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2019
Authors: Zhao H., Johnston W.R., Baker D.N., Li X, Ni B, et al.
Title: Characterization and Evolution of Radiation Belt Electron Energy Spectra Based on the Van Allen Probes Measurements
Abstract: 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. . .
Date: 05/2019 Publisher: Journal of Geophysical Research: Space Physics DOI: 10.1029/2019JA026697 Available at: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2019JA026697
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Authors: Zhao H., Baker D N, Li X, Jaynes A. N., and Kanekal S G
Title: The Effects of Geomagnetic Storms and Solar Wind Conditions on the Ultrarelativistic Electron Flux Enhancements
Abstract: 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 stor. . .
Date: 03/2019 Publisher: Journal of Geophysical Research: Space Physics Pages: 1948 - 1965 DOI: 10.1029/2018JA026257 Available at: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018JA026257
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2018
Authors: Zhao H., Baker D N, Li X, Jaynes A. N., and Kanekal S G
Title: The Acceleration of Ultrarelativistic Electrons During a Small to Moderate Storm of 21 April 2017
Abstract: 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 acc. . .
Date: 06/2018 Publisher: Geophysical Research Letters DOI: 10.1029/2018GL078582 Available at: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018GL078582
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Authors: Zhao H., Friedel R H W, Chen Y., Reeves G D, Baker D N, et al.
Title: An empirical model of radiation belt electron pitch angle distributions based on Van Allen Probes measurements
Abstract: Based on over 4 years of Van Allen Probes measurements, an empirical model of radiation belt electron equatorial pitch angle distribution (PAD) is constructed. The model, developed by fitting electron PADs with Legendre polynomials, provides the statistical PADs as a function of L‐shell (L=1 – 6), magnetic local time (MLT), electron energy (~30 keV – 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° pitch angle (PA) and lowe. . .
Date: 04/2018 Publisher: Journal of Geophysical Research: Space Physics DOI: 10.1029/2018JA025277 Available at: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018JA025277
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Authors: Jaynes A. N., Ali A. F., Elkington S R, Malaspina D. M., Baker D N, et al.
Title: Fast diffusion of ultra-relativistic electrons in the outer radiation belt: 17 March 2015 storm event
Abstract: Inward radial diffusion driven by ULF waves has long been known to be capable of accelerating radiation belt electrons to very high energies within the heart of the belts, but more recent work has shown that radial diffusion values can be highly event‐specific and mean values or empirical models may not capture the full significance of radial diffusion to acceleration events. Here we present an event of fast inward radial diffusion, occurring during a period following the geomagnetic storm of 17 March 2015. Ultra‐relativistic electrons up to ∼8 MeV are accelerated in the absence of intense higher‐frequency plasma waves, indicating an acceleration event in the core of the outer belt driven primarily or entirely by ULF wave‐driven diffusion. We examine this fast diffusion rate alon. . .
Date: 09/2018 Publisher: Geophysical Research Letters DOI: 10.1029/2018GL079786 Available at: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018GL079786
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Authors: Selesnick R. S., Baker D N, Kanekal S G, Hoxie V C, and Li X
Title: Modeling the Proton Radiation Belt With Van Allen Probes Relativistic Electron-Proton Telescope Data
Abstract: An empirical model of the proton radiation belt is constructed from data taken during 2013–2017 by the Relativistic Electron-Proton Telescopes on the Van Allen Probes satellites. The model intensity is a function of time, kinetic energy in the range 18–600 MeV, equatorial pitch angle, and L shell of proton guiding centers. Data are selected, on the basis of energy deposits in each of the nine silicon detectors, to reduce background caused by hard proton energy spectra at low L. Instrument response functions are computed by Monte Carlo integration, using simulated proton paths through a simplified structural model, to account for energy loss in shielding material for protons outside the nominal field of view. Overlap of energy channels, their wide angular response, and changing satellit. . .
Date: 01/2018 Publisher: Journal of Geophysical Research: Space Physics DOI: 10.1002/2017JA024661 Available at: http://onlinelibrary.wiley.com/doi/10.1002/2017JA024661/full
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2017
Authors: Li L. Y., Yu J., Cao J. B., Yang J. Y., Li X, et al.
Title: Roles of whistler-mode waves and magnetosonic waves in changing the outer radiation belt and the slot region
Abstract: Using the Van Allen Probe long-term (2013 – 2015) observations and quasi-linear simulations of wave-particle interactions, we examine the combined or competing effects of whistler-mode waves (chorus or hiss) and magnetosonic (MS) waves on energetic (<0.5 MeV) and relativistic (>0.5 MeV) electrons inside and outside the plasmasphere. Although whistler-mode chorus waves and MS waves can singly or jointly accelerate electrons from the hundreds of keV energy to the MeV energy in the low-density trough, most of the relativistic electron enhancement events are best correlated with the chorus wave emissions outside the plasmapause. Inside the plasmasphere, intense plasmaspheric hiss can cause the net loss of relativistic electrons via persistent pitch angle scattering, regardless of whether. . .
Date: 04/2017 Publisher: Journal of Geophysical Research: Space Physics DOI: 10.1002/2016JA023634 Available at: http://onlinelibrary.wiley.com/doi/10.1002/2016JA023634/full
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Authors: Schiller Q., Tu W., Ali A. F., Li X, Godinez H. C., et al.
Title: Simultaneous event-specific estimates of transport, loss, and source rates for relativistic outer radiation belt electrons
Abstract: The most significant unknown regarding relativistic electrons in Earth's outer Van Allen radiation belt is the relative contribution of loss, transport, and acceleration processes within the inner magnetosphere. Detangling each individual process is critical to improve the understanding of radiation belt dynamics, but determining a single component is challenging due to sparse measurements in diverse spatial and temporal regimes. However, there are currently an unprecedented number of spacecraft taking measurements that sample different regions of the inner magnetosphere. With the increasing number of varied observational platforms, system dynamics can begin to be unraveled. In this work, we employ in situ measurements during the 13–14 January 2013 enhancement event to isolate transport,. . .
Date: 03/2017 Publisher: Journal of Geophysical Research: Space Physics DOI: 10.1002/2016JA023093 Available at: http://doi.wiley.com/10.1002/2016JA023093
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Authors: Zhao H., Baker D N, Califf S., Li X, Jaynes A. N., et al.
Title: Van Allen Probes measurements of energetic particle deep penetration into the low L region (L<4) during the storm on 8 April 2016
Abstract: Using measurements from the Van Allen Probes, a penetration event of 10s – 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 – 100s of keV electron penetration is completely different from 10s of keV protons and cannot be well explained by inward radial diffus. . .
Date: 11/2017 Publisher: Journal of Geophysical Research: Space Physics DOI: 10.1002/2017JA024558 Available at: http://doi.wiley.com/10.1002/2017JA024558http://onlinelibrary.wiley.com/wol1/doi/10.1002/2017JA024558/fullpdf
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2016
Authors: Selesnick R. S., Baker D N, Jaynes A. N., Li X, Kanekal S G, et al.
Title: Inward diffusion and loss of radiation belt protons
Abstract: Radiation belt protons in the kinetic energy range 24 to 76 MeV are being measured by the Relativistic Electron Proton Telescope on each of the two Van Allen Probes. Data have been processed for the purpose of studying variability in the trapped proton intensity during October 2013 to August 2015. For the lower energies (≲32 MeV), equatorial proton intensity near L = 2 showed a steady increase that is consistent with inward diffusion of trapped solar protons, as shown by positive radial gradients in phase space density at fixed values of the first two adiabatic invariants. It is postulated that these protons were trapped with enhanced efficiency during the 7 March 2012 solar proton event. A model that includes radial diffusion, along with known trapped proton source and loss processes, s. . .
Date: 03/2016 Publisher: Journal of Geophysical Research: Space Physics Pages: n/a - n/a DOI: 10.1002/2015JA022154 Available at: http://onlinelibrary.wiley.com/wol1/doi/10.1002/2015JA022154/full
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Authors: Califf S., Li X, Wolf R. A., Zhao H., Jaynes A. N., et al.
Title: Large-amplitude electric fields in the inner magnetosphere: Van Allen Probes observations of subauroral polarization streams
Abstract: 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. . .
Date: 05/2016 Publisher: Journal of Geophysical Research: Space Physics DOI: 10.1002/2015JA022252 Available at: http://doi.wiley.com/10.1002/2015JA022252
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Authors: Foster J. C., Erickson P. J., Baker D N, Jaynes A. N., Mishin E. V., et al.
Title: Observations of the impenetrable barrier, the plasmapause, and the VLF bubble during the 17 March 2015 storm
Abstract: Van Allen Probes observations during the 17 March 2015 major geomagnetic storm strongly suggest that VLF transmitter-induced waves play an important role in sculpting the earthward extent of outer zone MeV electrons. A magnetically confined bubble of very low frequency (VLF) wave emissions of terrestrial, human-produced origin surrounds the Earth. The outer limit of the VLF bubble closely matches the position of an apparent barrier to the inward extent of multi-MeV radiation belt electrons near 2.8 Earth radii. When the VLF transmitter signals extend beyond the eroded plasmapause, electron loss processes set up near the outer extent of the VLF bubble create an earthward limit to the region of local acceleration near L = 2.8 as MeV electrons are scattered into the atmospheric loss cone.
Date: 06/2016 Publisher: Journal of Geophysical Research: Space Physics Pages: 5537 - 5548 DOI: 10.1002/jgra.v121.610.1002/2016JA022509 Available at: http://doi.wiley.com/10.1002/2016JA022509
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Authors: Kanekal S G, Baker D N, Fennell J. F., Jones A., Schiller Q., et al.
Title: Prompt acceleration of magnetospheric electrons to ultrarelativistic energies by the 17 March 2015 interplanetary shock
Abstract: Trapped electrons in Earth's outer Van Allen radiation belt are influenced profoundly by solar phenomena such as high-speed solar wind streams, coronal mass ejections (CME), and interplanetary (IP) shocks. In particular, strong IP shocks compress the magnetosphere suddenly and result in rapid energization of electrons within minutes. It is believed that the electric fields induced by the rapid change in the geomagnetic field are responsible for the energization. During the latter part of March 2015, a CME impact led to the most powerful geomagnetic storm (minimum Dst = −223 nT at 17 March, 23 UT) observed not only during the Van Allen Probe era but also the entire preceding decade. Magnetospheric response in the outer radiation belt eventually resulted in elevated levels of energized ele. . .
Date: 08/2016 Publisher: Journal of Geophysical Research: Space Physics Pages: 7622 - 7635 DOI: 10.1002/2016JA022596 Available at: http://doi.wiley.com/10.1002/2016JA022596
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Authors: Schiller Q., Kanekal S G, Jian L. K., Li X, Jones A., et al.
Title: Prompt injections of highly relativistic electrons induced by interplanetary shocks: A statistical study of Van Allen Probes observations
Abstract: We conduct a statistical study on the sudden response of outer radiation belt electrons due to interplanetary (IP) shocks during the Van Allen Probes era, i.e., 2012 to 2015. Data from the Relativistic Electron-Proton Telescope instrument on board Van Allen Probes are used to investigate the highly relativistic electron response (E > 1.8 MeV) within the first few minutes after shock impact. We investigate the relationship of IP shock parameters, such as Mach number, with the highly relativistic electron response, including spectral properties and radial location of the shock-induced injection. We find that the driving solar wind structure of the shock does not affect occurrence for enhancement events, 25% of IP shocks are associated with prompt energization, and 14% are associated wi. . .
Date: 12/2016 Publisher: Geophysical Research Letters DOI: 10.1002/2016GL071628 Available at: http://onlinelibrary.wiley.com/doi/10.1002/2016GL071628/full
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Authors: Zhao H., Li X, Baker D N, Claudepierre S G, Fennell J. F., et al.
Title: Ring current electron dynamics during geomagnetic storms based on the Van Allen Probes measurements
Abstract: 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 densi. . .
Date: 04/2016 Publisher: Journal of Geophysical Research: Space Physics Pages: 3333 - 3346 DOI: 10.1002/2016JA022358 Available at: http://doi.wiley.com/10.1002/2016JA022358
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2015
Authors: Zhao H., Li X, Baker D N, Fennell J. F., Blake J B, et al.
Title: The evolution of ring current ion energy density and energy content during geomagnetic storms based on Van Allen Probes measurements
Abstract: 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. . .
Date: 08/2015 Publisher: Journal of Geophysical Research: Space Physics DOI: 10.1002/2015JA021533 Available at: http://doi.wiley.com/10.1002/2015JA021533
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Authors: Blum L. W., Halford A., Millan R., Bonnell J. W., Goldstein J, et al.
Title: Observations of coincident EMIC wave activity and duskside energetic electron precipitation on 18-19 January 2013
Abstract: Electromagnetic ion cyclotron (EMIC) waves have been suggested to be a cause of radiation belt electron loss to the atmosphere. Here simultaneous, magnetically conjugate measurements are presented of EMIC wave activity, measured at geosynchronous orbit and on the ground, and energetic electron precipitation, seen by the Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) campaign, on two consecutive days in January 2013. Multiple bursts of precipitation were observed on the duskside of the magnetosphere at the end of 18 January and again late on 19 January, concurrent with particle injections, substorm activity, and enhanced magnetospheric convection. The structure, timing, and spatial extent of the waves are compared to those of the precipitation during both days to det. . .
Date: 07/2015 Publisher: Geophysical Research Letters DOI: 10.1002/2015GL065245 Available at: http://doi.wiley.com/10.1002/2015GL065245
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Authors: Kanekal S G, Baker D N, Henderson M G, Li W, Fennell J. F., et al.
Title: Relativistic electron response to the combined magnetospheric impact of a coronal mass ejection overlapping with a high-speed stream: Van Allen Probes observations
Abstract: During early November 2013, the magnetosphere experienced concurrent driving by a coronal mass ejection (CME) during an ongoing high-speed stream (HSS) event. The relativistic electron response to these two kinds of drivers, i.e., HSS and CME, is typically different, with the former often leading to a slower buildup of electrons at larger radial distances, while the latter energizing electrons rapidly with flux enhancements occurring closer to the Earth.We present a detailed analysis of the relativistic electron response including radial profiles of phase space density as observed by both MagEIS and REPT instruments on the Van Allen Probes mission. Data from the MagEIS instrument establishes the behavior of lower energy (<1MeV) electrons which span both intermediary and seed populations du. . .
Date: 09/2015 Publisher: Journal of Geophysical Research: Space Physics DOI: 10.1002/2015JA021395 Available at: http://doi.wiley.com/10.1002/2015JA021395
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Authors: Jaynes A.N., Baker D.N., Singer H.J., Rodriguez J.V., Loto'aniu T.M., et al.
Title: Source and Seed Populations for Relativistic Electrons: Their Roles in Radiation Belt Changes
Abstract: Strong enhancements of outer Van Allen belt electrons have been shown to have a clear dependence on solar wind speed and on the duration of southward interplanetary magnetic field. However, individual case study analyses also have demonstrated that many geomagnetic storms produce little in the way of outer belt enhancements and, in fact, may produce substantial losses of relativistic electrons. In this study, focused upon a key period in August-September 2014, we use GOES geostationary orbit electron flux data and Van Allen Probes particle and fields data to study the process of radiation belt electron acceleration. One particular interval, 13-22 September, initiated by a short-lived geomagnetic storm and characterized by a long period of primarily northward IMF, showed strong depletion of. . .
Date: 07/2015 Publisher: Journal of Geophysical Research: Space Physics DOI: 10.1002/2015JA021234 Available at: http://doi.wiley.com/10.1002/2015JA021234
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Authors: Li X, Selesnick R. S., Baker D N, Jaynes A. N., Kanekal S G, et al.
Title: Upper limit on the inner radiation belt MeV electron Intensity
Abstract: No instruments in the inner radiation belt are immune from the unforgiving penetration of the highly energetic protons (10s of MeV to GeV). The inner belt proton flux level, however, is relatively stable, thus for any given instrument, the proton contamination often leads to a certain background noise. Measurements from the Relativistic Electron and Proton Telescope integrated little experiment (REPTile) on board Colorado Student Space Weather Experiment (CSSWE) CubeSat, in a low Earth orbit, clearly demonstrate that there exist sub-MeV electrons in the inner belt because of their flux level is orders of magnitude higher than the background, while higher energy electron (>1.6 MeV) measurements cannot be distinguished from the background. Detailed analysis of high-quality measurements from . . .
Date: 01/2015 Publisher: Journal of Geophysical Research: Space Physics DOI: 10.1002/2014JA020777 Available at: http://doi.wiley.com/10.1002/2014JA020777
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2014
Authors: Zhao H., Li X, Blake J B, Fennell J. F., Claudepierre S G, et al.
Title: Characteristics of pitch angle distributions of 100 s keV electrons in the slot region and inner radiation belt
Abstract: 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 90∘), cap (exceedingly peaking narrowly around 90∘) and 90∘-minimum (lower flux at 90∘) PADs. By examining the characteristics of the PADs of ~460 keV electrons for over a year, we find that the 90∘-minimum PADs are generally present in the inner belt (L < 2), while normal PADs dominate at .L ~3.5. . .
Date: 11/2014 Publisher: Journal of Geophysical Research: Space Physics DOI: 10.1002/2014JA020386 Available at: http://doi.wiley.com/10.1002/2014JA020386
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Authors: Jaynes A. N., Li X, Schiller Q. G., Blum L. W., Tu W., et al.
Title: Evolution of relativistic outer belt electrons during an extended quiescent period
Abstract: To effectively study steady loss due to hiss-driven precipitation of relativistic electrons in the outer radiation belt, it is useful to isolate this loss by studying a time of relatively quiet geomagnetic activity. We present a case of initial enhancement and slow, steady decay of 700 keV - 2 MeV electron populations in the outer radiation belt during an extended quiescent period from ~15 December 2012 - 13 January 2013. We incorporate particle measurements from a constellation of satellites, including the Colorado Student Space Weather Experiment (CSSWE) CubeSat, the Van Allen Probes twin spacecraft, and THEMIS, to understand the evolution of the electron populations across pitch angle and energy. Additional data from calculated phase space density (PSD), as well as hiss and chorus w. . .
Date: 12/2014 Publisher: Journal of Geophysical Research: Space Physics DOI: 10.1002/2014JA020125 Available at: http://doi.wiley.com/10.1002/2014JA020125
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Authors: Baker D N, Jaynes A. N., Li X, Henderson M G, Kanekal S G, et al.
Title: Gradual diffusion and punctuated phase space density enhancements of highly relativistic electrons: Van Allen Probes observations
Abstract: The dual-spacecraft Van Allen Probes mission has provided a new window into mega electron volt (MeV) particle dynamics in the Earth's radiation belts. Observations (up to E ~10 MeV) show clearly the behavior of the outer electron radiation belt at different timescales: months-long periods of gradual inward radial diffusive transport and weak loss being punctuated by dramatic flux changes driven by strong solar wind transient events. We present analysis of multi-MeV electron flux and phase space density (PSD) changes during March 2013 in the context of the first year of Van Allen Probes operation. This March period demonstrates the classic signatures both of inward radial diffusive energization and abrupt localized acceleration deep within the outer Van Allen zone (L ~4.0 ± 0.5). Thi. . .
Date: 03/2014 Publisher: Geophysical Research Letters Pages: 1351 - 1358 DOI: 10.1002/2013GL058942 Available at: http://doi.wiley.com/10.1002/2013GL058942
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Authors: Baker D N, Jaynes A. N., Hoxie V C, Thorne R M, Foster J. C., et al.
Title: An impenetrable barrier to ultrarelativistic electrons in the Van Allen radiation belts
Abstract: Early observations1, 2 indicated that the Earth’s Van Allen radiation belts could be separated into an inner zone dominated by high-energy protons and an outer zone dominated by high-energy electrons. Subsequent studies3, 4 showed that electrons of moderate energy (less than about one megaelectronvolt) often populate both zones, with a deep ‘slot’ region largely devoid of particles between them. There is a region of dense cold plasma around the Earth known as the plasmasphere, the outer boundary of which is called the plasmapause. The two-belt radiation structure was explained as arising from strong electron interactions with plasmaspheric hiss just inside the plasmapause boundary5, with the inner edge of the outer radiation zone corresponding to the minimum plasmapause location6. Re. . .
Date: 11/2014 Publisher: Nature Pages: 531 - 534 DOI: 10.1038/nature13956 Available at: http://www.nature.com/doifinder/10.1038/nature13956
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Authors: Selesnick R. S., Baker D N, Jaynes A. N., Li X, Kanekal S G, et al.
Title: Observations of the inner radiation belt: CRAND and trapped solar protons
Abstract: Measurements of inner radiation belt protons have been made by the Van Allen Probes Relativistic Electron-Proton Telescopes as a function of kinetic energy (24 to 76 MeV), equatorial pitch angle, and magnetic L shell, during late-2013 and early-2014. A probabilistic data analysis method reduces background from contamination by higher energy protons. Resulting proton intensities are compared to predictions of a theoretical radiation belt model. Then trapped protons originating both from cosmic ray albedo neutron decay (CRAND) and from trapping of solar protons are evident in the measured distributions. An observed double-peaked distribution in L is attributed, based on the model comparison, to a gap in the occurrence of solar proton events during the 2007 to 2011 solar minimum. Equatorial. . .
Date: 08/2014 Publisher: Journal of Geophysical Research: Space Physics DOI: 10.1002/2014JA020188 Available at: http://doi.wiley.com/10.1002/2014JA020188
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Authors: Zhao H., Li X, Blake J B, Fennell J. F., Claudepierre S G, et al.
Title: Peculiar pitch angle distribution of relativistic electrons in the inner radiation belt and slot region
Abstract: 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° 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° minimum PADs is that off 90° electrons are preferentially heated by chorus waves just out. . .
Date: 04/2014 Publisher: Geophysical Research Letters Pages: 2250 - 2257 DOI: 10.1002/2014GL059725 Available at: http://doi.wiley.com/10.1002/2014GL059725
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Authors: Califf S., Li X, Blum L., Jaynes A., Schiller Q., et al.
Title: THEMIS measurements of quasi-static electric fields in the inner magnetosphere
Abstract: 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 stro. . .
Date: 10/2014 Publisher: Journal of Geophysical Research: Space Physics DOI: 10.1002/2014JA020360 Available at: http://doi.wiley.com/10.1002/2014JA020360
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2013
Authors: Wygant J R, Bonnell J W, Goetz K, Ergun R E, Mozer F S, et al.
Title: The Electric Field and Waves (EFW) Instruments on the Radiation Belt Storm Probes Mission
Abstract: The Electric Fields and Waves (EFW) Instruments on the two Radiation Belt Storm Probe (RBSP) spacecraft (recently renamed the Van Allen Probes) are designed to measure three dimensional quasi-static and low frequency electric fields and waves associated with the major mechanisms responsible for the acceleration of energetic charged particles in the inner magnetosphere of the Earth. For this measurement, the instrument uses two pairs of spherical double probe sensors at the ends of orthogonal centripetally deployed booms in the spin plane with tip-to-tip separations of 100 meters. The third component of the electric field is measured by two spherical sensors separated by ∼15 m, deployed at the ends of two stacer booms oppositely directed along the spin axis of the spacecraft. The instrume. . .
Date: 11/2013 Publisher: Space Science Reviews DOI: 10.1007/s11214-013-0013-7 Available at: http://link.springer.com/article/10.1007%2Fs11214-013-0013-7
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Authors: Li X, Schiller Q., Blum L., Califf S., Zhao H., et al.
Title: First Results from CSSWE CubeSat: Characteristics of Relativistic Electrons in the Near-Earth Environment During the October 2012 Magnetic Storms
Abstract: 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°) 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°) 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. . .
Date: 10/2013 Publisher: Journal of Geophysical Research: Space Physics DOI: 10.1002/2013JA019342 Available at: http://doi.wiley.com/10.1002/2013JA019342
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Authors: Baker D N, Hoxie V C, Jaynes A., Kale A., Kanekal S G, et al.
Title: James Van Allen and His Namesake NASA Mission
Abstract: In many ways, James A. Van Allen defined and “invented” modern space research. His example showed the way for government-university partners to pursue basic research that also served important national and international goals. He was a tireless advocate for space exploration and for the role of space science in the spectrum of national priorities.
Date: 12/2013 Publisher: Eos, Transactions American Geophysical Union Pages: 469 - 470 DOI: 10.1002/eost.v94.4910.1002/2013EO490001 Available at: http://doi.wiley.com/10.1002/eost.v94.49http://doi.wiley.com/10.1002/2013EO490001
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Authors: Baker D N, Kanekal S G, Hoxie V C, Henderson M G, Li X, et al.
Title: A Long-Lived Relativistic Electron Storage Ring Embedded in Earth's Outer Van Allen Belt
Abstract: Since their discovery more than 50 years ago, Earth’s Van Allen radiation belts have been considered to consist of two distinct zones of trapped, highly energetic charged particles. The outer zone is composed predominantly of megaelectron volt (MeV) electrons that wax and wane in intensity on time scales ranging from hours to days, depending primarily on external forcing by the solar wind. The spatially separated inner zone is composed of commingled high-energy electrons and very energetic positive ions (mostly protons), the latter being stable in intensity levels over years to decades. In situ energy-specific and temporally resolved spacecraft observations reveal an isolated third ring, or torus, of high-energy (>2 MeV) electrons that formed on 2 September 2012 and persisted largely unc. . .
Date: 04/2013 Publisher: Science Pages: 186-190 DOI: 10.1126/science.1233518 Available at: http://www.sciencemag.org/content/340/6129/186
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Authors: Blum L. W., Schiller Q., Li X, Millan R., Halford A., et al.
Title: New conjunctive CubeSat and balloon measurements to quantify rapid energetic electron precipitation
Abstract: Relativistic electron precipitation into the atmosphere can contribute significant losses to the outer radiation belt. In particular, rapid narrow precipitation features termed precipitation bands have been hypothesized to be an integral contributor to relativistic electron precipitation loss, but quantification of their net effect is still needed. Here we investigate precipitation bands as measured at low earth orbit by the Colorado Student Space Weather Experiment (CSSWE) CubeSat. Two precipitation bands of MeV electrons were observed on 18–19 January 2013, concurrent with precipitation seen by the 2013 Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) campaign. The newly available conjugate measurements allow for a detailed estimate of the temporal and spatial fea. . .
Date: 11/2013 Publisher: Geophysical Research Letters Pages: 5833 - 5837 DOI: 10.1002/2013GL058546 Available at: http://doi.wiley.com/10.1002/2013GL058546
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Authors: Baker D N, Kanekal S G, Hoxie V C, Batiste S, Bolton M, et al.
Title: The Relativistic Electron-Proton Telescope (REPT) Instrument on Board the Radiation Belt Storm Probes (RBSP) Spacecraft: Characterization of Earth’s Radiation Belt High-Energy Particle Populations
Abstract: Particle acceleration and loss in the million electron Volt (MeV) energy range (and above) is the least understood aspect of radiation belt science. In order to measure cleanly and separately both the energetic electron and energetic proton components, there is a need for a carefully designed detector system. The Relativistic Electron-Proton Telescope (REPT) on board the Radiation Belt Storm Probe (RBSP) pair of spacecraft consists of a stack of high-performance silicon solid-state detectors in a telescope configuration, a collimation aperture, and a thick case surrounding the detector stack to shield the sensors from penetrating radiation and bremsstrahlung. The instrument points perpendicular to the spin axis of the spacecraft and measures high-energy electrons (up to ∼20 MeV) with exc. . .
Date: 11/2013 Publisher: Space Science Reviews Pages: 337-381 DOI: 10.1007/s11214-012-9950-9 Available at: http://link.springer.com/article/10.1007%2Fs11214-012-9950-9
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Authors: Spence H E, Reeves G D, Baker D N, Blake J B, Bolton M, et al.
Title: Science Goals and Overview of the Energetic Particle, Composition, and Thermal Plasma (ECT) Suite on NASA’s Radiation Belt Storm Probes (RBSP) Mission
Abstract: The Radiation Belt Storm Probes (RBSP)-Energetic Particle, Composition, and Thermal Plasma (ECT) suite contains an innovative complement of particle instruments to ensure the highest quality measurements ever made in the inner magnetosphere and radiation belts. The coordinated RBSP-ECT particle measurements, analyzed in combination with fields and waves observations and state-of-the-art theory and modeling, are necessary for understanding the acceleration, global distribution, and variability of radiation belt electrons and ions, key science objectives of NASA’s Living With a Star program and the Van Allen Probes mission. The RBSP-ECT suite consists of three highly-coordinated instruments: the Magnetic Electron Ion Spectrometer (MagEIS), the Helium Oxygen Proton Electron (HOPE) senso. . .
Date: 11/2013 Publisher: Space Science Reviews Pages: 311-336 DOI: DOI: 10.1007/s11214-013-0007-5 Available at: http://link.springer.com/article/10.1007%2Fs11214-013-0007-5
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2006
Authors: Sarris T, Li X, and Temerin M
Title: Simulating radial diffusion of energetic (MeV) electrons through a model of fluctuating electric and magnetic fields
Abstract: In the present work, a test particle simulation is performed in a model of analytic Ultra Low Frequency, ULF, perturbations in the electric and magnetic fields of the Earth's magnetosphere. The goal of this work is to examine if the radial transport of energetic particles in quiet-time ULF magnetospheric perturbations of various azimuthal mode numbers can be described as a diffusive process and be approximated by theoretically derived radial diffusion coefficients. In the model realistic compressional electromagnetic field perturbations are constructed by a superposition of a large number of propagating electric and consistent magnetic pulses. The diffusion rates of the electrons under the effect of the fluctuating fields are calculated numerically through the test-particle simulation as a. . .
Date: 10/2006 Publisher: Annales Geophysicae Pages: 2583 - 2598 DOI: 10.5194/angeo-24-2583-2006 Available at: http://www.ann-geophys.net/24/2583/2006/angeo-24-2583-2006.html
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2004
Authors: Baker D N, Kanekal S G, Li X, Monk S P, Goldstein J, et al.
Title: An extreme distortion of the Van Allen belt arising from the ‘Hallowe’en’ solar storm in 2003
Abstract: The Earth's radiation belts—also known as the Van Allen belts1—contain high-energy electrons trapped on magnetic field lines2, 3. The centre of the outer belt is usually 20,000–25,000 km from Earth. The region between the belts is normally devoid of particles2, 3, 4, and is accordingly favoured as a location for spacecraft operation because of the benign environment5. Here we report that the outer Van Allen belt was compressed dramatically by a solar storm known as the ‘Hallowe'en storm’ of 2003. From 1 to 10 November, the outer belt had its centre only ~10,000 km from Earth's equatorial surface, and the plasmasphere was similarly displaced inwards. The region between the belts became the location of high particle radiation intensity. This remarkable deformation of the entire. . .
Date: 12/2004 Publisher: Nature Pages: 878 - 881 DOI: 10.1038/nature03116 Available at: http://www.nature.com/nature/journal/v432/n7019/full/nature03116.html
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