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We describe a new data product combining pitch angle resolved electron flux measurements from the Radiation Belt Storm Probes (RBSP) Energetic Particle Composition and Thermal Plasma (ECT) suite on the National Aeronautics and Space Administration’s Van Allen Probes. We describe the methodology used to combine each of the data sets and produce a consistent set of pitch-angle-resolved spectra for the entire Van Allen Probes mission. Three-minute-averaged flux spectra are provided spanning energies from 15 eV up to 20 MeV. This new data product offers researchers a consistent cross calibrated data set to explore the particle dynamics of the inner magnetosphere across a wide range of energies. [ABSTRACT FROM AUTHOR]

We present a statistical study of density cavities observed in the inner magnetosphere by the Van Allen Probes during four one-month periods: February 2013, July 2013, January 2014, and June 2014. These periods were chosen to allow the survey of all magnetic local times. We find that density cavities are a recurrent feature of the density profiles of in situ measurements in the inner magnetosphere. We further investigate the correlation between the density cavities and the enhancement of fluxes of warm ions with energies of 10–100 eV. The results show that warm ion flux enhancements associated with the density cavities were observed more frequently for H+, then for He+ and the least frequently for O+. The occurrences of the associated flux enhancements were increased when considering only the cavities inside the plasmasphere. Possible mechanisms responsible for the observed warm ion flux enhancements and the role of density cavities on these ion flux enhancements are discussed [ABSTRACT FROM AUTHOR]

Direct comparisons between RBSP (Van Allen Probes or Radiation Belt Storm Probes) and TWINS (Two Wide-angle Imaging Neutral-atom Spectrometers) for the main phase of two storms, March 17 and October 7, 2015, showed agreement between the in situ ion measurements and the ion spectra from the deconvolved energetic neutral-atom (ENA) measurements, except when O+ ions were significant. Spatial evolution of individual energy peaks in the ion spectra is studied using TWINS data. O+ ions are seen to result in intense peaks at 5--10 keV/amu in the TWINS ion spectra. These ion populations are confined to low L shells (L < 5) and localized in the premidnight sector. When H+ ions are significant, the low energy peaks (<25 keV/amu) are found to be less intense than the high energy peaks (>25 keV/amu), located at L > 4 and localized within the premidnight sector. During times of rapidly varying AE indices, two spatially distinct peaks, between 3--5RE and 6--8RE, are observed for the ions with energies >25 keV/amu. The outer peak appears for a few hours and fades while the inner peak is more stable. These structures are found to be consistent with particle injections observed in the RBSP data. When double peaked structures are swept off, low energy ions accumulate in the premidnight to midnight sectors whereas high energy ions are located premidnight to postmidnight sectors. Faster drift orbits of >25 keV/ amu ions may cause this kind of distribution. [ABSTRACT FROM AUTHOR]

We utilize 17 years of combined Van Allen Probes and Arase data to statistically analyze the response of the inner magnetosphere to the orientation of the interplanetary magnetic field (IMF) By component. Past studies have demonstrated that the IMF By component introduces a similarly oriented By component into the magnetosphere. However, these studies have tended to focus on field lines in the magnetotail only reaching as close to the Earth as the geosynchronous orbit. By exploiting data from these inner magnetospheric spacecraft, we have been able to investigate the response at radial distances of <7RE. When subtracting the background magnetic field values, provided by the T01 and IGRF magnetic field models, we find that the IMF By component does affect the configuration of the magnetic field lines in the inner magnetosphere. This control is observed throughout the inner magnetosphere, across both hemispheres, all radial distances, and all magnetic local time sectors. The ratio of IMF By to the observed By residual, also known as the "penetration efficiency," is found to be ~0.33. The IMF Bz component is found to increase, or inhibit, this control depending upon its orientation. [ABSTRACT FROM AUTHOR]

We investigate the rare events of sudden appearances of relativistic electrons (>700 keV), which are normally confined to the Van Allen belts, in the slot region. The frequency of occurrence of these events is on average 1-2 per year. To cope with the scarcity of events, in this study, we examine 21 years of trapped relativistic electron fluxes available from the POES and MetOp Space Environment Monitor (SEM-2). Our statistical analysis shows that these events can occur even during moderate geomagnetic activity. Occurrence of these events correlates with high-speed solar winds or interplanetary coronal mass ejections depending on the phase of the solar cycle. A strong correlation of these events is found with ultra-low frequency (ULF) wave activity, which can be used to predict these events with more than 75% accuracy. [ABSTRACT FROM AUTHOR]

Radiation belt electrons have a complicated relationship with geomagnetic activity. We select electron measurements from 7 years of DEMETER and 6 years of Van Allen Probes data during geomagnetic storms to conduct statistical analysis focusing on the correlation between electron flux and Dst index. We report, for the first time, an upper limit of electron fluxes observed by both satellites throughout the inner and outer belts across a wide energy range from ~100s keV to multi-MeV. The upper flux limit is determined at different L's and energies, for example, 1.9 X 107/cm²-s-sr-MeV at 470 keV at L = 1.5 and 3.6 X 105/cm²-s-sr-MeV at 3.4 MeV at L = 4 (Van Allen Probes). We present the energy spectra of the electron flux upper limit at different L shells and find the measured upper flux limit to be at least three times higher than the predicted flux from the AE8/AE9 models, although the spectral shape is remarkably similar. We show that the average flux with an applied time lag is better correlated with the Dst index and that the time lag optimizing the correlation coefficient is larger at lower L and at higher energies. These findings present the underlying challenges to model the dynamic variation of relativistic electrons in the inner magnetosphere and are important information for space weather considerations. [ABSTRACT FROM AUTHOR]

Ring current decay during storm recovery phase may be affected by different loss processes. In this study, we have investigated the lifetimes of ring current ions (H+ and O+) of energies from 1 keV to several hundred keV at L shell from 3 to 6 during the storm recovery phase through a statistical survey. The observational data of 48 geomagnetic storms from March 2013 to May 2019 are collected based on Van Allen Probe observations. We find that (1) the observed lifetimes of H+ and O+ in general increase with L shell and (2) the lifetimes of H+ is short than that of O+ when E < ~50 keV while the situation is reversed when E > ~50 keV. In addition, we have made use of the charge exchange theory, combined with previous experimental results on the charge exchange cross section and two distribution models of neutral hydrogen atoms in the exosphere, so as to directly estimate the ring current ions decay caused by charge exchange mechanism only. Through the comparison between the model predictions of charge exchange lifetime and the observed lifetimes, we find that (3) the observed lifetimes are in general consistent with model results, which confirms that charge exchange is a dominant loss mechanism of ring current ions during storm recovery phase. [ABSTRACT FROM AUTHOR]

Fast magnetosonic (MS) waves are excited by the ring distribution of energetic protons preferably when the ring velocity (VR) is within a factor of 2 above or below the local Alfvén speed (VA). Here we examine the global distributions of MS waves and proton rings with 0.5VA = VR = 2VA based on 64 months (from October 25, 2012 to February 28, 2018) of Van Allen Probes observations. The statistical results show that MS waves are present over a broad region of L = 1.2-6.0 and 00-24 magnetic local time (MLT), with a higher occurrence rate at L = 2.5-5.5 on the dayside. Proton rings occur mainly on the dayside of L > 5.0. During active geomagnetic periods, both MS waves and proton rings occur more frequently and extend to low L-shells. The current results provide the further observational evidence that MS waves can be excited by proton rings at a distant region and propagate to low L-shells. [ABSTRACT FROM AUTHOR]

Using 7 years of data from the Helium, Oxygen, Proton, and Electron instrument on the Van Allen Probes, equatorial pitch angle distributions (PADs) of 1-50 keV electrons in Earth's inner magnetosphere are investigated statistically. An empirical model of electron equatorial PADs as a function of radial distance, magnetic local time, geomagnetic activity, and electron energy is constructed using the method of Legendre polynomial fitting. Model results show that most equatorial PADs of 1-10s of keV electrons in Earth's inner magnetosphere are pancake PADs, and the lack of butterfly PADs is likely due to their relatively flat or positive flux radial gradients at higher altitudes. During geomagnetically quiet times, more anisotropic distributions of 1-10s of keV electrons at dayside than nightside are observed, which could be responsible for moderate chorus wave activities at dayside during quiet times as reported by previous studies. During active times, the anisotropy of 1-10s of keV electrons significantly enhances, consistent with the enhanced chorus wave activity during active times and suggesting the critical role of 1-10s of keV electrons in generating chorus waves in Earth's inner magnetosphere. Different enhanced anisotropy patterns of different energy electrons are also observed during active times: at R > ~4 RE, keV electrons are more anisotropic at dawn to noon, while 10s of keV electrons have larger anisotropy at midnight to dawn. These differences, combined with the statistical distribution of chorus waves shown in previous studies, suggest the differential roles of electrons with different energies in generating chorus waves with different properties. [ABSTRACT FROM AUTHOR]

In this study, ion data from the Helium, Oxygen, Proton, and Electron (HOPE) spectrometers onboard Van Allen Probes reveal the existence of wedge‐like structures of O+, He+, and H+ ions deep in the inner magnetosphere. The behaviors of the wedge‐like structures in terms of temporal evolution, spatial distribution, upper energy limit, as well as dependence on solar wind and different geomagnetic indices are investigated from both event studies of several consecutive orbits on 3 February 2013 and the subsequent statistical analyses using 4 years of data. Unlike the dominant distribution at L=4–8 in the dayside observed by the polar orbit satellites in previous studies, the wedge‐like structures deep in the equatorial plane of the inner magnetosphere are found mostly at the Mcllwain L shells of L=2–5 and have a preferential location in the duskside and nightside. The O+ and He+ structures can extend to smaller L shells with higher upper energy limits than the H+ structures, while the upper energy limits of all these particle species show a similar variation tendency with respect to magnetic local time (MLT) and L. Observations indicate that these wedge‐like structures are probably attributed to fresh substorm injections from the outer region. Key Points: Van Allen Probe observations reveal the existence of wedge‐like ion structures mainly at the L shells of L=2–5Structures have a preferential location in the duskside and nightside, and the O+ and He+ structures have higher upper energy limits than H+The formation of these structures are probably attributed to substorm injections drifting earthward from the outer region [ABSTRACT FROM AUTHOR]

Recent observations in the nightside inner magnetosphere have identified a series of wedge‐like spectral structures in the energy‐time spectrograms of oxygen, helium, and hydrogen ion fluxes. Although the shapes and distributions of these structures have been characterized by case and statistical studies, their formation mechanism remains unclear. Here we utilize a particle tracing model to reproduce the wedge‐like structures successively observed by the twin Van Allen Probes. The model suggests that these structures originate from intermittent substorm injection, and it is the accessibility region of these injected ions that determines their shapes. This mechanism is similar to the formation of another kind of structures, the inner magnetospheric nose‐like structures, except that the wedge‐like structures are separated from the tail population by the discontinuation of ion injections. This scenario is also supported by the distribution statistics of wedge‐like structures, which provides new insights into the dynamics of the magnetotail‐inner magnetosphere coupled system. Plain Language Summary: In Earth's inner magnetosphere, multiple plasma populations interact with one another to render a highly coupled system, with its dynamic evolution also subject to magnetospheric convection and magnetotail particle injections. The interplay of these complicated processes is manifested in spacecraft observations by various spectral structures of particle fluxes, and the understanding of their generation provides unique insights into the underlying processes. Here we focus on a specific type of spectral structures in the nightside inner magnetosphere, named the wedge‐like structures. These structures are characterized by enhanced fluxes of oxygen, helium, and hydrogen ions at increasing energies (from a few eV to several keV) with decreasing equatorial distance to the Earth. Based on a particle tracing model, we successfully reproduce the structure characteristics observed by NASA's (National Aeronautics and Space Administration) twin Van Allen Probes. The model suggests that these structures originate from transient injections associated with substorm activities, and their shapes are determined by the energy‐dependent ion accessibility into the inner magnetosphere. This scenario of wedge‐like structure formation is consistent with both case and statistical results, which sheds new light on our current understanding of the inner magnetospheric dynamics and its coupling process with the magnetotail plasma sheet. Key Points: The wedge‐like ion structures in the nightside inner magnetosphere are reproduced with a simple modelThe ions within the wedge originate from transient injections associated with substorm activitiesThe wedges are formed by the slow yet deep penetration of injected ions with specific magnetic moments [ABSTRACT FROM AUTHOR]

The paper describes a modified smooth approximation method for smoothing the dependence of the equivalent fluence of charged particles of the Earth's Van Allen radiation belts on time and position in near-Earth space. It is shown that this method can be effectively used to solve the problems of minimizing radiation-induced degradation of spacecraft solar cell arrays. The latter problems involve the variation of the trajectory that the spacecraft follows during an electric propulsion transfer from an initial orbit to a geostationary one. [ABSTRACT FROM AUTHOR]

The Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) mission provided long‐term measurements of 10s of megaelectron volt (MeV) inner belt (L < 2) protons (1992–2009) as did the Polar‐orbiting Operational Environmental Satellite‐18 (POES‐18, 2005 to present). These long‐term measurements at low‐Earth orbit (LEO) showed clear solar cycle variations which anticorrelate with sunspot number. However, the magnitude of the variation is much greater than the solar cycle variation of galactic cosmic rays (>GeV) that are regarded as a source of these trapped protons. Furthermore, the proton fluxes and their variations sensitively depend on the altitude above the South Atlantic Anomaly (SAA) region. With respect to protons (>36 MeV) mirroring near the magnetic equator, both POES measurements and simulations show no obvious solar cycle variations at L > 1.2. This is also confirmed by recent measurements from the Van Allen Probes (2012–2019), but there are clear solar cycle variations and a strong spatial gradient of the proton flux below L = 1.2. A direct comparison between measurements and simulations leads to the conclusion that energy loss of trapped protons due to collisions with free and bound electrons in the ionosphere and atmosphere is the dominant mechanism for the strong spatial gradient and solar cycle variation of the inner belt protons. This fact is also key of importance for spacecraft and instrument design and operation in near‐Earth space. Key Points: The solar cycle variation of inner belt protons measured by LEO satellites is mainly due to losses from atmospheric collisionsInner belt proton intensity and variation measured at LEO sensitively depend on the altitude above the SAAMeasurements and simulations reveal a strong spatial gradient of trapped protons mirroring near the magnetic equator below L = 1.2 [ABSTRACT FROM AUTHOR]

The Van Allen Probes Mission consists of two identical spacecraft flying in highly elliptical orbits, with perigee altitudes originally near 600 km. During the low‐altitude periods of the orbits, the spacecrafts are immersed in a region of high‐density atomic oxygen. Atomic oxygen is known to change and degrade the properties of spacecraft surfaces (Banks et al., 2004), such as those of the Van Allen Probes Electric Field and Waves (EFW) instrument. The consistency of the sensor surfaces in EFW is important because the mechanisms used to ensure the collection of high‐quality electric field measurements requires that the photoemission properties of each sensor are uniform and stable. Oxidation or erosion of the sensor surfaces could limit the instrument's ability to balance the currents produced by both the plasma electrons and the controlled bias current applied to the sensors and thus to properly operate the device. We have modeled the atomic oxygen exposure to the spacecraft to help understand the potential impact it has had on the sensors. We have calculated the fluence (time‐integrated flux) of atomic oxygen particles that have collided with the spacecrafts over the entire course of the mission. We have also looked at the distribution of atomic oxygen flux over time to further analyze malfunctions in the sensor readings at different points along the course of the mission. Additionally, we have investigated how different surfaces of the spacecraft are affected differently due to their orientation with respect to the spacecraft's motion. Key Points: The Van Allen Probes are exposed to high‐density atomic oxygen close to perigee that is known to degrade spacecraft surfacesThe atomic oxygen fluence is modeled to explore the extent of degradation, when the most exposure occurred, and how it can be mitigatedThe calculations can be replicated for past/future missions given their orbit parameters [ABSTRACT FROM AUTHOR]

The flux of energetic electrons in the outer radiation belt shows a high variability. The interactions of electrons with very low frequency (VLF) chorus waves play a significant role in controlling the flux variation of these particles. Quantifying the effects of these interactions is crucially important for accurately modeling the global dynamics of the outer radiation belt and to provide a comprehensive description of electron flux variations over a wide energy range (from the source population of 30 keV electrons up to the relativistic core population of the outer radiation belt). Here, we use a synthetic chorus wave model based on a combined database compiled from the Van Allen Probes and Cluster spacecraft VLF measurements to develop a comprehensive parametric model of electron lifetimes as a function of L‐shell, electron energy, and geomagnetic activity. The wave model takes into account the wave amplitude dependence on geomagnetic latitude, wave normal angle distribution, and variations of wave frequency with latitude. We provide general analytical formulas to estimate electron lifetimes as a function of L‐shell (for L = 3.0 to L = 6.5), electron energy (from 30 keV to 2 MeV), and geomagnetic activity parameterized by the AE index. The present model lifetimes are compared to previous studies and analytical results and also show a good agreement with measured lifetimes of 30 to 300 keV electrons at geosynchronous orbit. Plain Language Summary: The space surrounding our planet is full of charged particles trapped in donut‐shaped belts called the Van Allen radiation belts that encircle the Earth. These charged particles can cause significant malfunctions and unexpected failures to spacecraft electronics. The intensity of the radiation belts vary as these energetic particles interact with very low frequency (VLF) waves such as chorus waves. Therefore, it is crucially important to accurately quantify the wave‐particle interactions for accurately modeling and forecasting the global dynamics of the Van Allen radiation belts. Predicting the magnitude and duration of potentially hazardous conditions could help satellite operators to switch off nonessential satellite electronic systems to reduce malfunctions and unexpected failures only during the most dangerous periods. In this study, we use a chorus wave model based on multi‐satellite wave measurements to calculate the lifetimes of these charged particles in the Earth's Van Allen radiation belts. We then compare our results with previous studies, analytical results, and measured data. Key Points: A comprehensive parametric model of electron lifetime in the outer radiation belt has been developedModel lifetimes are compared with analytical results and measured dataThe model provides accurate estimates of electron lifetimes over a wide range of energies as a function of geomagnetic activity [ABSTRACT FROM AUTHOR]

The "zebra stripes" are peaks and valleys commonly present in the spectrograms of energetic particles trapped in the Earth's inner belt and slot region. Several theories have been proposed over the years to explain their generation, structure, and evolution. Yet, the plausibility of various theories has not been tested due to a historical lack of ground truth, including in situ electric field measurements. In this work, we leverage the new visibility offered by the database of Van Allen Probes electric drift measurements to reveal the conditions associated with the generation of zebra stripe patterns. Energetic electron fluxes by the Radiation Belt Storm Probes Ion Composition Experiment between 1 January 2013 and 31 December 2015 are systematically analyzed to determine 370 start times associated with the generation of zebra stripes. Statistical analyses of these events reveal that the zebra stripes are usually created during substorm onset, a time at which prompt penetration electric fields are present in the plasmasphere. All the pieces of experimental evidence collected are consistent with a scenario in which the prompt penetration electric field associated with substorm onset leads to a sudden perturbation of the trapped particle drift motion. Subsequent drift echoes constitute the zebra stripes. This study exemplifies how the analysis of trapped particle dynamics in the inner belt and slot region provides complementary information on the dynamics of plasmaspheric electric fields. It is the first time that the signature of prompt penetration electric fields is detected in near‐equatorial electric field measurements below L = 3. Key Points: An algorithm is designed to analyze the zebra stripes present in the spectrograms of energetic electrons trapped below L = 3A superposed epoch analysis of 370 events is performed to determine the experimental conditions associated with zebra stripe generationExperimental evidence suggests that prompt penetration electric fields associated with substorm onsets routinely generate zebra stripes [ABSTRACT FROM AUTHOR]

In this study we examine the ability of protons of solar origin to access the near‐equatorial inner magnetosphere. Here we examine four distinct solar proton events from 20–200 MeV, concurrent with both quiet time and storm time conditions using proton data from the ACE satellite in the solar wind upstream of Earth and data from the Relativistic Electron Proton Telescope (REPT) instrument aboard Van Allen Probes. We examine the direct flux correspondence between interplanetary space and the inner magnetosphere. Small substructures in interplanetary space are observable in the REPT flux profiles, which can penetrate down to L values of ≤4. Furthermore, there are orbit‐to‐orbit variations in the west‐to‐east anisotropic flux ratios. The anisotropic flux ratios are used as a proxy for cutoff energies and display cutoff variations with L shell and energy. The dependence of the anisotropic flux ratio on Dst values is shown. The results paint a picture of highly dynamic spatial and temporal proton cutoff rigidities in the near‐equatorial inner magnetosphere. Key Points: Small flux substructures in interplanetary space are observed in the inner magnetosphereWest‐to‐east flux ratios are used as a proxy for cutoff location and energyWest‐to‐east flux ratios are highly dynamic from orbit to orbit and respond quickly to magnetospheric changes [ABSTRACT FROM AUTHOR]

Whistler mode waves observed in the Earth's inner magnetosphere at frequencies between about 0.5 and 4 kHz which exhibit a nearly periodic time modulation of the wave intensity are called quasiperiodic (QP) emissions. Conjugate measurements of QP events at several different locations can be used to estimate their spatial extent and spatiotemporal variability. Results obtained using conjugate QP measurements provided by the ground‐based station Kannuslehto (L≈5.5) and the Van Allen Probes spacecraft (L shells between about 1.1 and 6.5) between September 2012 and November 2017 are presented. Altogether, 26 simultaneously detected events were analyzed. The event modulation periods and frequency‐time structures were generally the same at all observation points. Spatial separations of the spacecraft and the ground‐based station during conjugate observations are typically within about 40° in azimuth and from about 1 to 3 in L shell. RBSP consistently observes events at lower L shells than Kannuslehto, with the event occurrence primarily inside of the plasmasphere. Ratios of Poynting fluxes observed by the spacecraft and on the ground are used to evaluate event intensity variations related to the spacecraft position. It is found that the intensity decreases considerably both at low L shells and outside of the plasmasphere. Finally, an event containing a gap in its frequency‐time structure related to a sudden change of its properties is analyzed in detail. Key Points: Spatial separations during conjugate observations are typically within 40° in azimuth and from about 1 to 3 in L shellSpacecraft and ground‐based intensity ratios demonstrate an intensity decrease both close to the plasmapause and at low L shellsEvent with a gap and spectral structure change was explained as two immediately following events [ABSTRACT FROM AUTHOR]

Understanding the dynamic evolution of relativistic electrons in the Earth's radiation belts during both storm and nonstorm times is a challenging task. The U.S. National Science Foundation's Geospace Environment Modeling (GEM) focus group "Quantitative Assessment of Radiation Belt Modeling" has selected two storm time and two nonstorm time events that occurred during the second year of the Van Allen Probes mission for in‐depth study. Here, we perform simulations for these GEM challenge events using the 3D Versatile Electron Radiation Belt code. We set up the outer L* boundary using data from the Geostationary Operational Environmental Satellites and validate the simulation results against satellite observations from both the Geostationary Operational Environmental Satellites and Van Allen Probe missions for 0.9‐MeV electrons. Our results show that the position of the plasmapause plays a significant role in the dynamic evolution of relativistic electrons. The magnetopause shadowing effect is included by using last closed drift shell, and it is shown to significantly contribute to the dropouts of relativistic electrons at high L*. We perform simulations using four different empirical radial diffusion coefficient models for the GEM challenge events, and the results show that these simulations reproduce the general dynamic evolution of relativistic radiation belt electrons. However, in the events shown here, simulations using the radial diffusion coefficients from Brautigam and Albert (2000) produce the best agreement with satellite observations. Key Points: We investigate the effect of different plasmapause positions and radial diffusion coefficients during the four GEM challenge eventsIncluding the magnetopause shadowing effect by using the last closed drift shell helps to reproduce the dropoutsThe three‐dimensional Versatile Electron Radiation Belt code reproduces the general dynamics of relativistic electrons during GEM challenge events [ABSTRACT FROM AUTHOR]

We present global statistical models of both wave amplitude and wave normal angle (WNA) of plasmaspheric hiss using Van Allen Probe‐A observations. They utilize the time history of solar wind parameters, that is, interplanetary magnetic field BZ and solar wind speed, and the AE index for each measurement of hiss waves as inputs. The solar wind parameter‐based model generally results in higher performance than using only the AE index as an input. Both observations and model results reveal a clear dependence of hiss wave distribution on the magnetic local time (MLT): Higher amplitudes with field‐aligned (<30o) WNAs occur more frequently on the dayside than on the nightside. Such a tendency does not depend on magnetic latitude (MLAT), but slightly larger WNAs with a relatively low amplitude frequently appear for larger MLAT (>10o). We also examine how significantly the electron loss rates in the slot region can be changed by incorporating the model output of hiss waves into a diffusive transport simulation. Simulation results show that during a typical timescale (roughly a couple of days) of a corotating interaction region‐driven storm, the nightside hiss waves with larger WNA (>30o) do not contribute to the electron loss in the slot region due to their low amplitude and large WNA, while dayside hiss with WNAs less than 30o and comparatively higher amplitudes leads to a fast drop in flux, especially for electrons of a few hundred keV. Key Points: We develop statistical models of both wave amplitude and wave normal angle of plasmaspheric hissModel calculation results show a clear day‐night asymmetry of the wave distributionDayside hiss predominantly contributes to electron loss in the slot region compared to nightside hiss [ABSTRACT FROM AUTHOR]

We investigate the shapes of the inner boundary of the outer radiation belt during different geomagnetic storm phases using energetic electron observations from Van Allen Probes. The case of two consecutive but isolated storms in April 2016 shows that (a) the inner boundary, as a function of L shell and energy, exhibits a "V‐shaped" form with the energetic electrons showing a kappa‐like energy spectrum (electron flux steeply falling with increasing energies), whereas it is in a "S‐shaped" form as the energetic electrons show a reversed energy spectrum (electron flux going up with increasing energies from hundreds of keV to ∼1 MeV); (b) the boundary is abruptly transformed from S to V shape during the storm main phase and retains in V shape for several days until it evolves into S shape during the late recovery phase. The main statistical results from 37 isolated geomagnetic storms between 2013 and 2017 present that (a) the more SYM‐H drops, the closest to Earth the transition from V to S shape starts, with a linear correlation coefficient of ∼0.7; (b) the minimum energy at which the transition starts is between 100 and 550 keV (typically, less than 250 keV); (c) the transition from V to S shape typically occurs in the plasmasphere. Key Points: The S‐shaped inner boundary is abruptly transformed from V shape in storm main phase but reoccurs in several days in the late recovery phaseThe radial position where the transition from V to S shape starts shows a negative correlation with the intensity of magnetic stormsThe minimum energy where the transition from V to S shape starts is 100–550 keV, and the transition typically occurs in the plasmasphere [ABSTRACT FROM AUTHOR]

The Van Allen radiation belts in the magnetosphere have been extensively studied using models based on radial diffusion theory, which is derived from a quasi‐linear approach with prescribed inner and outer boundary conditions. The 1D diffusion model requires the knowledge of a diffusion coefficient and an electron loss timescale, which is typically parameterized in terms of various quantities such as the spatial (L) coordinate or a geomagnetic index (e.g., Kp). These terms are typically empirically derived, not directly measurable, and hence are not known precisely, due to the inherent nonlinearity of the process and the variable boundary conditions. In this work, we demonstrate a probabilistic approach by inferring the values of the diffusion and loss term parameters, along with their uncertainty, in a Bayesian framework, where identification is obtained using the Van Allen Probe measurements. Our results show that the probabilistic approach statistically improves the performance of the model, compared to the empirical parameterization employed in the literature. Key Points: We present the first application of Bayesian parameter estimation to the problem of quasi‐linear radial diffusion in the radiation beltThe Bayesian approach allows the problem to be cast in probabilistic terms and for ensemble simulations to be runAn improved accuracy is demonstrated when compared against standard deterministic models [ABSTRACT FROM AUTHOR]

Forecasting relativistic electron fluxes at geostationary Earth orbit (GEO) has been a long‐term goal of the scientific community, and significant advances have been made in the past, but the relation to the interior of the radiation belts, that is, to lower L‐shells, is still not clear. In this work we have identified 60 relativistic electron enhancement events at GEO to study the radial response of outer belt fluxes and the correlation between the fluxes at GEO and those at lower L‐shells. The enhancement events occurred between 1 October 2012 and 31 December 2017 and were identified using Geostationary Operational Environmental Satellite (GOES) 15 >2 MeV fluxes at GEO, which we have used to characterize the radial response of the radiation belt, by comparing to fluxes measured by the Van Allen probes Energetic Particle, Composition and Thermal Plasma Suite Relativistic Electron‐Proton Telescope (ECT‐REPT) between 2.55.0 and generally similar for L>4.5. Post‐enhancement maximum fluxes show a remarkable correlation for all L>4.0 although the magnitude of the pre‐existing fluxes on the outer belt plays a significant role and makes the ratio of pre‐enhancement to post‐enhancement fluxes less predictable in the region 4.0

Using the global Lagrangian version of the piecewise parabolic method‐magnetohydrodynamic (PPMLR‐MHD) model, we simulate two consecutive storms in December 2015, a moderate storm on 14–15 December and a strong storm on 19–22 December, and calculate the radial diffusion coefficients (DLL) from the simulated ultralow frequency waves. We find that even though the strong storm leads to more enhanced Bz and Eφ power than the moderate storm, the two storms share in common a lot of features on the azimuthal mode structure and power spectrum of ultralow frequency waves. For both storms, the total Bz and Eφ power is better correlated with the solar wind dynamic pressure in the storm initial phase and more correlated with AE index in the recovery phase. Bz wave power is shown to be mostly distributed in low mode numbers, while Eφ power spreads over a wider range of modes. Furthermore, the Bz and Eφ power spectral densities are found to be higher at higher L regions, with a stronger L dependence in the Bz spectra. The estimated DLL based on MHD fields shows that inside the magnetopause, the contribution from electric fields is larger than or comparable to that from magnetic fields, and our event‐specific MHD‐based DLL can be smaller than some previous empirical DLL estimations by more than an order of magnitude. At last, by validating against in situ observations from Magnetospheric Multiscale spacecraft, our MHD results are found to generally well reproduce the total Bz fields and wave power for both storms, while the Eφ power is underestimated in the MHD simulations. Key Points: PPMLR global MHD modeling is used to obtain the ULF wave characteristics during the 14–15 December 2015 and 19–22 December 2015 stormsThe estimated ULF wave mode structure and power spectrum are used to physically quantify the radial diffusion coefficients DLLMHD well produced the background magnetic field and ULF Bz power observed by MMS but underestimated the observed Eφ power [ABSTRACT FROM AUTHOR]

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Waves from nine major ground‐based very low frequency (VLF) transmitters are modeled from their sources to 660 km altitude with a full‐wave code, which reliably treats transionospheric attenuation, and then ray‐and‐power traced throughout the plasmasphere. Lightning‐generated whistlers, previously modeled at 660 km altitude, are ray‐and‐power traced throughout the plasmasphere as well. The resulting profiles of electric and magnetic fields, including wave normal angles, are organized by L value. Two versions of a realistic plasmaspheric density model are used, and ducted as well as nonducted propagation are treated. Results are compared to empirical models based on near‐equatorial measurements by Van Allen Probes. A companion paper will evaluate resonant interactions of these waves with radiation belt electrons. Key Points: Waves from VLF transmitters and lightning are modeled from their sources to the plasmasphereResults are compared to empirical models based on near‐equatorial measurements by Van Allen ProbesA companion paper evaluates their resonant interactions with radiation belt electrons [ABSTRACT FROM AUTHOR]

Electron fluxes (especially at energies E > 0.8 and 2 MeV) have been measured for many years by sensors on board the Geostationary Operational Environmental Satellite (GOES). These long‐term data (nominally at L ~ 6.6) have become a mainstay for monitoring the Earth's radiation environment. We have carried out a study directly comparing the comprehensive radiation belt particle measurements from the National Aeronautics and Space Administration dual‐spacecraft Van Allen Probes (Radiation Belt Storm Probes) sensor systems with selected GOES operational data. The Van Allen Probes have measured the properties of radiation belt electrons virtually continuously from September 2012 to 2018. We make statistical comparisons of Van Allen Probes electron data near L = 6 with concurrent daily averages of equivalent GOES‐15 flux values. We also compare Van Allen Probes data at various other L values and at a much broader range of particle energies with the more limited baseline GOES‐15 values. These comparisons inform us about the relative calibrations between the scientific and operational systems and also allow us to assess how well GOES data correlate with radiation belt behavior well away from the geostationary orbit location. We find that GOES daily average flux values can be a factor of 100 (or more) below the corresponding Van Allen Probes daily averaged fluxes at L = 6.0. This is due to the fact that GOES at stationary orbit often has excursions over large ranges of L space during a given day and strong radial gradients exist in this region of the magnetosphere. These results indicate that it is crucial to augment geosynchronous GOES observations with observations in the core of the outer belt (L ≲5.0). Key Points: Comparison of operational GOES data with NASA science dataStatistical study of radiation belt properties over 6 yearsStudy shows radial range and energy range where GOES can monitor radiation belts [ABSTRACT FROM AUTHOR]

We demonstrate indirect measurement of the electron flux outside space vehicles designed to carry crew, using a pixel detector situated inside the space craft. While the vehicle walls shield the detector from directly seeing the electrons, we show that bremsstrahlung generated in the shielding is visible and furthermore that fluorescence induced by this bremsstrahlung in components of the detector generates a sharply peaked characteristic signature. We show that measurements from the Exploration Test Flight 1 of the Orion Multi‐Purpose Crew Vehicle and from the International Space Station correlate very well with AE9‐based modeled electron flux predictions and with measurements from other instruments on the International Space Station and on the Polar Operational Environmental Satellites. We also present detailed Monte Carlo simulations performed with FLUKA, confirming the mechanism generating our electron signal. Key Points: Indirect measurement of the electron flux outside space vehicles designed to carry crew [ABSTRACT FROM AUTHOR]

High‐energy trapped particles in the radiation belts constitute potential threats to the functionality of satellites as they enter into those regions. In the inner radiation belt, the characteristics of high‐energy (>20 MeV) protons variations during geomagnetic activity times have been studied by implementing 4‐year (2013–2016) observations of the Van Allen probes. An empirical formula has been used to remove the satellite orbit effect, by which proton fluxes have been normalized to the geomagnetic equator. Case studies show that the region of L < 1.7 is relatively stable, while L > 1.7 is more dynamic and the most significant variation of proton fluxes occurs at L = 2.0. The 4‐year survey at L = 2.0 indicates that for every geomagnetic storm, sharp descent in proton fluxes is accompanied by the corresponding depression of SYM‐H index, with a one‐to‐one correspondence, regardless of the storm intensity. Proton flux dropouts are synchronous with SYM‐H reduction with similar short timescales. Our observational results reveal that high‐energy protons in the inner radiation belt are very dynamic, especially for the outer zone of the inner belt, which is beyond our previous knowledge. Plain Language Summary: The inner radiation belt, extending from 1,000 km to 2 Re above the Earth, is filled with a lot of high‐energy protons. These protons may affect the safety of the satellite as they are traveling into this region. Therefore, understanding the behavior of the proton belt is very important regarding the functionality of satellites. Early studies have found that protons in the inner radiation belt are stable over very long timescales. A few studies were focused on variations of inner zone protons during the huge geomagnetic storms. Van Allen Probes provide a great opportunity to explore whether geomagnetic activities affect the inner zone. New and fascinating phenomenon has been found that proton fluxes have prompt responses and one‐to‐one correspondence to geomagnetic disturbances, regardless of the storm intensity. This has not been reported before. Our results reveal that the high‐energy protons in the inner radiation belt are very dynamic, especially for the outer zone of the inner belt, which is contrast to the common knowledge. Key Points: Proton fluxes are normalized to geomagnetic equator for eliminating satellite orbit effectHigh‐energy protons in the inner radiation belt are very dynamic, especially for the outer zone of the inner belt, not stable as previous knowledgeProton flux has prompt responses to geomagnetic disturbances with one‐to‐one correspondence, regardless of geomagnetic storm intensity [ABSTRACT FROM AUTHOR]

With 64‐month magnetic data from Van Allen Probes, we have studied not only the global distribution, wave normal angle (θ), and ellipticity (ε) of electromagnetic ion cyclotron (EMIC) waves, but also the dependence of their occurrence rates and magnetic amplitudes on the AE* index (the mean value of AE index over previous 1 hr). Our results show that H+ band waves are preferentially detected at 5 ≤ L ≤ 6.5, in the noon sector. They typically have small θ (<30°) and weakly left‐hand polarization but become more oblique and linearly polarized at larger magnetic latitudes or L‐shells. With the increase of AE* index, their occurrence rate significantly increases in the noon sector, and their source region extends to dusk sector. He+ band waves usually occur in the predawn and morning sectors at 3 ≤ L ≤ 4.5. They generally have moderate θ (30 °  − 40°) and left‐hand polarization and also become more oblique and linearly polarized at larger latitudes or L‐shells. There is a clear enhancement of occurrence rate and amplitude during active geomagnetic periods, especially in the dusk and evening sectors. O+ band waves mainly occur at 3 ≤ L ≤ 4 in the predawn sector. They have either very small θ (<20°) or very large θ (>50°), and typically linear or weakly right‐hand polarization. During active periods, they mostly occur at the midnight sector and L < 3.5. As a valuable supplement to previous statistical studies, our result provides not only a more compresentive EMIC wave model for evaluating their effects on the radiation belt, but also detailed observational constraints on generation mechanisms of EMIC waves. Key Points: A statistical study of EMIC waves is performed using long‐term Van Allen Probes data in the inner magnetosphereWe have presented the distributions of occurrence rate, wave normal angle, and ellipticity for H+, He+, and O+ band wavesThe dependences of the occurrence rate and magnetic amplitude on the AE* index are also studied [ABSTRACT FROM AUTHOR]

The Van Allen Probes have observed both symmetric and asymmetric bipolar electric field structures in the Earth's inner magnetosphere. In general, the symmetric bipolar structures are identified as electron‐phase space holes, whereas the asymmetric structures are interpreted as electron acoustic double layers (EADLs). The generation mechanism of these EADLs is not entirely understood yet. We have modeled the EADLs observed on 13 November 2012 by Van Allen Probe‐B. We performed a fluid simulation of the EADLs and tracked their formation and evolution in the simulation. We found that the localized depletion and enhancement in the electron populations act as a perturbation to excite the symmetric bipolar electron acoustic solitary waves, which later evolve into the EADLs. The Ponderomotive force is found to be the main driver behind transformation of the symmetric electron acoustic solitary waves to EADLs via formation of the electron acoustic shocks. Key Points: Fluid simulation is performed to study the formation and evolution of asymmetric solitary waves in the Earth's inner magnetospherePonderomotive force modulates the symmetric solitary wave pulses into the asymmetric pulses of double layersElectron acoustic shock‐type waves play an important role in the formation of electron acoustic double layers [ABSTRACT FROM AUTHOR]

On 23 February 2014, Van Allen Probes sensors observed quite strong electromagnetic ion cyclotron (EMIC) waves in the outer dayside magnetosphere. The maximum amplitude was more than 14 nT, comparable to 7% of the magnitude of the ambient magnetic field. The EMIC waves consisted of a series of coherent rising tone emissions. Rising tones are excited sporadically by energetic protons. At the same time, the probes detected drastic fluctuations in fluxes of MeV electrons. It was found that the electron fluxes decreased by more than 30% during the 1 min following the observation of each EMIC rising tone emissions. Furthermore, it is concluded that the flux reduction is a nonadiabatic (irreversible) process since holes in the particle flux levels appear as drift echoes with energy dispersion. We examine the process of electron pitch angle scattering by nonlinear wave trapping due to anomalous cyclotron resonance with EMIC rising tone emissions. The energy range of precipitated electrons agrees with the presumed energy for the threshold amplitude for nonlinear wave trapping. This is the first report of rapid precipitation (<1 min) of relativistic electrons by EMIC rising tone emissions and their drift echoes in time observed by spacecraft. Key Points: Relativistic electrons disappear following intense EMIC rising tone in a few minutesDrift echoes of localized electron depletion are detectedThis is the direct observation of rapid loss process by the nonlinear EMIC scattering [ABSTRACT FROM AUTHOR]

Relativistic electron flux responses in the inner magnetosphere are investigated for 28 magnetic storms driven by corotating interaction region (CIR) and 27 magnetic storms driven by coronal mass ejection (CME), using data from the Relativistic Electron‐Proton Telescope instrument on board Van Allen Probes from October 2012 to May 2017. In this present study we analyze the role of CIRs and CMEs in electron dynamics by sorting the electron fluxes in terms of averaged solar wind parameters, L‐values, and energies. The major outcomes from our study are the following: (i) At L = 3 and E = 3.4 MeV, for >70% cases the electron flux remains stable, while at L = 5, for ~82% cases it changes with the geomagnetic conditions. (ii) At L = 5, ~53% of the CIR storms and 30% of the CME storms show electron flux increase. (iii) At a given L‐value, the tendency for the electron flux variation diminishes with the increasing energies for both categories of storms. (iv) In case of CIR‐driven storms, the electron flux changes are associated with changes in Vsw and Sym‐H. (v) At L ~ 3, CME storms show increased electron flux, while at L ~ 5, CIR storms are responsible for the electron flux enhancements. (vi) During CME‐ and CIR‐driven storms, distinct electron flux variations are observed at L = 3 and L = 5. Key Points: At L = 5, 53% of the CIR storms and 30% of the CME storms show electron flux increaseRelativistic electron flux variations at L = 5 is largely independent of geomagnetic storm strength but strongly depends upon averaged Vsw and IMF BzAt L = 3, >71% geomagnetic storms show no remarkable electron flux variations, irrespective of the storm driver [ABSTRACT FROM AUTHOR]

Near equatorial (fast) magnetosonic waves, characterized by high magnetic compressibility, are whistler‐mode emissions destabilized by proton shell/ring distributions. In the past, substorm proton injections are widely known to intensify magnetosonic waves in the inner magnetosphere. Here we report the unexpected observations by the Van Allen Probes of the magnetosonic wave quenching associated with the substorm proton injections under both high‐ and low‐density conditions. The enhanced proton thermal pressure distorted the background magnetic field configuration and the cold plasma density distribution. The reduced phase velocities locally allowed the weak growth or even damping of magnetosonic waves. Meanwhile, the spatially irregularly varying refractive indices might suppress the cumulative growth of magnetosonic waves. For intense injections, this wave quenching region could extend over 2 hr in magnetic local time and 0.5 Earth radii in radial distance. These results provide a new understanding of the generation and distribution of magnetosonic waves. Plain Language Summary: Magnetosonic waves are the near‐equatorially confined electromagnetic emissions between the proton gyrofrequency and the lower hybrid frequency in the magnetosphere. Theoretical and observational studies have demonstrated the potential of the magnetosonic waves to accelerate the radiation belt electrons. It is therefore important to understand the generation process and the spatiotemporal distribution of the magnetosonic waves. The substorm activities injecting hot protons into the inner magnetosphere are conventionally considered to intensify the magnetosonic waves. By analyzing the wave and particle data of the Van Allen Probes, we find a magnetosonic wave quenching region related to the distortion of the background magnetic field configuration and the cold plasma density distribution closely following the substorm injection front. We propose two possible causes for this phenomenon: (1) local weak growth or even damping of the magnetosonic waves with the reduced phase velocities and (2) suppression of the magnetosonic wave cumulative growth in the disturbed medium. This new finding may help refine the modeling of the magnetosonic waves and then the radiation belt electrons. Key Points: The enhanced thermal pressure of hot protons can distort the geomagnetic field configuration and the cold plasma density distributionThe magnetosonic wave quenching region can emerge closely following the substorm injection front under both high‐ and low‐density conditionsThe magnetosonic wave quenching region can extend over 2 hr in magnetic local time and 0.5 Earth radii in radial distance [ABSTRACT FROM AUTHOR]

In this study using Van Allen Probe wave observations we investigate the statistical properties of exohiss waves, which are structureless whistler mode waves observed outside the plasmapause. The exohiss waves are identified based on the cold electron number density, frequency distribution, ellipticity, and wave normal angle. The statistical analysis on exohiss wave properties shows that exohiss waves prefer to occur over 3

We explore the impact on the outer radiation belt of significant time‐integrated ap events of continuously elevated geomagnetic activity, making use of Van Allen Probes measurements in 2013–2017. We show that most high peaks of 1.8‐MeV electron flux occur during the 10 days immediately following such events, with stronger events leading to higher flux. Events larger than 800 nT·hr are always followed by a high peak of flux in the heart of the outer radiation belt. They lead to 10‐day‐averaged 1.8‐MeV electron fluxes of the order of 106 e/cm2/sr/s/MeV in this region and 8×104 e/cm2/sr/s/MeV near geostationary orbit. Accordingly, they can be considered as reliable precursors of high peaks of 2‐MeV electron flux throughout the outer belt. We demonstrate that the flux peaks following such significant events can be parameterized by different geomagnetic indices at different radial locations in 2013–2017. Comparisons between the corresponding modeled flux peaks and observations show a reasonable agreement for both the timing, duration, and level of the peaks with 2015 data from the Van Allen Probes and also with 2002–2012 data from Global Positioning System satellites and 2003–2016 data from Geostationary Operational Environmental Satellites, provided that dropouts are taken into account via an appropriate threshold on solar wind dynamic pressure. Plain Language Summary: Considering significant events of prolonged and elevated geomagnetic activity measured at middle‐latitude stations, we examine their effects on megaelectron volt electron fluxes that represent an important hazard for satellites throughout the outer radiation belt. Using measurements performed by the Van Allen Probes in 2013–2017, we find that high peaks of 1.8‐MeV electron flux appear in the immediate aftermath of all events in the heart of the outer radiation belt and last about 10 days. Such events can therefore be considered as reliable precursors of high flux peaks. The statistical dependence of flux peaks on different measures of precursor event strength and radial distance is obtained. The reliability of such a description over the long term is tested by comparisons with earlier measurements from other satellites in 2002–2012, further taking dropouts into account. These results should be useful for better understanding the effects of geomagnetic disturbances on megaelectron volt electrons and to improve forecasts of the timing and level of such peaks of 2‐MeV electron flux. Key Points: Significant time‐integrated ap geomagnetic events produce high peaks of 2‐MeV electron flux throughout the outer radiation beltThe 2‐MeV electron flux peaks produced by Int(ap) events are parameterized by event strength and radial distance based on Van Allen Probes dataThis model of 2‐MeV electron flux is consistent with earlier GPS and GOES measurements when dropouts are taken into account [ABSTRACT FROM AUTHOR]

In this study, rapid loss of relativistic radiation belt electrons at low L* values (2.4–3.2) during a strong geomagnetic storm on 22 June 2015 is investigated along with five possible loss mechanisms. Both the particle and wave data are obtained from the Van Allen Probes. Duskside H+ band electromagnetic ion cyclotron (EMIC) waves were observed during a rapid decrease of relativistic electrons with energy above 5.2 MeV occurring outside the plasmasphere during extreme magnetopause compression. Lower He+ composition and enriched O+ composition are found compared to typical values assumed in other studies of cyclotron resonant scattering of relativistic electrons by EMIC waves. Quantitative analysis demonstrates that even with the existence of He+ band EMIC waves, it is the H+ band EMIC waves that are likely to cause the depletion at small pitch angles and strong gradients in pitch angle distributions of relativistic electrons with energy above 5.2 MeV at low L values for this event. Very low frequency wave activity at other magnetic local time can be favorable for the loss of relativistic electrons at higher pitch angles. An illustrative calculation that combines the nominal pitch angle scattering rate due to whistler mode chorus at high pitch angles with the H+ band EMIC wave loss rate at low pitch angles produces loss on time scale observed at L=2.4–3.2. At high L values and lower energies, radial loss to the magnetopause is a viable explanation. Key Points: Greater than 5.2‐MeV electron loss predominantly occurs outside the plasmasphere during the extreme magnetopause compression event on 22 June 2015Relativistic electron loss is colocated with H+ band EMIC waves at low LFive possible loss mechanisms are considered to explain the fast net loss of relativistic electrons, including H+ EMIC wave scattering [ABSTRACT FROM AUTHOR]

Fast magnetosonic (MS) waves can play an important role in the evolution of the inner magnetosphere. However, there is still not an effective method to quantitatively identify such waves for observations of the Van Allen Probes reasonably. In this paper, we used Van Allen Probes data from 18 September 2012 to 30 September 2014 to find a more comprehensive automatic detection algorithm for fast MS waves through statistical analysis of the major properties, including the planarity, ellipticity, and wave normal angle of whole fluctuations using the singular value decomposition method. According to a control variate method, we find an obvious difference between fast MS waves and other waves in the statistical distribution of their major properties. After eliminating the influence of background noises, by excluding fluctuations at L < 1.8, we set up an automatic detection algorithm applied to fast MS waves, that is, smaller than 0.2 for the absolute value of wave ellipticity, larger than 70° for the wave normal angle, with frequency range of 2 Hz to 1.5 fLHR (fLHR is the local lower hybrid resonance frequency). Finally, we have checked the planarity to verify availability of this method and tested this completely automatic method on the Van Allen Probes data and found some results consistent with previous studies. Inside the plasmapause, we found that there is a more obviously favorable occurrence of MS waves at dusk sector with increasing magnetic latitudes. Key Points: This paper quantitatively demonstrates the differences in propagating properties of major plasma waves in the inner magnetosphereThis paper provides an automatic detection algorithm applied to fast magnetosonic wavesThis paper dispalys the global distributions of MS waves identified by our method [ABSTRACT FROM AUTHOR]

A Pc5 wave was simultaneously observed in the ionosphere by EKB radar and in the magnetosphere by both Van Allen Probe spacecraft within a substorm activity. The wave was located in the nightside, in 1.5‐ to 3‐hr magnetic local time sector, and in the region corresponding to the magnetic shells with maximal distances 4.6–7.8 Earth's radii. As it was found using both the radar and spacecraft data, the wave had frequency of about 1.8 mHz and azimuthal wave number m≈−10; that is, the wave was westward propagating. The EKB radar data revealed the equatorward wave propagating in the ionosphere, which corresponded to the earthward propagation in the magnetosphere. Furthermore, the field‐aligned magnetic component was approximately 2 times larger than both transverse components and accompanied by antiphase pressure oscillations; that is, the wave is compressional and diamagnetic. According to both radar and spacecraft measurements, among two transverse magnetic components, the dominant one was the poloidal. The wave was possibly driven by substorm‐injected energetic protons registered by the spacecraft: the proton fluxes were modulated with the wave frequency at energies of about 90 keV, which corresponded to the energy of the drift wave‐particle resonance. The wave frequency was much lower than the minimal frequency of the field line resonance calculated using the spacecraft data. We conclude that the wave is not the Alfvén mode, but some kind of compressional wave, for example, the drift‐compressional mode. Key Points: The 1.8‐mHz wave with m≈−10 was simultaneously observed in the ionosphere with a radar and in the magnetosphere with two spacecraftThe wave was compressional and diamagnetic, had poloidal transverse polarization, and propagated westward‐equatorward in the ionosphereThe wave frequency was much lower than the minimal frequency of the field line resonance estimated using the spacecraft data [ABSTRACT FROM AUTHOR]

In this study, access of solar energetic protons to the inner magnetosphere on 11 September 2017 is investigated by computing the reverse particle trajectories with the Dartmouth geomagnetic cutoff code (Kress et al., 2010). The maximum and minimum cutoff rigidity at each point along the orbit of Van Allen Probe A is numerically computed by extending the code to calculate cutoff rigidity for particles coming from arbitrary direction. Pulse height analyzed (PHA) data have the advantage of providing individual particle energies and effectively excluding background high‐energy proton contamination. This technique is adopted to study the cutoff locations for solar protons with different energy. The results demonstrate that cutoff latitude is lower for solar protons with higher energy, consistent with low‐altitude vertical cutoffs. Both the observations and numerical results show that proton access into the inner magnetosphere depends strongly on angle between particle arrival direction and magnetic west. The numerical result is approximately consistent with the observation that the energy of almost all solar protons stays above the minimum cutoff rigidity. Key Points: Pulse height analyzed data from the Van Allen Probes REPT instrument is compared with model solar proton geomagnetic cutoffsThe Dartmouth cutoff code is extended to calculate cutoff energy for particles arriving from arbitrary directions at the spinning spacecraftGeosynchronous transfer orbit measurements and calculated cutoff extend models previously applied to low‐altitude and geosynchronous cutoffs [ABSTRACT FROM AUTHOR]

Properties of banded, no‐gap, lower band only, and upper band only whistler mode waves (0.1–0.8fce) outside the plasmasphere are investigated using Van Allen Probes data. Our analysis shows that no‐gap whistler waves have higher occurrence rate at morning side and dayside, while banded and lower band only waves have higher occurrence rate between midnight and dawn. We also find that the occurrence rate of no‐gap whistler waves peaks at magnetic latitude |MLAT|∼8–10°, while banded waves have higher occurrence rate near the equator for |MLAT|≲6°. The wave normal angle distributions of these four groups of waves are similar to previous results. The distinct local time and latitudinal distribution of no‐gap and banded whistler mode waves is critical to further understand the formation mechanism of the power minimum at half electron gyrofrequency. Key Points: No‐gap whistler waves have the highest occurrence rate in the morning side and banded waves in the predawn sectorNo‐gap whistler waves have the highest occurrence rate off equator and banded waves near equatorThe wave normal angle distribution of both types of waves is similar to previous findings [ABSTRACT FROM AUTHOR]

Quasi thermal fluctuations in the Langmuir/upper‐hybrid frequency range are pervasively observed in space plasmas including the radiation belt and the ring current region of inner magnetosphere as well as the solar wind. The quasi thermal noise spectroscopy may be employed in order to determine the electron density and temperature as well as to diagnose the properties of energetic electrons when direct measurements are not available. However, when employing the technique, one must carefully take the spacecraft orientation into account. The present paper takes the upper‐hybrid and multiple harmonic—or (n + 1/2)fce—emissions measured by the Van Allen Probes as an example in order to illustrate how the spacecraft antenna geometrical factor can be incorporated into the theoretical interpretation. This method can in principle be applied to other spacecraft, including the Parker Solar Probe. Key Points: Quasi thermal noise can be used as a diagnostic tool for space plasmasUpper‐hybrid quasi thermal noise detected by the Van Allen Probes are analyzed with spacecraft orientation taken into accountThe present method may be extended for the Parker Solar Probe mission [ABSTRACT FROM AUTHOR]

We investigate the response of the outer Van Allen belt electrons to various types of solar wind and magnetospheric disturbances. We use electron phase space density calculations as well as concurrent Pc5 and chorus wave activity observations in the outer belt during the Van Allen Probes era to compare 20 electron enhancement and 8 depletion events. Results indicate that the combined effect of magnetopause shadowing and outward diffusion driven by Pc5 waves is present in both groups of events. Furthermore, in the case of enhancement events, the synergy of enhanced seed population levels and chorus activity—due to the enhanced substorm activity—can effectively replenish the losses of relativistic electrons, while inward diffusion can further accelerate them. Key Points: The number of events with PSD enhancements is L* and μ dependentLoss, as a combined effect of magnetopause shadowing and outward diffusion, is a common feature in both depletion and enhancement eventsThe synergy of enhanced seed population and chorus activity is what distinguishes relativistic electrons enhancements from depletions [ABSTRACT FROM AUTHOR]

In this paper we report a rare and fortunate event of fast magnetosonic (MS, also called equatorial noise) waves modulated by compressional ultralow frequency (ULF) waves measured by Van Allen Probes. The characteristics of MS waves, ULF waves, proton distribution, and their potential correlations are analyzed. The results show that ULF waves can modulate the energetic ring proton distribution and in turn modulate the MS generation. Furthermore, the variation of MS intensities is attributed to not only ULF wave activities but also the variation of background parameters, for example, number density. The results confirm the opinion that MS waves are generated by proton ring distribution and propose a new modulation phenomenon. Key Points: The PSD of ring proton is antiphase modulated by ULF waveA rare and fortunate MS wave modulated by ULF wave is reported for the first timeLinear theory and perpendicular propagation account for the modulation of MS [ABSTRACT FROM AUTHOR]

We study the response of the outer Van Allen radiation belt during an intense magnetic storm on 15–22 February 2014. Four interplanetary coronal mass ejections (ICMEs) arrived at Earth, of which the three last ones were interacting. Using data from the Van Allen Probes, we report the first detailed investigation of electron fluxes from source (tens of kiloelectron volts) to core (megaelectron volts) energies and possible loss and acceleration mechanisms as a response to substructures (shock, sheath and ejecta, and regions of shock‐compressed ejecta) in multiple interacting ICMEs. After an initial enhancement induced by a shock compression of the magnetosphere, core fluxes strongly depleted and stayed low for 4 days. This sustained depletion can be related to a sequence of ICME substructures and their conditions that influenced the Earth's magnetosphere. In particular, the main depletions occurred during a high‐dynamic pressure sheath and shock‐compressed southward ejecta fields. These structures compressed/eroded the magnetopause close to geostationary orbit and induced intense and diverse wave activity in the inner magnetosphere (ULF Pc5, electromagnetic ion cyclotron, and hiss) facilitating both effective magnetopause shadowing and precipitation losses. Seed and source electrons in turn experienced stronger variations throughout the studied interval. The core fluxes recovered during the last ICME that made a glancing blow to Earth. This period was characterized by a concurrent lack of losses and sustained acceleration by chorus and Pc5 waves. Our study highlights that the seemingly complex behavior of the outer belt during interacting ICMEs can be understood by the knowledge of electron dynamics during different substructures. Key Points: Detailed response of the outer belt to substructures in a complex solar wind driver is investigatedMost substructures in the interacting ICMEs here deplete the core radiation belt population but inject source electronsCore electrons were enhanced during sustained chorus and Pc5 activity and lack of losses [ABSTRACT FROM AUTHOR]

Using data from the Relativistic Electron Proton Telescope on the Van Allen Probes, the effects of geomagnetic storms and solar wind conditions on the ultrarelativistic electron (E > ~3 MeV) flux enhancements in the outer radiation belt, especially regarding their energy dependence, are investigated. It is showed that, statistically, more intense geomagnetic storms are indeed more likely to cause flux enhancements of ~1.8‐ to 7.7‐MeV electrons, though large variations exist. As the electron energy gets higher, the probability of flux enhancement gets lower. To shed light on which conditions of the storms are preferred to cause ultrarelativistic electron flux enhancement, detailed superposed epoch analyses of solar wind parameters and geomagnetic indices during moderate and intense storms with/without flux enhancements of different energy electrons are conducted. The results suggest that the storms with higher solar wind speed, sustained southward interplanetary magnetic field Bz, lower solar wind number density, higher solar wind Ey, and elevated and sustained substorm activity are more likely to cause ultrarelativistic electron flux enhancements in the outer belt. Comparing results of different energy electrons, the solar wind speed and AE index are the two parameters mostly correlated with the energy‐dependent acceleration of ultrarelativistic electrons: Storms with higher solar wind speed and elevated and sustained substorm activity are more likely to cause flux enhancement of ultrarelativistic electrons with higher energies. This suggests the important roles of inward radial diffusion as well as the source and seed populations provided by substorms on the energy‐dependent acceleration of ultrarelativistic electrons. Key Points: Probability of ultrarelativistic electron flux enhancement is higher during more intense storms and for electrons with lower energiesStorms with high solar wind speed, low number density, and sustained southward Bz are likely to cause ultrarelativistic electron enhancementHigh solar wind speed and sustained substorm activity are most influential to energy‐dependent acceleration of ultrarelativistic electrons [ABSTRACT FROM AUTHOR]