Found 41 entries in the Bibliography.
Showing entries from 1 through 41
Abstract 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 hours 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. This article is protected by copyright. All rights reserved.
Zhao, Hong; Sarris, Theodore; Li, Xinlin; Weiner, Max; Huckabee, Isabela; Baker, Daniel; Jaynes, Allison; Kanekal, Shrikanth; Elkington, Scot; Barani, Mohammad; Tu, Weichao; Liu, Wenlong; Zhang, Dianjun; Hartinger, Michael;
Published by: Journal of Geophysical Research: Space Physics Published on: 07/2021
YEAR: 2021   DOI: https://doi.org/10.1029/2021JA029284
Abstract 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 MagEIS onboard RBSP-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 SYM-H and 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.
Published by: Journal of Geophysical Research: Space Physics Published on: 06/2021
YEAR: 2021   DOI: https://doi.org/10.1029/2021JA029385
Abstract In the outer radiation belt, localized ULF waves can interact with energetic electrons by drift resonance, leading to quasiperiodic oscillations. The oscillations in the pitch angle spectrum can be characterized by either boomerang-shaped or straight stripes. Previous studies have shown that boomerang-shaped stripes evolve from straight ones when electrons drift away from the localized wave interaction region. Based on the time-of-flight technique on the pitch angle-dependent drift velocity, the origin can be remotely identified from the pitch angle dispersion. We report 27 straight stripe events and 86 boomerang-shaped events observed by Van Allen Probes from 2013/01/01 to 2017/12/31. Statistical study shows a good coincidence between the locations of straight ones and traceback regions from boomerang-shaped ones. These locations, mainly located in noon-to-dusk region, coincide well with the plasmaspheric plumes. Thus localized ULF waves trapped in the plume may result in the preference of localized ULF waves-electron interactions at noon-to-dusk region.
Published by: Geophysical Research Letters Published on: 05/2021
YEAR: 2021   DOI: https://doi.org/10.1029/2021GL093377
Abstract A radial diffusion model directly driven by the solar wind is developed to reproduce MeV electron variations between L=2-12 (L is L* in this study) from October 2012 to April 2015. The radial diffusion coefficient, internal source rate, quick loss due to EMIC waves, and slow loss due to hiss waves are all expressed in terms of the solar wind speed, dynamic pressure, and interplanetary magnetic field (IMF). The model achieves a prediction efficiency (PE) of 0.45 at L=5 and 0.51 at L=4 after converting the electron phase space densities to differential fluxes and comparing with Van Allen Probes measurements of 2 MeV and 3 MeV electrons at L=5 and L=4, respectively. Machine learning techniques are used to tune parameters to get higher PE. By tuning parameters for every 60-day period, the model obtains PE values of 0.58 and 0.82 at L=5 and L=4, respectively. Inspired by these results, we divide the solar wind activity into three categories based on the condition of solar wind speed, IMF Bz, and dynamic pressure, and then tune these three sets of parameters to obtain the highest PE. This experiment confirms that the solar wind speed has the greatest influence on the electron flux variations, particularly at higher L, while the dynamic pressure has more influence at lower L. Also, the PE at L=4 is mostly higher than those at L=5, suggesting that the electron loss due to the magnetopause shadowing combined with the outward radial diffusion is not well captured in the model. This article is protected by copyright. All rights reserved.
Published by: Journal of Geophysical Research: Space Physics Published on: 05/2021
YEAR: 2021   DOI: https://doi.org/10.1029/2020JA028988
Abstract Energy spectra of ring current protons are crucial to understanding the ring current dynamics. Based on high-quality Van Allen Probes RBSPICE measurements, we investigate the global distribution of the reversed proton energy spectra using the 2013-2019 RBSPICE datasets. The reversed proton energy spectra are characterized by the distinct flux minima around 50 - 100 keV and flux maxima around 200 - 400 keV. Our results show that the reversed proton energy spectrum is prevalent inside the plasmasphere, with the occurrence rates > 90\% at L ∼2 - 4 during geomagnetically quiet periods. Its occurrence also manifests a significant decrease trend with increasing L-shell and enhanced geomagnetic activity. It is indicated that the substorm-associated and/or convection processes are likely to lead to the disappearances of the reversed spectra. These results provide important clues for exploring the underlying physical mechanisms responsible for the formation and evolution of reversed proton energy spectra.
Published by: Geophysical Research Letters Published on: 01/2021
YEAR: 2021   DOI: https://doi.org/10.1029/2020GL091559
Using seven years of data from the HOPE 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. This article is protected by copyright. All rights reserved.
Published by: Journal of Geophysical Research: Space Physics Published on: 12/2020
YEAR: 2020   DOI: https://doi.org/10.1029/2020JA028322
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.
Published by: Journal of Geophysical Research: Space Physics Published on: 08/2020
YEAR: 2020   DOI: https://doi.org/10.1029/2020JA028198
Earth s slot region, lying between the outer and inner radiation belts, has been identified as due to a balance between inward radial diffusion and pitch angle (PA) scattering induced by waves. However, recent satellite observations and modeling studies indicate that cosmic ray albedo neutron decay (CRAND) may also play a significant role in energetic electron dynamics in the slot region. In this study, using a drift-diffusion-source model, we investigate the relative contribution of all significant waves and CRAND to the dynamics of energetic electrons in the slot region during July 2014, an extended period of quiet geomagnetic activity. The bounce-averaged PA diffusion coefficients from three types of waves (hiss, lightning-generated whistlers [LGW], and very low frequency [VLF] transmitters) are calculated based on quasi-linear theory, while the CRAND source follows the results in Xiang et al. (2019, https://doi.org/10.1029/2018GL081730). The simulation results indicate that both LGW and VLF transmitter waves can enhance loss and weaken the top hat PA distribution induced by hiss waves. For 470 keV electrons at L = 2.5, simulation results without CRAND show a much quicker decrease than observations from the Van Allen Probes. After including CRAND, simulated electron flux variations reproduce satellite observations, suggesting that CRAND is an important source for hundreds of keV electrons in the slot region during quiet times. The balance between the CRAND source and loss due to wave-particle interactions provides a lower limit to relativistic electron fluxes in the slot region, which can act as an important reference point for instrument calibration when a true background level is warranted.
Published by: Journal of Geophysical Research: Space Physics Published on: 08/2020
YEAR: 2020   DOI: https://doi.org/10.1029/2020JA028042
Ultralow frequency (ULF) wave-particle interactions play a significant role in the radiation belt dynamic process, during which drift resonance can accelerate and transport energetic electrons in the outer radiation belt. Observations of wave-electron drift resonance are characterized by quasiperiodic straight or “boomerang-shaped” stripes in the pitch angle spectrogram. Here we present an ULF wave event on 1 December 2015, during which both kinds stripes were observed by Van Allen Probes A and B, respectively. Using the time-of-flight technique based on the pitch angle dependence of electron drift velocities, the “boomerang-shaped” stripes are inferred to originate from straight stripes at the time and location covered by Probe B. Given that straight stripes were indeed observed by Probe B, our observations strongly support the charged particle interacting with azimuthally localized ULF waves. A new method is provided to identify the location of ULF wave-particle interaction on the basis of remote observations of electron flux modulations.
Published by: Geophysical Research Letters Published on: 07/2020
YEAR: 2020   DOI: https://doi.org/10.1029/2020GL087960
Coherent electron flux oscillations of hundreds of keV are often observed by the Van Allen Probes in the magnetosphere during quiet times in association with ultralow frequency (ULF) waves. They are observed in the form of periodic flux fluctuations, with a drift frequency that is energy dependent, but are not associated with drift echoes following storm- or substorm-related energetic particle injections. Instead, they are associated with the resonant interaction of electrons with ULF waves and are an indication of ongoing electron radial diffusion. To investigate details of such flux oscillations, particle-tracing simulations are conducted under the effect of realistic, broadband ULF electric and consistent magnetic fluctuations. Virtual detectors are simulated along spacecraft orbits and the results are compared to measurements. Through a parametric study, it is found that the width of electron energy channels is a critical parameter affecting the observed amplitude of flux oscillations, with narrower energy channel widths enabling the observation of higher-amplitude flux oscillations; this potentially explains why such features were not observed regularly before the Van Allen Probes era, as previous spacecraft generally had lower energy resolution, which only enabled the observation of large-amplitude drift echoes following a storm or substorm. Results are confirmed using the Magnetic Electron Ion Spectrometer (MagEIS) ultrahigh energy resolution data. Energy width effects are quantified through a parametric simulation study that matches flux oscillation observations during a period that is characterized by extremely quiet conditions, where the Van Allen Probes observed flux oscillations over multiple days.
Published by: Journal of Geophysical Research: Space Physics Published on: 05/2020
YEAR: 2020   DOI: https://doi.org/10.1029/2020JA027798
In this study, we present Van Allen Probe observations showing that seed (hundreds of keV) and core ( 1 MeV) electrons can resonate with ultra-low-frequency (ULF) wave modes with distinctive m values simultaneously. An unusual electron energy spectrogram with double-banded resonant structure was recorded by energetic particle, composition, and thermal plasma (ECT)-magnetic electron ion spectrometer (MagEIS) and, meanwhile, boomerang stripes in pitch angle spectrogram appeared at the lower energy band. A localized drift resonance with m = 10 wave component was responsible for the resonant band peaked at ∼200 keV while a global drift resonance with m = 3 component gave rise to the upper band resonance peaked at ∼1 MeV. Time-Of-Flight on boomerang stripes suggested that the localized drift resonance with ∼200 keV electrons was confined within the plasmaspheric plume. Electron flux modulations were reproduced by numerical simulations in good consistency with the observations, supporting the scenario that localized and global drift resonance could coexist in the outer belt electron dynamics simultaneously.
Published by: Geophysical Research Letters Published on: 05/2020
YEAR: 2020   DOI: https://doi.org/10.1029/2020GL088019
Abstract The plasmasphere is a highly dynamic toroidal region of cold, dense plasma around Earth. Plasma waves exist both inside and outside this region and can contribute to the loss and acceleration of high energy outer radiation belt electrons. Early observational studies found an apparent correlation on long time scales between the observed inner edge of the outer radiation belt and the modeled innermost plasmapause location. More recent work using high-resolution Van Allen Probes data has found a more complex relationship. For this study, we determine the standoff distance of the location of maximum electron flux of the outer belt MeV electrons from the plasmapause following rapid enhancement events. We find that the location of the outer radiation belt based on maximum electron flux is consistently outside the plasmapause, with a peak radial standoff distance of ∆L ~ 1. We discuss the implications this result has for acceleration mechanisms.
Published by: Geophysical Research Letters Published on: 03/2020
YEAR: 2020   DOI: 10.1029/2020GL086991
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 × 107/cm2-s-sr-MeV at 470 keV at L = 1.5 and 3.6 × 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.
Published by: Journal of Geophysical Research: Space Physics Published on:
YEAR: 2020   DOI: https://doi.org/10.1029/2020JA028511
Published by: Journal of Geophysical Research: Space Physics Published on: 11/2019
YEAR: 2019   DOI: 10.1029/2019JA027331
In this report, the relationship between innermost plasmapause locations (Lpp) and initial electron enhancements during both storm and nonstorm (Dst > -30 nT) periods are examined using data from the Van Allen Probes. The geomagnetic storms are classified into coronal mass ejection (CME)-driven and corotating interaction region (CIR)-driven storms to explore their influences on the initial electron enhancements, respectively. We also study nonstorm time electron enhancements and observe frequent, sudden (within two consecutive orbital passes) <400-keV electron enhancements during quiet periods. Our analysis reveals an incredibly cohesive observation that holds regardless of electron energies (~30 keV\textendash2.5 MeV) or geomagnetic conditions: the innermost Lpp is the innermost boundary of the initial energetic electron enhancements. Interestingly, the quantified energy-dependent relationship of the sudden, intense energetic electron enhancements, with respect to the innermost Lpp, also exhibit a very similar trend during both storm and nonstorm periods. In summary, the goal of this report is to provide a comprehensive quantification of this consistent relationship under various geomagnetic conditions, which will also enable better forecast and specification of energetic electrons in the inner magnetosphere.
Published by: Journal of Geophysical Research: Space Physics Published on: 11/2019
YEAR: 2019   DOI: 10.1029/2019JA027412
The magnetospheric driver of strong thermal emission velocity enhancement (STEVE) is investigated using conjugate observations when Van Allen Probes\textquoteright footprint directly crossed both STEVE and stable red aurora (SAR) arc. In the ionosphere, STEVE is associated with subauroral ion drift features, including electron temperature peak, density gradient, and westward ion flow. The SAR arc at lower latitudes corresponds to regions inside the plasmapause with isotropic plasma heating, which causes redline-only SAR emission via heat conduction. STEVE corresponds to the sharp plasmapause boundary containing quasi-static subauroral ion drift electric field and parallel-accelerated electrons by kinetic Alfv\ en waves. These parallel electrons could precipitate and be accelerated via auroral acceleration processes powered by Alfv\ en waves propagating along the magnetic field with the plasmapause as a waveguide. The electron precipitation, superimposed on the heat conduction, could explain multiwavelength continuous STEVE emission. The green picket-fence emissions are likely optical manifestations of electron precipitation associated with wave structures traveling along the plasmapause.
Chu, Xiangning; Malaspina, David; Gallardo-Lacourt, Bea; Liang, Jun; Andersson, Laila; Ma, Qianli; Artemyev, Anton; Liu, Jiang; Ergun, Robert; Thaller, Scott; Akbari, Hassanali; Zhao, Hong; Larsen, Brian; Reeves, Geoffrey; Wygant, John; Breneman, Aaron; Tian, Sheng; Connors, Martin; Donovan, Eric; Archer, William; MacDonald, Elizabeth;
Published by: Geophysical Research Letters Published on: 11/2019
YEAR: 2019   DOI: 10.1029/2019GL082789
Using energetic particle and wave measurements from the Van Allen Probes, Polar Orbiting Environmental Satellites (POES), and Geostationary Operational Environmental Satellite (GOES), the acceleration mechanism of ultrarelativistic electrons (>3 MeV) in the center of the outer radiation belt is investigated statistically. A superposed epoch analysis is conducted using 19 storms, which caused flux enhancements of 1.8\textendash7.7 MeV electrons. The evolution of electron phase space density radial profile suggests an energy-dependent acceleration of ultrarelativistic electrons in the outer belt. Especially, for electrons with very high energies (~7 MeV), prevalent positive phase space density radial gradients support inward radial diffusion being responsible for electron acceleration in the center of the outer belt (L*~3\textendash5) during most enhancement events in the Van Allen Probes era. We propose a two-step acceleration process to explain the acceleration of ~7 MeV electrons in the outer belt: intense and sustained chorus waves locally energize core electron populations to ultrarelativistic energies at high L region beyond the Van Allen Probes\textquoteright apogee, followed by inward radial diffusion which further energizes these populations to even higher energies. Statistical results of chorus wave activity inferred from POES precipitating electron measurements as well as core electron populations observed by the Van Allen Probes and GOES support this hypothesis.
Published by: Journal of Geophysical Research: Space Physics Published on: 10/2019
YEAR: 2019   DOI: 10.1029/2019JA027111
Auroral kilometric radiation (AKR) can potentially produce serious damage to space-borne systems by accelerating trapped radiation belt electrons to relativistic energies. Here we examine the global occurrences of AKR emissions in radiation belts based on Van Allen Probes observations from 1 October 2012 to 31 December 2016. The statistical results (1,848 events in total) show that AKR covers a broad region of L= 3\textendash6.5 and 00\textendash24 magnetic local time (MLT), with a higher occurrence on the nightside (20\textendash24 MLT and 00\textendash04 MLT) within L= 5\textendash6.5. All the AKR events are observed to be accompanied with suprathermal (\~1 keV) electron flux enhancements. During active geomagnetic periods, both AKR occurrences and electron injections tend to be more distinct, and AKR emission extends to the dayside. The current study shows that AKR emissions from the remote sources are closely associated with electron injections.
Published by: Geophysical Research Letters Published on: 07/2019
YEAR: 2019   DOI: 10.1029/2019GL083944
Using Van Allen Probe Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) wave observations from September 2012 to May 2018, we statistically investigate the distributions of power-weighted wave normal angle (WNA) of fast magnetosonic (MS) waves from L = 2\textendash6 within \textpm15\textdegree geomagnetic latitudes. The spatial distributions show that the MS WNAs are mainly confined within 87\textendash89\textdegree near the geomagnetic equator and decrease with increasing magnetic latitude. Further quantitative investigation demonstrates that the WNAs normally distribute as a mixture of two Gaussian distributions ranging from 85\textdegree to 88\textdegree, and the tangent of it can decrease as a Kappa distribution function when the waves propagate to higher latitudes. Our study completes the survey of spatial distributions of MS WNAs and provides quantitative dependence of the WNA distribution on the magnetic latitude in the inner magnetosphere, which can be readily useful in future global simulations of radiation belt particle dynamics.
Published by: Journal of Geophysical Research: Space Physics Published on: 07/2019
YEAR: 2019   DOI: 10.1029/2019JA026556
Based on the measurements of ~100-keV to 10-MeV electrons from the Magnetic Electron Ion Spectrometer (MagEIS) and Relativistic Electron and Proton Telescope (REPT) on the Van Allen Probes, the radiation belt electron energy spectra characterization and evolution have been investigated systematically. The results show that the majority of radiation belt electron energy spectra can be represented by one of three types of distributions: exponential, power law, and bump-on-tail (BOT). The exponential spectra are generally dominant in the outer radiation belt outside the plasmasphere, power law spectra usually appear at high L-shells during injections of lower-energy electrons, and BOT spectra commonly dominate inside the plasmasphere at L>2.5 during relatively quiet times. The main features of three types of energy spectra have also been revealed. Specifically, for the BOT energy spectrum, the energy of local flux maximum usually ranges from approximately hundreds of keV to several MeV and the energy of local flux minimum varies from ~100 keV to ~MeV, both increasing as L-shell decreases, confirming the plasmaspheric hiss wave scattering to be the main mechanism forming the BOT energy spectra. Statistical results using 4-year observations from the Van Allen Probes on the relation between energy spectra and plasmapause location also show that the plasmasphere plays a critical role in shaping radiation belt electron energy spectrum: the peak location of BOT energy spectra is ~1 L-shell inside the minimum plasmapause, where BOT energy spectra mostly form in ~1\textendash2 days as a result of hiss wave scattering.
Published by: Journal of Geophysical Research: Space Physics Published on: 05/2019
YEAR: 2019   DOI: 10.1029/2019JA026697
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.
Published by: Journal of Geophysical Research: Space Physics Published on: 03/2019
YEAR: 2019   DOI: 10.1029/2018JA026257
In addition to clarifying morphological structures of the Earth\textquoterights radiation belts, it has also been a major achievement of the Van Allen Probes mission to understand more thoroughly how highly relativistic and ultrarelativistic electrons are accelerated deep inside the radiation belts. Prior studies have demonstrated that electrons up to energies of 10 megaelectron volts (MeV) can be produced over broad regions of the outer Van Allen zone on timescales of minutes to a few hours. It often is seen that geomagnetic activity driven by strong solar storms (i.e., coronal mass ejections, or CMEs) almost inexorably leads to relativistic electron production through the intermediary step of intense magnetospheric substorms. In this study, we report observations over the 6-year period 1 September 2012 to 1 September 2018. We focus on data about the relativistic and ultrarelativistic electrons (E>=5 MeV) measured by the Relativistic Electron-Proton Telescope sensors on board the Van Allen Probes spacecraft. This work portrays the radiation belt acceleration, transport, and loss characteristics over a wide range of geomagnetic events. We emphasize features seen repeatedly in the data (three-belt structures, \textquotedblleftimpenetrable\textquotedblright barrier properties, and radial diffusion signatures) in the context of acceleration and loss mechanisms. We especially highlight solar wind forcing of the ultrarelativistic electron populations and extended periods when such electrons were absent. The analysis includes new display tools showing spatial features of the mission-long time variability of the outer Van Allen belt emphasizing the remarkable dynamics of the system.
Published by: Journal of Geophysical Research: Space Physics Published on: 03/2019
YEAR: 2019   DOI: 10.1029/2018JA026259
Using Van Allen Probes\textquoteright observations and established plasmapause location (Lpp) models, we investigate the relationship between the location of the initial enhancement (IE) of energetic electrons and the innermost (among all magnetic local time sectors) Lpp over five intense storm periods. Our study reveals that the IE events for 30 keV to 2MeV electrons always occurred outside of the innermost Lpp. On average, the inner extent of the IE events (LIE) for <800 keV electrons was closer to the innermost Lpp when compared to the LIE for >800 keV electrons that was found consistently at ~1.5 RE outside of the innermost Lpp. The IE of 10s keV electrons was observed before the IE of 100s keV electrons, and the IE of >800 keV electrons was observed on average 12.6\textpm2.3 hours after the occurrence of the earliest IE event. In addition, we report an overall electron (~30 keV to ~2 MeV) flux increase outside the plasmasphere during the selected storm periods, in contrast to the little change of energy spectrum evolution inside the plasmasphere; this demonstrates the important role of the plasmasphere in shaping energetic electron dynamics. Our investigation of the LIE-Lpp relationship also provides insights into the underlying physical processes responsible for the dynamics of tens keV to >MeV electrons.
Published by: Journal of Geophysical Research: Space Physics Published on: 10/2018
YEAR: 2018   DOI: 10.1029/2018JA026074
The ultrarelativistic electrons (E > ~3 MeV) in the outer radiation belt received limited attention in the past due to sparse measurements. Nowadays, the Van Allen Probes measurements of ultrarelativistic electrons with high energy resolution provide an unprecedented opportunity to study the dynamics of this population. In this study, using data from the Van Allen Probes, we report significant flux enhancements of ultrarelativistic electrons with energies up to 7.7 MeV during a small to moderate geomagnetic storm. The underlying physical mechanisms are investigated by analyzing and simulating the evolution of electron phase space density. The results suggest that during this storm, the acceleration mechanism for ultrarelativistic electrons in the outer belt is energy-dependent: local acceleration plays the most important role in the flux enhancements of ~3\textendash5 MeV electrons, while inward radial diffusion is the main acceleration mechanism for ~7 MeV electrons at the center of the outer radiation belt.
Published by: Geophysical Research Letters Published on: 06/2018
YEAR: 2018   DOI: 10.1029/2018GL078582
Oblique whistler mode waves have been suggested to play an important role in radiation belt electron dynamics. Recently, Fu et al.  proposed that highly oblique lower band whistler waves could be generated by nonlinear three-wave resonance. Here we present the first observational evidence of such process, using Van Allen Probes data, where an oblique lower band chorus wave is generated by two quasi-parallel waves through nonlinear three-wave interaction. The wave resonance condition is satisfied even in the presence of frequency chirping of one of the pump waves. Different from the simulation results of Fu et al. , simultaneous particle data do not show a plateau in the electron distribution, which could be due to the very weak intensity of the generated waves. These results should help to better understand the generation of oblique waves in the inner magnetosphere and their relative roles in energetic electron dynamics.
Published by: Geophysical Research Letters Published on: 06/2018
YEAR: 2018   DOI: 10.1029/2018GL078765
Based on over 4 years of Van Allen Probes measurements, an empirical model of radiation belt electron equatorial pitch angle distribution (PAD) is constructed. The model, developed by fitting electron PADs with Legendre polynomials, provides the statistical PADs as a function of L-shell (L=1 \textendash 6), magnetic local time (MLT), electron energy (~30 keV \textendash 5.2 MeV), and geomagnetic activity (represented by the Dst index), and is also the first empirical PAD model in the inner belt and slot region. For MeV electrons, model results show more significant day-night PAD asymmetry of electrons with higher energies and during disturbed times, which is caused by geomagnetic field configuration and flux radial gradient changes. Steeper PADs with higher fluxes around 90\textdegree pitch angle (PA) and lower fluxes at lower PAs for higher energy electrons and during active times are also present, which could be due to EMIC wave scattering. For 100s of keV electrons, cap PADs are generally present in the slot region during quiet times and their energy-dependent features are consistent with hiss wave scattering, while during active times, cap PADs are less significant especially at outer part of slot region, which could be due to the complex energizing and transport processes. 90\textdegree-minimum PADs are persistently present in the inner belt and appear in the slot region during active times, and minima at 90\textdegree PA are more significant for electrons with higher energies, which could be a critical evidence in identifying the underlying physical processes responsible for the formation of 90\textdegree-minimum PADs.
Published by: Journal of Geophysical Research: Space Physics Published on: 04/2018
YEAR: 2018   DOI: 10.1029/2018JA025277
During the 13-14 November 2012 storm, Van Allen Probe A simultaneously observed a 10-h period of enhanced chorus (including quasi-parallel and oblique propagation components) and relativistic electron fluxes over a broad range of L = 3-6 and MLT=2 - 10 within a complete orbit cycle. By adopting a Gaussian fit to the observed wave spectra, we obtain the wave parameters and calculate the bounce-averaged diffusion coefficients. We solve the Fokker-Planck diffusion equation to simulate flux evolutions of relativistic (1.8-4.2 MeV) electrons during two intervals when Probe A passed the location L = 4.3 along its orbit. The simulating results show that chorus with combined quasi-parallel and oblique components can produce a more pronounced flux enhancement in the pitch angle range \~45o-80o, consistent well with the observation. The current results provide the first evidence on how relativistic electron fluxes vary under the drive of almost continuously distributed chorus with both quasi-parallel and oblique components within a complete orbit of Van Allen Probe.
Published by: Geophysical Research Letters Published on: 02/2018
YEAR: 2018   DOI: 10.1002/2017GL075894
Using measurements from the Van Allen Probes, a penetration event of 10s \textendash 100s of keV electrons and 10s of keV protons into the low L-shells (L<4) is studied. Timing and magnetic local time (MLT) differences of energetic particle deep penetration are unveiled and underlying physical processes are examined. During this event, both proton and electron penetrations are MLT-asymmetric. The observed MLT difference of proton penetration is consistent with convection of plasma sheet protons, suggesting enhanced convection during geomagnetic active times to be the cause of energetic proton deep penetration during this event. The observed MLT difference of 10s \textendash 100s of keV electron penetration is completely different from 10s of keV protons and cannot be well explained by inward radial diffusion, convection of plasma sheet electrons, or transport of trapped electrons by enhanced convection electric field represented by the Volland-Stern model or a uniform dawn-dusk electric field model based on the electric field measurements. It suggests that the underlying physical mechanism responsible for energetic electron deep penetration, which is very important for fully understanding energetic electron dynamics in the low L-shells, should be MLT-localized.
Published by: Journal of Geophysical Research: Space Physics Published on: 11/2017
YEAR: 2017   DOI: 10.1002/2017JA024558
The objective of this study is to investigate the relationship between the levels of electron flux oscillations and radial diffusion for different Phase Space Density (PSD) gradients, through observation and particle tracing simulations under the effect of model Ultra Low Frequency (ULF) fluctuations. This investigation aims to demonstrate that electron flux oscillation is associated with and could be used as an indicator of ongoing radial diffusion. To this direction, flux oscillations are observed through the Van Allen Probes\textquoteright MagEIS energetic particle detector; subsequently, flux oscillations are produced in a particle tracing model that simulates radial diffusion by using model magnetic and electric field fluctuations that are approximating measured magnetic and electric field fluctuations as recorded by the Van Allen Probes\textquoteright EMFISIS and EFW instruments, respectively. The flux oscillation amplitudes are then correlated with Phase Space Density gradients in the magnetosphere and with the ongoing radial diffusion process.
Published by: Journal of Geophysical Research: Space Physics Published on: 06/2017
YEAR: 2017   DOI: 10.1002/2016JA023741
The Van Allen Probes have reported frequent flux enhancements of 100s keV electrons in the slot region, with lower energy electrons exhibiting more dynamic behavior at lower L shells. Also, in situ electric field measurements from the Combined Release and Radiation Effects Satellite, Time History of Events and Macroscale Interactions during Substorms (THEMIS), and the Van Allen Probes have provided evidence for large-scale electric fields at low L shells during active times. We study an event on 19 February 2014 where hundreds of keV electron fluxes were enhanced by orders of magnitude in the slot region and electric fields of 1\textendash2 mV/m were observed below L = 3. Using a 2-D guiding center particle tracer and a simple large-scale convection electric field model, we demonstrate that the measured electric fields can account for energization of electrons up to at least 500 keV in the slot region through inward radial transport.
Published by: Journal of Geophysical Research: Space Physics Published on: 02/2017
YEAR: 2017   DOI: 10.1002/2016JA023657
Multiband electromagnetic ion cyclotron (EMIC) waves can drive efficient scattering loss of radiation belt relativistic electrons. However, it is statistically uncommon to capture the three bands of EMIC waves concurrently. Utilizing data from the Electric and Magnetic Field Instrument Suite and Integrated Science magnetometer onboard Van Allen Probe A, we report the simultaneous presence of three (H+, He+, and O+) emission bands in an EMIC wave event, which provides an opportunity to look into the combined scattering effect of all EMIC emissions and the relative roles of each band in diffusing radiation belt relativistic electrons under realistic circumstances. Our quantitative results, obtained by quasi-linear diffusion rate computations and 1-D pure pitch angle diffusion simulations, demonstrate that the combined resonant scattering by the simultaneous three-band EMIC waves is overall dominated by He+ band wave diffusion, mainly due to its dominance over the wave power (the mean wave amplitudes are approximately 0.4 nT, 1.6 nT, and 0.15 nT for H+, He+, and O+ bands, respectively). Near the loss cone, while 2\textendash3 MeV electrons undergo pitch angle scattering at a rate of the order of 10-6\textendash10-5 s-1, 5\textendash10 MeV electrons can be diffused more efficiently at a rate of the order of 10-3\textendash10-2 s-1, which approaches the strong diffusion level and results in a moderately or heavily filled loss cone for the atmospheric loss. The corresponding electron loss timescales (i.e., lifetimes) vary from several days at the energies of ~2 MeV to less than 1 h at ~10 MeV. This case study indicates the leading contribution of He+ band waves to radiation belt relativistic electron losses during the coexistence of three EMIC wave bands and suggests that the roles of different EMIC wave bands in the relativistic electron dynamics should be carefully incorporated in future modeling efforts.
Published by: Journal of Geophysical Research: Space Physics Published on: 05/2016
YEAR: 2016   DOI: 10.1002/2016JA022483
The subauroral polarization stream (SAPS) is an important magnetosphere-ionosphere (MI) coupling phenomenon that impacts a range of particle populations in the inner magnetosphere. SAPS studies often emphasize ionospheric signatures of fast westward flows, but the equatorial magnetosphere is also affected through strong radial electric fields in the dusk sector. This study focuses on a period of steady southward interplanetary magnetic field (IMF) during the 29 June 2013 geomagnetic storm where the Van Allen Probes observe a region of intense electric fields near the plasmapause over multiple consecutive outbound duskside passes. We show that the large-amplitude electric fields near the equatorial plane are consistent with SAPS by investigating the relationship between plasma sheet ion and electron boundaries, associated field-aligned currents, and the spatial location of the electric fields. By incorporating high-inclination DMSP data we demonstrate the spatial and temporal variability of the SAPS region, and we suggest that discrete, earthward-propagating injections are driving the observed strong electric fields at low L shells in the equatorial magnetosphere. We also show the relationship between SAPS and plasmasphere erosion, as well as a possible correlation with flux enhancements for 100 s keV electrons.
Published by: Journal of Geophysical Research: Space Physics Published on: 05/2016
YEAR: 2016   DOI: 10.1002/2015JA022252
Based on comprehensive measurements from Helium, Oxygen, Proton, and Electron Mass Spectrometer Ion Spectrometer, Relativistic Electron-Proton Telescope, and Radiation Belt Storm Probes Ion Composition Experiment instruments on the Van Allen Probes, comparative studies of ring current electrons and ions are performed and the role of energetic electrons in the ring current dynamics is investigated. The deep injections of tens to hundreds of keV electrons and tens of keV protons into the inner magnetosphere occur frequently; after the injections the electrons decay slowly in the inner belt but protons in the low L region decay very fast. Intriguing similarities between lower energy protons and higher-energy electrons are also found. The evolution of ring current electron and ion energy densities and energy content are examined in detail during two geomagnetic storms, one moderate and one intense. The results show that the contribution of ring current electrons to the ring current energy content is much smaller than that of ring current ions (up to ~12\% for the moderate storm and ~7\% for the intense storm), and <35 keV electrons dominate the ring current electron energy content at the storm main phases. Though the electron energy content is usually much smaller than that of ions, the enhancement of ring current electron energy content during the moderate storm can get to ~30\% of that of ring current ions, indicating a more dynamic feature of ring current electrons and important role of electrons in the ring current buildup. The ring current electron energy density is also shown to be higher at midnight and dawn while lower at noon and dusk.
Published by: Journal of Geophysical Research: Space Physics Published on: 04/2016
YEAR: 2016   DOI: 10.1002/2016JA022358
To improve our understanding of the role of electromagnetic ion cyclotron (EMIC) waves in radiation belt electron dynamics, we perform a comprehensive analysis of EMIC wave-induced resonant scattering of outer zone relativistic (>0.5 MeV) electrons and resultant electron loss time scales with respect to EMIC wave band, L shell, and wave normal angle model. The results demonstrate that while H+-band EMIC waves dominate the scattering losses of ~1\textendash4 MeV outer zone relativistic electrons, it is He+-band and O+-band waves that prevail over the pitch angle diffusion of ultrarelativistic electrons at higher energies. Given the wave amplitude, EMIC waves at higher L shells tend to resonantly interact with a larger population of outer zone relativistic electrons and drive their pitch angle scattering more efficiently. Obliquity of EMIC waves can reduce the efficiency of wave-induced relativistic electron pitch angle scattering. Compared to the frequently adopted parallel or quasi-parallel model, use of the latitudinally varying wave normal angle model produces the largest decrease in H+-band EMIC wave scattering rates at pitch angles < ~40\textdegree for electrons > ~5 MeV. At a representative nominal amplitude of 1 nT, EMIC wave scattering produces the equilibrium state (i.e., the lowest normal mode under which electrons at the same energy but different pitch angles decay exponentially on the same time scale) of outer belt relativistic electrons within several to tens of minutes and the following exponential decay extending to higher pitch angles on time scales from <1 min to ~1 h. The electron loss cone can be either empty as a result of the weak diffusion or heavily/fully filled due to approaching the strong diffusion limit, while the trapped electron population at high pitch angles close to 90\textdegree remains intact because of no resonant scattering. In this manner, EMIC wave scattering has the potential to deepen the anisotropic distribution of outer zone relativistic electrons by reshaping their pitch angle profiles to \textquotedbllefttop-hat.\textquotedblright Overall, H+-band and He+-band EMIC waves are most efficient in producing the pitch angle scattering loss of relativistic electrons at ~1\textendash2 MeV. In contrast, the presence of O+-band EMIC waves, while at a smaller occurrence rate, can dominate the scattering loss of 5\textendash10 MeV electrons in the entire region of the outer zone, which should be considered in future modeling of the outer zone relativistic electron dynamics.
Published by: Journal of Geophysical Research: Space Physics Published on: 09/2015
YEAR: 2015   DOI: 10.1002/2015JA021466
Enabled by the comprehensive measurements from the MagEIS, HOPE, and RBSPICE instruments onboard Van Allen Probes in the heart of the radiation belt, the relative contributions of ions with different energies and species to the ring current energy density and their dependence on the phases of geomagnetic storms are quantified. The results show that lower energy (<50 keV) protons enhance much more often and also decay much faster than higher energy protons. During the storm main phase, ions with energies < 50 keV contribute more significantly to the ring current than those with higher energies; while the higher energy protons dominate during the recovery phase and quiet times. The enhancements of higher energy proton fluxes as well as energy content generally occur later than those of lower energy protons, which could be due to the inward radial diffusion. For the March 29, 2013 storm we investigated in detail, the contribution from O+ is ~25\% of the ring current energy content during the main phase, and the majority of that comes from < 50 keV O+. This indicates that even during moderate geomagnetic storms the ionosphere is still an important contributor to the ring current ions. Using the Dessler-Parker-Sckopke relation, the contributions of ring current particles to the magnetic field depression during this geomagnetic storm are also calculated. The results show that the measured ring current ions contribute about half of the Dst depression.
Published by: Journal of Geophysical Research: Space Physics Published on: 08/2015
YEAR: 2015   DOI: 10.1002/2015JA021533
Fifteen months of pitch angle resolved Van Allen Probes REPT measurements of differential electron flux are analyzed to investigate the characteristic variability of the pitch angle distribution (PAD) of radiation belt ultra-relativistic (>2 MeV) electrons during storm conditions and during the long-term post-storm decay. By modeling the ultra-relativistic electron pitch angle distribution as sinn α, where α is the equatorial pitch angle, we examine the spatio-temporal variations of the n-value. The results show that in general n-values increase with the level of geomagnetic activity. In principle, ultra-relativistic electrons respond to geomagnetic storms by becoming more peaked at 90\textdegree pitch angle with n-values of 2\textendash3 as a supportive signature of chorus acceleration outside the plasmasphere. High n-values also exist inside the plasmasphere, being localized adjacent to the plasmapause and exhibiting energy dependence, which suggests a significant contribution from EMIC waves scattering. During quiet periods, n-values generally evolve to become small, i.e., 0\textendash1. The slow and long-term decays of the ultra-relativistic electrons after geomagnetic storms, while prominent, produce energy and L-shell dependent decay timescales in association with the solar and geomagnetic activity and wave-particle interaction processes. At lower L shells inside the plasmasphere, the decay timescales τd for electrons at REPT energies are generally larger, varying from tens of days to hundreds of days, which can be mainly attributed to the combined effect of hiss induced pitch angle scattering and inward radial diffusion. As L shell increases to L ~ 3.5, a narrow region exists (with a width of ~0.5 L) where the observed ultra-relativistic electrons decay fastest, possibly resulting from efficient EMIC wave scattering. As L shell continues to increase, τd generally becomes larger again, indicating an overall slower loss process by waves at high L shells. Our investigation based upon the sinn α function fitting and the estimate of decay timescale offers a convenient and useful means to evaluate the underlying physical processes that play a role in driving the acceleration and loss of ultra-relativistic electrons and to assess their relative contributions.
Published by: Journal of Geophysical Research: Space Physics Published on: 05/2015
YEAR: 2015   DOI: 10.1002/2015JA021065
Local acceleration via whistler wave and particle interaction plays a significant role in particle dynamics in the radiation belt. In this work we explore gyroresonant wave-particle interaction and quasi-linear diffusion in different magnetic field configurations related to the 17 March 2013 storm. We consider the Earth\textquoterights magnetic dipole field as a reference and compare the results against nondipole field configurations corresponding to quiet and stormy conditions. The latter are obtained with the ring current-atmosphere interactions model with a self-consistent magnetic field (RAM-SCB), a code that models the Earth\textquoterights ring current and provides a realistic modeling of the Earth\textquoterights magnetic field. By applying quasi-linear theory, the bounce- and Magnetic Local Time (MLT)-averaged electron pitch angle, mixed-term, and energy diffusion coefficients are calculated for each magnetic field configuration. For radiation belt (\~1 MeV) and ring current (\~100 keV) electrons, it is shown that at some MLTs the bounce-averaged diffusion coefficients become rather insensitive to the details of the magnetic field configuration, while at other MLTs storm conditions can expand the range of equatorial pitch angles where gyroresonant diffusion occurs and significantly enhance the diffusion rates. When MLT average is performed at drift shell L=4.25 (a good approximation to drift average), the diffusion coefficients become quite independent of the magnetic field configuration for relativistic electrons, while the opposite is true for lower energy electrons. These results suggest that, at least for the 17 March 2013 storm and for L≲4.25, the commonly adopted dipole approximation of the Earth\textquoterights magnetic field can be safely used for radiation belt electrons, while a realistic modeling of the magnetic field configuration is necessary to describe adequately the diffusion rates of ring current electrons.
Published by: Journal of Geophysical Research: Space Physics Published on: 04/2015
YEAR: 2015   DOI: 10.1002/2014JA020858
The pitch angle distribution (PAD) of energetic electrons in the slot region and inner radiation belt received little attention in the past decades due to the lack of quality measurements. Using the state-of-art pitch-angle-resolved data from the Magnetic Electron Ion Spectrometer (MagEIS) instrument onboard the Van Allen Probes, a detailed analysis of 100 s keV electron PADs below L = 4 is performed, in which the PADs is categorized into three types: normal (flux peaking at 90o), cap (exceedingly peaking narrowly around 90o) and 90o-minimum (lower flux at 90o) PADs. By examining the characteristics of the PADs of ~460 keV electrons for over a year, we find that the 90o-minimum PADs are generally present in the inner belt (L < 2), while normal PADs dominate at .L ~3.5 - 4. In the region between, 90o-minimum PADs dominate during injection times and normal PADs dominate during quiet times. Cap PADs appear mostly at the decay phase of storms in the slot region and are likely caused by the pitch angle scattering of hiss waves. Fitting the normal PADs into sinnα form, the parameter n is much higher below L = 3 than that in the outer belt and relatively constant in the inner belt but changes significantly in the slot region (2 < L < 3) during injection times. As for the 90o-minimum PADs, by performing a detailed case study, we find in the slot region this type of PAD is likely caused by chorus wave heating, butthis mechanism can hardly explain the formation of 90o-minimum PADs at the center of inner belt.
Published by: Journal of Geophysical Research: Space Physics Published on: 11/2014
YEAR: 2014   DOI: 10.1002/2014JA020386
We use four years of THEMIS double-probe measurements to offer, for the first time, a complete picture of the dawn-dusk electric field covering all local times and radial distances in the inner magnetosphere based on in situ equatorial observations. This study is motivated by the results from the CRRES mission, which revealed a local maximum in the electric field developing near Earth during storm times, rather than the expected enhancement at higher L shells that is shielded near Earth as suggested by the Volland-Stern model. The CRRES observations were limited to the dusk side, while THEMIS provides complete local time coverage. We show strong agreement with the CRRES results on the dusk side, with a local maximum near L =4 for moderate levels of geomagnetic activity and evidence of strong electric fields inside L =3 during the most active times. The extensive dataset from THEMIS also confirms the day/night asymmetry on the dusk side, where the enhancement is closest to Earth in the dusk-midnight sector, and is farther away closer to noon. A similar, but smaller in magnitude, local maximum is observed on the dawn side near L =4. The noon sector shows the smallest average electric fields, and for more active times, the enhancement develops near L =7 rather than L =4. We also investigate the impact of the uncertain boom-shorting factor on the results, and show that while the absolute magnitude of the electric field may be underestimated, the trends with geomagnetic activity remain intact.
Published by: Journal of Geophysical Research: Space Physics Published on: 10/2014
YEAR: 2014   DOI: 10.1002/2014JA020360
The relativistic electrons in the inner radiation belt have received little attention in the past due to sparse measurements and unforgiving contamination from the inner belt protons. The high-quality measurements of the Magnetic Electron Ion Spectrometer instrument onboard Van Allen Probes provide a great opportunity to investigate the dynamics of relativistic electrons in the low L region. In this letter, we report the newly unveiled pitch angle distribution (PAD) of the energetic electrons with minima at 90\textdegree near the magnetic equator in the inner belt and slot region. Such a PAD is persistently present throughout the inner belt and appears in the slot region during storms. One hypothesis for 90\textdegree minimum PADs is that off 90\textdegree electrons are preferentially heated by chorus waves just outside the plasmapause (which can be at very low L during storms) and/or fast magnetosonic waves which exist both inside and outside the plasmasphere.
Published by: Geophysical Research Letters Published on: 04/2014
YEAR: 2014   DOI: 10.1002/2014GL059725
Measurements from the Relativistic Electron and Proton Telescope integrated little experiment (REPTile) on board the Colorado Student Space Weather Experiment (CSSWE) CubeSat mission, which was launched into a highly inclined (65\textdegree) low Earth orbit, are analyzed along with measurements from the Relativistic Electron and Proton Telescope (REPT) and the Magnetic Electron Ion Spectrometer (MagEIS) instruments aboard the Van Allen Probes, which are in a low inclination (10\textdegree) geo-transfer-like orbit. Both REPT and MagEIS measure the full distribution of energetic electrons as they traverse the heart of the outer radiation belt. However, due to the small equatorial loss cone (only a few degrees), it is difficult for REPT and MagEIS to directly determine which electrons will precipitate into the atmosphere, a major radiation belt loss process. REPTile, a miniaturized version of REPT, measures the fraction of the total electron population that has small enough equatorial pitch angles to reach the altitude of CSSWE, 480 km \texttimes 780 km, thus measuring the precipitating population as well as the trapped and quasi-trapped populations. These newly available measurements provide an unprecedented opportunity to investigate the source, loss, and energization processes that are responsible for the dynamic behavior of outer radiation belt electrons. The focus of this paper will be on the characteristics of relativistic electrons measured by REPTile during the October 2012 storms; also included are long-term measurements from the Solar Anomalous and Magnetospheric Particle Explorer to put this study into context.
Published by: Journal of Geophysical Research: Space Physics Published on: 10/2013
YEAR: 2013   DOI: 10.1002/2013JA019342