6, and the respective source radial dimensions to about 0.6–1.2 Earth radii. The frequencies of the events are confined below half of the equatorial electron gyrofrequency in the determined source regions. Finally, it is shown that individual QP elements exhibit a fine inner structure corresponding to the wave bouncing between the hemispheres. Key Points: Energetic electron precipitation bursts corresponding to quasiperiodic emission peaks are identifiedInteraction regions occur at L‐shells between about 4 and 6 and have dimensions of about 0.6–1.2 Earth radiiIndividual wave elements exhibit a fine inner structure corresponding to the wave bouncing between the hemispheres [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]

The Earth's magnetosphere is mainly a collisionless plasma system with non‐Maxwellian particle distributions, which are often fitted by the kappa function. While the Maxwell distribution function is described by two parameters (density and temperature), the kappa distribution function has three parameters: density, core energy, and the kappa index that characterizes the slope at high energies. In this study, we fitted an ion flux measured by the five Time History of Events and Macroscale Interactions during Substorms satellites in the Earth's magnetosphere during quiet geomagnetic conditions by a single kappa distribution function. The data were constrained to the following regions: from seven Earth Radii (7 RE) to the Earth's magnetopause at the dayside, and up to 20 RE for other magnetic local time sectors. The two‐dimensional spatial distribution of the fitted parameters and their radial dependencies were analyzed. Regions of high and low κ‐parameter are selected and their formation is discussed. Key Points: Ion spectra obtained from the Time History of Events and Macroscale Interactions during Substorms satellite mission near the equatorial plane are fitted by a kappa distribution functionDistribution of three fitting parameters (density, core energy, value of k characterizing the spectra slope at high energies) are obtainedRegion of increased k index at geocentric distances 8–10 RE is selected [ABSTRACT FROM AUTHOR]

The evolution of the low‐latitude magnetosphere over the substorm cycle is reconstructed based on a new high‐resolution 3D representation of the magnetic field and nearest‐neighbor data mining. The study covers radial distances 2.5–25RE and employs a record‐large pool of spacecraft data taken during 1995–2019. The magnetospheric state is quantified by four indices, representing the ground geomagnetic activity and its temporal trends in the entire ±90° range of geomagnetic latitude: the SuperMAG SMR, the midlatitude positive bay MPB, the auroral SML, and the polar cap PC index. The developed technique has been tested on specific substorm events, with the results presented in the form of 5‐min cadence diagrams and animations of the magnetic field line configurations and electric current distributions. In all the analyzed events, the initial intensification and radial expansion of the inner tail current is accompanied by a gradual stretching of the magnetic field, followed by its sudden collapse, dramatic depletion of the current beyond R∼12RE, and a large‐scale dipolarization of the field around the time of MPB peak, after which the system recovers and tends to its pre‐substorm state. Plain Language Summary: The dynamical structure of the Earth's magnetosphere during geomagnetic substorms is reconstructed, based on (a) a large multi‐year database of satellite data taken during the last quarter century, (b) a pool of concurrent ground geomagnetic activity indices, covering full range of latitudes, (c) a new magnetic field model with enhanced spatial resolution, and (d) an advanced "nearest‐neighbor" data mining approach. Based on a synthesis of the above methods and data, we explore the ability of our approach to extract maximum information from past observations and reproduce the principal phases of magnetospheric substorms in terms of time sequences of the magnetic field and electric current diagrams, from the beginning to active and recovery phase of the disturbance. Key Points: A new high‐resolution B‐field representation combined with dynamical data mining reveals magnetosphere behavior on the substorm‐time scaleFull cycle of magnetosphere evolution is reconstructed based on 25‐year archive of satellite data and a set of ground‐based activity indicesInitial growth of the distant magnetotail current and its subsequent sudden collapse during the substorm onset are consistently reproduced [ABSTRACT FROM AUTHOR]