Bibliography

Characterization and Evolution of Radiation Belt Electron Energy Spectra Based on the Van Allen Probes Measurements

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.

Zhao, H.; Johnston, W.R.; Baker, D.N.; Li, X.; Ni, B.; Jaynes, A.N.; Kanekal, S.G.; Blake, J.B.; Claudepierre, S.G.; Reeves, G.D.; Boyd, A.J.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 05/2019

YEAR: 2019     DOI: 10.1029/2019JA026697

Bump-on-tail energy spectrum; Energy spectrum; Exponential energy spectrum; Plasmapause; Power law energy spectrum; radiation belt electrons; Van Allen Probes

An empirical model of radiation belt electron pitch angle distributions based on Van Allen Probes measurements

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.

Zhao, H.; Friedel, R.; Chen, Y.; Reeves, G.; Baker, D.; Li, X.; Jaynes, A.; Kanekal, S.; Claudepierre, S.; Fennell, J.; Blake, J.; Spence, H.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 04/2018

YEAR: 2018     DOI: 10.1029/2018JA025277

Empirical Model; Geomagnetic storms; inner belt and slot region; Pitch angle distribution; radiation belt electrons; Van Allen Probes

Resonant Scattering of Radiation Belt Electrons by Off-Equatorial Magnetosonic Waves

Fast magnetosonic (MS) waves are commonly regarded as electromagnetic waves that are characteristically confined within \textpm3\textdegree of the geomagnetic equator. We report two typical off-equatorial MS events observed by Van Allen Probes, that is, the 8 May 2014 event that occurred at the geomagnetic latitudes of 7.5\textdegree\textendash9.2\textdegree both inside and outside the plasmasphere with the wave amplitude up to 590 pT and the 9 January 2014 event that occurred at the latitudes of\textemdash(15.7\textdegree\textendash17.5\textdegree) outside the plasmasphere with a smaller amplitude about 81 pT. Detailed test particle simulations quantify the electron resonant scattering rates by the off-equatorial MS waves to find that they can cause the pitch angle scattering and momentum diffusion of radiation belt electrons with equatorial pitch angles < ~75\textdegree or < ~58\textdegree (depending on the wave latitudinal coverage) on timescales of a day. Subsequent two-dimensional Fokker-Planck diffusion simulations indicate that the strong off-equatorial MS waves are capable of efficiently transporting high pitch angle electrons to lower pitch angles to facilitate the formation of radiation belt electron butterfly distributions for a broad energy range from ~100 keV to >1 MeV within an hour. Our study clearly demonstrates that the presence of off-equatorial MS waves, in addition to equatorial MS waves, can contribute importantly to the dynamical variations of radiation belt electron fluxes and their pitch angle distribution.

Ni, Binbin; Zou, Zhengyang; Fu, Song; Cao, Xing; Gu, Xudong; Xiang, Zheng;

Published by: Geophysical Research Letters      Published on: 02/2018

YEAR: 2018     DOI: 10.1002/grl.v45.310.1002/2017GL075788

butterfly pitch angle distributions; off-equatorial MS waves; radiation belt electrons; Van Allen Probes

Multiple-satellite observation of magnetic dip event during the substorm on 10 October, 2013

We present a multiple-satellite observation of the magnetic dip event during the substorm on October 10, 2013. The observation illustrates the temporal and spatial evolution of the magnetic dip and gives a compelling evidence that ring current ions induce the magnetic dip by enhanced plasma beta. The dip moves with the energetic ions in a comparable drift velocity and affects the dynamics of relativistic electrons in the radiation belt. In addition, the magnetic dip provides a favorable condition for the EMIC wave generation based on the linear theory analysis. The calculated proton diffusion coefficients show that the observed EMIC wave can lead to the pitch angle scattering losses of the ring current ions, which in turn partially relax the magnetic dip in the observations. This study enriches our understanding of magnetic dip evolution and demonstrates the important role of the magnetic dip for the coupling of radiation belt and ring current.

He, Zhaoguo; Chen, Lunjin; Zhu, Hui; Xia, Zhiyang; Reeves, G.; Xiong, Ying; Xie, Lun; Cao, Yong;

Published by: Geophysical Research Letters      Published on: 09/2017

YEAR: 2017     DOI: 10.1002/2017GL074869

EMIC wave; magnetic dip; radiation belt electrons; Ring current ions; Van Allen Probes

On the relation between radiation belt electrons and solar wind parameters/geomagnetic indices: Dependence on the first adiabatic invariant and L*

The relation between radiation belt electrons and solar wind/magnetospheric processes is of particular interest due to both scientific and practical needs. Though many studies have focused on this topic, electron data from Van Allen Probes with wide L shell coverage and fine energy resolution, for the first time, enabled this statistical study on the relation between radiation belt electrons and solar wind parameters/geomagnetic indices as a function of first adiabatic invariant μ and L*. Good correlations between electron phase space density (PSD) and solar wind speed, southward IMF Bz, SYM-H, and AL indices are found over wide μ and L* ranges, with higher correlation coefficients and shorter time lags for low-μ electrons than high-μ electrons; the anticorrelation between electron PSD and solar wind proton density is limited to high-μ electrons at high L*. The solar wind dynamic pressure has dominantly positive correlation with low-μ electrons and negative correlation with high-μ electrons at different L*. In addition, electron PSD enhancements also correlate well with various solar wind/geomagnetic parameters, and for most parameters this correlation is even better than that of electron PSD while the time lag is also much shorter. Among all parameters investigated, AL index is shown to correlate the best with electron PSD enhancements, with correlation coefficients up to ~0.8 for low-μ electrons (time lag ~ 0 day) and ~0.7 for high-μ electrons (time lag ~ 1\textendash2 days), suggesting the importance of seed and source populations provided by substorms in radiation belt electron PSD enhancements.

Published by: Journal of Geophysical Research: Space Physics      Published on: 01/2017

YEAR: 2017     DOI: 10.1002/2016JA023658

Geomagnetic storms; magnetospheric substorms; Phase space density; radiation belt electron content; radiation belt electrons; Solar wind; Van Allen Probes

High-resolution in situ observations of electron precipitation-causing EMIC waves

Electromagnetic ion cyclotron (EMIC) waves are thought to be important drivers of energetic electron losses from the outer radiation belt through precipitation into the atmosphere. While the theoretical possibility of pitch angle scattering-driven losses from these waves has been recognized for more than four decades, there have been limited experimental precipitation observations to support this concept. We have combined satellite-based observations of the characteristics of EMIC waves, with satellite and ground-based observations of the EMIC-induced electron precipitation. In a detailed case study, supplemented by an additional four examples, we are able to constrain for the first time the location, size, and energy range of EMIC-induced electron precipitation inferred from coincident precipitation data and relate them to the EMIC wave frequency, wave power, and ion band of the wave as measured in situ by the Van Allen Probes. These observations will better constrain modeling into the importance of EMIC wave-particle interactions.

Rodger, Craig; Hendry, Aaron; Clilverd, Mark; Kletzing, Craig; Brundell, James; Reeves, Geoffrey;

Published by: Geophysical Research Letters      Published on: 11/2015

YEAR: 2015     DOI: 10.1002/grl.v42.2210.1002/2015GL066581

EMIC waves; energetic electron precipitation; radiation belt electrons; Van Allen Probes; wave-particle interactions

Near-Earth Injection of MeV Electrons associated with Intense Dipolarization Electric Fields: Van Allen Probes observations

Substorms generally inject 10s-100s keV electrons, but intense substorm electric fields have been shown to inject MeV electrons as well. An intriguing question is whether such MeV electron injections can populate the outer radiation belt. Here we present observations of a substorm injection of MeV electrons into the inner magnetosphere. In the pre-midnight sector at L\~5.5, Van Allen Probes (RBSP)-A observed a large dipolarization electric field (50mV/m) over \~40s and a dispersionless injection of electrons up to \~3 MeV. Pitch angle observations indicated betatron acceleration of MeV electrons at the dipolarization front. Corresponding signals of MeV electron injection were observed at LANL-GEO, THEMIS-D, and GOES at geosynchronous altitude. Through a series of dipolarizations, the injections increased the MeV electron phase space density by one order of magnitude in less than 3 hours in the outer radiation belt (L>4.8). Our observations provide evidence that deep injections can supply significant MeV electrons.

Dai, Lei; Wang, Chi; Duan, Suping; He, Zhaohai; Wygant, John; Cattell, Cynthia; Tao, Xin; Su, Zhenpeng; Kletzing, Craig; Baker, Daniel; Li, Xinlin; Malaspina, David; Blake, Bernard; Fennell, Joseph; Claudepierre, Seth; Turner, Drew; Reeves, Geoffrey; Funsten, Herbert; Spence, Harlan; Angelopoulos, Vassilis; Fruehauff, Dennis; Chen, Lunjin; Thaller, Scott; Breneman, Aaron; Tang, Xiangwei;

Published by: Geophysical Research Letters      Published on: 07/2015

YEAR: 2015     DOI: 10.1002/2015GL064955

electric fields; radiation belt electrons; substorm dipolarization; substorm injection; Van Allen Probes

Relativistic electron scattering by magnetosonic waves: Effects of discrete wave emission and high wave amplitudes

In this paper, we study relativistic electron scattering by fast magnetosonic waves. We compare results of test particle simulations and the quasi-linear theory for different spectra of waves to investigate how a fine structure of the wave emission can influence electron resonant scattering. We show that for a realistically wide distribution of wave normal angles theta (i.e., when the dispersion delta theta >= 0.5 degrees), relativistic electron scattering is similar for a wide wave spectrum and for a spectrum consisting in well-separated ion cyclotron harmonics. Comparisons of test particle simulations with quasi-linear theory show that for delta theta > 0.5 degrees, the quasi-linear approximation describes resonant scattering correctly for a large enough plasma frequency. For a very narrow h distribution (when delta theta >= 0.05 degrees), however, the effect of a fine structure in the wave spectrum becomes important. In this case, quasi-linear theory clearly fails in describing accurately electron scattering by fast magnetosonic waves. We also study the effect of high wave amplitudes on relativistic electron scattering. For typical conditions in the earth\textquoterights radiation belts, the quasi-linear approximation cannot accurately describe electron scattering for waves with averaged amplitudes > 300 pT. We discuss various applications of the obtained results for modeling electron dynamics in the radiation belts and in the Earth\textquoterights magnetotail. (C) 2015 AIP Publishing LLC.

Artemyev, A.; Mourenas, D.; Agapitov, O.; Krasnoselskikh, V.;

Published by: Physics of Plasmas      Published on: 06/2015

YEAR: 2015     DOI: 10.1063/1.4922061

chorus waves; CLUSTER SPACECRAFT; equatorial noise; MAGNETIC-FIELD; PLASMA; Quasi-linear diffusion; radiation belt electrons; RESONANT SCATTERING; Van Allen Probes; WHISTLER-MODE WAVES