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Radial diffusion in planetary radiation belts is a dominant transport mechanism resulting in the energization and losses of charged particles by large-scale electromagnetic fluctuations. In this work we exploit the extensive radial diffusion coefficients (D LL) database created in the framework of the Horizon 2020 SafeSpace project, which spans 9 years of hourly D LL coefficients, to investigate the spatio-temporal distributions of the coefficients. Our results indicate that the radial distribution of the magnetic and electric component of the D LL , as well as their sum, the total D LL , can be well described by a power law function of L* in the [4.3–7.7] range. We show that the L*-dependent spectral index varies significantly and is far from constant as assumed and implemented in many semi-empirical models. We examine the quasi-periodic behavior of the radial profiles of the D LL throughout most of the 24th Solar cycle, which the data cover, and find an approximately 420 days dominant periodicity. This periodic behavior is linked (in terms of cross-wavelet analysis) with solar activity, nevertheless, its origin remains unclear. The uncovered features are important for understanding D LL behavior and drivers as well as for current and future modelling efforts. [ABSTRACT FROM AUTHOR]

Using GOES magnetometer data spanning over 30 yr, we study the distribution of Pc5 ultralow-frequency waves at geosynchronous orbit. The CLEAN algorithm was applied to detect narrowband Pc5 waves. This method was first used in astronomical radio interferometry, but has since been applied to many other areas. The algorithm allows for identification of multiple individual peaks within the same power spectral density. We found close to 30,000 events in each magnetic field component in field-aligned coordinates. With some exceptions, results were in agreement with previous studies. Wave occurrence along magnetic local time (MLT) was higher in the 18–23 MLT sector. However, the largest accumulated wave amplitudes were observed on the dayside, peaking close to local noon. As expected, wave amplitudes were larger during more active solar wind conditions. For the radial and parallel components, amplitudes were maximum at the interplanetary magnetic field clock angle of −30° to −45°. The resultant database of Pc5 ultralow-frequency wave events can be used to constrain radiation belt models, while the CLEAN method provides a unique technique to automatically detect narrowband plasma waves in the magnetosphere and beyond. [ABSTRACT FROM AUTHOR]

The Electron Loss and Fields Investigation with a Spatio-Temporal Ambiguity-Resolving option (ELFIN-STAR, or heretoforth simply: ELFIN) mission comprises two identical 3-Unit (3U) CubeSats on a polar (∼93∘ inclination), nearly circular, low-Earth (∼450 km altitude) orbit. Launched on September 15, 2018, ELFIN is expected to have a >2.5 year lifetime. Its primary science objective is to resolve the mechanism of storm-time relativistic electron precipitation, for which electromagnetic ion cyclotron (EMIC) waves are a prime candidate. From its ionospheric vantage point, ELFIN uses its unique pitch-angle-resolving capability to determine whether measured relativistic electron pitch-angle and energy spectra within the loss cone bear the characteristic signatures of scattering by EMIC waves or whether such scattering may be due to other processes. Pairing identical ELFIN satellites with slowly-variable along-track separation allows disambiguation of spatial and temporal evolution of the precipitation over minutes-to-tens-of-minutes timescales, faster than the orbit period of a single low-altitude satellite (Torbit ∼ 90 min). Each satellite carries an energetic particle detector for electrons (EPDE) that measures 50 keV to 5 MeV electrons with Δ E/E < 40% and a fluxgate magnetometer (FGM) on a ∼72 cm boom that measures magnetic field waves (e.g., EMIC waves) in the range from DC to 5 Hz Nyquist (nominally) with 1 MeV. This broad energy range of precipitation indicates that multiple waves are providing scattering concurrently. Many observed events show significant backscattered fluxes, which in the past were hard to resolve by equatorial spacecraft or non-pitch-angle-resolving ionospheric missions. These observations suggest that the ionosphere plays a significant role in modifying magnetospheric electron fluxes and wave-particle interactions. Routine data captures starting in February 2020 and lasting for at least another year, approximately the remainder of the mission lifetime, are expected to provide a very rich dataset to address questions even beyond the primary mission science objective.

Magnetospheric relativistic electrons are accelerated during substorms and strong convection events that occur during high-intensity long-duration continuous auroral electrojet activity (HILDCAA) events, associated with solar wind high-speed streams (coming from coronal holes). From an analysis of ∼2–20 MeV electrons at L ∼ 2–7 measured by the Van Allen Probe satellite, it is shown that ∼3.4–4.1 days long HILDCAA events are characterized by ∼7.2 MeV electron acceleration in the L ∼ 4.0–6.0 region, which occurs ∼2.9–3.4 days after the onset of HILDCAA. The dominant acceleration process is due to wave–particle interactions between magnetospheric electromagnetic chorus waves and substorm-injected ∼100 keV electrons. The longer the HILDCAA and chorus last, the higher the maximum energy of the accelerated relativistic electrons. The acceleration to higher and higher energies is due to a bootstrap mechanism.

A comprehensive theoretical model for a homogeneous plasma system of hot, tenuous Maxwellian ring-distributed protons and the cold background of Maxwellian ions and electrons is used to study the resonant instabilities of magnetosonic (MS) waves. The perpendicular velocity integrals associated with the Maxwellian ring distribution have no analytical solution, hence, are solved numerically, while the parallel velocity integrals are solved analytically by invoking series expansion of the plasma dispersion function. For the plasma parameters relevant to Earth’s inner magnetosphere, the theoretical model generates MS waves with frequencies from 5 times the local proton cyclotron frequency to above the lower hybrid frequency for a propagation angle of 89.5°. Hydrogen (H ^+ ) band electromagnetic ion cyclotron waves are also excited by the Maxwellian ring protons for the same set of plasma parameters. A detailed analysis reveals that a sufficiently large ring velocity, as well as a smaller perpendicular and parallel thermal velocity, can enhance the MS wave growth. The study also explores the role of background plasma parameters in modulating the waves. The present theoretical model reproduces the harmonics of the MS waves observed by the Van Allen Probes in the Earth’s inner magnetosphere. Further, the model can generate MS waves in other plasma environments, e.g., Mars, where the presence of ring protons has been established by MAVEN in connection with the MS wave observations.

The excitation of electrostatic and/or electromagnetic waves in the plasma universe is often associated with anisotropic velocity distributions of charged particles. In Earth's inner magnetosphere, this anisotropy can gradually develop as particles injected from the magnetotail drift around the Earth at different speeds depending on their energy and pitch angle. Here, we show that the perpendicular‐moving and bouncing ions can be separated more abruptly near the injection front. These pitch‐angle filters are localized magnetic dip structures formed by the diamagnetic behavior of the injected particles, which can trap perpendicular‐moving ions and allow bouncing ions to overtake. The resulting ion anisotropy facilitates the rapid generation of electromagnetic ion cyclotron (EMIC) waves, which in turn can largely reshape the Van Allen radiation belts. This scenario is examined by case and statistical observations, together with numerical simulations that reproduce most of the observational signatures, to support the causal relationship between magnetic dips, anisotropic ion distributions, and localized excitation of EMIC waves. Our study highlights the important roles of magnetic dips in the inner magnetospheric dynamics, as pitch‐angle filters of the injected ions and traveling hotspots of EMIC wave activities. Plain Language Summary: In Earth's Van Allen radiation belts, energetic particles are magnetically trapped to form a hazardous environment for spacecraft and astronauts. These particles can stay trapped for years before they are scattered toward the atmosphere by resonant interactions with plasma waves. Among these waves, the electromagnetic ion cyclotron (EMIC) waves are particularly important in the removal of energetic ions and relativistic electrons. The EMIC wave excitation is often associated with strong ion anisotropy, which gradually develops as particles injected from the magnetotail drift around the Earth at different speeds depending on their energy and pitch angle. Here, we propose a new scenario of localized EMIC wave excitation, in which the injected perpendicular‐moving and bouncing ions are abruptly separated by their associated diamagnetic dips, to develop strong ion anisotropy inside the dip structure and generate EMIC waves. This scenario is validated via case and statistical observations, together with numerical simulations that reproduce most of the observational signatures, which reveals the importance of the magnetic dips as traveling hotspots of EMIC wave activities. These insights indicate the localized and dynamical removal of energetic particles and provide key revision to the prevailing radiation belt models. Key Points: We perform case and statistical study on the association between magnetic dips, anisotropic proton distributions and EMIC wavesMagnetic dips serve as reversed pitch‐angle filters to trap the perpendicular‐moving protons and allow the bouncing protons to overtakeThe pitch‐angle filter leads to a higher ion anisotropy therein, which favors the localized excitation of electromagnetic ion cyclotron wave [ABSTRACT FROM AUTHOR]

Recent studies have reported the simultaneous observation of Magnetosonic (MS) waves and Electromagnetic ion cyclotron (EMIC) waves. In this study, a detailed survey of the concurrent EMIC waves and MS waves is performed using the Van Allen Probes observations from 2012 to 2017. The results suggest that most of the concurrent EMIC waves and MS waves are H+ band EMIC waves and MS waves. The favorable geomagnetic conditions for the concurrent EMIC waves and MS waves are AE* = 500 nT inside the plasmapause and AE* = 100 nT outside the plasmapause. Although EMIC waves are generally stronger than MS waves, stronger MS waves than EMIC waves are more likely to occur during moderate and active geomagnetic times. Our study is consistent with the most recent research, and provides more detailed information that helps us to deepen our understanding of the concurrent EMIC waves and MS waves. Plain Language Summary Electromagnetic ions cyclotron (EMIC) waves and magnetosonic (MS) waves are two common wave mode in the Earth radiation belts and play important roles in radiation belt dynamics. Recent studies have reported observations of concurrent EMIC waves and MS waves and conducted a brief statistic survey. To deepen our understanding of the concurrent EMIC waves and MS waves, this study aims to comprehensively analyze the statistical occurrence pattern and associated global distribution of concurrent EMIC waves and MS waves. Our study demonstrated that most of the concurrent EMIC waves and MS waves are H+ band EMIC waves and MS waves, which reveals the deep relationship between MS waves and the concurrent H+ band EMIC waves. Our study also demonstrated that the concurrent EMIC waves are generally stronger than MS waves, while the wave amplitudes of MS waves can exceed EMIC waves during geomagnetically turbulent times. This result indicates that the combined scattering effect of the concurrent EMIC waves and MS waves could vary significantly under different geomagnetic conditions. [ABSTRACT FROM AUTHOR]

Precipitating auroral and radiation belt electrons are considered to play an important part in the natural forcing of the middle atmosphere with a possible impact on the climate system. Recent studies suggest that this forcing is underestimated in current chemistry-climate models. The HEPPA III intercomparison experiment is a collective effort to address this point. In this study, we apply electron ionization rates from three data-sets in four chemistry-climate models during a geomagnetically active period in April 2010. Results are evaluated by comparison with observations of nitric oxide (NO) in the mesosphere and lower thermosphere. Differences between the ionization rate data-sets have been assessed in a companion study. In the lower thermosphere, NO densities differ by up to one order of magnitude between models using the same ionization rate data-sets due to differences in the treatment of NO formation, model climatology, and model top height. However, a good agreement in the spatial and temporal variability of NO with observations lends confidence that the electron ionization is represented well above 80 km. In the mesosphere, the averages of model results from all chemistry-climate models differ consistently with the differences in the ionization-rate data-sets, but are within the spread of the observations, so no clear assessment on their comparative validity can be provided. However, observed enhanced amounts of NO in the mid-mesosphere below 70 km suggest a relevant contribution of the high-energy tail of the electron distribution to the hemispheric NO budget during and after the geomagnetic storm on April 6. [ABSTRACT FROM AUTHOR]

Precipitating auroral and radiation belt electrons are considered an important part of the natural forcing of the climate system. Recent studies suggest that this forcing is underestimated in current chemistryclimate models. The High Energy Particle Precipitation in the Atmosphere III intercomparison experiment is a collective effort to address this point. Here, eight different estimates of medium energy electron (MEE) (>30 keV) ionization rates are assessed during a geomagnetic active period in April 2010. The objective is to understand the potential uncertainty related to the MEE energy input. The ionization rates are all based on the Medium Energy Proton and Electron Detector (MEPED) on board the NOAA/POES and EUMETSAT/MetOp spacecraft series. However, different data handling, ionization rate calculations, and background atmospheres result in a wide range of mesospheric electron ionization rates. Although the eight data sets agree well in terms of the temporal variability, they differ by about an order of magnitude in ionization rate strength both during geomagnetic quiet and disturbed periods. The largest spread is found in the aftermath of enhanced geomagnetic activity. Furthermore, governed by different energy limits, the atmospheric penetration depth varies, and some differences related to latitudinal coverage are also evident. The mesospheric NO densities simulated with the Whole Atmospheric Community Climate Model driven by highest and lowest ionization rates differ by more than a factor of eight. In a follow-up study, the atmospheric responses are simulated in four chemistry-climate models (CCM) and compared to satellite observations, considering both the CCM structure and the ionization forcing. [ABSTRACT FROM AUTHOR]

The Van Allen Probes spacecraft flew through the inner radiation belt and the plasmasphere, probing the extremely dynamic coupling between the thermosphere, the ionosphere and the subauroral magnetosphere. Examining the electrodynamics of this region using Van Allen Probes data required that the ~1 mV/m electric field (E-field) of interest be measured in a region where the E-field due to spacecraft motion was ~100 mV/m. This means that the E-field, the magnetic field, and the spacecraft velocity had to be measured to better than 1%. The instruments on board the Van Allen Probes have achieved this accuracy, delivering reliable near equatorial E-field measurements even below three Earth radii. The objective of this commentary is to summarize the methodology developed over the years to maximize the accuracy and scientific return of these double probe E-field measurements below L = 3. [ABSTRACT FROM AUTHOR]

With new satellite constellations being launched into low Earth orbit, the growing use of medium Earth orbit for radio‐navigation and timing signals, slot region orbits for telecommunications and the introduction of electric propulsion to reach geostationary orbit, there is a growing need to develop services to protect satellites from space weather. Here we highlight two recent events in relation to satellite operations. We summarize 10 user needs that arose out of meetings with satellite operators, designers, underwriters and space agency staff. We present the satellite risk prediction and radiation forecast (SaRIF) system which is designed to meet most of these needs. The system uses real‐time data as input to the BAS radiation belt model (BAS‐RBM) to solve the Fokker Planck equation and provides a forecast of the electron flux throughout the outer radiation belt with 1‐hr resolution up to 24 hr ahead. The electron flux is used to calculate charging currents, and is combined with GOES near real time proton fluxes to calculate dose rate and total ionizing dose behind set levels of shielding for satellites in Medium Earth orbit, Geostationary orbit and slot region orbits. The results are compared against design standards and presented as risk indicators to forecast the risk of damage. The system works automatically and is updated every hour. We also present data and a best reconstruction of the radiation environment which are held in a searchable archive for satellite anomaly resolution. The SaRIF system is available via the European Space Agency space weather web portal. Plain Language Summary: With new satellite constellations being launched into low Earth orbit, the growing use of other orbits and the introduction of electric propulsion, we need to protect satellites from space weather. Here we highlight two recent events in relation to satellite operations. We summarize 10 user needs that arose out of meetings with satellite operators, designers, underwriters and space agency staff. We present the satellite risk prediction and radiation forecast (SaRIF) system which is designed to meet most of these needs. The system uses real‐time data as input to the BAS radiation belt model (BAS‐RBM) and provides a forecast of the electron flux throughout the outer radiation belt with 1‐hr resolution up to 24 hr ahead. The electron flux is used to calculate charging currents, and is combined with GOES near real time proton fluxes to calculate dose rate and total ionizing dose behind set levels of shielding for satellites on orbit. The results are compared against design standards and presented as risk indicators to forecast the risk of damage. The system works automatically and is updated every hour. We also present a best reconstruction of the radiation environment which are held in a searchable archive for satellite anomaly resolution. Key Points: The satellite risk prediction and radiation forecast (SaRIF) system forecasts the outer electron radiation up to 24 hr ahead, updated every hourRisk indicators are provided for four satellite orbits and can be compared against design standardsThe SaRIF system provides a searchable archive of data for anomaly resolution by satellite operators, designers and underwriters [ABSTRACT FROM AUTHOR]

Increasing numbers of satellites are orbiting through the Earth's radiation belts, and the range of orbits being commonly used is also growing. As a result, there is an increasing need for services to help protect satellites from space weather. The Satellite RIsk prediction and radiation Forecast (SaRIF) system provides reconstructions and forecasts of the high‐energy electron flux throughout the outer radiation belt and translates these predictions into charging currents, dose rates, total ionizing dose and risk indicators. SaRIF both informs satellite operators of current and expected conditions and provides a tool to aid in post‐event analysis. The reconstructions and forecasts are provided by the British Antarctic Survey Radiation Belt Model (BAS‐RBM) running as part of an automatic system using real‐time data to specify the boundary conditions and drive processes within the physics‐based model. If SaRIF is to provide a useful tool, then the accuracy of the reconstructions and forecasts needs to be understood. Here we assess the accuracy of the simulations for geostationary orbit by comparing the model output with measurements made by the GOES 14 spacecraft for the period March–September 2019. No GOES 14 data was used to create the reconstruction or forecasts. We show that, with some improvements to the original system, the reconstructions have a prediction efficiency of 0.82 for >800 keV electrons and 0.87 for >2 MeV electrons, with corresponding prediction efficiencies of 0.59 and 0.78 for the forecasts. Plain Language Summary: The number of satellites orbiting the Earth is steadily increasing. Many of these satellites pass through the Earth's radiation belts where they may encounter high‐energy electrons that could potentially damage the satellite. The levels of these high‐energy electrons can vary by orders of magnitude depending on conditions in space, but actual observations of these electrons from space are sparse. As a result, there in an increasing need for services that alert satellite operators to hazardous conditions and provide a means of understanding the radiation environment when a satellite anomaly has occurred. The Satellite Risk prediction and radiation Forecast (SaRIF) system provides one such service. SaRIF uses the British Antarctic Survey Radiation Belt Model (BAS‐RBM) and real‐time data to simulate the Earth's radiation belts, providing reconstructions of the state of the radiation belts in the past and forecasts 24 hr ahead. This paper assesses the accuracy of the reconstructions and forecasts by comparing them with data measured independently by the GOES 14 spacecraft. We show that for the period from 1 March to 1 September 2019 there is reasonable agreement between the model and measured data, and that the agreement increases when some improvements are introduced into the SaRIF modeling. Key Points: SaRIF provides forecasts and reconstructions of the high‐energy (E > ∼100 keV) electron flux across the radiation beltsPrediction efficiencies of 0.87 for reconstructions of >2 MeV electrons at geostationary orbitPrediction efficiencies of 0.78 for forecasts of >2 MeV electrons 1 day ahead at geostationary orbit [ABSTRACT FROM AUTHOR]

Radiation safety in the Earth's atmosphere is of particular importance to our living environment, especially at aviation altitudes. Aviation radiation has been long known to originate primarily from the galactic and solar system: galactic cosmic rays and solar energetic protons. Recent flight measurements by the Automated Radiation Measurements for Aerospace Safety experiment have uncovered another potential source for aviation radiation: Relativistic Electron Precipitation (REP) from the Van Allen radiation belts. REP can induce radiation at aviation altitudes through bremsstrahlung X‐ray production, which carries radiation down to the stratosphere and even the troposphere. In this study, using a suite of physics‐based Monte Carlo models, we characterize the effective radiation dose produced at altitudes between ground and low‐Earth‐orbit by relativistic precipitation electrons with energies between 100 keV and 10 MeV. We produce a lookup table of atmospheric radiation production that calculates the expected radiation dose for a given precipitation flux, spectrum, and pitch angle distribution. This lookup table provides results that are consistent with X‐ray measurements during radiation belt precipitation by balloon‐borne instruments in the stratosphere, and can be directly used to convert space‐borne measurements of precipitation fluxes into aviation radiation. This study represents our first attempt toward better understanding of REP's role in the atmospheric high‐altitude radiation environment. Plain Language Summary: Radiation safety at aviation altitudes is of particular importance to our living environment. Aviation radiation has been long known to be primarily produced by energetic particles originating from the galactic and solar system. However, recent flight measurements by the Automated Radiation Measurements for Aerospace Safety experiment have uncovered another potential source for aviation radiation: Relativistic Electron Precipitation (REP) from the Van Allen radiation belts. Through interaction with air molecules, REP can produce intense X‐rays and these X‐rays can penetrate into the stratosphere and even the troposphere, leading to radiation doses therein. In this study, we report a lookup table of atmospheric radiation production by REP, and this lookup table can be directly used to convert space‐borne measurements of precipitation fluxes into aviation radiation. This study represents our first attempt toward better understanding of REP's role in the atmospheric high‐altitude radiation environment. Key Points: We report a lookup table of atmospheric radiation production between ground and low‐Earth‐orbit by monoenergetic precipitation electronsWe explain how this lookup table can be utilized for rapid specification of radiation production by arbitrary precipitation energy spectrumWe validate this lookup table by comparing with balloon measurements of X‐ray spectrum in stratosphere during radiation belt precipitation [ABSTRACT FROM AUTHOR]

Properly characterizing fast relativistic electron losses in the terrestrial Van Allen belts remains a significant challenge for accurately simulating their dynamics. In particular, magnetopause shadowing losses can deplete the radiation belt within hours or even minutes, but can have long‐lasting impacts on the subsequent belt dynamics. By statistically analyzing seven years of data from the entire Van Allen Probes mission in the context of the last closed drift shell, here we show how these losses are much more organized and predictable than previously thought. Once magnetic storm electron dynamics are properly analyzed in terms of the location of the last closed drift shell, not only is the loss shown to be repeatable but its energy‐dependent spatio‐temporal evolution is also revealed to follow a very similar pattern from storm to storm. Employing an energy‐dependent ULF wave radial diffusion model, we further show for the first time how the similar and repeatable fractional loss of the pre‐storm electron population in each storm can be reproduced and explained. Empirical characterization of this loss may open a pathway toward improved radiation belt specification and forecast models. This is especially important since underestimates of loss can also create unrealistic sources in models, creating phantom electron radiation and leading to the prediction of an overly harsh radiation environment. Key Points: Magnetopause compression, characterized by inward motion of the last closed drift shell, results in rapid loss of radiation belt electronsThe associated loss develops in a similar and repeatable manner depleting the radiation belt by the same fraction in every event studiedThe L∗ and energy dependence of the loss is consistent with magnetopause shadowing losses enhanced by outward ULF wave radial diffusion [ABSTRACT FROM AUTHOR]

Based on Van Allen Probes observations, we report a prompt enhancement event of radiation belt electrons over a wide energy range from tens of keV to multiple meV spanning 2013 January 13–15. During this period, we also observe prolonged moderate substorm activities and intense whistler-mode chorus emissions. To differentiate the underlying mechanisms responsible for this prompt electron enhancement process, we investigate in detail the evolution of electron phase space densities (PSDs) for various values of the first and second adiabatic invariants ( μ and K ). The results show that tens to hundreds of keV electrons rapidly penetrated to L * < 4 during the substorm period, with the corresponding PSDs increasing by more than 3 orders of magnitude within about 1 day. Comparatively, the PSD enhancements of higher energy electrons are less significant and shift to higher L *. We find that the fast acceleration of hundreds of keV seed electrons to multi-meV electrons may be reasonably attributed to interactions with the concurrent chorus waves. Specifically, the electron PSD increases for μ ≥ 300 MeV G ^−1 become less pronounced as K increases, consistent with the pitch angle dependence of chorus-induced electron energy diffusion at high energies. Our results therefore provide clear observational evidence for the combined effect of substorm-induced injection and chorus wave scattering on the prompt enhancements of radiation belt electrons over a wide energy range within a couple of days.

Observational studies of the Earth’s radiation belts indicate that Alfvénic fluctuations in the frequency range of 2–25 mHz accelerate electrons to relativistic energies. For decades, statistical models of radiation belts have quantified the impact of Alfvénic waves in terms of quasi-linear diffusion. However, quasi-linear models are inadequate to quantify Alfvénic radial transport occurring on timescales comparable to the azimuthal drift period of 0.1–10 MeV electrons. With recent advances in observational methodologies offering coverage of the Earth’s radiation belts on fast timescales, a theoretical framework that distinguishes between fast and diffusive radial transport can be tested for the first time in situ. In this report, we present a drift-kinetic description of radial transport for planetary radiation belts. We characterize fast linear processes and determine the conditions under which higher-order effects become dynamically significant. In the linear regime, wave–particle interactions are categorized in terms of resonant and nonresonant responses. We demonstrate that the phenomenon of zebra stripes is nonresonant and can originate from injection events in the inner radiation belts. We derive a radial diffusion coefficient for a field model that satisfies Faraday’s law and that contains two terms: one scaling as L ^10 independent of the azimuthal number m , and a second scaling as m ^2 L ^6 . In the higher-order regime, azimuthally symmetric waves with properties consistent with in situ measurements can energize 10–100 keV electrons in less than a drift period. This process provides new evidence that acceleration by Alfvénic waves in radiation belts cannot be fully contained within diffusive models.

The Radiation Belt Storm Probes (RBSP) and the Arase satellites have different inclinations and sometimes they fly both near the equator and off the equator on the same magnetic field line simultaneously. Such conjunction events give us opportunities to compare the electron density at different latitudes. In this study, we analyzed the plasma waves observed by Arase and RBSP during the three conjunction events during and after the September 7, 2017 storm. The electron number density at the satellite positions was estimated from frequencies of the Upper Hybrid Resonance emissions obtained by the High Frequency Analyzer of the Plasma Wave Experiment onboard the Arase and the Waves instrument onboard the RBSP, respectively. During the three conjunction events, the satellites passed through the plume, inner trough (the narrow region with low electron density between the main body of the plasmasphere and the plume), plasmatrough with variable electron density, and partially refilled plasmasphere. The power‐law index m for the inner trough and plume was inferred to be 4–7 and ∼0, respectively. This is interpreted to mean that the trough was close to collisionless and the plume was relatively near diffusive equilibrium. In the plasmatrough with the varying density, both the high‐density and low‐density regions had m ∼ 0. The low‐density portion of this region may have a different origin from the inner trough, because of the different m indices. For the partially refilled plasmasphere in the storm recovery phase, the power‐law index m showed negative values, meaning that the density in the equatorial plane was higher than at higher latitudes. Plain Language Summary: The plasmasphere is a region filled with cold, dense ionized gas in geospace. The ionized gas mainly consists of protons, helium ions, oxygen ions, and electrons, which come from Earth's ionosphere and fill in magnetic flux tubes. The density distribution of the ionized gas along the flux tube provides important information to understand how the ions and electrons have been supplied from the ionosphere. Many satellites fly in the equatorial plane; hence, they do not provide information on the electron density along the field. The Radiation Belt Storm Probes and the Arase satellites have different inclinations and sometimes they simultaneously fly near the equator and off the equator on the same magnetic field line. Using electron densities observed by these satellites during the September 7, 2017 storm, we successfully estimated the electron density distribution along the field lines inside the partially refilled plasmasphere, outside of the plasmasphere, and in the tail‐like structure called a plume. Key Points: Using in situ measurements of the electron density from Arase and Radiation Belt Storm Probes, we estimated the density distribution along the magnetic field linesThe power‐law index of the electron density distribution was 4∼7, ∼0, and −2∼−1 for the trough, plume, and partially refilled plasmasphereThis is the first estimation of the power‐law index using the data from different spacecraft projects [ABSTRACT FROM AUTHOR]

Terrestrial lightning frequently serves as a loss mechanism for energetic electrons in the Van Allen radiation belts, leading to lightning‐induced electron precipitation (LEP). Regardless of the specific causes, energetic electron precipitation from the radiation belts in general has a significant influence on the ozone concentration in the stratosphere and mesosphere. The atmospheric chemical effects induced by LEP have been previously investigated using subionospheric VLF measurements at Faraday station, Antarctica (65.25°S, 64.27°W, L = 2.45). However, there exist large variations in the precipitation flux, ionization production, and occurrence rate of LEP events depending on the peak current of the parent lightning discharge, as well as the season, location, and intensity of the thunderstorm activity. These uncertainties motivate us to revisit the calculation of atmospheric chemical changes produced by LEP. In this study, we combine a well‐validated LEP model and first‐principles atmospheric chemical simulation, and investigate three intense storms in the year of 2013, 2015, and 2017 at the magnetic latitude of 50.9°, 32.1°, and 35.7°, respectively. Modeling results show that the LEP events in these storms can cumulatively drive significant changes in the NOx, HOx, and Ox concentration in the mesosphere. These changes are as high as ∼156%, ∼66%, and −5% at 75–85 km altitude, respectively, and comparable to the effects typically induced by other types of radiation belt electron precipitation events. Considering the high occurrence rate of thunderstorms around the globe, the long‐term global chemical effects produced by LEP events need to be properly quantified. Key Points: We quantify the atmospheric chemical response due to lightning‐induced electron precipitation (LEP) using first‐principles simulationsThe change in ozone concentration during one thunderstorm via LEP processes is comparable to other types of energetic particle precipitationThe long‐term global chemical effects produced by LEP events could be potentially important and need to be properly quantified [ABSTRACT FROM AUTHOR]

Energetic electrons inside Earth's Van Allen belts pose a major radiation threat to space‐borne electronics that often play vital roles in modern society. Ultra‐relativistic electrons with energies greater than or equal to two megaelectron‐volt (MeV) are of particular interest, and thus forecasting these ≥2 MeV electrons has a significant meaning to all space sectors. Here, we update the latest development of the predictive model for MeV electrons in the outer radiation belt. The new version, called PREdictive MEV Electron (PreMevE)‐2E, forecasts ultra‐relativistic electron flux distributions across the outer belt, with no need for in situ measurements of the trapped MeV electron population except at the geosynchronous orbit (GEO). Model inputs include precipitating electrons observed in low‐Earth‐orbits by NOAA satellites, upstream solar wind speeds and densities from solar wind monitors, as well as ultra‐relativistic electrons measured by one Los Alamos GEO satellite. We evaluated 32 supervised machine learning models that fall into four different classes of linear and neural network architectures, and successfully tested ensemble forecasting by using groups of top‐performing models. All models are individually trained, validated, and tested by in situ electron data from NASA's Van Allen Probes mission. It is shown that the final ensemble model outperforms individual models at most L‐shells, and this PreMevE‐2E model can provide 25‐h (∼1‐day) and 50‐h (∼2‐day) forecasts with high mean performance efficiency and correlation values. Our results also suggest that this new model is dominated by nonlinear components at L‐shells <∼4 for ultra‐relativistic electrons, different from the dominance of linear components for 1 MeV electrons as previously discovered. Plain Language Summary: High‐speed electrons inside Earth's outer Van Allen belt pose a major radiation threat to man‐made satellites by causing malfunction of space‐borne electronics. These electrons, especially those traveling at nearly the speed of light, are of particular interest due to their high penetrating ability, and thus forecasting ultra‐relativistic (with energies greater than or equal to 2 MeV) electrons has a significant meaning to all space sectors. Here, we update the latest development of the predictive model for megaelectron‐volt (MeV) electrons inside the Earth's outer radiation belt. This new version, called PREdictive MEV Electron (PreMevE)‐2E, focuses on forecasting ultra‐relativistic electron flux distributions across the outer radiation belt, with no need for local measurements of the trapped MeV electrons except at the geosynchronous orbit (GEO). Model inputs include electrons observed in low‐Earth‐orbits by NOAA satellites, upstream solar wind conditions from solar wind monitors, and ultra‐relativistic electrons measured by one Los Alamos GEO satellite. We evaluated 32 supervised machine learning models in four different classes, and successfully demonstrated the performance of the ensemble forecasting technique. All models are trained, validated, and tested by electron data from NASA's Van Allen Probes mission. This new PreMevE‐2E model provides 1‐ and 2‐day forecasts of incoming nearly light‐speed electrons with high statistical fidelity. Key Points: Machine‐learning based PREdictive MEV Electron (PreMevE) model is extended to predict ultra‐relativistic electron flux distributions during megaelectron‐volt (MeV) electron eventsThis new PreMevE‐2E model makes reliable 1‐ and 2‐day ensemble forecasts of ≥2 MeV electrons inside Earth's outer radiation beltNonlinear components play a major role in this new model at small L‐shells (<∼4) in contrast to previous PreMevE for 1 MeV electrons [ABSTRACT FROM AUTHOR]

We have studied the statistical properties of low‐energy proton (H+) and helium (He+) ion flux enhancements associated with electromagnetic ion cyclotron (EMIC) waves in the inner magnetosphere using Van Allen Probes data for 2013–2017. We identified 167 low‐energy ion flux enhancements when the EMIC waves occurred in a He‐band or in a multiple band (H‐band and He‐band) with strong He‐band and weak H‐band wave activity and found that most of them occurred from the noon to the premidnight sector near the magnetic equator just inside the plasmapause. Of 167 flux enhancement events, 68 exhibited only He+ flux enhancements, and 99 exhibited both H+ and He+ flux enhancements. The EMIC wave‐associated flux enhancement events are mostly energized in the direction perpendicular to the background magnetic field. When both H+ and He+ fluxes are simultaneously enhanced, the H+ flux events have a peak energy distributed in the range of 2–100 eV, and the peak energies of the He+ flux events are distributed in the 2–600 eV range, implying that the helium ions are more energized than the protons. The peak energies of only He+ flux enhancement without H+ flux enhancement are mostly distributed in a lower energy range, 2–10 eV. The energization of H+ and He+ ions can be explained by a linear plasma flow associated with EMIC waves. We suggest that the wave‐associated linear plasma motion is a likely mechanism to explain the observations. Key Points: Cold protons and helium ions are transversely heated by interactions with electromagnetic ion cyclotron (EMIC) waves in the inner magnetosphereCold helium ions are more energized than cold protons by EMIC wavesEMIC wave‐associated linear plasma motion is a likely mechanism for the cold particle acceleration [ABSTRACT FROM AUTHOR]

Radial diffusion is one of the dominant physical mechanisms driving acceleration and loss of radiation belt electrons. A number of parameterizations for radial diffusion coefficients have been developed, each differing in the data set used. Here, we investigate the performance of different parameterizations by Brautigam and Albert (2000), https://doi.org/10.1029/1999ja900344, Brautigam et al. (2005), https://doi.org/10.1029/2004ja010612, Ozeke et al. (2014), https://doi.org/10.1002/2013ja019204, Ali et al. (2015), https://doi.org/10.1002/2014ja020419; Ali et al. (2016), https://doi.org/10.1002/2016ja023002; Ali (2016), and Liu et al. (2016), https://doi.org/10.1002/2015gl067398 on long‐term radiation belt modeling using the Versatile Electron Radiation Belt (VERB) code, and compare the results to Van Allen Probes observations. First, 1‐D radial diffusion simulations are performed, isolating the contribution of solely radial diffusion. We then take into account effects of local acceleration and loss showing additional 3‐D simulations, including diffusion across pitch‐angle, energy, and mixed diffusion. For the L* range studied, the difference between simulations with Brautigam and Albert (2000), https://doi.org/10.1029/1999ja900344, Ozeke et al. (2014), https://doi.org/10.1002/2013ja019204, and Liu et al. (2016), https://doi.org/10.1002/2015gl067398 parameterizations is shown to be small, with Brautigam and Albert (2000), https://doi.org/10.1029/1999ja900344 offering the smallest averaged (across multiple energies) absolute normalized difference with observations. Using the Ali et al. (2016), https://doi.org/10.1002/2016ja023002 parameterization tended to result in a lower flux than both the observations and the VERB simulations using the other coefficients. We find that the 3‐D simulations are less sensitive to the radial diffusion coefficient chosen than the 1‐D simulations, suggesting that for 3‐D radiation belt models, a similar result is likely to be achieved, regardless of whether Brautigam and Albert (2000), https://doi.org/10.1029/1999ja900344, Ozeke et al. (2014), https://doi.org/10.1002/2013ja019204, and Liu et al. (2016), https://doi.org/10.1002/2015gl067398 parameterizations are used. Key Points: 3‐D simulations using different radial diffusion coefficients, except Ali et al. (2016), produce similar resultsUsing Ali et al. (2016) DLL, simulated flux is significantly lower than observations3‐D modeling with Brautigam and Albert (2000) DLL results in a slightly smaller normalized difference (averaged over energies) to observations [ABSTRACT FROM AUTHOR]

We present a global survey of energetic electron precipitation from the equatorial magnetosphere due to hiss waves in the plasmasphere and plumes. Using Van Allen Probes measurements, we calculate the pitch angle diffusion coefficients at the bounce loss cone, and evaluate the energy spectrum of precipitating electron flux. Our ∼6.5‐year survey shows that, during disturbed times, hiss inside the plasmasphere primarily causes the electron precipitation at L > 4 over 8 h < MLT < 18 h, and hiss waves in plumes cause the precipitation at L > 5 over 8 h < MLT < 14 h and L > 4 over 14 h < MLT < 20 h. The precipitating energy flux increases with increasing geomagnetic activity, and is typically higher in the plasmaspheric plume than the plasmasphere. The characteristic energy of precipitation increases from ∼20 keV at L = 6–∼100 keV at L = 3, potentially causing the loss of electrons at several hundred keV. Plain Language Summary: Hiss is a plasma wave with a broad frequency range spanning from tens to several thousand Hz, commonly observed in the dayside plasmasphere and plumes of the Earth's magnetosphere, and plays an important role in the loss of energetic electrons. Subject to the interaction with hiss waves, the radiation belt electrons precipitate from the equatorial magnetosphere into the Earth's upper atmosphere, potentially changing the ionospheric conductance and chemistry. Using the measurements of hiss waves and electrons from the ∼6.5‐year data of Van Allen Probes, we perform a global survey of electron precipitation due to hiss waves in the magnetosphere. Our survey indicates that hiss waves mostly cause the energetic electron precipitation in the dayside plasmasphere and in the afternoon sector of the plume. The precipitation is more significant during disturbed geomagnetic conditions than quiet times, and the precipitating flux is higher in the plasmaspheric plume than the plasmasphere. The characteristic energy of precipitating electrons increases with decreasing distance from the Earth. Although the average precipitating electron flux due to hiss is lower than that of chorus, the average energy of precipitation is higher, potentially causing the loss of relativistic electrons in the radiation belts. Key Points: During disturbed times, hiss waves cause enhanced electron precipitation in the dayside plasmasphere and plume in the afternoon sectorThe average total precipitating energy flux due to hiss waves reaches 0.3–1 erg/cm2/s at L > 4.5 and 8 h < MLT < 18 h when AE∗ > 500 nTAverage precipitating flux is higher in the plume than plasmasphere, and the precipitation energy increases with decreasing L shell [ABSTRACT FROM AUTHOR]

We report plasma wave observations of equatorial magnetosonic waves at integer harmonics of the local gyrofrequency of doubly ionized helium (He++). The waves were observed by Van Allen Probe A on 08 Feb 2014 when the spacecraft was in the afternoon magnetic local time sector near L≈4 inside of the plasmasphere. Analysis of the complementary in‐situ energetic ion measurements (1–300 keV) reveals the presence of a helium ion ring distribution centered near 30 keV. Theoretical linear growth rate calculations suggest that the local plasma and field conditions can support the excitation of the magnetosonic waves from the unstable ring distribution. This represents the first report of the generation of magnetosonic equatorial noise via a ring distribution in energetic He++ ions in the near‐Earth space plasma environment. Plain Language Summary: In nature, an object displaced from its stable equilibrium typically responds by oscillating at its natural frequency (e.g., a plucked guitar string). The ionized gas (plasma) in outer space where Earth‐orbiting satellites fly is no different. In music, we have octaves or the overtone series of pure tones, where each frequency is an integer multiple of a fundamental. In this study, we report on such "musical notes" in space, but instead of sound waves propagating through air (music), these musical notes are propagating in the plasma in space. And while music is heard because of the sound waves compressing air, these space plasma waves are the result of subatomic particles oscillating about (and cannot be heard). The oscillating particles are helium atoms that have two of their electrons stripped away by the violent processes that occur near the Sun's surface. These particles have made their way through interplanetary space all the way to Earth where they are detected by sophisticated sensors on NASA's Van Allen Probe satellites. Key Points: Equatorial magnetosonic waves are observed at harmonics of the He++ gyrofrequency near L = 4 on the daysideParticle observations suggest the presence of a helium ion ring distribution at energies ∼30–300 keVLinear growth rate calculations indicate that the MS waves are generated from the unstable ring distribution [ABSTRACT FROM AUTHOR]

Radiation belt electrons undergo frequent acceleration, transport, and loss processes under various physical mechanisms. One of the most prevalent mechanisms is radial diffusion, caused by the resonant interactions between energetic electrons and ULF waves in the Pc4‐5 band. An indication of this resonant interaction is believed to be the appearance of periodic flux oscillations. In this study, we report long‐lasting, drift‐periodic flux oscillations of relativistic and ultrarelativistic electrons with energies up to ∼7.7 MeV in the outer radiation belt, observed by the Van Allen Probes mission. During this March 2017 event, multi‐MeV electron flux oscillations at the electron drift frequency appeared coincidently with enhanced Pc5 ULF wave activity and lasted for over 10 h in the center of the outer belt. The amplitude of such flux oscillations is well correlated with the radial gradient of electron phase space density (PSD), with almost no oscillation observed near the PSD peak. The temporal evolution of the PSD radial profile also suggests the dominant role of radial diffusion in multi‐MeV electron dynamics during this event. By combining these observations, we conclude that these multi‐MeV electron flux oscillations are caused by the resonant interactions between electrons and broadband Pc5 ULF waves and are an indicator of the ongoing radial diffusion process during this event. They contain essential information of radial diffusion and have the potential to be further used to quantify the radial diffusion effects and aid in a better understanding of this prevailing mechanism. Key Points: Long‐lasting, drift‐periodic flux oscillations of 1.8–7.7 MeV electrons in the radiation belt are reported using Van Allen Probes dataFlux oscillations appeared coincidently with enhanced Pc5 ULF waves with amplitude well correlated with phase space density radial gradientThese flux oscillations indicate the resonant interactions between electrons and broadband Pc5 ULF waves [ABSTRACT FROM AUTHOR]

Here we perform a statistical analysis of low frequency ultra‐low‐frequency (ULF) waves (∼ mHz ‐ Hz) in the Earth's inner magnetosphere excluding electromagnetic ion cyclotron waves concurrently observed. We use the magnetic field data from the two Van Allen Probes during their first magnetic local time (MLT) revolution that cover the periods of coronal mass ejections. The major results of our analysis are as follows. (a) Spectra of both the transverse and compressional ULF waves are well approximated by the power‐laws in the ∼ mHz ‐ Hz frequency range. (b) There are two sources of the low frequency ULF waves: an internal magnetospheric source and an external source outside of the magnetosphere. (c) The average transverse power in the 6–24 h MLT sector dominates that in the 0–6 h sector, whereas the compressional power in the 12–24 h sector dominates that in the 0–12 h sector. (d) The average powers of transverse and compressional ULF waves in the plasmasphere dominate the average powers in the high L shell region of 5.5≲L≲6.5, and there is a deep power minimum in the intermediary region of 3.5≲L≲4.5. (e) The compressional ULF wave power has a maximum in the near equatorial region, whereas the transverse power has a minimum there. (f) A wave energy cascade from low frequency ULF waves into the higher frequency range of EMIC waves (∼ Hz) supplies the nonthermal seed fluctuations from which EMIC waves can then grow due to instabilities of the energetic magnetospheric ions. Key Points: Power‐laws approximate well the spectra of transverse and compressional ultra‐low‐frequency (ULF) waves in the Pc 1–5 frequency rangeStatistics of low frequency ULF waves in the Earth's inner magnetosphere during coronal mass ejection conditions in the upstream solar wind are presentedCascade from low frequency ULF waves into the higher frequency range supplies seed fluctuations for electromagnetic ion cyclotron wave growth due to instabilities [ABSTRACT FROM AUTHOR]

After the launch of Van Allen Probes, the three‐belt structures of ultra‐relativistic electrons are discovered. In this study, we investigate the three‐belt structures of sub‐MeV electrons, which may form under different mechanism compared with those of ultra‐relativistic electrons and are worth in‐depth analysis. Based on the differential flux data from Magnetic Electron Ion Spectrometer onboard Radiation Belt Storm Probes‐B satellite, we find 54 events, in which two comparable peaks of sub‐MeV electron fluxes and a slot appear where there should be the outer radiation belt. Through the statistical analysis, the three‐belt structures of sub‐MeV electrons are found to be closely related to symmetric H‐component (SYM‐H) and Auroral Electrojet (AE) indices. The 2‐day SYM‐H minimum and AE maximum before the event have a linear trend with the remnant belt and the "second slot" locations. The L‐values of the remnant belt and the "second slot" of different energy electrons decrease as energy increases in general and show interesting characteristics during their temporal evolution. Moreover, the lifetime of the remnant belt of different energy electrons increases as energy increases. We find similarities and differences between sub‐MeV and ultra‐relativistic electrons three‐belt events, which provides a new perspective in three‐belt structure study. Plain Language Summary: An important result of the Van Allen Probes mission was that the ultra‐relativistic electron (>2MeV) radiation belts, containing an inner belt, a slot region and an outer belt, often show a three‐belt structure. That is, two belts and a slot region appear where there should be the outer radiation belt. The dominate mechanism to form the structure is still under debate. Recently, such structure for sub‐MeV electrons was also reported in a case study. Since the physical properties of these two types of electrons are different, the characteristics and mechanism to form the structure may be different. In this work, we show that common occurrence and general characteristics of sub‐MeV electron three‐belt events. The location, formation, and temporal evolution of the structure are influenced by electron energy and magnetospheric conditions reflected by geomagnetic indices. We have also determined the relatively short lifetime of the intermediate belt (remnant belt) and explained why three‐belt events for sub‐MeV electrons are less visible. Key Points: We report for the first time the main statistical characteristics of sub‐MeV three‐belt events using Van Allen Probe dataThe occurrence and the location of the remnant belt for the sub‐MeV electrons are affected by electron energy and geomagnetic activitiesThe lifetime of sub‐MeV electrons in the remnant belt is much smaller, explaining why these three‐belt events are less visible [ABSTRACT FROM AUTHOR]

The spatial scales of whistler‐mode waves, determined by their generation process, propagation, and damping, are important for assessing the scaling and efficiency of wave‐particle interactions affecting the dynamics of the radiation belts. We use multi‐point wave measurements by two Van Allen Probes in 2013–2019 covering all MLTs at L = 2–6 to investigate the spatial extent of active regions of chorus and hiss waves, their wave amplitude distribution in the source/generation region, and the scales of chorus wave packets, employing a time‐domain correlation technique to the spacecraft approaches closer than 1,000 km, which happened every 70 days in 2012–2018 and every 35 days in 2018–2019. The correlation of chorus wave power dynamics using is found to remain significant up to inter‐spacecraft separations of 400–750 km transverse to the background magnetic field direction, consistent with previous estimates of the chorus wave packet extent. Our results further suggest that the chorus source region can be slightly asymmetrical, more elongated in either the azimuthal or radial direction, which could also explain the aforementioned two different scales. An analysis of average chorus and hiss wave amplitudes at separate locations similarly shows the reveals different radial and azimuthal extents of the corresponding wave active regions, complementing previous results based on THEMIS spacecraft statistics mainly at larger L > 6. Both the chorus source region scale and the chorus active region size appear smaller inside the outer radiation belt (at L < 6) than at higher L‐shells. Key Points: We use Van Allen Probes filter bank VLF wave data (2013–2019) to investigate the spatial extent of active regions of chorus and hiss wavesThe correlation of chorus wave power is found to remain significant up to inter‐spacecraft separations of 400–750 km transverse to B0The chorus source scale and the chorus active region size appear smaller inside the outer radiation belt (at L<6) than at higher L [ABSTRACT FROM AUTHOR]

The access of solar energetic protons into the inner magnetosphere on September 7–8, 2017 is investigated by following reversed proton trajectories to compute the proton cutoff energy using the Dartmouth geomagnetic cutoff code (Kress et al., 2010, https://doi.org/10.1029/2009sw000488). The cutoff energies for protons coming from the west and east direction, the minimum and maximum cutoff energy respectively, are calculated every 5 min along the orbit of Van Allen Probes using TS07 and the Lyon‐Fedder‐Mobarry (LFM) MHD magnetic field model. The result shows that the cutoff energy increases significantly as the radial distance decreases, and that the cutoff energy decreases with the building up of the ring current during magnetic storms. Solar wind dynamic pressure also affects cutoff suppression (Kress et al., 2004, https://doi.org/10.1029/2003gl018599). The LFM‐RCM model shows stronger suppression of cutoff energy than TS07 during strong solar wind driving conditions. The simulation result is compared with proton flux measurements, showing consistent variation of the cutoff location during the September 7–8, 2017 geomagnetic storm. Key Points: The Dartmouth geomagnetic cutoff code is updated with TS07 and LFM MHD magnetic field snapshotsThe proton cutoff energy is calculated during the September 7–8, 2017 SEP eventComparison with data shows better correlation using LFM than TS04 and TS07 [ABSTRACT FROM AUTHOR]

Following the end of the Van Allen Probes mission, the Arase satellite offers a unique opportunity to continue in‐situ radiation belt and ring current particle measurements into the next solar cycle. In this study we compare spin‐averaged flux measurements from the MEPe, HEP‐L, HEP‐H, and XEP‐SSD instruments on Arase with those from the MagEIS and REPT instruments on the Van Allen Probes, calculating Pearson correlation coefficient and the mean ratio of fluxes at L* conjunctions between the spacecraft. Arase and Van Allen Probes measurements show a close agreement over a wide range of energies, observing a similar general evolution of electron flux, as well as average, peak, and minimum values. Measurements from the two missions agree especially well in the 3.6 = L* ≤ 4.4 range where Arase samples similar magnetic latitudes to Van Allen Probes. Arase tends to record higher flux for energies <670 keV with longer decay times after flux enhancements, particularly for L* < 3.6. Conversely, for energies >1.4 MeV, Arase flux measurements are generally lower than those of Van Allen Probes, especially for L* > 4.4. The correlation coefficient values show that the >1.4 MeV flux from both missions are well correlated, indicating a similar general evolution, although flux magnitudes differ. We perform a preliminary intercalibration between the two missions using the mean ratio of the fluxes as an energy‐ and L*‐ dependent intercalibration factor. The intercalibration factor improves agreement between the fluxes in the 0.58–1 MeV range. Key Points: MEPe, HEPH, HEPL, XEPSSD, and MAGEIS/REPT show a good correlation at energies above 300 keV and 3 < L* < 4.6Flux measurements at the same energy from the two missions are highly comparable in magnitudeIntercalibration via energy‐ and L*‐dependent factors improves the agreement between Arase and RBSP [ABSTRACT FROM AUTHOR]

Whistler mode chorus waves play an important role in the radiation belt dynamics, which usually appear as discrete elements with frequency sweeping. Finer structure analysis shows that a chorus element is composed of several frequency‐sweeping subelements, and such two‐level structures can be successfully reproduced by modeling based on nonlinear theories. Previous observations and models suggest that an element and its subelements should have the same frequency‐sweep direction. However, we here present two unexpected chorus rising tone events within which the subelements exhibit clearly reversed, falling frequency‐sweep. Moreover, the subelements consist of several wave packets that also show falling frequency‐sweep features. The three‐level structured chorus elements are distinctly different from all the reported observations and seem to bring challenges to the existing theories. We propose a possible scenario that the falling tone subelements are formed by nonlinear process with much shorter timescale and the starting frequency of each subelement is controlled by fast varying electron distribution. This study may inspire more studies toward a thorough understanding of the chorus generation process. Plain Language Summary: Whistler mode chorus is a very important plasma wave in the magnetosphere. The frequency‐time spectrogram of chorus waves shows discrete elements with frequency sweeping. Wave data analysis with higher time resolution shows that the frequency‐sweeping element is composed of several frequency‐sweeping subelements. Such two‐level structures have been successfully reproduced by modeling based on nonlinear models. It is widely accepted so far by the researchers that the subelements should have the same direction of frequency sweep with the whole element that they formed. However, in this study, we report two unexpected chorus rising tone events in which the frequency‐sweep direction of subelements is clearly opposite. Moreover, we find that the subelements consist of even finer structures that have the same frequency‐sweep direction as the subelements. Such a three‐level chorus wave structure is distinctly different from all the previous observations and seems to contradict the existing theories. We propose a possible scenario that the falling tone subelements are formed by nonlinear process with much shorter timescale and the starting frequency of each subelement is controlled by fast varying linear electron distribution. Our finding provides new insight into the chorus generation and will be of high interest to the communities of space physics. Key Points: Reversed frequency sweep of subelements in rising tone chorus are observed in the upper bandThe unusual chorus rising tones show a three‐level structure with each subelement containing multiple hyper‐subpacketsThe duration and sweep rate of the unusual elements are respectively comparable with those of the typical cases [ABSTRACT FROM AUTHOR]

We present the results of a multi‐point and multi‐instrument study of electromagnetic ion cyclotron (EMIC) waves and related energetic proton precipitation during a substorm. We analyze the data from Arase (ERG) and Van Allen Probes (VAPs) A and B spacecraft for an event of 16 and 17 UT on December 1, 2018. VAP‐A detected an almost dispersionless injection of energetic protons related to the substorm onset in the night sector. Then the proton injection was detected by VAP‐B and further by Arase, as a dispersive enhancement of energetic proton flux. The proton flux enhancement at every spacecraft coincided with the EMIC wave enhancement or appearance. This data show the excitation of EMIC waves first inside an expanding substorm wedge and then by a drifting cloud of injected protons. Low‐orbiting NOAA/POES and MetOp satellites observed precipitation of energetic protons nearly conjugate with the EMIC wave observations in the magnetosphere. The proton pitch‐angle diffusion coefficient and the strong diffusion regime index were calculated based on the observed wave, plasma, and magnetic field parameters. The diffusion coefficient reaches a maximum at energies corresponding well to the energy range of the observed proton precipitation. The diffusion coefficient values indicated the strong diffusion regime, in agreement with the equality of the trapped and precipitating proton flux at the low‐Earth orbit. The growth rate calculations based on the plasma and magnetic field data from both VAP and Arase spacecraft indicated that the detected EMIC waves could be generated in the region of their observation or in its close vicinity. Plain Language Summary: Electromagnetic ion cyclotron (EMIC) waves are believed to play a significant role in the dynamics of energetic protons and relativistic electrons in the Earth's magnetosphere. The properties of these waves are being intensively studied. We consider the conditions of the EMIC wave generation and the dynamics of the wave source during a substorm event using a unique configuration of three spacecraft (Arase and two Van Allen Probes). All spacecraft were at approximately the same distance from the Earth, forming a chain across the evening local time sector. Analyzing parameters of the wave generation obtained from in situ measured proton distribution function, we came to the conclusion that the waves could be generated within the substorm area, sometimes close to, but not necessary at the spacecraft location. As the substorm expands in longitude, the EMIC wave source exhibits a longitudinal drift. When substorm expansion stops, the wave generation region expands due to the magnetic drift of protons injected during the substorm. The observed wave properties show that the waves are able to precipitate energetic protons into the atmosphere. This is confirmed by observations of low orbiting satellites measuring proton precipitating fluxes. Key Points: Westward propagation of the EMIC wave generation region is due to both the substorm expansion and azimuthal drift of injected protonsStrong pitch‐angle diffusion regime is confirmed by observations of proton fluxes at low altitude and the diffusion coefficient calculationThe diffusion coefficient maximum corresponds well to the energy range of the observed proton precipitation [ABSTRACT FROM AUTHOR]

Three new methods for estimating a ratio of the ultralow frequency (ULF; 1–100 mHz) wave equatorial electric field amplitude in the Earth's magnetosphere to ground magnetic field amplitudes for field line resonances (FLR) are described. These methods use ratios of the time series extrema, ratios of the envelope waveform and the ratio of the spectral amplitude at the FLR frequency. These methods were applied to four ULF resonance intervals; three detected by the Van Allen Probe A spacecraft and one detected by the POLAR spacecraft. The intervals were conjoined with the CARISMA and IMAGE ground magnetometer arrays. The spectral ratio results for the Van Allen Probe intervals were approximately twice to three times the ratios estimated from the two time series based methods. The POLAR interval showed similar values across all three methods. The differences are attributed to broadband frequency signals that modify the time series amplitudes, while the spectral method avoids these off‐resonant frequencies. Based on the results of this study, a spectral based method for calculating the ratio at the FLR frequency is best. Plain Language Summary: A reliable method to remote sense the amplitude of electric fields of natural frequencies in space using ground magnetic fields would remove difficulties in obtaining sufficient spatial information of fields that are thought to energize electrons to high energies. One recently proposed method maps ground magnetic field variations, b, to electric field variations, e, in space via the ratio, e/b. However, there are different ways to calculate e/b and the uncertainties of this metric have not been reported. This paper presents a study of three methods used to calculate e/b and the uncertainty limits; a ratio of the peaks/troughs of the fields, ratios of each point of the envelope waveform of the fields, and a ratio of the spectral amplitude of the frequency where resonance occurs. These methods were applied to electric field data obtained from two satellites with corresponding magnetic field data taken from ground magnetic field detectors at the footprint of the resonant magnetic field line. It was found that the frequency‐based method gave two to three times larger values and smaller uncertainty values compared with the other methods. Based on the results of this study, the frequency‐based method is preferred for estimating e/b. Key Points: Three methods for estimating the ratio of ultralow frequency electric fields in space to magnetic fields on the ground, e/b, are presentedRatio values for 106 field line resonant events were calculate from spacecraft and ground data over September 25, 2012 to February 25, 2014The uncertainties in e/b for using the spectral method were consistently smaller than the other methods [ABSTRACT FROM AUTHOR]

Accurate measurements of trapped energetic electron fluxes are of major importance for the studies of the complex nature of radiation belts and the characterization of space radiation environment. The harmonization of measurements between different instruments increases the accuracy of scientific studies and the reliability of data‐driven models that treat the specification of space radiation environment. An intercalibration analysis of the energetic electron flux measurements of the Magnetic Electron Ion Spectrometer (MagEIS) and the Relativistic Electron‐Proton Telescope (REPT) instruments on‐board the Van Allen Probes (VAP) Mission versus the measurements of the Extremely High Energy Electron Experiment (XEP) unit on‐board Arase satellite is presented. The performed analysis demonstrates a remarkable agreement between the majority of MagEIS and XEP measurements and suggests the rescaling of MagEIS HIGH unit and of REPT measurements for the treatment of flux spectra discontinuities. The proposed adjustments were validated successfully using measurements from ESA Environmental Monitoring Unit (EMU) on‐board GSAT0207 and the Standard Radiation Monitor (SREM) on‐board INTEGRAL. The derived results lead to the harmonization of science‐class experiments on‐board VAP (2012–2019) and Arase (2017–) and propose the use of the data sets as reference in a series of space weather and space radiation environment developments. Plain Language Summary: Accurate measurements of trapped energetic electron fluxes are of major importance for the studies of the complex nature of radiation belts and the characterization of space radiation environment. The harmonization of measurements between different instruments increases the accuracy of scientific studies and the reliability of data‐driven models that treat the specification of space radiation environment. This study leads to the harmonization of the relativistic electron flux data sets from the cornerstone US and Japanese Radiation Belt missions. The derived results are validated by European Space Agency radiation monitors and can be used in a series of applications and developments. Key Points: Cross‐calibration analysis between Arase/XEP and RBSP/MagEIS & REPT measurements and validation using two different ESA radiation monitorsHarmonization of science‐class energetic electron flux measurements define reference data set(s) for the outer Van Allen beltReference data sets to be used for the harmonization of radiation monitors and the update/validation of climatological and space weather models [ABSTRACT FROM AUTHOR]

As a common boundary layer that distinctly separates the regions of high-density plasmasphere and low-density plasmatrough, the plasmapause is essential to comprehend the dynamics and variability of the inner magnetosphere. Using the machine learning framework PyTorch and high- quality Van Allen Probes data set, we develop a neural network model to predict the global dynamic variation of the plasmapause location, along with the identification of 6,537 plasmapause crossing events during the period from 2012 to 2017. To avoid the overfitting and optimize the model generalization, 5,493 events during the period from September 2012 to December 2015 are adopted for division into the training set and validation set in terms of the 10-fold cross-validation method, and the remaining 1,044 events are used as the test set. The model parameterized by only AE or Kp index can reproduce the plasmapause locations similar to those modeled using all five considered solar wind and geomagnetic parameters. Model evaluation on the test set indicates that our neural network model is capable of predicting the plasmapause location with the lowest RMSE. Our model can also produce a smooth magnetic local time variation of the plasmapause location with good accuracy, which can be incorporated into global radiation belt simulations and space weather forecasts under a variety of geomagnetic conditions. [ABSTRACT FROM AUTHOR]

We investigate the timing and relative influence of VLF in the chorus frequency range observed by the DEMETER spacecraft and ULF wave activity from ground stations on daily changes in electron flux (0.23 to over 2.9 MeV) observed by the HEO‐3 spacecraft. At each L‐shell, we use multiple regression to investigate the effects of each wave type and each daily lag independent of the others. We find that reduction and enhancement of electrons occur at different timescales. Chorus power spectral density and ULF wave power are associated with immediate electron decreases on the same day but with flux enhancement 1–2 days later. ULF is nearly always more influential than chorus on both increases and decreases of flux, although chorus is often a significant factor. There was virtually no difference in correlations of ULF Pc3, Pc4, or Pc5 with electron flux. A synergistic interaction between chorus and ULF waves means that enhancement is most effective when both waves are present, pointing to a two‐step process where local acceleration by chorus waves first energizes electrons which are then brought to even higher energies by inward radial diffusion due to ULF waves. However, decreases in flux due to these waves act additively. Chorus and ULF waves combined are most effective at describing changes in electron flux at >1.5 MeV. At lower L (2–3), correlations between ULF and VLF (likely hiss) with electron flux were low. The most successful models, over L = 4–6, explained up to 47.1% of the variation in the data. Plain Language Summary: Low‐frequency electromagnetic waves in the Earth's radiation belt are associated with immediate (same day) decreases in the electron population, but appear to increase the number of electrons 1–2 days later. This is most prominent in the outer radiation belt, at an equatorial distance of 4–6 Earth radii. Key Points: VLF and ULF waves are associated with same day electron (>1.5 MeV) decreases but with flux enhancement 1–2 days laterULF waves are nearly always more influential than chorus, but electron enhancement is most effective when both wave types are presentBoth VLF and ULF waves are more influential at L 4–6 than at lower altitudes [ABSTRACT FROM AUTHOR]

Proton anisotropy in velocity space has been generally accepted as a major parameter for exciting electromagnetic ion cyclotron (EMIC) waves. In this study, we estimate the proton anisotropy parameter as defined by the linear resonance theory using data from the Van Allen Probes mission. Our investigation uses the measurements of the inner magnetosphere (L < 6) from January 2013 to February 2018. We find that the proton anisotropy is always clearly limited by an upper bound and it well follows an inverse relationship with the parallel proton β (the ratio of the plasma pressure to the magnetic pressure) within a certain range. This upper bound exists over wide spatial regions, AE conditions, and resonance energies regardless of the presence of EMIC waves. EMIC waves occur when the anisotropy lies below but close to this upper bound within a narrow plasma β range: The lower cutoff β is due to an excessively high anisotropy threshold and the upper cutoff β is possibly due to the predominant role of a faster‐growing mirror mode instability. We also find that the anisotropy during the observed EMIC waves is unstable, leading to the linear ion cyclotron instability. This result implies that the upper bound of the anisotropy is due to nonlinear processes. Key Points: Proton anisotropy has a clear upper bound that follows an inverse relationship with the plasma beta within a certain rangeEMIC waves occur with an anisotropy below but close to the upper bound within a narrower plasma beta rangeThe anisotropy associated with EMIC waves is unstable to the linear instability, requiring a nonlinear process to set the upper bound [ABSTRACT FROM AUTHOR]

We have conducted a statistical study of toroidal mode standing Alfvén waves detected by the Van Allen Probes spacecraft in the dayside inner magnetosphere, with an emphasis on the nodal structure of the fundamental through fifth harmonics. We developed a technique to accurately assign harmonic mode numbers to peaks in the power spectra of the electric (Eν) and magnetic (Bϕ) field components of toroidal waves and then determine the spectral intensities of Eν and Bϕ and the coherence and cross‐phase between these field components for each harmonic. The magnetic latitude (MLAT) dependence of these quantities was statistically examined to determine the location of the nodes. In addition to the equatorial nodes located close to the equator (MLAT = 0), we identified several nodes away from the equator within the MLAT range from −20° to +20°. We found that the Eν‐Bϕ cross‐phase is very close to ±90° except near the nodes, indicating that the fixed‐end approximation is appropriate in modeling dayside toroidal waves. Noting that the node latitudes depend on the distribution of the mass density (ρ) along the background magnetic field, we inferred the distribution from the nodes observed at L = 4–6. If we adopt a model field line mass density (ρ) distribution of the form ρ ∝ (1/r)α, where r is geocentric distance to the field line and α is a free parameter, the statistically determined node latitudes indicate that α∼1.5 is appropriate for both the plasmasphere and the plasmatrough. Plain Language Summary: Interaction of the solar wind plasma with the outer boundary of Earth's magnetosphere results in compressional magnetohydrodynamic waves in the 1–100 mHz band that propagate into the magnetosphere. Each geomagnetic field line is stimulated by the waves and executes its own discrete eigenmode oscillations because the field line is tied to the rigid ionosphere. The oscillations contain valuable information on how the plasma mass is distributed along the field line. Unlike the mass distribution on a guitar string, which is uniform, the mass distribution of geomagnetic field lines changes with distance from the ionosphere. The mass distribution affects various magnetospheric phenomena but is difficult to directly determine using particle experiments on spacecraft. Fortunately, we can estimate the mass distribution from the location of the nodes of the field line eigenmode oscillations, which can be determined using spacecraft observations of the electric and magnetic fields of the oscillations. In this study, we use data from the Van Allen Probes spacecraft to demonstrate that the node location can be determined for the fundamental through fifth harmonics and describe how this information is used to estimate the field line mass density distribution. Key Points: We determine the power and phase of the electric and magnetic fields of toroidal waves detected by spacecraftWe determine the node latitudes of five harmonics from the latitude dependence of the power and phaseWe compare the node latitudes with theoretical models to infer the field line mass distribution [ABSTRACT FROM AUTHOR]

This study considers the impact of electron precipitation from Earth's radiation belts on atmospheric composition using observations from the NASA Van Allen Probes and NSF Focused Investigations of Relativistic Electron Burst Intensity, Range, and Dynamics (FIREBIRD II) CubeSats. Ratios of electron flux between the Van Allen Probes (in near‐equatorial orbit in the radiation belts) and FIREBIRD II (in polar low Earth orbit) during spacecraft conjunctions (2015–2017) allow an estimate of precipitation into the atmosphere. Total Radiation Belt Electron Content, calculated from Van Allen Probes RBSP‐ECT MagEIS data, identifies a sustained 10‐day electron loss event in March 2013 that serves as an initial case study. Atmospheric ionization profiles, calculated by integrating monoenergetic ionization rates across the precipitating electron flux spectrum, provide input to the NCAR Whole Atmosphere Community Climate Model in order to quantify enhancements of atmospheric HOx and NOx and subsequent destruction of O3 in the middle atmosphere. Results suggest that current APEEP parameterizations of radiation belt electrons used in Coupled Model Intercomparison Project may underestimate the duration of events as well as higher energy electron contributions to atmospheric ionization and modeled NOx concentrations in the mesosphere and upper stratosphere. Plain Language Summary: High‐energy particles precipitating into the atmosphere from space affect the chemistry and composition of Earth's atmosphere. While there is significant understanding about the atmospheric impacts of auroral electrons, solar protons, and galactic cosmic rays, the effects of electrons from the near‐Earth Van Allen radiation belts remain uncertain. This study helps quantify electrons precipitating into the atmosphere by comparing measurements within the radiation belts from the NASA Van Allen Probes spacecraft to observations from the low‐altitude NSF Focused Investigations of Relativistic Electron Burst Intensity, Range, and Dynamics (FIREBIRD II) CubeSats. Global atmospheric model simulations quantify the impact of estimated electron precipitation on the ionization and chemical composition of Earth's atmosphere. Results from an initial case study using this new method suggest that electrons from the radiation belts may produce more atmospheric ionization at lower altitudes and for longer duration than currently recommended estimates, potentially affecting the chemistry of ozone in the middle atmosphere and as a consequence influencing atmospheric heating and dynamics. Key Points: Conjunctions between Van Allen Probes and FIREBIRD II enable novel estimates of atmospheric electron precipitationEstimates of electron precipitation from Van Allen Probes suggest CMIP6 may underestimate atmospheric ionization from 60 to 70 kmDirect production of NOx by precipitating electrons during March 2013 using this new method suggest 40% enhancements from 60 to 70 km [ABSTRACT FROM AUTHOR]

Van Allen Probes observations of ion spectra often show a sustained gap within a very narrow energy range throughout the full orbit. To understand their formation mechanism, we statistically investigate the characteristics of the narrow gaps for oxygen ions and find that they are most frequently observed near the noon sector with a peak occurrence rate of over 30%. The magnetic moment (µ) of the oxygen ions in the gap shows a strong dependence on magnetic local time (MLT), with higher and lower µ values in the morning and afternoon sectors, respectively. Moreover, we find through superposed epoch analysis that the gap formation also depends on geomagnetic conditions. Those gaps formed at lower magnetic moments (µ < 3,000 keV/G) are associated with stable convection electric fields, which enable magnetospheric ions to follow a steady drift pattern that facilitates the gap formation by corotational drift resonance. On the other hand, gaps with higher µ values are statistically preceded by a gradual increase of geomagnetic activity. We suggest that ions within the gap were originally located inside the Alfven layer following closed drift paths, before they were transitioned into open drift paths as the convection electric field was enhanced. The sunward drift of these ions, with very low fluxes, forms a drainage void in the dayside magnetosphere manifested as the sustained gap in the oxygen spectrum. This scenario is supported by particle-tracing simulations, which reproduce most of the observed characteristics and therefore provide new insights into inner magnetospheric dynamics. [ABSTRACT FROM AUTHOR]

The impact of radial diffusion in storm time radiation belt dynamics is well-debated. In this study we quantify the changes and variability in radial diffusion coefficients during geomagnetic storms. A statistical analysis of Van Allen Probes data (2012-2019) is conducted to obtain measurements of the magnetic and electric power spectral densities for Ultra Low Frequency (ULF) waves, and corresponding radial diffusion coefficients. The results show global wave power enhancements occur during the storm main phase, and continue into the recovery phase. Local time asymmetries show sources of wave power are both external solar wind driving and internal sources from coupling with ring current ions and substorms. Wave power enhancements are also observed at low L values (L < 4). The accessibility of wave power to low L is attributed to a depression of the Alfvén continuum. The increased wave power drives enhancements in both the magnetic and electric field diffusion coefficients by more than an order of magnitude. Significant variability in diffusion coefficients is observed, with values ranging over several orders of magnitude. A comparison to the Kp parameterized empirical model of Ozeke et al. (2014) is conducted and indicates important differences during storm times. Although the electric field diffusion coefficient is relatively well described by the empirical model, the magnetic field diffusion coefficient is approximately ~10 times larger than predicted. We discuss how differences could be attributed to data set limitations and assumptions. Alternative storm-time radial diffusion coefficients are provided as a function of L* and storm phase. [ABSTRACT FROM AUTHOR]

Drift-bounce resonance between ultra-low-frequency (ULF) waves and ring current ions has been widely studied, because of its important role in ring current acceleration and relevant geomagnetic activities. To identify drift-bounce resonance in observations, 180° phase shifts across resonant pitch angle have been proposed as diagnostic signatures. This study, however, presents observations that suggest this criterion may be invalid when phase space density (PSD) distributions vary nonmonochromatically with energy. We identified 14 ULF wave-ion interaction cases from 2-years Van Allen Probes data. In these cases, 180° phase shifts across pitch angle are observed at particular energies. Near these energies, pitch angle-dependent PSD bump-on-tail distributions were also observed. As a result, at fixed energies, the sign of ion PSD energy gradient changes across pitch angle, which then can result in the observed 180° phase shift. Based on the observations, we suggest 180° phase shifts across pitch angle can also result from pitch angle-dependent bump-on-tail distributions, which should be taken into account in future ULF wave-ion interaction studies. [ABSTRACT FROM AUTHOR]

We present, for the first time, a plasmaspheric hiss event observed by the Van Allen probes in response to two successive interplanetary (IP) shocks occurring within an interval of ~2 h on December 19, 2015. The first shock arrived at 16:16 UT and caused disappearance of hiss for ~30 min. Combined effect of plasmapause crossing, significant Landau damping by suprathermal electrons and their gradual removal by magnetospheric compression led to the disappearance of hiss. Calculation of electron phase space density and linear wave growth rates showed that the shock did not change the growth rate of whistler waves within the core frequency range of plasmaspheric hiss (0.1-0.5 kHz) during this interval making conditions unfavorable for the generation of hiss. The recovery began at ~16:45 UT which is attributed to an enhancement in local plasma instability initiated by the first shock-induced substorm and additional possible contribution from chorus waves. This time, the wave growth rate peaked within the core frequency range (~350 Hz). The second shock arrived at 18:02 UT and generated patchy hiss persisting up to ~19:00 UT. It is shown that an enhanced growth rate and additional contribution from shock-induced poloidal Pc5 mode (periodicity ~240 s) ultralow frequency (ULF) waves resulted in the excitation of hiss waves during this period. The hiss wave amplitudes were found to be additionally modulated by background plasma density and fluctuating plasmapause location. The investigation highlights the important roles of IP shocks, substorms, ULF waves, and background plasma density in the variability of plasmaspheric hiss. [ABSTRACT FROM AUTHOR]

Space instruments rely heavily on modeling to predict and understand the instrument response, enabling a determination of the capabilities and resolution. The Bayesian approach provides a framework to incorporate prior knowledge and propagate uncertainty to predict the instrument response. We present an empirical Bayes model for the end-to-end performance of the Helium, Oxygen, Proton, and Electron mass spectrometers aboard the Van Allen Probes mission. In this model, we use a combination of external modeling, laboratory calibration, and expert opinion to construct the time-of-flight spectra and demonstrate good agreement with on-orbit data. The empirical Bayes model is applied to explore doubly charged ions and carbon, nitrogen, oxygen discrimination during the Van Allen Probes mission. [ABSTRACT FROM AUTHOR]

In this study, analytical approximation is used to solve the wave equations near the plasmapause boundary layer in order to examine the validity of ray tracing approach for fast magnetosonic (MS) wave propagations, and then analytical solutions for electromagnetic fields of MS waves through the plasmapause boundary layer are provided for the first time. Both theoretical calculations from the analytical expressions and observations of Van Allen Probes have indicated that electric fields of MS waves decrease rapidly but magnetic fields increase rapidly when propagating across the plasmapause boundary layer from the outside. Considering effects of the plasmapause width and wave frequency, parameter analysis has shown that when the width of the plasmapause boundary layer is narrow in comparison with the wavelength of MS waves, a significant part of waves will be reflected. In these circumstances, the WKB approximation and then ray tracing method might become invalid, and analytical approach provided in this paper could be utilized to solve the wave equation. [ABSTRACT FROM AUTHOR]