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The Adolescent Brain Cognitive Development (ABCD) Study®is a 10-year longitudinal study of children recruited at ages 9 and 10. A battery of neuroimaging tasks are administered biennially to track neurodevelopment and identify individual differences in brain function. This study reports activation patterns from functional MRI (fMRI) tasks completed at baseline, which were designed to measure cognitive impulse control with a stop signal task (SST; N= 5,547), reward anticipation and receipt with a monetary incentive delay (MID) task (N= 6,657) and working memory and emotion reactivity with an emotional N-back (EN-back) task (N= 6,009). Further, we report the spatial reproducibility of activation patterns by assessing between-group vertex/voxelwise correlations of blood oxygen level-dependent (BOLD) activation. Analyses reveal robust brain activations that are consistent with the published literature, vary across fMRI tasks/contrasts and slightly correlate with individual behavioral performance on the tasks. These results establish the preadolescent brain function baseline, guide interpretation of cross-sectional analyses and will enable the investigation of longitudinal changes during adolescent development.

The state of genome-wide chromatin accessibility in cells, tissues, or organisms can be investigated with a technique called assay for transposase-accessible chromatin using sequencing (ATAC-seq). ATAC-seq is a powerful approach for profiling the epigenomic landscape of cells using very low input materials. Analysis of chromatin accessibility data allows for prediction of gene expression and identification of regulatory elements such as potential enhancers and specific transcription-factor binding sites. Here, we describe an optimized ATAC-seq protocol for the preparation of isolated nuclei and subsequent next-generation sequencing from whole embryos and tissues of the African turquoise killifish (Nothobranchius furzeri). Importantly, we provide an overview of a pipeline for processing and analyzing ATAC-seq data from killifish.

Two wave packets of second harmonic poloidal Pc 4 waves with a wave frequency of ~7 mHz were detected by Van Allen Probe A at a radial distance of ~5.8 REand magnetic local time of 13 hr near the magnetic equator, where plasmaspheric refilling was in progress. Proton butterfly distributions with energy dispersions were also measured at the same time; the proton fluxes at 10–30 keV oscillated with the same frequency as the Pc 4 waves. Using the ion sounding technique, we find that the Pc 4 waves propagated eastward with an azimuthal wave number (mnumber) of ~220 and ~260 for each wave packet, respectively. Such eastward propagating high‐m(m> 100) waves were seldom reported in previous studies. The condition of drift‐bounce resonance is well satisfied for the estimated mnumbers in both events. Proton phase space density was also examined to understand the wave excitation mechanism. We obtained temporal variations of the energy and radial gradient of the proton phase space density and find that temporal intensification of the radial gradient can generate the two wave packets. The cold electron density around the spacecraft apogee was >100 cm−3in the present events, and hence the eigenfrequency of the Pc 4 waves became lower. This causes the increase of the mnumber which satisfies the resonance condition of drift‐bounce resonance for 10–30 keV protons and meets the condition for destabilization due to gyrokinetic effect. The first direct observation of drift‐bounce resonance that excites eastward propagating second harmonic poloidal waves with high mnumberThese waves are excited by intensification of radial gradient of proton phase space density due to substorm injectionCold electrons also contribute to the wave excitation by increasing the mnumber to satisfy gyro‐kinetic destabilization condition

Using Van Allen Probe observations of the inner magnetosphere during geomagnetic storms driven by interplanetary coronal mass ejections (ICMEs) and corotating interaction regions (CIRs), we characterize the impact of these drivers on the storm‐time ring current development. Using 25 ICME‐ and 35 CIR‐driven storms, we have determined the ring current pressure development during the prestorm, main, early‐recovery, and late‐recovery storm phases, as a function of magnetic local time, L shell and ion species (H+, He+, and O+) over the 100‐ to 600‐keV energy range. Consistent with previous results, we find that during the storm main phase, most of the ring current pressure in the inner magnetosphere is contributed by particles on open drift paths drifting duskward leading to a strong partial ring current. The largest difference between the ICME and CIR ring current responses during the storm main and early‐recovery phases is the difference in the response of the <~55‐keV O+to these drivers. While the H+pressure response shows similar source and convection patterns for ICME and CIR storms, the O+pressure response is significantly stronger for ICME storms. The ICME O+pressure increases more strongly than H+with decreasing L and peaks at lower L shells than H+. During the storm main and early‐recovery phases, most of the ring current pressure is contributed by particles on open drift pathsThe largest difference between ICME and CIR responses in the main and early recovery phase is the <~55‐keV O+pressure contributionDuring the storm main and early‐recovery phases, the plasma beta increases to values greater than ~1 for L> 4 for ICMEs and L> 4.5 for CIRs

We describe a new data product combining the spin‐averaged electron flux measurements from the Radiation Belt Storm Probes (RBSP) Energetic Particle Composition and Thermal Plasma (ECT) suite on the National Aeronautics and Space Administration's Van Allen Probes. We describe the methodology used to combine each of the data sets and produce a consistent set of spectra for September 2013 to the present. Three‐minute‐averaged flux spectra are provided spanning energies from 15 eV up to 20 MeV. This new data product provides additional utility to the ECT data and offers a consistent cross calibrated data set for researchers interested in examining the dynamics of the inner magnetosphere across a wide range of energies. A new combined electron flux data product for the Van Allen Probes mission is describedResults from cross calibration of the RBSP‐ECT instrument suite are presentedThis data product represents the first ever complete electron spectra throughout the inner magnetosphere from tens of electron volts to tens of megaelectron volts

Electrons with energies in the keV range play an important role in the dynamics of the inner magnetosphere. Therefore, accurately modeling electron fluxes in this region is of great interest. However, these calculations constitute a challenging task since the lifetimes of electrons that are available have limitations. In this study, we simulate electron fluxes in the energy range of 20 eV to 100 keV to assess how well different electron loss models can account for the observed electron fluxes during the Geospace Environment Modelling Challenge Event of the 2013 St. Patrick's Day storm. Three models (Case 1, Case 2, and Case 3) of electron lifetimes due to wave‐induced pitch angle scattering are used to compute the fluxes, which are compared with measurements from the Van Allen Probes. The three models consider electron losses due to interactions with whistler mode hiss waves inside the plasmasphere and with whistler mode chorus waves outside the plasmasphere. The Case 1 (historical) model produces excessive loss at low L shells before and after the storm, suggesting that it overestimates losses due to hiss during quiet times. During the storm main phase and early recovery all three models show good agreement with the observations, indicating that losses due to chorus during disturbed times are, in general, well accounted for by the models. Furthermore, the more recent Case 2 and Case 3 models show overall better agreement with the observed fluxes. Energetic electrons play a key role in the dynamics of the inner magnetosphere. Consequently, it is of great interest to estimate the electron fluxes in this region. Since the electron lifetimes available to date have limitations, an accurate estimation has been a challenging task. This study aims to assess how well different electron loss models can account for the observed electron fluxes during a geomagnetic storm. Three models of electron losses due to interactions with magnetospheric waves are used in the flux calculations, and the results are then compared with fluxes observed by the Van Allen Probes mission. Our results show that the historical model overestimates the losses at low altitudes before the storm and in the late recovery phase of the storm, indicating that the losses due to interactions with hiss waves in the model are too strong. Furthermore, we find that the more recent models used, based on empirical wave models, are overall in better agreement with the observations over the entire storm. Three models of electron lifetimes due to wave‐induced pitch angle scattering in the magnetosphere are comparedElectron flux computed with the lifetimes based on recent empirical wave models show the best agreement with Van Allen Probes observationsThe historical model produces fluxes that are lower than the observed at low L shells, suggesting that it overestimates hiss losses

The relationship between dispersive Alfvén waves (DAWs), magnetospheric activity, and O+ion outflow/energy density is examined using measurements from the Van Allen Probes. We show that correlated DAW activity and O+outflow/energization is a characteristic feature of the inner magnetosphere during active conditions and during storms persists for several hours over large L‐shell and azimuthal ranges of the plasma sheet. Though enhanced during substorm and storm active periods, these correlated features are most intense during geomagnetic storms. Comparisons show a linear relationship between DAW electric (and magnetic) field energy density and outflowing O+energy. Statistical measurements from a large number of storms also reveal a linear relationship between DAW energy density and gross enhancements in energetic O+energy densities. These observations support the notion that DAWs play an important role in the energization of O+ions into and within the inner magnetosphere. Geomagnetic storms are major disturbances in the Earth's magnetosphere, during which the particle content and pressure in the magnetosphere increase considerably. Much of the pressure increase is due to singly charged oxygen ions that come from the ionosphere. How this happens is not clear. Analyzing satellite observations, we found evidence suggesting that a particular type of low‐frequency electromagnetic wave called a dispersive Alfvén wave may be playing a key role. These waves are found to be more prevalent and intense in the magnetosphere during storms. Oxygen ion energies are shown to increase with increasing intensities of these waves. The oxygen ion contribution to pressure also increases in association with intensified wave activity. These observations support the notion that the waves energize and heat oxygen ions into and within the magnetosphere over extended periods of time, which leads to significant magnetospheric pressure increases. Dispersive Alfven wave energy density is enhanced during substorms and geomagnetic stormsOxygen ion outflow energy and energy density are correlated with dispersive Alfven wave energy densityObservations support modeling that dispersive Alfven waves enhance oxygen ion energy density in the inner magnetosphere

We analyze a database of Dynamics Explorer‐1 (DE‐1) Retarding Ion Mass Spectrometer densities and temperatures to yield the first explicit measure of how cold ion concentration depends on temperature. We find that cold H+and He+concentrations have very weak dependence on temperature, but cold O+ion concentration increases steeply as these ions become warmer. We demonstrate how this result can aid in analyzing composition data from other satellites without spacecraft potential mitigation, by applying the result to an example using data from the Van Allen Probes mission. Measurement of light ion concentrations above 1 electron volt (eV) are a reasonable proxy for the concentrations of colder (eV) ions. Warmer O+ion concentrations may be extrapolated to colder temperatures using our fit to the statistical distribution versus temperature. Fractional H+and He+ion concentrations have extremely weak dependence on temperatureFractional OO+ion concentration increases with temperatureFor light ions, concentrations above 1 eV are a proxy for colder ions

Whistler mode chorus waves influence the dynamics of the Earth's radiation belts and the inner magnetosphere through gyroresonant wave particle interactions. Chorus waves are generated by anisotropic hot electrons from a few to tens of keV, called source electrons, which have increased access from the nightside plasma sheet to the inner magnetosphere during geomagnetic storms. The primary drivers of geomagnetic storms are coronal mass ejections (CMEs) and corotating interaction regions (CIRs). Through differences in their characteristic physical parameters, they can each impact the nightside plasma sheet differently. Using Van Allen Probes observations, we have conducted a superposed epoch analysis of chorus wave activity and source electron development across all local times between L= 2–6 during 25 CME‐ and 35 CIR‐driven storms. The superposed epoch analysis shows that chorus wave power follows the storm phase‐dependent access of the source electron population. Chorus waves and source electrons are observed on the dawnside during the main phase, when open drift path access via eastward convective drift from the plasma sheet is enhanced. During the recovery phase, chorus waves and source electrons are observed at all magnetic local times with low intensities, exemplifying the formation of a weak symmetric, trapped electron population. A linear theory approximation for wave growth from source electron observations shows that increased wave growth follows the enhanced source electrons during each storm phase. CME and CIR storms display similar behavior and levels of average wave power; however, chorus wave activity reaches lower L‐shells during CME storms on average. A Van Allen Probes superposed epoch analysis of source electrons and chorus waves in MLT/Lduring CME‐ and CIR‐driven stormsThe local time distribution of chorus wave power reflects storm phase‐dependent access and trapping of ones to tens of keV source electronsStrongest wave power and source access is during the main phase near dawn; transitions to weaker waves across all MLT in the recovery phase

Substorms are a highly variable process, which can occur as an isolated event or as part of a sequence of multiple substorms (compound substorms). In this study we identify how the low‐energy population of the ring current and subsequent energization varies for isolated substorms compared to the first substorm of a compound event. Using observations of H+and O+ions (1 eV to 50 keV) from the Helium Oxygen Proton Electron instrument onboard Van Allen Probe A, we determine the energy content of the ring current in L‐MLT space. We observe that the ring current energy content is significantly enhanced during compound substorms as compared to isolated substorms by ∼20–30%. Furthermore, we observe a significantly larger magnitude of energization (by ∼40–50%) following the onset of compound substorms relative to isolated substorms. Analysis suggests that the differences predominantly arise due to a sustained enhancement in dayside driving associated with compound substorms compared to isolated substorms. The strong solar wind driving prior to onset results in important differences in the time history of the magnetosphere, generating significantly different ring current conditions and responses to substorms. The observations reveal information about the substorm injected population and the transport of the plasma in the inner magnetosphere. Quantitative estimates of ring current energy for compound and isolated substorms are shownThe energy content and postonset enhancement is larger for compound compared to isolated substormsSolar wind coupling is a key driver for differences in the ring current between isolated and compound substorms

Electromagnetic ion cyclotron (EMIC) waves and magnetosonic waves are commonly observed in the Earth's magnetosphere associated with enhanced ring current activity. Using wave and ion measurements from the Van Allen Probes, we identify clear correlations between the hydrogen‐ and helium‐band EMIC waves with the enhancement of trapped helium and oxygen ion fluxes, respectively. We calculate the diffusion coefficients of different ion species using quasi‐linear theory to understand the effects of resonant scattering by EMIC waves. Our calculations indicate that EMIC waves can cause pitch angle scattering loss of several keV to hundreds of keV ions, and heating of tens of eV to several keV helium and oxygen ions by hydrogen‐ and helium‐band EMIC waves, respectively. Moreover, we found that magnetosonic waves can cause the resonant heating of thermal protons. Our study indicates the importance of energy transfer from the EMIC and magnetosonic waves to ions with different species at thermal energies. Electromagnetic ion cyclotron (EMIC) waves and magnetosonic waves are plasma waves commonly observed in the Earth's magnetosphere. The EMIC waves and magnetosonic waves are associated with the ions drifting around the Earth. We use satellite observations of the plasma waves and ions to study the relation between EMIC or magnetosonic waves with ions at thermal energies. The measurement clearly indicates the correlation between the enhancement of proton, helium, and oxygen fluxes with magnetosonic waves, hydrogen‐band EMIC waves, and helium‐band EMIC waves, respectively. We use the physics‐based model to quantify the possible heating of ions with different species by different wave modes. Our calculation suggests that the protons, helium, and oxygen ions at thermal energies could be effectively heated by magnetosonic waves, hydrogen‐band EMIC waves, and helium‐band EMIC waves, respectively. The model results explain the measured correlation between the ions and waves. Our study suggests that the EMIC and magnetosonic waves which are generated by the ions in ring current could transfer energy to the lower energy ions. EMIC waves in different frequency bands and magnetosonic waves are correlated with the enhancement of ion fluxes of different speciesEMIC and magnetosonic waves could heat different ion species at thermal energies through resonant interactionField‐aligned thermal ions are accelerated to larger pitch angles and trapped ion fluxes increase due to the EMIC and magnetosonic waves

Based on the measurements of ~100‐keV to 10‐MeV electrons from the Magnetic Electron Ion Spectrometer (MagEIS) and Relativistic Electron and Proton Telescope (REPT) on the Van Allen Probes, the radiation belt electron energy spectra characterization and evolution have been investigated systematically. The results show that the majority of radiation belt electron energy spectra can be represented by one of three types of distributions: exponential, power law, and bump‐on‐tail (BOT). The exponential spectra are generally dominant in the outer radiation belt outside the plasmasphere, power law spectra usually appear at high L‐shells during injections of lower‐energy electrons, and BOT spectra commonly dominate inside the plasmasphere at L>2.5 during relatively quiet times. The main features of three types of energy spectra have also been revealed. Specifically, for the BOT energy spectrum, the energy of local flux maximum usually ranges from approximately hundreds of keV to several MeV and the energy of local flux minimum varies from ~100 keV to ~MeV, both increasing as L‐shell decreases, confirming the plasmaspheric hiss wave scattering to be the main mechanism forming the BOT energy spectra. Statistical results using 4‐year observations from the Van Allen Probes on the relation between energy spectra and plasmapause location also show that the plasmasphere plays a critical role in shaping radiation belt electron energy spectrum: the peak location of BOT energy spectra is ~1 L‐shell inside the minimum plasmapause, where BOT energy spectra mostly form in ~1–2 days as a result of hiss wave scattering. Most radiation belt electron energy spectra can be represented by one of three distributions: exponential, power law, and bump‐on‐tailExponential spectra dominate outside the plasmasphere; power law spectra often appear at high L during injections of lower‐energy electronsBump‐on‐tail spectra dominate inside the plasmasphere at L<2.5 and are caused by hiss wave scattering

We employ Magnetospheric Multiscale, Geostationary Operational Environmental and Los Alamos National Laboratory satellites, as well as the ground magnetometer networks over Greenland and North America to study a substorm on 9 August 2016 between 9 and 10 UT. We found that during the substorm two earthward flows, whose dipolarization‐injection fronts exceeded 6.5 and 4 Earth's radii (RE) in YGSM, impinged and rebounded from Earth's dipolar field lines at L=6–7 downtail, where Lis the McIlwain number. The impingements and rebounds ended with a substorm current system of downward R1 and upward R2 currents which grew to azimuthally cover the whole North American continent. At the fronts, regions of enhanced negative j·E′were formed and peaked toward the end of the impingements. These regions appeared to be conjugate with eastward moving aurora (along the growth phase arc and together with eastward drifting energetic electrons at geosynchronous equatorial orbit), which manifests ionospheric Ohmic losses. Two broad (6.5 and 4 REin YGSM) dipolarization‐injection fronts impinged and rebounded from Earth's dipolar field lines near GEO during the same substormDownward R1 and upward R2 currents grew to azimuthally cover the whole North American continent in the course of the substormRegions of negative j·E′(generator) peaked toward the end of the impingements and were conjugate to R2 auroral current (load)

Whistler mode waves are important for precipitating energetic electrons into Earth's upper atmosphere, while the quantitative effect of each type of whistler mode wave on electron precipitation is not well understood. In this letter, we evaluate energetic electron precipitation driven by three types of whistler mode waves: plume whistler mode waves, plasmaspheric hiss, and exohiss observed outside the plasmapause. By quantitatively analyzing three conjunction events between Van Allen Probes and POES/MetOp satellites, together with quasi‐linear calculation, we found that plume whistler mode waves are most effective in pitch angle scattering loss, particularly for the electrons from tens to hundreds of keV. Our new finding provides the first direct evidence of effective pitch angle scattering driven by plume whistler mode waves and is critical for understanding energetic electron loss process in the inner magnetosphere. We suggest the effect of plume whistler mode waves be accurately incorporated into future radiation belt modeling. Electron precipitation into Earth's upper atmosphere is an important loss mechanism of energetic electrons trapped in the inner magnetosphere. Although whistler mode waves are known to be effective in producing electron precipitation through pitch angle scattering, the relative roles of various whistler mode waves that play in electron losses are unclear. In this letter, we quantitatively analyze conjunction events, where Van Allen Probes observed various whistler mode waves near the equator and Low‐Earth‐Orbiting satellites detected electron precipitation approximately along the same magnetic field line but at low altitudes. By combining the satellite data analysis and quasi‐linear theory, we found that whistler mode waves observed in plumes are very effective in scattering energetic electrons, which are ultimately lost through interacting with the neutral atmosphere. Our new finding provides the direct evidence that plume whistler mode waves play an important role in energetic electron precipitation, which is crucial for understanding energetic electron loss process in the Earth's inner magnetosphere. Three types of whistler mode waves are observed during conjunction events between Van Allen Probes and POES/MetOpThese whistler mode waves include plume whistler mode waves, plasmaspheric hiss, and exohissPlume whistler mode waves are very effective in producing energetic electron precipitation (from tens to hundreds of keV)

Electromagnetic ion cyclotron (EMIC) waves can drive precipitation of tens of keV protons and relativistic electrons, and are a potential candidate for causing radiation belt flux dropouts. In this study, we quantitatively analyze three cases of EMIC‐driven precipitation, which occurred near the dusk sector observed by multiple Low‐Earth‐Orbiting (LEO) Polar Operational Environmental Satellites/Meteorological Operational satellite programme (POES/MetOp) satellites. During EMIC wave activity, the proton precipitation occurred from few tens of keV up to hundreds of keV, while the electron precipitation was mainly at relativistic energies. We compare observations of electron precipitation with calculations using quasi‐linear theory. For all cases, we consider the effects of other magnetospheric waves observed simultaneously with EMIC waves, namely, plasmaspheric hiss and magnetosonic waves, and find that the electron precipitation at MeV energies was predominantly caused by EMIC‐driven pitch angle scattering. Interestingly, each precipitation event observed by a LEO satellite extended over a limited L shell region (ΔL~ 0.3 on average), suggesting that the pitch angle scattering caused by EMIC waves occurs only when favorable conditions are met, likely in a localized region. Furthermore, we take advantage of the LEO constellation to explore the occurrence of precipitation at different L shells and magnetic local time sectors, simultaneously with EMIC wave observations near the equator (detected by Van Allen Probes) or at the ground (measured by magnetometers). Our analysis shows that although EMIC waves drove precipitation only in a narrow ΔL, electron precipitation was triggered at various locations as identified by POES/MetOp over a rather broad region (up to ~4.4 hr MLT and ~1.4 Lshells) with similar patterns between satellites. We show three cases of proton and relativistic electron precipitation observed simultaneously with EMIC wavesEMIC‐driven precipitation was observed by POES/MetOp satellites at different locations over a broad L‐MLT regionEach precipitation event extended over ΔL~ 0.3 on average, showing that wave‐driven pitch angle scattering is localized

Highly energetic electrons are trapped in the magnetic field of Earth’s radiation belts. The physical mechanisms driving the dynamics of the Van Allen belts can be understood from the electron’s energy spectrum, which is believed to be steeply falling with increasing energy. This view has been prevalent for the past 60 years since the energy spectra were first measured. Here, we report the observation of a reversed energy spectrum with abundant high-energy and fewer low-energy electrons spanning from hundreds of keV to around two MeV in electron energy in data collected with NASA’s Van Allen Probes. We find that this spectrum dominates inside the plasmasphere—a dense cold plasma region co-rotating with the Earth. Using two-dimensional Fokker–Planck diffusion simulations with a time-dependent, data-driven model of hiss waves in the plasmasphere, we demonstrate that the formation of the reversed spectrum is explained by the scattering of hiss waves. The results have important implications for understanding the distributions of charged particles and wave–particle interactions in magnetized plasmas throughout the solar system and beyond.

We study electron behavior in the outer radiation belts during the 16 July 2017 storm sudden commencement (SSC), in which prompt intensification of ultrarelativistic electron fluxes was observed at around L= 4.8 by Van Allen Probe B immediately after an interplanetary shock. The electron fluxes in multiple energy channels show clear oscillations in the Pc5 frequency range, although the oscillation characteristics are quite different in different energy channels. At energies above ∼1 MeV, the oscillation periods were very close to the electron drift period, which resembles an energy spectrogram evolution expected for an energetic particle injection event and its drift echoes. At lower energies, however, the oscillation periods hardly depended on the energy: They were very close to the ultralow frequency (ULF) wave period derived from electric field measurements (about 250 s according to wavelet analysis). These complex signatures are consistent with the picture of drift resonance between electrons and short‐lived ULF waves with low azimuthal wave numbers. Good agreement between the observations and numerical simulations confirms that shock‐induced global‐scale ULF waves can efficiently accelerate outer belt ultrarelativistic electrons up to 3.4 MeV over a time scale shorter than 1 hr. The flux of ultrarelativistic electrons enhanced by an order of magnitude within 1 hr after SSC on 16 July 2017Drift resonance between 3.4‐MeV electrons and shock‐induced ULF waves is confirmedThe behavior of nonresonant particles in response to the shock indicates the scenario of drift resonance with damping ULF waves

Ring current electrons (1–100 keV) have received significant attention in recent decades, but many questions regarding their major transport and loss mechanisms remain open. In this study, we use the four‐dimensional Versatile Electron Radiation Belt code to model the enhancement of phase space density that occurred during the 17 March 2013 storm. Our model includes global convection, radial diffusion, and scattering into the Earth's atmosphere driven by whistler‐mode hiss and chorus waves. We study the sensitivity of the model to the boundary conditions, global electric field, the electric field associated with subauroral polarization streams, electron loss rates, and radial diffusion coefficients. The results of the code are almost insensitive to the model parameters above 4.5 RERE, which indicates that the general dynamics of the electrons between 4.5 REand the geostationary orbit can be explained by global convection. We found that the major discrepancies between the model and data can stem from the inaccurate electric field model and uncertainties in lifetimes. We show that additional mechanisms that are responsible for radial transport are required to explain the dynamics of ≥40‐keV electrons, and the inclusion of the radial diffusion rates that are typically assumed in radiation belt studies leads to a better agreement with the data. The overall effect of subauroral polarization streams on the electron phase space density profiles seems to be smaller than the uncertainties in other input parameters. This study is an initial step toward understanding the dynamics of these particles inside the geostationary orbit. The dynamics of the ring current electrons is a competition between loss and transport processes in the Earth's inner magnetosphere. These processes remain poorly understood due to difficulties of in situ particle measurements. Given the scarcity of satellite data, numerical modeling is a powerful approach that allows us to gain a deeper insight into the behavior of the ring current electrons. In this work, we investigate which processes dominate the dynamics of these particles within the geostationary orbit. We use the four‐dimensional Versatile Electron Radiation Belt code to model electron loss and transport processes during 17 March 2013 geomagnetic storm. To understand the significance of the model uncertainty, we run an ensemble of simulations with different model parameters and compare results with the Van Allen Probe satellite observations. We show that the global convective electric and magnetic fields control the transport of the ring current electrons inside the geostationary orbit. This work is a basis for future studies, which will be extended further away from the Earth and include more comprehensive plasma wave models. Ring current electron dynamics within geostationary orbit is modeled and the sensitivity of the model to the input parameters is exploredGlobal convective electron transport from geostationary orbit to 4.5 REcan explain Van Allen Probe observationsModel results below 4.5 REare most sensitive to the electric field and electron lifetimes

We describe a new, more accurate procedure for estimating and removing inner zone background contamination from Van Allen Probes Magnetic Electron Ion Spectrometer (MagEIS) radiation belt measurements. This new procedure is based on the underlying assumption that the primary source of background contamination in the electron measurements at Lshells less than three, energetic inner belt protons, is relatively stable. Since a magnetic spectrometer can readily distinguish between foreground electrons and background signals, we are able to exploit the proton stability to construct a model of the background contamination in each MagEIS detector by only considering times when the measurements are known to be background dominated. We demonstrate, for relativistic electron measurements in the inner zone, that the new technique is a significant improvement upon the routine background corrections that are used in the standard MagEIS data processing, which can “overcorrect” and therefore remove real (but small) electron fluxes. As an example, we show that the previously reported 1‐MeV injection into the inner zone that occurred in June of 2015 was distributed more broadly in Land persisted in the inner zone longer than suggested by previous estimates. Such differences can have important implications for both scientific studies and spacecraft engineering applications that make use of MagEIS electron data in the inner zone at relativistic energies. We compare these new results with prior work and present more recent observations that also show a 1‐MeV electron injection into the inner zone following the September 2017 interplanetary shock passage. All measurements suffer from error, which can arise from a variety of sources, including from the instrument itself (“noise”), as well as measured signals that are not the intended observation (“background”). When measurement errors can be quantified and accounted for, measurement accuracy, precision, and uncertainty can all be improved. This often leads to new discoveries in science. This work describes a new technique for quantifying and mitigating measurement error due to backgrounds in the Earth's Van Allen radiation belts. This allows us to measure the inner radiation belt with an increased level of accuracy and precision, enabling new scientific understanding. Specifically, we find that the inner radiation belt is longer lived and of greater intensity than suggested by previous work. Such findings are important scientifically, as they provide ground truth for radiation belt models and allow us to test various theories regarding the growth and decay of the inner radiation belt. This work is also important from a practical standpoint, as it helps improve statistical models that are used for spacecraft design, to determine how best to shield spacecraft and sensitive electronics from the damaging effects of the inner radiation belt. A new background correction algorithm for relativistic inner zone electrons is developedWe find important differences versus the standard algorithm, with several new/clarified features revealedData from the new algorithm should be used for quantitative inner zone studies at energies >0.7 MeV

A statistical study was conducted of Earth's radiation belt electron response to geomagnetic storms using NASA's Van Allen Probes mission. Data for electrons with energies ranging from 30 keV to 6.3 MeV were included and examined as a function of L‐shell, energy, and epoch time during 110 storms with SYM‐H≤−50 nT during September 2012 to September 2017 (inclusive). The radiation belt response revealed clear energy and L‐shell dependencies, with tens of keV electrons enhanced at all L‐shells (2.5 ≤ L ≤ 6) in all storms during the storm commencement and main phase and then quickly decaying away during the early recovery phase, low hundreds of keV electrons enhanced at lower L‐shells (~3 ≤ L ≤ ~4) in upward of 90% of all storms and then decaying gradually during the recovery phase, and relativistic electrons throughout the outer belt showing main phase dropouts with subsequent and generally unpredictable levels of replenishment during the recovery phase. Compared to prestorm levels, electrons with energies >1 MeV also revealed a marked increase in likelihood of a depletion at all L‐shells through the outer belt (3.5 ≤ L ≤ 6). Additional statistics were compiled revealing the storm time morphology of the radiation belts, confirming the aforementioned qualitative behavior. Considering storm drivers in the solar wind: storms driven by coronal mass ejection (CME) shocks/sheaths and CME ejecta only are most likely to result in a depletion of >1‐MeV electrons throughout the outer belt, while storms driven by full CMEs and stream interaction regions are most likely to produce an enhancement of MeV electrons at lower (L < ~5) and higher (L > ~4.5) L‐shells, respectively. CME sheaths intriguingly result in a distinct enhancement of ~1‐MeV electrons around L~5.5, and on average, CME sheaths and stream interaction regions result in double outer belt structures. A statistical model of electron radiation belt response to storms as a function of energy and L‐shell is developedStorm‐time morphology of the electron radiation belts is qualitatively predictableResults are better organized when the solar wind drivers of storms are identified

The evolution of the radiation belts in L‐shell (L), energy (E), and equatorial pitch angle (α0) is analyzed during the calm 11‐day interval (4–15 March) following the 1 March 2013 storm. Magnetic Electron and Ion Spectrometer (MagEIS) observations from Van Allen Probes are interpreted alongside 1D and 3D Fokker‐Planck simulations combined with consistent event‐driven scattering modeling from whistler mode hiss waves. Three (L, E, α0) regions persist through 11 days of hiss wave scattering; the pitch angle‐dependent inner belt core (L~ <2.2 and E< 700 keV), pitch angle homogeneous outer belt low‐energy core (L> ~5 and E~ < 100 keV), and a distinct pocket of electrons (L~ [4.5, 5.5] and E~ [0.7, 2] MeV). The pitch angle homogeneous outer belt is explained by the diffusion coefficients that are roughly constant for α0~ <60°, E> 100 keV, 3.5 < L< Lpp~ 6. Thus, observed unidirectional flux decays can be used to estimate local pitch angle diffusion rates in that region. Top‐hat distributions are computed and observed at L~ 3–3.5 and E= 100–300 keV. We study the evolution of the radiation belts during quiet geomagnetic times from satellite observations and numerical codes. We reach a global understanding of the trapped electrons variation with time, space, energy, and pitch angle (the angle of the velocity vector with the magnetic field). We exhibit three stable regions, which are less sensitive to scattering from hiss waves, while, on the other hand, hiss causes flux decay over 12 days that forms the slot region between the inner and outer belt. The existing theory explains why the outer belt electron decay is independent of pitch angle but dependent upon energy. This implies that satellite observations can reveal local pitch angle diffusion rates, themselves intimately connected with the wave properties. Thus, a connection is made between observed wave properties and observed/computed scattered electron flux, consistent with theory. Regions where the flux is pitch angle dependent are isolated in the low‐energy slot region where we show that the real shape is a smoothed version of the ideal top‐hat distribution computed from theory. The impact of this work is improved understanding of the belt evolution for space weather prediction, with a proposed event‐driven method that accurately (within ×2) predicts the electron flux decay after storms. Global computations of the (L, E, α0) structure of the evolving radiation belt during quiet times agree well with observationsThe inner belt decay is pitch angle dependent, while the outer belt is much more homogeneous with two distinct (L, E) regionsThe homogeneity of the pitch angle diffusion coefficient due to hiss waves explains the uniform outer belt decay and why 1D and 3D simulations agree

Nitric oxide infrared (5.3 µm) radiative cooling plays an important role in Earth's upper atmospheric energy balance during space weather events. Its behavior during a HILDCAA event following a geomagnetic storm can be more complicated than what is observed in an isolated geomagnetic storm (without HILDCAA). In order to understand this contrast, two moderate (SYM‐Hmin−80 nT; April 2005 (event‐1) and −65 nT; December 2006 (event‐2)), and an intense (Dstmin−150 nT; May 2000 (event‐3)) geomagnetic storms followed by HILDCAA events are considered for this study. It is found that, despite the larger solar wind‐magnetospheric energy coupling during the main phases of event‐1 and event‐2, the NO radiative cooling (NORC) and energetic electron precipitation (EEP) rates are larger during the HILDCAA phase. TIE‐GCM simulations indicate that the NORC pattern can be mainly attributed to variation in temperature, [O], and [NO] during HILDCAA. A larger rates of EEP due to the continuous substorm activity contributed to an additional ionization in D‐region and E‐region of the northern polar ionosphere during HILDCAA period of event‐2. This work is the first attempt to understand the NORC and EEP fluctuations during HILDCAA events preceded by geomagnetic events like event‐1 and event‐2. This study presents a comprehensive investigation of NORC and energetic electron (>27.93 keV) precipitation rate during HILDCAA event preceded by a moderate geomagnetic storm. An atmospheric chemistry‐based NORC emission model is used to understand the variation of NORC during intense geomagnetic storm (event‐3). The enhanced solar wind‐magnetosphere‐thermosphere energetics, and subsequent NO radiative cooling in lower thermosphere during extreme space weather events regulate the energy equilibrium of Earth's upper atmosphere. Enhance Joule heating and energetic particle precipitation in high‐latitude regions during these events result in temperature and compositional changes in mesosphere and lower thermosphere (MLT) region of the Earth's atmosphere. Enhanced production of NO and its subsequent emission of 5.3 μm make it a natural thermostat in lower thermosphere which cools the MLT region during geomagnetic activity. The enhanced EEP during these events can also enhance the D‐region and E‐region ionospheric densities by ionizing the Earth's upper atmosphere. In this work, we use Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) observation and specified dynamics (SD) Whole Atmosphere Community Climate Model with Thermosphere and Ionosphere eXtension (WACCM‐X) estimation to investigate NO radiative cooling in lower thermosphere during geomagnetic storm followed by High Intensity Long Duration Continuous Auroral Activity events (HILDCAA). Due to complicated energy coupling between solar wind‐magnetosphere‐thermosphere during these events, the temporal variations in NO radiative cooling can also be complicated. It is established in this study that NO radiative cooling correlates more with the thermospheric Joule heating than the solar wind‐magnetosphere energy coupling during such events. Thermospheric composition changes (mainly [O], [NO], and temperature), which are pivotal for NO radiative cooling, have also been investigated with the help of Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIE‐GCM 2.0) output data sets. Enhanced substorm activity during moderate storm following a HILDCAA event correlates well with the EEP rate of >27.93 keV and enhanced D‐region and E‐region ionospheric electron densities. NO 5.3 μm radiative emissions in the lower thermosphere are maximum during the HILDCAA events preceded by moderate stormsThe modeled (TIE‐GCM and SD WACCM‐X) NO cooling emission (5.3 μm) pattern is in agreement with the SABER observation during considered moderate stormsEnhanced EEP during HILDCAA events preceded by moderate storms enhances D‐region and E‐region electron densities in polar region NO 5.3 μm radiative emissions in the lower thermosphere are maximum during the HILDCAA events preceded by moderate storms The modeled (TIE‐GCM and SD WACCM‐X) NO cooling emission (5.3 μm) pattern is in agreement with the SABER observation during considered moderate storms Enhanced EEP during HILDCAA events preceded by moderate storms enhances D‐region and E‐region electron densities in polar region

The stimulation of electromagnetic ion cyclotron (EMIC) waves by a magnetospheric compression is perhaps the closest thing to a controlled experiment that is currently possible in magnetospheric physics, in that one prominent factor that can increase wave growth acts at a well‐defined time. We present a detailed analysis of EMIC waves observed in the outer dayside magnetosphere by the four Magnetosphere Multiscale (MMS) spacecraft, Van Allen Probe A, and GOES 13 and by four very high latitude ground magnetometer stations in the western hemisphere before, during, and after a modest interplanetary shock on 14 December 2015. Analysis shows several features consistent with current theory, as well as some unexpected features. During the most intense MMS wave burst, which began ~ 1 min after the end of a brief magnetosheath incursion, independent transverse EMIC waves with orthogonal linear polarizations appeared simultaneously at all four spacecraft. He++band EMIC waves were observed by MMS inside the magnetosphere, whereas almost all previous studies of He++band EMIC waves observed them only in the magnetosheath and magnetopause boundary layers. Transverse EMIC waves also appeared at Van Allen Probe A and GOES 13 very near the times when the magnetic field compression reached their locations, indicating that the compression lowered the instability threshold to allow for EMIC wave generation throughout the outer dayside magnetosphere. The timing of the EMIC waves at both MMS and Van Allen Probe A was consistent with theoretical expectations for EMIC instabilities based on characteristics of the proton distributions observed by instruments on these spacecraft. MMS observed a burst of independent transverse EMIC waves with orthogonal linear polarizations following the shockWave onsets at both MMS and Van Allen Probe A were consistent with theoretical expectations based on particle observationsA rarely observed minimum in wave power at the gyrofrequency of He++ions was present both before and after the shock

The substorm process releases large amounts of energy into the magnetospheric system, although where the energy is transferred to and how it is partitioned remains an open question. In this study, we address whether the substorm process contributes a significant amount of energy to the ring current. The ring current is a highly variable region, and understanding the energization processes provides valuable insight into how substorm‐ring current coupling may contribute to the generation of storm conditions and provide a source of energy for wave driving. In order to quantify the energy input into the ring current during the substorm process, we analyze Radiation Belt Storm Probes Ion Composition Experiment and Helium Oxygen Proton Electron ion flux measurements for H+, O+, and He+. The energy content of the ring current is estimated and binned spatially for Land magnetic local time. The results are combined with an independently derived substorm event list to perform a statistical analysis of variations in the ring current energy content with substorm phase. We show that the ring current energy is significantly higher in the expansion phase compared to the growth phase, with the energy enhancement persisting into the substorm recovery phase. The characteristics of the energy enhancement suggest the injection of energized ions from the tail plasma sheet following substorm onset. The local time variations indicate a loss of energetic H+ions in the afternoon sector, likely due to wave‐particle interactions. Overall, we find that the average energy input into the ring current is ∼9% of the previously reported energy released during substorms. The Earth's near‐space environment is populated by energetic charged particles, whose motion is largely controlled by the global geomagnetic field. This region, known as the magnetosphere, is highly dynamic and variable, strongly coupled to the solar wind (a continuous stream of charged particles outflowing from the Sun). At times, the Earth's magnetic field can become highly distorted and release a large amount of energy into the magnetospheric system. This process is termed a substorm, and the release of energy has significant consequences for the structure of the region and the characteristics of the plasma within it. The amount of energy that is transferred to the magnetospheric particle population remains to be fully understood. In this study, we use spacecraft measurements of highly energetic particles observed by the Van Allen Probes between 2012 and 2017. Using a statistical approach, we quantify the magnitude of the energy input into the particle population due to a typical substorm. Furthermore, we investigate the location of the energy enhancements, providing an insight into how energy is transported throughout the magnetospheric system. This study explored the spatial variations of the average ring current energy content for H+, O+, and He+ ionsThe ring current energy content increases following substorm onset with strong spatial dependencesApproximately 9% of the energy released in a typical substorm is transferred to the ring current

A magnetic conjunction between Van Allen Probes spacecraft and the Balloon Array for Radiation‐belt Relativistic Electron Losses (BARREL) reveals the simultaneous occurrence of broadband Alfvénic fluctuations and multi‐timescale modulation of enhanced atmospheric X‐ray bremsstrahlung emission. The properties of the Alfvénic fluctuations are used to build a model for pitch angle scattering in the outer radiation belt on electron gyro‐radii scale field structures. It is shown that this scattering may lead to the transport of electrons into the loss cone over an energy range from hundreds of keV to multi‐MeV on diffusive timescales on the order of hours. This process may account for modulation of atmospheric X‐ray fluxes observed from balloons and constitute a significant loss process for the radiation belts. Energetic particle populations in Earth's near‐space environment pose existential risks to spacecraft supporting our modern way of life. Understanding the enigmatic rapid variability of these populations is required to mitigate these risks. Coordinated observation from National Aeronautics and Space Administration (NASA)'s Van Allen Probes and Balloon Array for Radiation‐belt Relativistic Electron Losses has discovered a new mechanism through which energetic electrons in Earth's outer radiation belt may be depleted. This mechanism arises through the action of fine‐scale oscillations in Earth's magnetic field that are driven to large amplitudes in space weather events known as geomagnetic storms. These oscillations, known as kinetic Alfven waves, disrupt the orbit of energetic electrons around the Earth so that they precipitate into Earth's atmosphere. Estimates based on observations and theory suggest this mechanism is a significant driver of radiation belt depletion. Broadband electromagnetic waves drive radiation belt electrons into the atmosphereFinite gyro‐radii effects and drift‐bounce resonances modify pitch angle distributions of radiation belt electronsBroadband electromagnetic waves may be significant unrecognized drivers of atmospheric X‐ray emission and radiation belt depletion

Exohiss is a low‐frequency structureless whistler‐mode emission potentially contributing to the precipitation loss of radiation belt electrons outside the plasmasphere. Exohiss is usually considered the plasmaspheric hiss leaked out of the dayside plasmapause. However, the evolution of exohiss after the leakage has not been fully understood. Here we report the prompt enhancements of exohiss waves following substorm injections observed by Van Allen Probes. Within several minutes, the energetic electron fluxes around 100 keV were enhanced by up to 5 times, accompanied by an up to 10‐time increase of the exohiss wave power. These substorm‐injected electrons are shown to produce a new peak of linear growth rate in the exohiss band (< 0.1fce). The corresponding path‐integrated growth rate of wave power within 10° latitude of the magnetic equatorial plane can reach 13.4, approximately explaining the observed enhancement of exohiss waves. These observations and simulations suggest that the substorm‐injected energetic electrons could amplify the preexisting exohiss waves.

Magnetospheric plasma waves play a significant role in ring current and radiation belt dynamics, leading to pitch angle scattering loss and/or stochastic acceleration of the particles. During a non‐storm time dropout event on 24 September 2013, intense electromagnetic ion cyclotron (EMIC) waves were detected by Van Allen Probe A (Radiation Belt Storm Probes‐A). We quantitatively analyze a conjunction event when Van Allen Probe A was located approximately along the same magnetic field line as MetOp‐01, which detected simultaneous precipitation of >30 keV protons and energetic electrons over an unexpectedly broad energy range (>~30 keV). Multipoint observations together with quasi‐linear theory provide direct evidence that the observed electron precipitation at higher energy (>~700 keV) is primarily driven by EMIC waves. However, the newly observed feature of the simultaneous electron precipitation extending down to ~30 keV is not supported by existing theories and raises an interesting question on whether EMIC waves can scatter such low‐energy electrons. Energetic electrons can move from the magnetosphere into the Earth's upper atmosphere and cause chemical changes in the atmosphere leading to ozone reduction. The present paper studies the physical process that causes such electron precipitation. When a charged particle interacts with a plasma wave, its trajectory can be altered such that the particle falls into the upper atmosphere, but this process occurs only for a specific range of particle energy. In this study, we use one satellite (MetOp‐01) orbiting in the upper atmosphere (altitude ~800 km) that can detect particle precipitation, and a Van Allen Probes satellite, which provides wave measurements in the equatorial magnetosphere. During the electron precipitation detected by MetOp‐01, a Van Allen Probes satellite observed strong electromagnetic ion cyclotron (EMIC) waves. Multipoint satellite observations together with quasi‐linear theory provide a direct evidence that the observed electron precipitation is primarily driven by EMIC waves. Another new interesting finding is that the precipitation occurs not only for electrons at high energies (>~1 MeV) but also at low energies (down to ~30 keV). This newly observed feature is not supported by existing theories and raises an interesting question whether EMIC waves can interact with such low‐energy electrons as well. Strong electromagnetic ion cyclotron (EMIC) waves were observed during a non‐storm time electron dropout eventSimultaneous particle precipitation was observed for >30 keV protons and energetic electrons in a broad energy range (>~30 keV)Quasi‐linear theory shows that EMIC waves dominate precipitation of high‐energy electrons but underestimates low‐energy electron precipitation

Oblique whistler mode waves have been suggested to play an important role in radiation belt electron dynamics. Recently, X. Fu et al. (2017, https://doi.org/10.1002/2017GL074411) proposed that highly oblique lower band whistler waves could be generated by nonlinear three‐wave resonance. Here we present the first observational evidence of such process, using Van Allen Probes data, where an oblique lower band chorus wave is generated by two quasi‐parallel waves through nonlinear three‐wave interaction. The wave resonance condition is satisfied even in the presence of frequency chirping of one of the pump waves. Different from the simulation results of X. Fu et al. (2017), simultaneous particle data do not show a plateau in the electron distribution, which could be due to the very weak intensity of the generated waves. These results should help to better understand the generation of oblique waves in the inner magnetosphere and their relative roles in energetic electron dynamics. Oblique lower band whistler mode waves play an important role in the energetic electron dynamics in the magnetosphere. Recent research has also tried to understand how these highly oblique waves are generated. This paper presents the observations which directly confirm a recently proposed generation mechanism of oblique waves via nonlinear wave‐wave interaction. In this theory, an oblique whistler wave, called the daughter wave, can obtain energy from the coupling with two parent whistler waves. Our observation shows that the generated oblique whistler waves also exhibit frequency chirping, caused by the frequency chirping of one of the parent waves. However, the observed waves are too weak to effectively scatter or energize electrons. Our study should be useful to further understand how the oblique waves are excited and their effects on energetic electron dynamics in the inner magnetosphere. Oblique lower band chorus generated by nonlinear three‐wave interaction was observedThis study provides the first direct evidence for the theory proposed by Fu et alThe electron phase space density does not show a plateau because the generated wave is too weak

We survey radiation belt enhancement events during the Van Allen Probes era to determine what mechanism is the dominant cause of enhancements and where it is most effective. Two primary mechanisms have been proposed: (1) betatron/Fermi acceleration due to the Earthward radial transport of electrons, which produces monotonic gradients in phase space density (PSD), and (2) “local acceleration” due to gyro/Landau resonant interaction with electromagnetic waves, which produces radially localized growing peaks in PSD. To differentiate between these processes, we examine radial profiles of PSD in adiabatic coordinates using data from the Van Allen Probes and Time History of Events and Macroscale Interactions during Substorms satellites for 80 outer belt enhancement events from October 2012 to April 2017 This study shows that local acceleration is the dominant acceleration mechanism for MeV electrons in the outer belt, with 87% of the enhancement events exhibiting growing peaks. The strong correlation of the location of these with geomagnetic activity further supports this conclusion. The radiation belts are regions of space around the Earth that are known for their high‐energy particle populations. One of the key open questions to understanding how the radiation belts evolve is as follows: How do these particles get accelerated to such high energies? To address this question, two types of acceleration processes have been proposed. The first process takes particles located far from the Earth and moves them inward, energizing the particles as they move into the stronger magnetic field close to the Earth. The second process takes particles located in the heart of the radiation belts and energizes via interactions with electromagnetic waves in a process known as local acceleration. Differentiating between these processes has been historically difficult, and only recently are quality observations available for a long‐enough time period to understand when and where and how often each process is dominant. In this paper, we examine observations of the radiation belts from the last 4.5 years and show that local acceleration is the dominant acceleration process in the outer radiation belt. Local acceleration is the dominant acceleration mechanism for most MeV electron enhancements in the outer beltLocal acceleration is observed for both storm time and non–storm time eventsThe radial location of phase space density peaks are well correlated with geomagnetic activity and the plasmapause location

We present Van Allen Probe observations of electromagnetic ion cyclotron (EMIC) waves triggered solely due to individual substorm‐injected ions in the absence of storms or compressions of the magnetosphere during 9 August 2015. The time at which the injected ions are observed directly corresponds to the onset of EMIC waves at the location of Van Allen Probe A (L= 5.5 and 18:06 magnetic local time). The injection was also seen at geosynchronous orbit by the Geostationary Operational Environmental Satellite and Los Alamos National Laboratory spacecraft, and the westward(eastward) drift of ions(electrons) was monitored by Los Alamos National Laboratory spacecraft at different local times. The azimuthal location of the injection was determined by tracing the injection signatures backward in time to their origin assuming a dipolar magnetic field of Earth. The center of this injection location was determined to be close to ∼20:00 magnetic local time. Geostationary Operational Environmental Satellite and ground magnetometer responses confirm substorm onset at approximately the same local time. The observed EMIC wave onsets at Van Allen Probe were also associated with a magnetic field decrease. The arrival of anisotropic ions along with the decrease in the magnetic field favors the growth of the EMIC wave instability based on linear theory analysis. Substorm ion injections can trigger EMIC waves; a decrease in the magnetic field magnitude is associated with the arrival of injected ionsInjection signatures are observed at GOES, Van Allen Probe A, and LANL spacecraft; energy and pitch angle dispersion are observed by Van Allen ProbeDrift paths of injected ions/electrons from various LANL spacecraft are traced back to find their injection location at  20:00 MLT

To better understand rapid enhancements of the seed populations (hundreds of keV electrons) in the heart of the Earth's outer radiation belt (L*~ 3.5–5.0) during different geomagnetic activities, we investigate three enhancement events measured by Van Allen Probes in detail. Observations of the fluxes and the pitch angle distributions of energetic electrons are analyzed to determine rapid enhancements of the seed populations. Our study shows that three specified processes associated with substorm electron injections can lead to rapid enhancements of the seed populations, and the electron energy increases up to 342 keV. In the first process, substorm electron injections accompanied by the transient and intense substorm electric fields can directly lead to rapid enhancements of the seed populations in the heart of the outer radiation belt. In the second process, the substorm injected electrons are first trapped in the outer radiation belt and subsequently transported into L*< 4.5 by the convection electric field. In the third process, the lower energy electrons are first injected at L*~ 5.3 and then undergo drift resonance with ultralow‐frequency waves. These accelerated electrons by ultralow‐frequency waves are further transported into L*< 4.5 due to the convection electric field. This process is consistent with the radial diffusion. Our results suggest that these specified processes are important for understanding the dynamics of the seed populations in the heart of the outer radiation belt. Three specified different processes can lead to rapid enhancements of the seed populations in the heart of the Earth's outer radiation beltThe resonant acceleration associated with ULF waves is important for enhancements of the seed populations in L*~ 3.5–5.0These specified processes are important for understanding the dynamics of the seed populations in L*~ 3.5–5.0

Using the total radiation belt electron content calculated from Van Allen Probe phase space density, the time‐dependent and global response of the outer radiation belt during storms is statistically studied. Using phase space density reduces the impacts of adiabatic changes in the main phase, allowing a separation of adiabatic and nonadiabatic effects and revealing a clear modality and repeatable sequence of events in storm time radiation belt electron dynamics. This sequence exhibits an important first adiabatic invariant (μ)‐dependent behavior in the seed (150 MeV/G), relativistic (1,000 MeV/G), and ultrarelativistic (4,000 MeV/G) populations. The outer radiation belt statistically shows an initial phase dominated by loss followed by a second phase of rapid acceleration, while the seed population shows little loss and immediate enhancement. The time sequence of the transition to the acceleration is also strongly μdependent and occurs at low μfirst, appearing to be repeatable from storm to storm. The Earth's outer radiation belt is a region of near‐Earth space composed of highly energetic electrons. Typically, the outer radiation belt is in a quiet state; however, during geomagnetic storms the outer radiation belt becomes extremely dynamic. During these storms rapid changes in the number of energetic electrons in the outer radiation belt can lead to satellite failures. Our new research has found a level repeatability in storm time outer radiation belt dynamics not previously appreciated and offers important insights into radiation belt modeling and forecasting. This new work can be used to mitigate the negative effects radiation belt electrons can have on satellite infrastructure. Statistical assessment of the global and repeatable response of the outer radiation belt to stormsUsing radiation belt content derived from phase space density reduces impacts from adiabatic changes revealing a clear sequence of eventsThe statistical response is characterized by a fast genuine‐loss dominated phase followed by a clear acceleration phase

On 13 April 2013, the ACE spacecraft detected arrival of an interplanetary shock at 2250 UT, which is followed by the passage of the sheath region of an interplanetary coronal mass ejection (ICME) for a prolonged (18‐hr) period. The polarity of interplanetary magnetic field Bz was northward inside the magnetic cloud region of the ICME. The ring current (SYM‐H) index did not go below −7 nT during this event suggesting the absence of a typical geomagnetic storm. The responses of the global ionospheric electric field associated with the passage of the ICME sheath region have been investigated using incoherent scatter radar measurements of Jicamarca and Arecibo (postmidnight sector) along with the variations of equatorial electrojet strength over India (day sector). It is found that westward and eastward prompt penetration (PP) electric fields affected ionosphere over Jicamarca/Arecibo and Indian sectors, respectively, during 0545–0800 UT. The polarities of the PP electric field perturbations over the day/night sectors are consistent with model predictions. In fact, DP2‐type electric field perturbations with ∼40‐min periodicity are found to affect the ionosphere over both the sectors for about 2.25 hr during the passage of the ICME sheath region. This result shows that SYM‐H index may not capture the full geoeffectivenss of the ICME sheath‐driven storms and suggests that the PP electric field perturbations should be evaluated for geoeffectiveness of ICME when the polarity of interplanetary magnetic field Bz is northward inside the magnetic cloud region of the ICME. Effects of ICME sheath region is observed on equatorial ionosphere in the absence of a typical stormPrompt penetration electric field can capture the geoeffectiveness of an ICME eventThe geoeffectivenss of an ICME can be efficiently evaluated by the PP/OS electric field effects on the global ionosphere

Based on over 4 years of Van Allen Probes measurements, an empirical model of radiation belt electron equatorial pitch angle distribution (PAD) is constructed. The model, developed by fitting electron PADs with Legendre polynomials, provides the statistical PADs as a function of L‐shell (L= 1–6), magnetic local time, electron energy (~30 keV to 5.2 MeV), and geomagnetic activity (represented by the Dstindex) and is also the first empirical PAD model in the inner belt and slot region. For megaelectron volt electrons, model results show more significant day‐night PAD asymmetry of electrons with higher energies and during disturbed times, which is caused by geomagnetic field configuration and flux radial gradient changes. Steeper PADs with higher fluxes around 90° pitch angle and lower fluxes at lower pitch angles for higher‐energy electrons and during active times are also present, which could be due to electromagnetic ion cyclotron wave scattering. For hundreds of kiloelectron volt electrons, cap PADs are generally present in the slot region during quiet times and their energy‐dependent features are consistent with hiss wave scattering, while during active times, cap PADs are less significant especially at outer part of slot region, which could be due to the complex energizing and transport processes. The 90°‐minimum PADs are persistently present in the inner belt and appear in the slot region during active times, and minima at 90° pitch angle are more significant for electrons with higher energies, which could be a critical evidence in identifying the underlying physical processes responsible for the formation of 90°‐minimum PADs. Megaelectron volt electrons have steeper pitch angle distributions (PAD) during active times and for higher‐energy electrons, suggesting EMIC wave scatteringCap PADs are generally present in the slot region during quiet times with energy‐dependent features consistent with hiss wave scatteringThe 90°‐minimum PADs are persistently present in the inner belt, with more significant minima at 90° PA for electrons with higher energies

Whistler mode chorus waves are particularly important in outer radiation belt dynamics due to their key role in controlling the acceleration and scattering of electrons over a very wide energy range. The efficiency of wave-particle resonant interactions is defined by whistler wave properties which have been described by the approximation of plane linear waves propagating through the cold plasma of the inner magnetosphere. However, recent observations of extremely high-amplitude whistlers suggest the importance of nonlinear wave-particle interactions for the dynamics of the outer radiation belt. Oblique chorus waves observed in the inner magnetosphere often exhibit drastically nonsinusoidal (with significant power in the higher harmonics) waveforms of the parallel electric field, presumably due to the feedback from hot resonant electrons. We have considered the nature and properties of such nonlinear whistler waves observed by the Van Allen Probes and Time History of Events and Macroscale Interactions define during Substorms in the inner magnetosphere, and we show that the significant enhancement of the wave electrostatic component can result from whistler wave coupling with the beam-driven electrostatic mode through the resonant interaction with hot electron beams. Being modulated by a whistler wave, the electron beam generates a driven electrostatic mode significantly enhancing the parallel electric field of the initial whistler wave. We confirm this mechanism using a self-consistent particle-in-cell simulation. The nonlinear electrostatic component manifests properties of the beam-driven electron acoustic mode and can be responsible for effective electron acceleration in the inhomogeneous magnetic field. We consider the effects of induced scattering of the electromagnetic whistler wave to the electrostatic electron acoustic wave (observed as field-aligned electric field bursts). The main discussed effect is based on the coupling of the slightly oblique whistler wave and a beam-driven electron acoustic wave observed as “nonlinear whistler waves”. The wave interaction as the result produces the whistler wave and the rapidly steepening acoustic electrostatic wave with the same phase (and the same k and frequency). Then, because the two different modes are the result of the interaction, the following dynamics of the waves in the inhomogeneous magnetic field is different: the whistler wave phase velocity depends on the background magnetic field magnitude but the acoustic mode propagate with the constant phase velocity. This dynamics leads to the waves phase differences and explains the fact that the observed in the experiment whistler and electrostatic bursts usually have actually random phase shift. To confirm this, we studied the dynamics of these waves in the inhomogeneous magnetic field system making use of the particle-in-cell simulation, reproduced all steps of the modes conversion, and confirmed that the dynamics in the inhomogeneous plasma system leads to the observed effects. Nonlinear electrostatic steepening of whistler waves - the induced scattering to electron acoustic wavesResonant electron beam is highly modulated by a weakly oblique whistler waveModulated electron beam initiates the electron acoustic wave turned into the electrostatic nonlinear mode phase related to the whistler

Observations during the main phase of geomagnetic storms reveal an anticorrelation between the occurrence of broadband low-frequency electromagnetic waves and outer radiation belt electron flux. We show that the drift-bounce motion of electrons in the magnetic field of these waves leads to rapid electron transport. For observed spectral energy densities it is demonstrated that the wave magnetic field can drive radial diffusion via drift-bounce resonance on time scales less than a drift orbit. This process may provide outward transport sufficient to account for electron “dropouts” during storm main phase and more generally modulate the outer radiation belt during geomagnetic storms. Kinetic eigenmodes of the geomagnetic field may deplete the outer radiation belt during the main phase of geomagnetic stormsDrift-bounce resonances with the wave magnetic field can drive radial diffusion on time scales less than a drift periodBroadband electromagnetic waves commonly observed in the inner magnetosphere during geomagnetic storms may be an important unrecognized driver of radiation belt dynamics

We simulate the radiation belt electron flux enhancements during selected Geospace Environment Modeling (GEM) challenge events to quantitatively compare the major processes involved in relativistic electron acceleration under different conditions. Van Allen Probes observed significant electron flux enhancement during both the storm time of 17–18 March 2013 and non–storm time of 19–20 September 2013, but the distributions of plasma waves and energetic electrons for the two events were dramatically different. During 17–18 March 2013, the SYM‐Hminimum reached −130 nT, intense chorus waves (peak Bw~140 pT) occurred at 3.5 < L< 5.5, and several hundred keV to several MeV electron fluxes increased by ~2 orders of magnitude mostly at 3.5 < L< 5.5. During 19–20 September 2013, the SYM‐Hremained higher than −30 nT, modestly intense chorus waves (peak Bw~80 pT) occurred at L> 5.5, and electron fluxes at energies up to 3 MeV increased by a factor of ~5 at L> 5.5. The two electron flux enhancement events were simulated using the available wave distribution and diffusion coefficients from the GEM focus group Quantitative Assessment of Radiation Belt Modeling. By comparing the individual roles of local electron heating and radial transport, our simulation indicates that resonant interaction with chorus waves is the dominant process that accounts for the electron flux enhancement during the storm time event particularly near the flux peak locations, while radial diffusion by ultralow‐frequency waves plays a dominant role in the enhancement during the non–storm time event. Incorporation of both processes reasonably reproduces the observed location and magnitude of electron flux enhancement. Energetic electron fluxes are enhanced during storm and non–storm time events, but wave and electron structures are dramatically differentLocal heating by whistler mode chorus wave is the major contributor to the flux enhancement near the peak during 17–18 March 2013Radial diffusion by ultralow‐frequency wave is the major contributor to the observed flux enhancement during 19–20 September 2013

Using the Van Allen Probes/Helium, Oxygen, Proton, and Electron mass spectrometer, we perform a case study of the temporal evolution of ion spectral structures observed in the energy range of 1 to ~50 keV throughout the geomagnetic storm of 2 October 2013. The ion spectral features are observed near the inner edge of the plasma sheet and are signatures of fresh transport from the plasma sheet into the inner magnetosphere. We find that the characteristics of the ion structures are determined by the intensity of the convection electric field. Prior to the beginning of the storm, the plasma sheet inner edge exhibits narrow nose spectral structures that vary little in energy across Lvalues. Ion access to the inner magnetosphere during these times is limited to the nose energy bands. As convection is enhanced and large amounts of plasma are injected from the plasma sheet during the main phase of the storm, ion access occurs at a wide energy range, as no nose structures are observed. As the magnetosphere recovers from the storm, single noses and then multiple noses are observed once again. We use a model of ion drift and losses due to charge exchange to simulate the ion spectra and gain insight into the main observed features. The characteristics of the ion spectral structures are determined by the intensity of the convection electric field throughout the stormIon access is limited to narrow energy bands before and after the storm, while it occurs over a wide energy range during the stormThe ion range of access is more clearly species dependent before and after the storm, owing to charge exchange effects

There was a geomagnetic storm on 6–8 March 2016, in which Van Allen Probes A and B separated by ∼2.5 h measured increase of relativistic electrons with energies approximately several hundred keV to 1 MeV. Simultaneously, chorus waves were measured by both Van Allen Probes and Magnetospheric Multiscale (MMS) mission. Some of the chorus elements were rising tones, possibly due to nonlinear effects. These measurements are compared with a nonlinear theory of chorus waves incorporating the inhomogeneity ratio and the field equation. From this theory, a chorus wave profile in time and one‐dimensional space is simulated. Test particle calculations are then performed in order to examine the energization rate of electrons. Some electrons are accelerated, although more electrons are decelerated. The measured time scale of the electron increase is inferred to be consistent with this nonlinear theory. Relativistic electron increase within 2.5 h was accompanied by chorus wave activities during a geomagnetic stormA spatiotemporal profile of nonlinear chorus wave packets is simulated through theoretical modelingTest particle calculations imply that the time scale of relativistic electron increase is consistent with the nonlinear theory

How relativistic electrons are lost is an important question surrounding the complex dynamics of the Earth's outer radiation belt. Radial loss to the magnetopause and local loss to the atmosphere are two main competing paradigms. Here on the basis of the analysis of a radiation belt storm event on 27 February 2014, we present new evidence for the electromagnetic ion cyclotron (EMIC) wave‐driven local precipitation loss of relativistic electrons in the heart of the outer radiation belt. During the main phase of this storm, the radial profile of relativistic electron phase space density was quasi‐monotonic, qualitatively inconsistent with the prediction of radial loss theory. The local loss at low Lshells was required to prevent the development of phase space density peak resulting from the radial loss process at high Lshells. The rapid loss of relativistic electrons in the heart of outer radiation belt was observed as a dip structure of the electron flux temporal profile closely related to intense EMIC waves. Our simulations further confirm that the observed EMIC waves within a quite limited longitudinal region were able to reduce the off‐equatorially mirroring relativistic electron fluxes by up to 2 orders of magnitude within about 1.5 h. New evidence for EMIC-driven rapid loss of radiation belt relativistic electrons is provided by an event studyQuasi‐monotonic radial profile of relativistic electron phase space density formed because of the combination of radial and local loss processesRelativistic electron flux showed a dip structure closely related to intense EMIC waves

Using the electron phase space density (PSD) data measured by Van Allen Probe A from January 2013 to April 2015, we investigate the effects of magnetospheric processes on relativistic electron dynamics in the Earth's outer radiation belt during 50 geomagnetic storms. A statistical study shows that the maximum electron PSDs for various μ(μ =630, 1096, 2290, and 3311 MeV/G) at L*~4.0 after the storm peak have good correlations with storm intensity (cc~0.70). This suggests that the occurrence and magnitude of geomagnetic storms are necessary for relativistic electron enhancements at the inner edge of the outer radiation belt (L* =4.0). For moderate or weak storm events (SYM‐Hmin> ~−100 nT) with weak substorm activity (AEmax< 800 nT) and strong storm events (SYM‐Hmin≤ ~−100 nT) with intense substorms (AEmax≥ 800 nT) during the recovery phase, the maximum electron PSDs for various μat different L*values (L* =4.0, 4.5, and 5.0) are well correlated with storm intensity (cc >0.77). For storm events with intense substorms after the storm peak, relativistic electron enhancements at L* =4.5 and 5.0 are observed. This shows that intense substorms during the storm recovery phase are crucial to relativistic electron enhancements in the heart of the outer radiation belt. Our statistics study suggests that magnetospheric processes during geomagnetic storms have a significant effect on relativistic electron dynamics. The occurrence and magnitude of geomagnetic storms are necessary for relativistic electron enhancements at the inner edge of the outer radiation beltIntense substorms after the storm peak are crucial to relativistic electron enhancements in the heart of the outer radiation beltMagnetospheric processes during geomagnetic storms have a significant effect on relativistic electron dynamics

We present a multiple‐satellite observation of the magnetic dip event during the substorm on 10 October 2013. The observation illustrates the temporal and spatial evolution of the magnetic dip and gives a compelling evidence that ring current ions induce the magnetic dip by enhanced plasma beta. The dip moves with the energetic ions in a comparable drift velocity and affects the dynamics of relativistic electrons in the radiation belt. In addition, the magnetic dip provides a favorable condition for the electromagnetic ion cyclotron (EMIC) wave generation based on the linear theory analysis. The calculated proton diffusion coefficients show that the observed EMIC wave can lead to the pitch angle scattering losses of the ring current ions, which in turn partially relax the magnetic dip in the observations. This study enriches our understanding of magnetic dip evolution and demonstrates the important role of the magnetic dip for the coupling of radiation belt and ring current. Multiple satellites observation of the magnetic dip is presentedThe magnetic dip is formed by and moves with energetic ionsThe magnetic dip induces the generation of EMIC waves, which in turn partially relax the magnetic dip by scattering ring current ions

A representative case study from the Van Allen Probes during a geomagnetic storm recovery phase reveals enhanced electron fluxes at intermediate pitch angles over energies from ~100 keV to 5 MeV coincident with broadband low‐frequency electromagnetic waves. The statistical properties of these waves are used to build a model for radial diffusion via drift‐bounce resonances in kinetic Alfvén eigenmodes/kinetic field‐line resonances. Estimated diffusion coefficients indicate timescales for radial transport on the order of hours in storm time events at energies from <100 keV to MeVs over equatorial pitch angles from the edge of the loss cone to nearly perpendicular to the geomagnetic field. The correlation of kinetic resonances with electron depletions and enhancements during storm main phase and recovery, and the rapid diffusion these waves drive, suggests that they may modulate the outer radiation belt. Kinetic resonances of the geomagnetic field drive radial transport of outer radiation belt electrons via drift‐bounce resonanceRadial diffusion in these waves drives depletion/enhancement of outer radiation belt electron fluxes over energies from ~100 keV to multiple MeVThese waves may play a role in the rapid variation of the outer radiation belt during geomagnetic storms

We present an analysis of “boomerang‐shaped” pitch angle evolutions of outer radiation belt relativistic electrons observed by the Van Allen Probes after the passage of an interplanetary shock on 7 June 2014. The flux at different pitch angles is modulated by Pc5 waves, with equatorially mirroring electrons reaching the satellite first. For 90° pitch angle electrons, the phase change of the flux modulations across energy exceeds 180° and increasingly tilts with time. Using estimates of the arrival time of particles of different pitch angles at the spacecraft location, a scenario is investigated in which shock‐induced ULF waves interact with electrons through the drift resonance mechanism in a localized region westward of the spacecraft. Numerical calculations on particle energy gain with the modified ULF wavefield reproduce the observed boomerang stripes and modulations in the electron energy spectrogram. The study of boomerang stripes and their relationship to drift resonance taking place at a location different from the observation point adds new understanding of the processes controlling the dynamics of the outer radiation belt. Boomerang‐shaped pitch angle evolutions of relativistic electrons in Pc5 band is observed for the first time in the outer radiation beltElectrons' drift resonating with azimuthally localized ULF waves can produce boomerang‐shaped pitch angle stripesNonlocal effects in drift‐resonant process on outer belt dynamics

We present simulations of the loss of radiation belt electrons by resonant pitch angle diffusion caused by whistler mode hiss waves for March 2013. Pitch angle diffusion coefficients are computed from the wave properties and the ambient plasma data obtained by the Van Allen Probes with a resolution of 8 h and 0.1 Lshell. Loss rates follow a complex dynamic structure, imposed by the wave and plasma properties. Hiss effects can be strong, with minimum lifetimes (of ~1 day) moving from energies of ~100 keV at L~ 5 up to ~2 MeV at L~ 2 and stop abruptly, similarly to the observed energy‐dependent inner belt edge. Periods when the plasmasphere extends beyond L~ 5 favor long‐lasting hiss losses from the outer belt. Such loss rates are embedded in a reduced Fokker‐Planck code and validated against Magnetic Electron and Ion Spectrometer observations of the belts at all energy. Results are complemented with a sensitivity study involving different radial diffusion and lifetime models. Validation is carried out globally at all Lshells and energies. The good agreement between simulations and observations demonstrates that hiss waves drive the slot formation during quiet times. Combined with transport, they sculpt the energy structure of the outer belt into an “S shape.” Low energy electrons (<0.3 MeV) are less subject to hiss scattering below L= 4. In contrast, 0.3–1.5 MeV electrons evolve in an environment that depopulates them as they migrate from L~ 5 to L~ 2.5. Ultrarelativistic electrons are not affected by hiss losses until L~ 2–3. Computations of daily pitch angle diffusion coefficients and electron lifetimes from properties of hiss waves observed in March 2013Good agreement found between MagEIS flux observations and 1‐D Fokker‐Planck simulations based on our hiss loss term for quiet timesCombined with transport, hiss waves loss drives the daily energy structure of the radiation belts, with a typical S‐shaped outer belt

Electrostatic electron cyclotron harmonic (ECH) waves generated by the electron loss cone distribution can produce efficient scattering loss of plasma sheet electrons, which has a significant effect on the dynamics in the outer magnetosphere. Here we report two ECH emission events around the same location L≈ 5.7–5.8, MLT ≈ 12 from Van Allen Probes on 11 February (event A) and 9 January 2014 (event B), respectively. The spectrum of ECH waves was centered at the lower half of the harmonic bands during event A, but the upper half during event B. The observed electron phase space density in both events is fitted by the subtracted bi‐Maxwellian distribution, and the fitting functions are used to evaluate the local growth rates of ECH waves based on a linear theory for homogeneous plasmas. ECH waves are excited by the loss cone instability of 50 eV–1 keV electrons in the lower half of harmonic bands in the low‐density plasmasphere in event A, and 1–10 keV electrons in the upper half of harmonic bands in a relatively high‐density region in event B. The current results successfully explain observations and provide a first direct evidence on how ECH waves are generated in the lower and upper half of harmonic frequency bands. Initial data reported on two ECH wave events around the same location but in the different frequency bandNumerical simulations show that ECH waves can be generated in the different half of harmonic frequency bandsThe background density and electron temperature are the main factors to yield the difference of wave spectrum

The Van Allen Probes observe generally two types of electrostatic solitary waves (ESW) contributing to the broadband electrostatic wave activity in the nightside inner magnetosphere. ESW with symmetric bipolarparallel electric field are electron phase space holes. The nature of ESW with asymmetric bipolar(and almost unipolar) parallel electric field has remained puzzling. To address their nature, we consider a particular event observed by Van Allen Probes to argue that during the broadband wave activity electrons with energy above 200 eV provide the dominant contribution to the total electron density, while the density of cold electrons (below a few eV) is less than a few tenths of the total electron density. We show that velocities of the asymmetric ESW are close to velocity of electron‐acoustic waves (existing due to the presence of cold and hot electrons) and follow the Korteweg‐de Vries (KdV) dispersion relation derived for the observed plasma conditions (electron energy spectrum is a power law between about 100 eV and 10 keV and Maxwellian above 10 keV). The ESW spatial scales are in general agreement with the KdV theory. We interpret the asymmetric ESW in terms of electron‐acoustic solitons and double layers (shocks waves). Electrostatic solitary waves with asymmetric bipolar parallel electric field contribute to broadband wave activity in inner magnetosphereAsymmetric ESW follow KdV dispersion relation and have spatial scales comparable to those of electron‐acoustic solitonsAsymmetric ESW are interpreted in terms of electron‐acoustic solitons and electron‐acoustic shock waves

Energetic (hundreds of keV) electrons in the radiation belt slot region have been found to exhibit the butterfly pitch angle distributions. Resonant interactions with magnetosonic and whistler-mode waves are two potential mechanisms for the formation of these peculiar distributions. Here we perform a statistical study of energetic electron pitch angle distribution characteristics measured by Van Allen Probes in the slot region during a 3 year period from May 2013 to May 2016. Our results show that electron butterfly distributions are closely related to magnetosonic waves rather than to whistler-mode waves. Both electron butterfly distributions and magnetosonic waves occur more frequently at the geomagnetically active times than at the quiet times. In a statistical sense, more distinct butterfly distributions usually correspond to magnetosonic waves with larger amplitudes and vice versa. The averaged magnetosonic wave amplitude is less than 5 pT in the case of normal and flat-top distributions with a butterfly index BI =1 but reaches ~50–95 pT in the case of distinct butterfly distributions with BI >1.3. For magnetosonic waves with amplitudes >50 pT, the occurrence rate of butterfly distribution is above 80%. Our study suggests that energetic electron butterfly distributions in the slot region are primarily caused by magnetosonic waves. Electron butterfly distributions occur more frequently at the geomagnetically active times than at the quite timesA positive relation between magnetosonic waves and electron butterfly distributionsNo close relation between whistler-mode waves and electron butterfly distributions

This study tries to bring out the fact that storm time substorms can compete and at times significantly contribute to the geomagnetically disturbed time prompt penetration electric field effects on low and equatorial latitudes. Observations of unusual equatorial plasma drift data from Jicamarca Unattended Long‐term Investigations of the Ionosphere and Atmosphere during two space weather events show that substorms can induce both eastward and westward penetration electric fields under steady southward interplanetary magnetic field (IMF Bz) conditions. During the first event on 2 January 2005, the enhancement of the daytime eastward electric field over Jicamarca due to substorm is found to be comparable with the Sqand interplanetary electric field (IEFy) generated electric fields combined. During the second event on 19 August 2006, the substorm is seen to weaken the daytime eastward field thereby inducing a westward field in spite of the absence of northward turning of IMF Bz(overshielding). The westward electric field perturbation in the absence of any overshielding events is observationally sparse and contrary to the earlier results. Further, the substorm‐induced field is found to be strong enough to compete or almost nullify the effects of storm time IEFy fields. This study also shows quantitatively that at times substorm contribution to the disturbed time prompt electric fields can be significant and thus should be taken into consideration in evaluating penetration events over low latitudes. Unusually large storm time plasma drifts are observed by JULIA despite small IEFy magnitudesSubstorms at times can contribute significantly to the disturbed time electric field in equatorial ionosphereSubstorms induce both eastward and westward electric fields over equatorial latitudes

Using the particle data measured by Van Allen Probe A from October 2012 to March 2016, we investigate in detail the radiation belt seed population and its association with the relativistic electron dynamics during 74 geomagnetic storms. The period of the storm recovery phase was limited to 72 h. The statistical study shows that geomagnetic storms and substorms play important roles in the radiation belt seed population (336 keV electrons) dynamics. Based on the flux changes of 1 MeV electrons before and after the storm peak, these storm events are divided into two groups of “large flux enhancement” and “small flux enhancement.” For large flux enhancement storm events, the correlation coefficients between the peak flux location of the seed population and those of relativistic electrons (592 keV, 1 MeV, 1.8 MeV, and 2.1 MeV) during the storm recovery phase decrease with electron kinetic energy, being 0.92, 0.68, 0.49, and 0.39, respectively. The correlation coefficients between the peak flux of the seed population and those of relativistic electrons are 0.92, 0.81, 0.75, and 0.73. For small flux enhancement storm events, the correlation coefficients between the peak flux location of the seed population and those of relativistic electrons are relatively smaller, while the peak flux of the seed population is well correlated with those of relativistic electrons (correlation coefficients >0.84). It is suggested that during geomagnetic storms there is a good correlation between the seed population and ≤1 MeV electrons and the seed population is important to the relativistic electron dynamics. Geomagnetic storms and substorms play important roles in the radiation belt seed population dynamicsThe radiation belt seed population is well correlated with ≤1 MeV electronsThe radiation belt seed population is important to the relativistic electron dynamics