Search

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

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

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

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

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)

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

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

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

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

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

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 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

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

Using the Van Allen Probe long‐term (2013–2015) observations and quasi‐linear simulations of wave‐particle interactions, we examine the combined or competing effects of whistler mode waves (chorus or hiss) and magnetosonic (MS) waves on energetic (<0.5 MeV) and relativistic (>0.5 MeV) electrons inside and outside the plasmasphere. Although whistler mode chorus waves and MS waves can singly or jointly accelerate electrons from the hundreds of keV energy to the MeV energy in the low‐density trough, most of the relativistic electron enhancement events are best correlated with the chorus wave emissions outside the plasmapause. Inside the plasmasphere, intense plasmaspheric hiss can cause the net loss of relativistic electrons via persistent pitch angle scattering, regardless of whether MS waves were present or not. The intense hiss waves not only create the energy‐dependent electron slot region but also remove a lot of the outer radiation belt electrons when the expanding dayside plasmasphere frequently covers the outer zone. Since whistler mode waves (chorus or hiss) can resonate with more electrons than MS waves, they play dominant roles in changing the outer radiation belt and the slot region. However, MS waves can accelerate the energetic electrons below 400 keV and weaken their loss inside the plasmapause. Thus, MS waves and plasmaspheric hiss generate different competing effects on energetic and relativistic electrons in the high‐density plasmasphere. Most relativistic electron enhancement events are correlated with intense chorus waves rather than MS waves outside the plasmapauseIntense hiss waves can cause the net loss of relativistic electrons in the plasmasphere, regardless of whether MS waves were present or notPlasmaspheric hiss also removes a lot of the outer radiation belt electrons when the expanding dayside plasmasphere covers the outer zone

We have studied the spatial location relative to the plasmapause (PP) of the most intense electromagnetic ion cyclotron (EMIC) waves observed on Van Allen Probes A and B during their first full precession in local time. Most of these waves occurred over an Lrange of from −1 to +2 RErelative to the PP. Very few events occurred only within 0.1 REof the PP, and events with a width in Lof < 0.2 REoccurred both inside and outside the PP. Wave occurrence was always associated with high densities of ring current ions; plasma density gradients or enhancements were associated with some events but were not dominant factors in determining the sites of wave generation. Storm main and recovery phase events in the dusk sector were often inside the PP, and dayside events during quiet times and compressions of the magnetosphere were more evenly distributed both inside and outside the PP. Superposed epoch analyses of the dependence of wave onset on solar wind dynamic pressure (Psw), the SME (SuperMAG auroral electrojet) index, and the SYM‐Hindex showed that substorm injections and solar wind compressions were temporally closely associated with EMIC wave onset but to an extent that varied with frequency band, magnetic local time, and storm phase, and location relative to the PP. The fact that increases in SME and Psw were less strongly correlated with events at the PP than with other events might suggest that the occurrence of those events was affected by the density gradient. A statistical and superposed epoch study of the 158 most intense EMIC wave events in the Van Allen Probes data set has been performedEMIC wave onsets were often temporally linked to recent particle injections and/or increases in solar wind pressurePlasma density gradients or enhancements were not dominant factors in determining the sites of wave generation

We present measurements of relativistic electrons (0.7–1.5 MeV) in the inner zone and slot region obtained by the Magnetic Electron and Ion Spectrometer (MagEIS) instrument on Van Allen Probes. The data presented are corrected for background contamination, which is primarily due to inner‐belt protons in these low‐Lregions. We find that ∼1 MeV electrons were transported into the inner zone following the two largest geomagnetic storms of the Van Allen Probes era to date, the March and June 2015 events. As ∼1 MeV electrons were not observed in Van Allen Probes data in the inner zone prior to these two events, the injections created a new inner belt that persisted for at least 1.5 years. In contrast, we find that electrons injected into the slot region decay on much faster timescales, approximately tens of days. Furthermore, we find no evidence of >1.5 MeV electrons in the inner zone during the entire time interval considered (April 2013 through September 2016). The energies we examine thus span a transition range in the steeply falling inner zone electron spectrum, where modest intensities are observed at 0.7 MeV, and no electrons are observed at 1.5 MeV. To validate the results obtained from the background corrected flux measurements, we also present detailed pulse‐height spectra from individual MagEIS detectors. These measurements confirm our results and also reveal low‐intensity inner zone and slot region electrons that are not captured in the standard background corrected data product. Finally, we briefly discuss efforts to refine the upper limit of inner zone MeV electron flux obtained in earlier work. ∼1 MeV electrons can be injected into the inner zone during large storms, forming new belts that decay very slowly0.7–1.5 MeV electrons that are transported into the slot region decay rapidly (∼10 days)We find no evidence of electrons >1.5 MeV in the inner zone during the interval considered (April 2013 through September 2016)

Recent studies have shown that chorus can efficiently accelerate the outer radiation belt electrons to relativistic energies. Chorus, previously often observed above 0.1 equatorial electron gyrofrequency fce, was generated by energetic electrons originating from Earth's plasma sheet. Chorus below 0.1 fcehas seldom been reported until the recent data from Van Allen Probes, but its origin has not been revealed so far. Because electron resonant energy can approach the relativistic level at extremely low frequency, relativistic effects should be considered in the formula for whistler mode wave growth rate. Here we report high‐resolution observations during the 14 October 2014 small storm and firstly demonstrate, using a fully relativistic simulation, that electrons with the high‐energy tail population and relativistic pitch angle anisotropy can provide free energy sufficient for generating chorus below 0.1 fce. The simulated wave growth displays a very similar pattern to the observations. The current results can be applied to Jupiter, Saturn, and other magnetized planets. Initial RBSP correlated data of ELF chorus waves and energetic electron fluxesA fully relativistic treatment is adopted to examine how ELF chorus is generatedELF chorus can be generated by the high‐energy tail and relativistic anisotropy

On 7 January 2005 (Ap=40) prompt penetration electric field perturbations of opposite polarities were observed over Thumba and Jicamarca on a few occasions during 13:45–16:30 UT. However, the electric field was found to be eastward during 14:45–15:30 UT over both Thumba and Jicamarca contrary to the general expectation wherein opposite polarities are expected at nearly antipodal points. On closer scrutiny, three important observational features are noticed during 14:10–15:15 UT. First, during 14:10–14:45 UT, despite increasing southward interplanetary magnetic field (IMF) Bzcondition, the already westward electric field over Thumba weakened (less westward) while the eastward electric field over Jicamarca intensified (more eastward). Second, the electric field not only became anomalously eastward over Thumba but also got intensified further during 14:45–15:00 UT similar to Jicamarca. Third, during 15:00–15:15 UT, despite IMF Bzremaining steadily southward, the eastward electric field continued to intensify over Thumba but weakened over Jicamarca. It is suggested that the changes in IMF Bycomponent under southward IMF Bzcondition are responsible for skewing the ionospheric equipotential patterns over the dip equator in such a way that Thumba came into the same DP2 cell as that of Jicamarca leading to anomalous electric field variations. Magnetic field measurements along the Indian and Jicamarca longitude sectors and changes in high‐latitude ionospheric convection patterns provide credence to this proposition. Thus, the present investigation shows that the variations in IMF Byare fundamentally important to understand the prompt penetration effects over low latitudes. Identical polarity of penetration electric field over Thumba and JicamarcaRole of IMF Byin determining polarity of prompt penetration electric fieldChanges in IMF Bydistort DP2 cells to bring antipodal stations (Thumba and Jicamarca) in the same cell

Accurate knowledge of the plasma fluxes in the inner magnetosphere is essential for both scientific and programmatic applications. Knowledge of the low‐energy electrons (approximately tens to hundreds of eV) in the inner magnetosphere is particularly important since these electrons are acted upon by various physical processes, accelerating the electrons to higher energies, and also causing their loss. However, measurements of low‐energy electrons are challenging, and as a result, this population has been somewhat neglected previously. This study concerns observations of low‐energy electrons made by the Helium Oxygen Proton Electron instrument on board the Van Allen Probes satellites and also observations from geosynchronous orbit made by the Magnetospheric Plasma Analyzer on board Los Alamos National Laboratory satellites. The fluxes of electrons from ~30 eV to 1 keV are quantified as a function of pitch‐angle, McIlwain L parameter, and local time for both quiet and active periods. Results indicate two sources for low‐energy electrons in this energy range: the low‐energy tail of the electron plasma sheet and the high‐energy tail of the dayside ionosphere. These populations are identified primarily as a result of their different pitch‐angle distributions. Field‐aligned outflows from the dayside ionosphere are observed at all L shells during quiet and active periods. Our results also demonstrate that the dayside electron field‐aligned fluxes at ~30 eV are particularly strong between L values of 6 and 7, indicating an enhanced source within the polar ionosphere. Low‐energy electrons (tens to hundreds of eV) originate from two main sources: the ionosphere and the plasma sheetLow‐energy electrons pervade the inner magnetosphere where they can drive wave‐particle interactionsFluxes of electrons from ~30 eV to 1 keV are quantified by pitch‐angle, L value, and local time for both quiet and active periods

We present cross‐scale magnetospheric observations of the 17 March 2015 (St. Patrick's Day) storm, by Time History of Events and Macroscale Interactions during Substorms (THEMIS), Van Allen Probes (Radiation Belt Storm Probes), and Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS), plus upstream ACE/Wind solar wind data. THEMIS crossed the bow shock or magnetopause 22 times and observed the magnetospheric compression that initiated the storm. Empirical models reproduce these boundary locations within 0.7 RE. Van Allen Probes crossed the plasmapause 13 times; test particle simulations reproduce these encounters within 0.5 RE. Before the storm, Van Allen Probes measured quiet double‐nose proton spectra in the region of corotating cold plasma. About 15 min after a 0605 UT dayside southward turning, Van Allen Probes captured the onset of inner magnetospheric convection, as a density decrease at the moving corotation‐convection boundary (CCB) and a steep increase in ring current (RC) proton flux. During the first several hours of the storm, Van Allen Probes measured highly dynamic ion signatures (numerous injections and multiple spectral peaks). Sustained convection after ∼1200 UT initiated a major buildup of the midnight‐sector ring current (measured by RBSP A), with much weaker duskside fluxes (measured by RBSP B, THEMIS a and THEMIS d). A close conjunction of THEMIS d, RBSP A, and TWINS 1 at 1631 UT shows good three‐way agreement in the shapes of two‐peak spectra from the center of the partial RC. A midstorm injection, observed by Van Allen Probes and TWINS at 1740 UT, brought in fresh ions with lower average energies (leading to globally less energetic spectra in precipitating ions) but increased the total pressure. The cross‐scale measurements of 17 March 2015 contain significant spatial, spectral, and temporal structure. Observations by THEMIS, Van Allen Probes, and TWINS contain much spatial, spectral, and temporal variationDuring main phase, all three missions measured two‐peak ion spectrum in center of partial ring currentEncounters with bow shock, magnetopause (by THEMIS) and plasmapause (RBSP) reproduced by models

Using observations from NASA's Van Allen Probes, we study the role of sudden particle enhancements at low L shells (SPELLS) as a source of inner radiation belt electrons. SPELLS events are characterized by electron intensity enhancements of approximately an order of magnitude or more in less than 1 day at L < 3. During quiet and average geomagnetic conditions, the phase space density radial distributions for fixed first and second adiabatic invariants are peaked at 2 < L < 3 for electrons ranging in energy from ~50 keV to ~1 MeV, indicating that slow inward radial diffusion is not the dominant source of inner belt electrons under quiet/average conditions. During SPELLS events, the evolution of electron distributions reveals an enhancement of phase space density that can exceed 3 orders of magnitude in the slot region and continues into the inner radiation belt, which is evidence that these events are an important—and potentially dominant—source of inner belt electrons. Electron fluxes from September 2012 through February 2016 reveal that SPELLS occur frequently (~2.5/month at 200 keV), but the number of observed events decreases exponentially with increasing electron energy for ≥100 keV. After SPELLS events, the slot region reforms due to slow energy‐dependent decay over several day time scales, consistent with losses due to interactions with plasmaspheric hiss. Combined, these results indicate that the peaked phase space density distributions in the inner electron radiation belt result from an “on/off,” geomagnetic‐activity‐dependent source from higher radial distances. Phase space density distributions of tens to hundreds of keV electrons in the inner belt are peaked in L* during quiet and average conditionsSudden flux enhancements during active conditions are the dominant source of hundreds of keV electrons in the inner beltThe occurrence of sudden flux enhancements at low L is strongly dependent on energy

During a weak geomagnetic storm (Ap = 15) on 24 December 2014, the penetration electric field perturbations over the Indian dip equatorial sector are found to be anomalous on a number of occasions during postsunset hours. The event is anomalous as the magnitude and polarity of penetration electric fields do not obey the existing paradigm. The penetration electric field perturbations are investigated using the vertical drifts derived from the CADI (Canadian Advanced Digital Ionosonde) measurements at Tirunelveli (8.7°N, 77.7°E, dip angle: 1.7°). During this event, we observed postsunset vertical drift of ∼42 m s−1not only at 18:10 LT but also ∼36 m s−1at ∼21:00 LT which is anomalous. Interestingly, the dawn‐dusk component of interplanetary electric field (IEFy) is relatively less (<2 mV/m) at ∼21:00 LT compared to the interval 19:30–20:30 LT (IEFy ∼3 mV/m). Despite that, the vertical drift observed over Tirunelveli is very close to zero or nominally upward during 19:30–20:30 LT. In addition, the downward drift just after 21:30 LT on this night is found to be exceptionally large (∼−60 m s−1). By combining vertical total electron content over the Indian sector with the OI 630.0 nm airglow intensity from Mt. Abu, chain of magnetometer and Los Alamos National Laboratory geosynchronous satellite particle measurements, it is suggested that the anomalous penetration electric field perturbations on this night arise from the effects of interplanetary magnetic field By and substorm. Variation in the zonal electric field in the equatorial ionosphere during postsunset hours is important to understand the plasma distribution over low latitudes and also generation of plasma irregularities. The changes in the ionospheric conditions over low/equatorial latitudes have implications for communication and navigational applications. Therefore, if ionospheric electric field over equatorial ionosphere behaves anomalously during space weather events, it will be difficult to model the low latitude ionosphere for scientific understanding and practical applications. In this investigation, we show that the less studied Y‐component of interplanetary magnetic field and substorm can significantly modulate the ionospheric electric field giving rise to anomalous response. Under southward interplanetary magnetic field (IMF) Bz conditions, anomalous electric field perturbations are observed over the equatorial ionosphereThe amplitude of electric field perturbations is more when the magnitude of IEFy is less and vice versaThese anomalous electric field perturbations are explained based on the effects of IMF By and substorm Under southward interplanetary magnetic field (IMF) Bz conditions, anomalous electric field perturbations are observed over the equatorial ionosphere The amplitude of electric field perturbations is more when the magnitude of IEFy is less and vice versa These anomalous electric field perturbations are explained based on the effects of IMF By and substorm

The auroral streamer is a type of auroral form commonly observed during geomagnetic substorms. Previous studies suggested a coupling between auroral streamers and channels of bursty bulk flows in the plasma sheet. However, whether one flow channel can map to multiple streamers is unclear. Here, we present an event containing consecutive auroral streamer subevents. The event features similar spatial and temporal development of auroral streamers in conjugate hemispheres. In most of the subevents, we observed that what an in‐space auroral camera saw as one streamer actually consisted of multiple streamers (i.e., a streamer bundle). A coordinated analysis of near‐earth injections and ionospheric currents suggests that one overall flow channel (or bubble) can map to a streamer bundle. Multiple streamer bundles and thus multiple overall flow channels can occur simultaneously at different local times. Evidence supports that the overall flow channel may consist of or split into several narrower flow channels and each maps to a streamer and causes a particle injection. Geomagnetic substorm is a complex energy release process involving many observable features in the geospace. Common substorm features include enhanced auroral activities and currents in the ionosphere, enhanced fluxes of energetic electrons and ions (injections), and localized bursty bulk flows (BBFs) in the nightside magnetosphere. It is well established that injection, BBF, and a type of aurora called auroral streamer are physically related. Here, we present an event when streamers form bundles in successive subevents and discuss how streamers and streamer bundles are related to BBF and injection. Auroral data show that these streamer bundles developed similarly in both hemispheres. We show observational evidence that a streamer bundle maps to an overall BBF and streamers within a bundle map to narrower BBFs. Similar spatial and temporal development of bundles of auroral streamers is observed in conjugate hemispheresA streamer bundle may map to an overall bubble in the plasma sheetStreamers within a streamer bundle may map to narrower bubbles Similar spatial and temporal development of bundles of auroral streamers is observed in conjugate hemispheres A streamer bundle may map to an overall bubble in the plasma sheet Streamers within a streamer bundle may map to narrower bubbles

Although lagged correlations have suggested influences of solar wind velocity (V) and number density (N), Bz, ultralow frequency (ULF) wave power, and substorms (as measured by the auroral electrojet (AE) index) on MeV electron flux at geosynchronous orbit over an impressive number of hours and days, a satellite's diurnal cycle can inflate correlations, associations between drivers may produce spurious effects, and correlations between all previous time steps may create an appearance of additive influence over many hours. Autoregressive‐moving average transfer function (ARMAX) multiple regressions incorporating previous hours simultaneously can eliminate cycles and assess the impact of parameters, at each hour, while others are controlled. ARMAX influences are an order of magnitude lower than correlations uncorrected for time behavior. Most influence occurs within a few hours, not the many hours suggested by correlation. A log transformation accounts for nonlinearities. Over all hours, solar wind velocity (V) and number density (N) show an initial negative impact, with longer term positive influences over the 9 (V) or 27 (N) hr. Bz is initially a positive influence, with a longer term (6 hr) negative effect. ULF waves impact flux in the first (positive) and second (negative) hour before the flux measurement, with further negative influences in the 12–24 hr before. AE (representing electron injection by substorms) shows only a short term (1 hr) positive influence. However, when only recovery and after‐recovery storm periods are considered (using stepwise regression), there are positive influences of ULF waves, AE, and V, with negative influences of Nand Bz. The influence of solar wind, waves, and substorms on high energy electrons at geosynchronous orbit can appear to occur over a number of hours and days. However, these long duration correlations may be due to diurnal cycles in satellite data, associations between the driving parameters, or correlations of each variable with itself over previous time steps. These extraneous correlations can be corrected for using autoregressive‐moving average multiple regression models including previous hours simultaneously. Once these are controlled, the correlations between possible driving parameters and high energy electrons are both lower and influential only over a few hours. Autoregressive‐moving average transfer function models show drivers of relativistic electron flux are influential only within a few hours or a day of flux changesContrary to simple correlation findings, influences are lower in magnitude and act more immediatelyStepwise multiple regression shows less cumulative effects of drivers in after‐storm periods than simple correlation would suggest Autoregressive‐moving average transfer function models show drivers of relativistic electron flux are influential only within a few hours or a day of flux changes Contrary to simple correlation findings, influences are lower in magnitude and act more immediately Stepwise multiple regression shows less cumulative effects of drivers in after‐storm periods than simple correlation would suggest

After 06:13 UT on 24 August 2005, an interplanetary shock triggers large‐amplitude ultralow‐frequency (ULF) waves (|δB| ≥ 15 nT) in the Pc4–Pc5 wave band (1.6–9 mHz) near the noon geosynchronous orbit (6.6 RE). The local and global effects of ULF waves on energetic particles are observed by five Los Alamos National Laboratory satellites at different magnetic local times. The large‐amplitude ULF waves cause the synchronous oscillations of energetic electrons and protons (≥75 keV) at the noon geosynchronous orbit. When the energetic particles have a negative phase space density radial gradient, they undergo rapid outward radial diffusion and loss in the wave activity region. In the particle drift paths without strong ULF waves, only the rapidly drifting energetic electrons (≥225 keV) display energy‐dispersive oscillations and flux decays, whereas the slowly drifting electrons (<225 keV) and protons (75–400 keV) have no ULF oscillation and loss feature. When the dayside magnetopause is compressed to the geosynchronous orbit, most of energetic electrons and protons are rapidly lost because of open drift trajectories. The global and multicomposition particle measurements demonstrate that the effect of ULF waves on nonlocal particle flux depends on the particle energy and species, whereas magnetopause shadowing effect is independent of the energetic particle species. For the rapidly drifting outer radiation belt particles (≥225 keV), nonlocal particle loss/acceleration processes could also change their fluxes in the entire drift trajectory in the absence of “Dsteffect” and substorm injection. Large‐amplitude ULF waves cause the oscillation and loss of local energetic particles (>75 keV) through rapid outward radial diffusionEffect of the large‐amplitude ULF waves on nonlocal particle flux depends on the particle energy and speciesMagnetopause shadowing effect is independent of the energetic particle species

We quantify the spatial relationship between the plasmapause and outer belt electrons for a 5 day period, 15–20 January 2013, by comparing locations of relativistic electron flux peaks to the plasmapause. A peak‐finding algorithm is applied to 1.8–7.7 MeV relativistic electron flux data. A plasmapause gradient finder is applied to wave‐derived electron number densities >10 cm−3. We identify two outer belts. Outer belt 1 is a stable zone of >3 MeV electrons located 1–2 REinside the plasmapause. Outer belt 2 is a dynamic zone of <3 MeV electrons within 0.5 REof the moving plasmapause. Electron fluxes earthward of each belt's peak are anticorrelated with cold plasma density. Belt 1 decayed on hiss timescales prior to a disturbance on 17 January and suffered only a modest dropout, perhaps owing to shielding by the plasmasphere. Afterward, the partially depleted belt 1 continued to decay at the initial rate. Belt 2 was emptied out by strong disturbance‐time losses but restored within 24 h. For global context we use a plasmapause test particle simulation and derive a new plasmaspheric index Fp, the fraction of a circular drift orbit inside the plasmapause. We find that the locally measured plasmapause is (for this event) a good proxy for the globally integrated opportunity for losses in cold plasma. Our analysis of the 15–20 January 2013 time interval confirms that high‐energy electron storage rings can persist for weeks or even months if prolonged quiet conditions prevail. This case study must be followed up by more general study (not limited to a 5 day period). Two outer belts, a dynamic zone near the plasmapause, and a stable zone deep within cold plasmaRelativistic electron flux earthward of the peak is anticorrelated with dense plasmaElectron lifetimes in stable outer belt are consistent with decay by plasmaspheric hiss

Three mechanisms have been proposed to explain relativistic electron flux depletions (dropouts) in the Earth's outer radiation belt during storm times: adiabatic expansion of electron drift shells due to a decrease in magnetic field strength, magnetopause shadowing and subsequent outward radial diffusion, and precipitation into the atmosphere (driven by EMIC wave scattering). Which mechanism predominates in causing electron dropouts commonly observed in the outer radiation belt is still debatable. In the present study, we evaluate the physical mechanism that may be primarily responsible for causing the sudden change in relativistic electron pitch angle distributions during a dropout event observed by Van Allen Probes during the main phase of the 27 February 2014 storm. During this event, the phase space density of ultrarelativistic (>1 MeV) electrons was depleted by more than 1 order of magnitude over the entire radial extent of the outer radiation belt (3 < L* < 5) in less than 6 h after the passage of an interplanetary shock. We model the electron pitch angle distribution under a compressed magnetic field topology based on actual solar wind conditions. Although these ultrarelativistic electrons exhibit highly anisotropic (peaked in 90°), energy‐dependent pitch angle distributions, which appear to be associated with the typical EMIC wave scattering, comparison of the modeled electron distribution to electron measurements indicates that drift shell splitting is responsible for this rapid change in electron pitch angle distributions. This further indicates that magnetopause loss is the predominant cause of the electron dropout right after the shock arrival. Drift shell splitting associated with the magnetopause shadowing can result in “top‐hat” distributions on the daysideDrift shell splitting significantly modulates particle dynamics at high L shellsDrift shell splitting is independent of particle species, but it leads to larger pitch angle anisotropies for multi‐MeV electrons

Mechanisms for electron injection, trapping, and loss in the near‐Earth space environment are investigated during the October 2012 “double‐dip” storm using our ring current‐atmosphere interactions model with self‐consistent magnetic field (RAM‐SCB). Pitch angle and energy scattering are included for the first time in RAM‐SCB using Land magnetic local time (MLT)‐dependent event‐specific chorus wave models inferred from NOAA Polar‐orbiting Operational Environmental Satellites (POES) and Van Allen Probes Electric and Magnetic Field Instrument Suite and Integrated Science observations. The dynamics of the source (approximately tens of keV) and seed (approximately hundreds of keV) populations of the radiation belts simulated with RAM‐SCB is compared with Van Allen Probes Magnetic Electron Ion Spectrometer observations in the morning sector and with measurements from NOAA 15 satellite in the predawn and afternoon MLT sectors. We find that although the low‐energy (E< 100 keV) electron fluxes are in good agreement with observations, increasing significantly by magnetospheric convection during both SYM‐Hdips while decreasing during the intermediate recovery phase, the injection of high‐energy electrons is underestimated by this mechanism throughout the storm. Local acceleration by chorus waves intensifies the electron fluxes at E≥50 keV considerably, and RAM‐SCB simulations overestimate the observed trapped fluxes by more than an order of magnitude; the precipitating fluxes simulated with RAM‐SCB are weaker, and their temporal and spatial evolutions agree well with POES/Medium Energy Proton and Electron Detectors data. First RAM‐SCB simulations with 2‐D (MLT‐ and L‐dependent) event‐specific chorus wave models inferred from LEO and RBSP dataThe measured low‐energy trapped and precipitating electron fluxes are well reproduced by magnetospheric convectionLocal acceleration by chorus waves intensifies the electron fluxes at energies E greater than approximately 50 keV and overestimates observations

We investigate the magnetic field dipolarization in the inner magnetosphere and its associated ion flux variations, using the magnetic field and energetic ion flux data acquired by the Van Allen Probes. From a study of 74 events that appeared at L= 4.5–6.6 between 1 October 2012 and 31 October 2013, we reveal the following characteristics of the dipolarization in the inner magnetosphere: (1) its time scale is approximately 5 min; (2) it is accompanied by strong magnetic fluctuations that have a dominant frequency close to the O+gyrofrequency; (3) ion fluxes at 20–50 keV are simultaneously enhanced with larger magnitudes for O+than for H+; (4) after a few minutes of the dipolarization, the flux enhancement at 0.1–5 keV appears with a clear energy‐dispersion signature only for O+; and (5) the energy‐dispersed O+flux enhancement appears in directions parallel or antiparallel to the magnetic field. From these characteristics, we discuss possible mechanisms that can provide selective acceleration to O+ions at >20 keV. We conclude that O+ions at L= 5.4–6.6 undergo nonadiabatic local acceleration caused by oscillating electric field associated with the magnetic fluctuations and/or adiabatic convective transport from the plasma sheet to the inner magnetosphere by the impulsive electric field. At L= 4.5–5.4, however, only the former acceleration is plausible. We also conclude that the field‐aligned energy‐dispersed O+ions at 0.1–5 keV originate from the ionosphere and are extracted nearly simultaneously to the onset of the dipolarization. Magnetic field dipolarization is commonly observed in the inner magnetosphere at L = 4.5‐6.6A large flux increase at 20‐50 keV and an energy‐dispersed flux at 0.1‐5 keV are observed for O+ionsLikely, acceleration mechanisms for O+ions and origins of the energy‐dispersed O+flux are discussed

Trapped electrons in Earth's outer Van Allen radiation belt are influenced profoundly by solar phenomena such as high‐speed solar wind streams, coronal mass ejections (CME), and interplanetary (IP) shocks. In particular, strong IP shocks compress the magnetosphere suddenly and result in rapid energization of electrons within minutes. It is believed that the electric fields induced by the rapid change in the geomagnetic field are responsible for the energization. During the latter part of March 2015, a CME impact led to the most powerful geomagnetic storm (minimum Dst= −223 nT at 17 March, 23 UT) observed not only during the Van Allen Probe era but also the entire preceding decade. Magnetospheric response in the outer radiation belt eventually resulted in elevated levels of energized electrons. The CME itself was preceded by a strong IP shock whose immediate effects vis‐a‐vis electron energization were observed by sensors on board the Van Allen Probes. The comprehensive and high‐quality data from the Van Allen Probes enable the determination of the location of the electron injection, timescales, and spectral aspects of the energized electrons. The observations clearly show that ultrarelativistic electrons with energies E> 6 MeV were injected deep into the magnetosphere at L≈ 3 within about 2 min of the shock impact. However, electrons in the energy range of ≈250 keV to ≈900 keV showed no immediate response to the IP shock. Electric and magnetic fields resulting from the shock‐driven compression complete the comprehensive set of observations that provide a full description of the near‐instantaneous electron energization. March 2015 IP shock event resulted in near‐instantaneous energization of electron to ultrarelativistic energiesElectrons energization was mostly pitch angle independentInjected‐electron spectra were harder than preinjection spectra at all pitch angles

We present a statistical analysis of phase space density data from the first 26 months of the Van Allen Probes mission. In particular, we investigate the relationship between the tens and hundreds of keV seed electrons and >1 MeV core radiation belt electron population. Using a cross‐correlation analysis, we find that the seed and core populations are well correlated with a coefficient of ≈0.73 with a time lag of 10–15 h. We present evidence of a seed population threshold that is necessary for subsequent acceleration. The depth of penetration of the seed population determines the inner boundary of the acceleration process. However, we show that an enhanced seed population alone is not enough to produce acceleration in the higher energies, implying that the seed population of hundreds of keV electrons is only one of several conditions required for MeV electron radiation belt acceleration. The seed population is strongly correlated with the core radiation belt populationThe strongest correlation occurs at a 10–15 h time lagThe seed population is subject to a threshold limit

In this study, we complement the notion of equilibrium states of the radiation belts with a discussion on the dynamics and time needed to reach equilibrium. We solve for the equilibrium states obtained using 1‐D radial diffusion with recently developed hiss and chorus lifetimes at constant values of Kp= 1, 3, and 6. We find that the equilibrium states at moderately low Kp, when plotted versus L shell (L) and energy (E), display the same interesting S shape for the inner edge of the outer belt as recently observed by the Van Allen Probes. The S shape is also produced as the radiation belts dynamically evolve toward the equilibrium state when initialized to simulate the buildup after a massive dropout or to simulate loss due to outward diffusion from a saturated state. Physically, this shape, intimately linked with the slot structure, is due to the dependence of electron loss rate (originating from wave‐particle interactions) on both energy and L shell. Equilibrium electron flux profiles are governed by the Biot number (τDiffusion/τloss), with large Biot number corresponding to low fluxes and low Biot number to large fluxes. The time it takes for the flux at a specific (L, E) to reach the value associated with the equilibrium state, starting from these different initial states, is governed by the initial state of the belts, the property of the dynamics (diffusion coefficients), and the size of the domain of computation. Its structure shows a rather complex scissorform in the (L, E) plane. The equilibrium value (phase space density or flux) is practically reachable only for selected regions in (L, E) and geomagnetic activity. Convergence to equilibrium requires hundreds of days in the inner belt for E> 300 keV and moderate Kp(≤3). It takes less time to reach equilibrium during disturbed geomagnetic conditions (Kp≥ 3), when the system evolves faster. Restricting our interest to the slot region, below L = 4, we find that only small regions in (L, E) space can reach the equilibrium value: E~ [200, 300] keV for L = [3.7, 4] at Kp= 1, E~[0.6, 1] MeV for L = [3, 4] at Kp= 3, and E~300 keV for L = [3.5, 4] at Kp= 6 assuming no new incoming electrons. The time to reach equilibrium value has a complex scissor shape in (L, E)Equilibrium value only reachable for selected and determined (L, E, Kp)Dynamics and equilibrium S shape of the belts as in VAP observations

Electromagnetic ion cyclotron (EMIC) waves have been proposed to cause efficient losses of highly relativistic (>1?MeV) electrons via gyroresonant interactions. Simultaneous observations of EMIC waves and equatorial electron pitch angle distributions, which can be used to directly quantify the EMIC wave scattering effect, are still very limited, however. In the present study, we evaluate the effect of EMIC waves on pitch angle scattering of ultrarelativistic (>1?MeV) electrons during the main phase of a geomagnetic storm, when intense EMIC wave activity was observed in situ (in the plasma plume region with high plasma density) on both Van Allen Probes. EMIC waves captured by Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes and on the ground across the Canadian Array for Real-time Investigations of Magnetic Activity (CARISMA) are also used to infer their magnetic local time (MLT) coverage. From the observed EMIC wave spectra and local plasma parameters, we compute wave diffusion rates and model the evolution of electron pitch angle distributions. By comparing model results with local observations of pitch angle distributions, we show direct, quantitative evidence of EMIC wave-driven relativistic electron losses in the Earth's outer radiation belt. Direct, quantitative evidence of EMIC wave-driven relativistic electron loss in the Earth's outer radiation belt is presentedEMIC wave scattering effects are quantified based on the observed EMIC wave spectra and local plasma parameters using quasi-linear theoryDual-probe analysis reveals wave-driven losses without being affected by magnetospheric reconfiguration or satellite movement