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Found 98 entries in the Bibliography.
Showing entries from 51 through 98
| 2017 |
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Simulated prompt acceleration of multi-MeV electrons by the 17 March 2015 interplanetary shock Prompt enhancement of relativistic electron flux at L = 3-5 has been reported from Van Allen Probes Relativistic Electron Proton Telescope (REPT) measurements associated with the 17 March 2015 interplanetary shock compression of the dayside magnetosphere. Acceleration by \~ 1 MeV is inferred on less than a drift time scale as seen in prior shock compression events, which launch a magetosonic azimuthal electric field impulse tailward. This impulse propagates from the dayside around the flanks accelerating electrons in drift resonance at the dusk flank. Such longitudinally localized acceleration events produce a drift echo signature which was seen at >1 MeV energy on both Van Allen Probe spacecraft, with sustained observations by Probe B outbound at L = 5 at 2100 MLT at the time of impulse arrival, measured by the Electric Fields and Waves instrument. MHD-test particle simulations are presented which reproduce drift echo features observed in the REPT measurements at Probe B, including the energy and pitch angle dependence of drift echoes observed. While the flux enhancement was short-lived for this event due to subsequent inward motion of the magnetopause, stronger events with larger electric field impulses, as observed in March 1991 and the Halloween 2003 storm, produce enhancements which can be quantified by the inward radial transport and energization determined by the induction electric field resulting from dayside compression. Hudson, Mary; Jaynes, Allison; Kress, Brian; Li, Zhao; Patel, Maulik; Shen, Xiaochen; Thaller, Scott; Wiltberger, Michael; Wygant, John; Published by: Journal of Geophysical Research: Space Physics Published on: 09/2017 YEAR: 2017 DOI: 10.1002/2017JA024445 17 March 2015; MeV electron acceleration; Radiation belt; test-particle simulation; Van Allen Probes |
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Observations from Magnetospheric MultiScale (~8 Re) and Van Allen Probes (~5 and 4 Re) show that the initial dayside response to a small interplanetary shock is a double-peaked dawnward electric field, which is distinctly different from the usual bipolar (dawnward and then duskward) signature reported for large shocks. The associated ExB flow is radially inward. The shock compressed the magnetopause to inside 8 Re, as observed by MMS, with a speed that is comparable to the ExB flow. The magnetopause speed and the ExB speeds were significantly less than the propagation speed of the pulse from MMS to the Van Allen Probes and GOES-13, which is consistent with the MHD fast mode. There were increased fluxes of energetic electrons up to several MeV. Signatures of drift echoes and response to ULF waves also were seen. These observations demonstrate that even very weak shocks can have significant impact on the radiation belts. Cattell, C.; Breneman, A.; Colpitts, C.; Dombeck, J.; Thaller, S.; Tian, S.; Wygant, J.; Fennell, J.; Hudson, M.; Ergun, Robert; Russell, C.; Torbert, Roy; Lindqvist, Per-Arne; Burch, J.; Published by: Geophysical Research Letters Published on: 08/2017 YEAR: 2017 DOI: 10.1002/2017GL074895 electric field response; interplanetary shock; magnetopause; Radiation belt; Van Allen Probes |
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Direct observation of generation and propagation of magnetosonic waves following substorm injection Magnetosonic whistler mode waves play an important role in the radiation belt electron dynamics. Previous theory has suggested that these waves are excited by the ring distributions of hot protons and can propagate radially and azimuthally over a broad spatial range. However, because of the challenging requirements on satellite locations and data-processing techniques, this theory was difficult to validate directly. Here we present some experimental tests of the theory on the basis of Van Allen Probes observations of magnetosonic waves following substorm injections. At higher L-shells with significant substorm injections, the discrete magnetosonic emission lines started approximately at the proton gyrofrequency harmonics, qualitatively consistent with the prediction of linear proton Bernstein mode instability. In the frequency-time spectrograms, these emission lines exhibited a clear rising tone characteristic with a long duration of 15-25 mins, implying the additional contribution of other undiscovered mechanisms. Nearly at the same time, the magnetosonic waves arose at lower L-shells without substorm injections. The wave signals at two different locations, separated by ΔL up to 2.0 and by ΔMLT up to 4.2, displayed the consistent frequency-time structures, strongly supporting the hypothesis about the radial and azimuthal propagation of magnetosonic waves. Su, Zhenpeng; Wang, Geng; Liu, Nigang; Zheng, Huinan; Wang, Yuming; Wang, Shui; Published by: Geophysical Research Letters Published on: 07/2018 YEAR: 2017 DOI: 10.1002/2017GL074362 Bernstein mode instability; magnetosonic waves; Radiation belt; rising tone; substorm injection; Van Allen Probes; Wave-particle interaction |
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The 17 March 2015 St. Patrick\textquoterights Day Storm is the largest geomagnetic storm to date of Solar Cycle 24, with a Dst of -223 nT. The magnetopause moved inside geosynchronous orbit under high solar wind dynamic pressure and strong southward IMF Bz causing loss, however a subsequent drop in pressure allowed for rapid rebuilding of the radiation belts. The 17 March 2013 storm also shows similar effects on outer zone electrons: first a rapid dropout due to inward motion of the magnetopause followed by rapid increase in flux above the pre-storm level early in the recovery phase and a slow increase over the next 12 days. These phases can be seen in temporal evolution of the electron phase space density measured by the ECT instruments on Van Allen Probes. Using the Lyon-Fedder-Mobarry global MHD model driven by upstream solar wind measurements, we simulated both St. Patrick\textquoterights Day 2013 and 2015 events, analyzing LFM electric and magnetic fields to calculate radial diffusion coefficients. These coefficients have been implemented in a radial diffusion code, using the measured electron phase space density following the local heating as the initial radial profile and outer boundary condition for subsequent temporal evolution over the next 12 days, beginning 18 March. Agreement with electron phase space density at 1000 MeV/G measured by the MagEIS component of the ECT instrument suite on Van Allen Probes was much improved using radial diffusion coefficients from the MHD simulations relative to coefficients parametrized by a global geomagnetic activity index. Li, Zhao; Hudson, Mary; Patel, Maulik; Wiltberger, Michael; Boyd, Alex; Turner, Drew; Published by: Journal of Geophysical Research: Space Physics Published on: 06/2017 YEAR: 2017 DOI: 10.1002/2016JA023846 March 2013; March 2015; radial diffusion; Radiation belt; Van Allen Probes |
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Using multisatellite measurements, a uniquely strong and long-lived relativistic electron slot region refilling event from November 2004 to January 2005 is investigated. This event occurred under remarkable interplanetary and magnetospheric conditions. Both empirically modeled and observationally estimated plasmapause locations demonstrate that the plasmasphere eroded significantly prior to the enhancement phase of this event. The estimated diffusion coefficients indicate that the radial diffusion due to ULF waves is insufficient to account for the observed enhancement of slot region electrons. However, the diffusion coefficients evaluated using the distribution of chorus wave intensities derived from low-altitude POES electron observations indicate that the local acceleration induced by chorus could account for the major feature of observed enhancement outside the plasmapause. When the plasmasphere recovered, the refilled slot region was enveloped inside the plasmapause. In the plasmasphere, while the efficiency of hiss scattering loss increases by including unusually low frequency hiss waves, the interaction with hiss alone cannot fully explain the decay of this event, especially at higher energies, which suggests that electromagnetic ion cyclotron waves contribute to the relativistic electron loss process at such low L shells for this refilling event. Through a comprehensive analysis on the basis of data analyses and numerical calculations, the present study sheds light on the underlying physics responsible for the unusual slot refilling by relativistic electrons, which exhibits the complexity of both radiation belt electron dynamics and associated wave-particle interactions. Yang, Xiaochao; Ni, Binbin; Yu, Jiang; Zhang, Yang; Zhang, Xiaoxin; Sun, Yueqiang; Published by: Journal of Geophysical Research: Space Physics Published on: 06/2017 YEAR: 2017 DOI: 10.1002/2016JA023204 Radiation belt; Relativistic electron; Slot region; Van Allen Probes |
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Dayside modulated relativistic electron\textquoterights butterfly pitch angle distributions (PADs) from \~200 keV to 2.6 MeV were observed by Van Allen Probe B at L = 5.3 on 15 November 2013. They were associated with localized magnetic dip driven by hot ring current ion (60\textendash100 keV proton and 60\textendash200 keV helium and oxygen) injections. We reproduce the electron\textquoterights butterfly PADs at satellite\textquoterights location using test particle simulation. The simulation results illustrate that a negative radial flux gradient contributes primarily to the formation of the modulated electron\textquoterights butterfly PADs through inward transport due to the inductive electric field, while deceleration due to the inductive electric field and pitch angle change also makes in part contribution. We suggest that localized magnetic field perturbation, which is a frequent phenomenon in the magnetosphere during magnetic disturbances, is of great importance for creating electron\textquoterights butterfly PADs in the Earth\textquoterights radiation belts. Xiong, Ying; Chen, Lunjin; Xie, Lun; Fu, Suiyan; Xia, Zhiyang; Pu, Zuyin; Published by: Geophysical Research Letters Published on: 05/2017 YEAR: 2017 DOI: 10.1002/2017GL072558 butterfly distribution; Radiation belt; ring current; Van Allen Probes |
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The hidden dynamics of relativistic electrons (0.7-1.5~MeV) in the inner zone and slot region We present measurements of relativistic electrons (0.7\textendash1.5 MeV) in the inner zone and slot region obtained by the Magnetic Electron and Ion Spectrometer (MagEIS) instrument on Van Allen Probes. The data presented are corrected for background contamination, which is primarily due to inner-belt protons in these low-L regions. We find that \~1 MeV electrons were transported into the inner zone following the two largest geomagnetic storms of the Van Allen Probes era to date, the March and June 2015 events. As \~1 MeV electrons were not observed in Van Allen Probes data in the inner zone prior to these two events, the injections created a new inner belt that persisted for at least 1.5 years. In contrast, we find that electrons injected into the slot region decay on much faster timescales, approximately tens of days. Furthermore, we find no evidence of >1.5 MeV electrons in the inner zone during the entire time interval considered (April 2013 through September 2016). The energies we examine thus span a transition range in the steeply falling inner zone electron spectrum, where modest intensities are observed at 0.7 MeV, and no electrons are observed at 1.5 MeV. To validate the results obtained from the background corrected flux measurements, we also present detailed pulse-height spectra from individual MagEIS detectors. These measurements confirm our results and also reveal low-intensity inner zone and slot region electrons that are not captured in the standard background corrected data product. Finally, we briefly discuss efforts to refine the upper limit of inner zone MeV electron flux obtained in earlier work. Claudepierre, S.; O\textquoterightBrien, T.; Fennell, J.; Blake, J.; Clemmons, J.; Looper, M.; Mazur, J.; Roeder, J.; Turner, D.; Reeves, G.; Spence, H.; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2017 YEAR: 2017 DOI: 10.1002/2016JA023719 Inner zone; particle detectors; Radiation belt; relativistic electrons; Slot region; Space weather; Van Allen Probes |
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The hidden dynamics of relativistic electrons (0.7-1.5~MeV) in the inner zone and slot region We present measurements of relativistic electrons (0.7\textendash1.5 MeV) in the inner zone and slot region obtained by the Magnetic Electron and Ion Spectrometer (MagEIS) instrument on Van Allen Probes. The data presented are corrected for background contamination, which is primarily due to inner-belt protons in these low-L regions. We find that \~1 MeV electrons were transported into the inner zone following the two largest geomagnetic storms of the Van Allen Probes era to date, the March and June 2015 events. As \~1 MeV electrons were not observed in Van Allen Probes data in the inner zone prior to these two events, the injections created a new inner belt that persisted for at least 1.5 years. In contrast, we find that electrons injected into the slot region decay on much faster timescales, approximately tens of days. Furthermore, we find no evidence of >1.5 MeV electrons in the inner zone during the entire time interval considered (April 2013 through September 2016). The energies we examine thus span a transition range in the steeply falling inner zone electron spectrum, where modest intensities are observed at 0.7 MeV, and no electrons are observed at 1.5 MeV. To validate the results obtained from the background corrected flux measurements, we also present detailed pulse-height spectra from individual MagEIS detectors. These measurements confirm our results and also reveal low-intensity inner zone and slot region electrons that are not captured in the standard background corrected data product. Finally, we briefly discuss efforts to refine the upper limit of inner zone MeV electron flux obtained in earlier work. Claudepierre, S.; O\textquoterightBrien, T.; Fennell, J.; Blake, J.; Clemmons, J.; Looper, M.; Mazur, J.; Roeder, J.; Turner, D.; Reeves, G.; Spence, H.; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2017 YEAR: 2017 DOI: 10.1002/2016JA023719 Inner zone; particle detectors; Radiation belt; relativistic electrons; Slot region; Space weather; Van Allen Probes |
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Oxygen cyclotron harmonic waves observed by the Van Allen Probes Fine structured multiple-harmonic electromagnetic emissions at frequencies around the equatorial oxygen cyclotron harmonics are observed by Van Allen Probe A outside the core plasmasphere (L~5) off the magnetic equator (MLAT~-7.5\textdegree) during a magnetic storm. We find that the multiple-harmonic emissions have their PSD peaks at 2~8 equatorial oxygen gyro-harmonics (f~nfO+, n=2~8) while the fundamental mode (n=1) is absent, implying that the harmonic waves are generated near the equator and propagate into the observation region. Additionally these electromagnetic emissions are linear polarized. Different from the equatorial noise emission propagating very obliquely, these emissions have moderate wave normal angles (about 40\textdegree~60\textdegree) which predominately become larger as the harmonic number increases. Considering their frequency and wave normal angle characteristics, it is suggested that these multiple-harmonic emissions might play an important role in the dynamic variation of radiation belt electrons. Xiongdong, Yu; Zhigang, Yuan; Dedong, Wang; Shiyong, Huang; Haimeng, Li; Tao, Yu; Zheng, Qiao; Published by: Science China: Earth Sciences Published on: 03/2017 YEAR: 2017 DOI: 10.1007/s11430-016-9024-3 Oxygen Cyclotron Harmonic Waves; Radiation belt; Ring current ions; Van Allen Probes |
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Energetic (hundreds of keV) electrons in the radiation belt slot region have been found to exhibit the butterfly pitch angle distributions. Resonant interactions with magnetosonic and whistler-mode waves are two potential mechanisms for the formation of these peculiar distributions. Here we perform a statistical study of energetic electron pitch angle distribution characteristics measured by Van Allen Probes in the slot region during a three-year period from May 2013 to May 2016. Our results show that electron butterfly distributions are closely related to magnetosonic waves rather than to whistler-mode waves. Both electron butterfly distributions and magnetosonic waves occur more frequently at the geomagnetically active times than at the quiet times. In a statistical sense, more distinct butterfly distributions usually correspond to magnetosonic waves with larger amplitudes and vice versa. The averaged magnetosonic wave amplitude is less than 5 pT in the case of normal and flat-top distributions with a butterfly index BI = 1 but reaches \~ 35\textendash95 pT in the case of distinct butterfly distributions with BI > 1.3. For magnetosonic waves with amplitudes >50 pT, the occurrence rate of butterfly distribution is above 80\%. Our study suggests that energetic electron butterfly distributions in the slot region are primarily caused by magnetosonic waves. Yang, Chang; Su, Zhenpeng; Xiao, Fuliang; Zheng, Huinan; Wang, Yuming; Wang, Shui; Spence, H.; Reeves, G.; Baker, D.; Blake, J.; Funsten, H.; Published by: Geophysical Research Letters Published on: 03/2017 YEAR: 2017 DOI: 10.1002/2017GL073116 butterfly distributions; Electron acceleration; Landau resonance; magnetosonic wave; Radiation belt; Van Allen Probes; Wave-particle interaction |
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The most significant unknown regarding relativistic electrons in Earth\textquoterights 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\textendash14 January 2013 enhancement event to isolate transport, loss, and source dynamics in a one-dimensional radial diffusion model. We then validate the results by comparing them to Van Allen Probes and Time History of Events and Macroscale Interactions during Substorms observations, indicating that the three terms have been accurately and individually quantified for the event. Finally, a direct comparison is performed between the model containing event-specific terms and various models containing terms parameterized by geomagnetic index. Models using a simple 3/Kp loss time scale show deviation from the event-specific model of nearly 2 orders of magnitude within 72 h of the enhancement event. However, models using alternative loss time scales closely resemble the event-specific model. Schiller, Q.; Tu, W.; Ali, A.; Li, X.; Godinez, H.; Turner, D.; Morley, S.; Henderson, M.; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2017 YEAR: 2017 DOI: 10.1002/2016JA023093 CubeSat; data assimilation; electron; event specific; Modeling; Radiation belt; Van Allen Probes |
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In September 2014 an unusually long-lasting (≳10 days) ultra-relativistic electron flux depletion occurred in the outer radiation belt despite ongoing solar wind forcing. We simulate this period using a ULF wave radial diffusion model, driven by observed ULF wave power coupled to flux variations at the outer boundary at L* = 5, including empirical electron loss models due to chorus and hiss wave scattering. Our results show that unexplained rapid main phase loss, that depletes the belt within hours, is essential to explain the observations. Such ultra-relativistic electron extinction decouples the prestorm and poststorm fluxes, revealing the subsequent belt dynamics to be surprisingly independent of prestorm flux. However, once this extinction is included, ULF wave transport and coupling to the outer boundary explain the extended depletion event and also the eventual flux recovery. Neither local acceleration nor ongoing losses from hiss or chorus wave scattering to the atmosphere are required. Ozeke, Louis; Mann, Ian; Murphy, Kyle; Sibeck, David; Baker, Daniel; Published by: Geophysical Research Letters Published on: 03/2017 YEAR: 2017 DOI: 10.1002/2017GL072811 radial diffusion; Radiation belt; ULF waves; ultrarelativistic; Van Allen Probes; wave-particle interactions |
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Diffusive scattering of electrons by electron holes around injection fronts Van Allen Probes have detected nonlinear electrostatic spikes around injection fronts in the outer radiation belt. These spikes include electron holes (EH), double layers, and more complicated solitary waves. We show that EHs can efficiently scatter electrons due to their substantial transverse electric fields. Although the electron scattering driven by EHs is diffusive, it cannot be evaluated via the standard quasi-linear theory. We derive analytical formulas describing local electron scattering by a single EH and verify them via test particle simulations. We show that the most efficiently scattered are gyroresonant electrons (crossing EH on a time scale comparable to the local electron gyroperiod). We compute bounce-averaged diffusion coefficients and demonstrate their dependence on the EH spatial distribution (latitudinal extent and spatial filling factor) and individual EH parameters (amplitude of electrostatic potential, velocity, and spatial scales). We show that EHs can drive pitch angle scattering of math formula5 keV electrons at rates 10-2-10-4 s-1 and, hence, can contribute to electron losses and conjugated diffuse aurora brightenings. The momentum and pitch angle scattering rates can be comparable, so that EHs can also provide efficient electron heating. The scattering rates driven by EHs at L shells L \~ 5\textendash8 are comparable to those due to chorus waves and may exceed those due to electron cyclotron harmonics. Vasko, I; Agapitov, O.; Mozer, F.; Artemyev, A.; Krasnoselskikh, V.; Bonnell, J.; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2017 YEAR: 2017 DOI: 10.1002/2016JA023337 electron holes; electron losses; injection; Radiation belt; solitary waves; Van Allen Probes |
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Prompt recovery of MeV (millions of electron Volts) electron populations in the poststorm core of the outer terrestrial radiation belt involves local acceleration of a seed population of energetic electrons in interactions with VLF chorus waves. Electron interactions during the generation of VLF rising tones are strongly nonlinear, such that a fraction of the relativistic electrons at resonant energies are trapped by waves, leading to significant nonadiabatic energy exchange. Through detailed examination of VLF chorus and electron fluxes observed by Van Allen Probes, we investigate the efficiency of nonlinear processes for acceleration of electrons to MeV energies. We find through subpacket analysis of chorus waveforms that electrons with initial energy of hundreds of keV to 3 MeV can be accelerated by 50 keV\textendash200 keV in resonant interactions with a single VLF rising tone on a time scale of 10\textendash100 ms. Foster, J.; Erickson, P.; Omura, Y.; Baker, D.; Kletzing, C.; Claudepierre, S.; Published by: Journal of Geophysical Research: Space Physics Published on: 01/2017 YEAR: 2017 DOI: 10.1002/2016JA023429 nonlinear acceleration; Radiation belt; Van Allen Probes; VLF chorus; wave-particle interactions |
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An interesting form of \textquotedblleftzipper-like\textquotedblright magnetosonic waves consisting of two bands of interleaved periodic rising-tone spectra was newly observed by the Van Allen Probes, the Time History of Events and Macroscale Interactions during Substorms (THEMIS), and the Magnetospheric Multiscale (MMS) missions. The two discrete bands are distinct in frequency and intensity; however, they maintain the same periodicity which varies in space and time, suggesting that they possibly originate from one single source intrinsically. In one event, the zipper-like magnetosonic waves exhibit the same periodicity as a constant-frequency magnetosonic wave and an electrostatic emission, but the modulation comes from neither density fluctuations nor ULF waves. A statistical survey based on 3.5 years of multisatellite observations shows that zipper-like magnetosonic waves mainly occur on the dawnside to noonside, in a frequency range between 10 fcp and fLHR. The zipper-like magnetosonic waves may provide a new clue to nonlinear excitation or modulation process, while its cause still remains to be fully understood. Li, J.; Bortnik, J.; Li, W.; Ma, Q.; Thorne, R.; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Wygant, J.; Breneman, A.; Thaller, S.; Funsten, H.; Mitchell, D.; Manweiler, J.; Torbert, R.; Le Contel, O.; Ergun, R.; Lindqvist, P.-A.; Torkar, K.; Nakamura, R.; Andriopoulou, M.; Russell, C.; Published by: Journal of Geophysical Research: Space Physics Published on: 01/2017 YEAR: 2017 DOI: 10.1002/2016JA023536 magnetosonic wave; Radiation belt; rising-tone; Van Allen Probes; zipper-like |
| 2016 |
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Electron holes in the outer radiation belt: Characteristics and their role in electron energization Van Allen Probes have detected electron holes (EHs) around injection fronts in the outer radiation belt. Presumably generated near equator, EHs propagate to higher latitudes potentially resulting in energization of electrons trapped within EHs. This process has been recently shown to provide electrons with energies up to several tens of keV and requires EH propagation up to rather high latitudes. We have analyzed more than 100 EHs observed around a particular injection to determine their kinetic structure and potential energy sources supporting the energization of trapped electrons. EHs propagate with velocities from 1000 to 20,000 km/s (a few times larger than the thermal velocity of the coldest background electron population). The parallel scale of observed EHs is from 0.3 to 3 km that is of the order of hundred Debye lengths. The perpendicular to parallel scale ratio is larger than one in a qualitative agreement with the theoretical scaling relation. The amplitudes of EH electrostatic potentials are generally below 100 V. We determine the properties of the electron population trapped within EHs by making use of the Bernstein-Green-Kruskal analysis and via analysis of EH magnetic field signatures. The density of the trapped electron population is on average 20\% of the background electron density. The perpendicular temperature of the trapped population is on average 300 eV and is larger for faster EHs. We show that energy losses of untrapped electrons scattered by EHs in the inhomogeneous background magnetic field may balance the energization of trapped electrons. Vasko, I; Agapitov, O.; Mozer, F.; Artemyev, A.; Drake, J.; Kuzichev, I.; Published by: Journal of Geophysical Research: Space Physics Published on: 12/2016 YEAR: 2016 DOI: 10.1002/2016JA023083 Electron acceleration; electron holes; injection; Radiation belt; solitary waves; Van Allen Probes |
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Prompt recovery of MeV electron populations in the post-storm core of the outer terrestrial radiation belt involves local acceleration of a seed population of energetic electrons in interactions with VLF chorus waves. Electron interactions during the generation of VLF rising tones are strongly non-linear, such that a fraction of the relativistic electrons at resonant energies are trapped by waves, leading to significant non-adiabatic energy exchange. Through detailed examination of VLF chorus and electron fluxes observed by Van Allen Probes, we investigate the efficiency of non-linear processes for acceleration of electrons to MeV energies. We find through subpacket analysis of chorus waveforms that electrons with initial energy 100s keV - 3 MeV can be accelerated by 50 keV - 200 keV in resonant interactions with a single VLF rising tone on a time scale of 10-100 msec. Foster, J.; Erickson, P.; Omura, Y.; Baker, D.; Kletzing, C.; Claudepierre, S.; Published by: Journal of Geophysical Research: Space Physics Published on: 12/2016 YEAR: 2016 DOI: 10.1002/2016JA023429 nonlinear acceleration; Radiation belt; Van Allen Probes; VLF chorus; wave particle interactions |
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The loss of protons in the outer part of the inner radiation belt (L = 2 to 3) during the 17 March 2015 geomagnetic storm was investigated using test particle simulations that follow full Lorentz trajectories with both magnetic and electric fields calculated from an empirical model. The simulation results presented here are compared with proton pitch angle measurements from the Van Allen Probe satellites Relativistic Electron Proton Telescope (REPT) instrument before and after the coronal mass ejection-shock-driven storm of 17\textendash18 March 2015, with minimum Dst =- 223 nT, the strongest storm of Solar Cycle 24, for four different energy ranges with 30, 38, 50, and 66 MeV mean energies. Two simulations have been run, one with an inductive electric field and one without. All four energy channels show good agreement with the Van Allen Probes REPT measurements for low L (L < 2.4) in both simulations but diverge for higher L values. The inclusion of the inductive electric field, calculated from the time-changing magnetic field, significantly improves the agreement between simulation and REPT measurements at L > 2.4. A previous study using the Highly Elliptical Orbiter 3 spacecraft also showed improved agreement when including the inductive electric field but was unable to compare effects on the pitch angle distributions. Engel, M.; Kress, B.; Hudson, M.; Selesnick, R.; Published by: Journal of Geophysical Research: Space Physics Published on: 11/2016 YEAR: 2016 DOI: 10.1002/2016JA023333 field line curvature scattering; inductive electric field; proton loss; Radiation belt; Van Allen Probes |
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We present observations of higher-frequency (~50\textendash2500 Hz, ~0.1\textendash0.7 fce) wave modes modulated at the frequency of colocated lower frequency (0.5\textendash2 Hz, on the order of fci) waves. These observations come from the Van Allen Probes Electric Field and Waves instrument\textquoterights burst mode data and represent the first observations of coupling between waves in these frequency ranges. The higher-frequency wave modes, typically whistler mode hiss and chorus or magnetosonic waves, last for a few to a few tens of seconds but are in some cases observed repeatedly over several hours. The higher-frequency waves are observed to be unmodulated before and after the presence of the electromagnetic ion cyclotron (EMIC) waves, but when the EMIC waves are present, the amplitude of the higher-frequency waves drops to the instrument noise level once every EMIC wave cycle. Such modulation could significantly impact wave-particle interactions such as acceleration and pitch angle scattering, which are crucial in the formation and depletion of the radiation belts. We present one case study with broadband, high-frequency waves observed to be modulated by EMIC waves repeatedly over a 2 h time span on both spacecraft. Finally, we show two additional case studies where other high-frequency wave modes exhibit similar modulation. Colpitts, C.; Cattell, C.; Engebretson, M.; Broughton, M.; Tian, S.; Wygant, J.; Breneman, A.; Thaller, S.; Published by: Geophysical Research Letters Published on: 11/2016 YEAR: 2016 DOI: 10.1002/2016GL071566 EMIC; Modulation; precipitation; Radiation belt; Van Allen Probes; wave; whistler |
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The 17\textendash18 March 2015 storm is the largest geomagnetic storm in the Van Allen Probes era to date. The Lyon-Fedder-Mobarry global MHD model has been run for this event using ARTEMIS data as solar wind input. The ULF wave power spectral density of the azimuthal electric field and compressional magnetic field is analyzed in the 0.5\textendash8.3 mHz range. The lowest three azimuthal modes account for 70\% of the total power during quiet times. However, during high activity, they are not exclusively dominant. The calculation of the radial diffusion coefficient is presented. We conclude that the electric field radial diffusion coefficient is dominant over the magnetic field coefficient by one to two orders of magnitude. This result contrasts with the dominant magnetic field diffusion coefficient used in most 3-D diffusion models. Li, Zhao; Hudson, Mary; Paral, Jan; Wiltberger, Michael; Turner, Drew; Published by: Journal of Geophysical Research: Space Physics Published on: 07/2016 YEAR: 2016 DOI: 10.1002/2016JA022508 March 2015; radial diffusion; radial diffusion coefficient; Radiation belt; ULF waves; Van Allen Probes |
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Inner zone and slot electron radial diffusion revisited Using recent data from NASA\textquoterights Van Allen Probes, we estimate the quiet time radial diffusion coefficients for electrons in the inner radiation belt (L < 3) with energies from ~50 to 750 keV. The observations are consistent with dynamics dominated by pitch angle scattering and radial diffusion. We use a coordinate system in which these two modes of diffusion are separable. Then we integrate phase space density over pitch angle to obtain a \textquotedblleftbundle content\textquotedblright that is invariant to pitch angle scattering, except for atmospheric loss. We estimate the effective radial diffusion coefficient from the temporal and radial variation of the bundle content. We show that our diffusion coefficients agree well with previously determined values obtained in the 1960s and 1970s and follow the form one expects for radial diffusion caused by exponentially decaying impulses in the large-scale electrostatic potential. O\textquoterightBrien, T.; Claudepierre, S.; Guild, T.; Fennell, J.; Turner, D.; Blake, J.; Clemmons, J.; Roeder, J.; Published by: Geophysical Research Letters Published on: 07/2016 YEAR: 2016 DOI: 10.1002/2016GL069749 Inner zone; radial diffusion; Radiation belt; Van Allen Probes |
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Statistical Properties of the Radiation Belt Seed Population We present a statistical analysis of phase space density data from the first 26 months of the Van Allen Probes mission. In particular we investigate the relationship between the 10s-100s keV seed electrons and >1 MeV core radiation belt electron population. Using a cross correlation analysis, we find that the seed and core populations are well correlated with a coefficient of ≈ 0.73 with a time lag of 10-15 hours. We present evidence of a seed population threshold that is necessary for subsequent acceleration. The depth of penetration of the seed population determines the inner boundary of the acceleration process. However, we show that an enhanced seed population alone is not enough to produce acceleration in the higher energies, implying that the seed population of 100s of keV electrons is only one of several conditions required for MeV electron radiation belt acceleration. Boyd, A.J.; Spence, H.E.; Huang, C.-L.; Reeves, G.; Baker, D.; Turner, D.L.; Claudepierre, S.; Fennell, J.; Blake, J.; Shprits, Y.Y.; Published by: Journal of Geophysical Research: Space Physics Published on: 07/2016 YEAR: 2016 DOI: 10.1002/2016JA022652 Phase space density; Radiation belt; seed population; Van Allen Probes |
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Here we examine the speed, strength, and depth of the coupling between dynamical variations of ultrarelativistic electron flux at the outer boundary and that in the heart of the outer radiation belt. Using ULF wave radial diffusion as an exemplar, we show how changing boundary conditions can completely change belt morphology even under conditions of identical wave power. In the case of ULF wave radial diffusion, the temporal dynamics of a new source population or a sink of electron flux at the outer plasma sheet boundary can generate a completely opposite response which reaches deep into the belt under identical ULF wave conditions. Very significantly, here we show that such coupling can occur on timescales much faster than previously thought. We show that even on timescales ~1 h, changes in the outer boundary electron population can dramatically alter the radiation belt flux in the heart of the belt. Importantly, these flux changes can at times occur on timescales much faster than the L shell revisit time obtained from elliptically orbiting satellites such as the Van Allen Probes. We underline the importance of such boundary condition effects when seeking to identify the physical processes which explain the dominant behavior of the Van Allen belts. Overall, we argue in general that the importance of temporal changes in the boundary conditions is sometimes overlooked in comparison to the pursuit of (ever) increasingly accurate estimates of wave power and other wave properties used in empirical representations of wave transport and diffusion rates. Published by: Journal of Geophysical Research: Space Physics Published on: 06/2016 YEAR: 2016 DOI: 10.1002/jgra.v121.610.1002/2016JA022647 |
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Structure and Evolution of Electron "Zebra Stripes" in the Inner Radiation Belt Zebra stripes\textquotedblright are newly found energetic electron energy-spatial (L shell) distributed structure with an energy between tens to a few hundreds keV in the inner radiation belt. Using high-quality measurements of electron fluxes from Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) on board the twin Van Allen Probes, we carry out case and statistical studies from April 2013 to April 2014 to study the structural and evolutionary characteristics of zebra stripes below L = 3. It is revealed that the zebra stripes can be transformed into evenly spaced patterns in the electron drift frequency coordinate: the detrended logarithmic fluxes in each L shell region can be well described by sinusoidal functions of drift frequency. The \textquotedblleftwave number\textquotedblright of this sinusoidal function, which corresponds to the reciprocal of the gap between two adjacent peaks in the drift frequency coordinate, increases in proportion to real time. Further, these structural and evolutionary characteristics of zebra stripes can be reproduced by an analytic model of the evolution of the particle distribution under a single monochromatic or static azimuthal electric field. It is shown that the essential ingredient for the formation of multiple zebra stripes is the periodic drift of particles. The amplitude of the zebra stripes shows a good positive correlation with Kp index, which indicates that the generation mechanism of zebra stripes should be related to geomagnetic activities Liu, Y.; Zong, Q.-G.; Zhou, X.-Z.; Foster, J.; Rankin, R.; Published by: Journal of Geophysical Research: Space Physics Published on: 05/2016 YEAR: 2016 DOI: 10.1002/2015JA022077 electric field; energetic electrons; particle dynamic; Radiation belt; Van Allen Probes; zebra stripes |
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Things we don\textquoterightt yet understand about solar driving of the radiation belts. This commentary explores how close we are to predicting the behavior of the radiations belts - the primary science objective of NASA\textquoterights Van Allen Probes mission. Starting with what we know or think we know about competing sources, enhancement, transport, and loss, I walk through recent papers that have improved our understanding and then focus on flux dropouts as one particular yardstick of success. I mention a new paradigm for electrons and the importance of reliably matching models and observations for different solar inputs. Although the case for prediction remains a work in progress, there are encouraging signs of progress. Published by: Journal of Geophysical Research: Space Physics Published on: 05/2016 YEAR: 2016 DOI: 10.1002/2016JA022472 |
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Formation of Energetic Electron Butterfly Distributions by Magnetosonic Waves via Landau Resonance Radiation belt electrons can exhibit different types of pitch angle distributions in response to various magnetospheric processes. Butterfly distributions, characterized by flux minima at pitch angles around 90\textdegree, are broadly observed in both the outer and inner belts and the slot region. Butterfly distributions close to the outer magnetospheric boundary have been attributed to drift shell splitting and losses to the magnetopause. However, their occurrence in the inner belt and the slot region has hitherto not been resolved. By analyzing the particle and wave data collected by the Van Allen Probes during a geomagnetic storm, we combine test particle calculations and Fokker-Planck simulations to reveal that scattering by equatorial magnetosonic waves is a significant cause for the formation of energetic electron butterfly distributions in the inner magnetosphere. Another event shows that a large-amplitude magnetosonic wave in the outer belt can create electron butterfly distributions in just a few minutes. Li, Jinxing; Ni, Binbin; Ma, Qianli; Xie, Lun; Pu, Zuyin; Fu, Suiyan; Thorne, R.; Bortnik, J.; Chen, Lunjin; Li, Wen; Baker, Daniel; Kletzing, Craig; Kurth, William; Hospodarsky, George; Fennell, Joseph; Reeves, Geoffrey; Spence, Harlan; Funsten, Herbert; Summers, Danny; Published by: Geophysical Research Letters Published on: 04/2016 YEAR: 2016 DOI: 10.1002/2016GL067853 butterfly distributions; energetic electrons; Landau resonance; magnetosonic waves; Radiation belt; Van Allen Probes |
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The Van Allen Probe observations during the recovery phase of a large storm that occurred on 17 March 2015 showed that the ultrarelativistic electrons at the inner boundary of the outer radiation belt (L* = 2.6\textendash3.7) exhibited butterfly pitch angle distributions, while the inner belt and the slot region also showed evidence of sub-MeV electron butterfly distributions. Strong magnetosonic waves were observed in the same regions and at the same time periods as these butterfly distributions. Moreover, when these magnetosonic waves extended to higher altitudes (L* = 4.1), the butterfly distributions also extended to the same region. Combining test particle calculations and Fokker-Planck diffusion simulations, we successfully reproduced the formation of the ultrarelativistic electron butterfly distributions, which primarily result from parallel acceleration caused by Landau resonance with magnetosonic waves. The coexistence of ultrarelativistic electron butterfly distributions with magnetosonic waves was also observed in the 24 June 2015 storm, providing further support that the magnetosonic waves play a key role in forming butterfly distributions. Li, Jinxing; Bortnik, Jacob; Thorne, Richard; Li, Wen; Ma, Qianli; Baker, Daniel; Reeves, Geoffrey; Fennell, Joseph; Spence, Harlan; Kletzing, Craig; Kurth, William; Hospodarsky, George; Angelopoulos, Vassilis; Blake, Bernard.; Published by: Journal of Geophysical Research: Space Physics Published on: 04/2016 YEAR: 2016 DOI: 10.1002/2016JA022370 butterfly distributions; Landau resonance; magnetosonic waves; Radiation belt; Van Allen Probes |
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Ultralow frequency (ULF) electromagnetic waves in Earth\textquoterights magnetosphere can accelerate charged particles via a process called drift resonance. In the conventional drift-resonance theory, a default assumption is that the wave growth rate is time-independent, positive, and extremely small. However, this is not the case for ULF waves in the real magnetosphere. The ULF waves must have experienced an earlier growth stage when their energy was taken from external and/or internal sources, and as time proceeds the waves have to be damped with a negative growth rate. Therefore, a more generalized theory on particle behavior during different stages of ULF wave evolution is required. In this paper, we introduce a time-dependent imaginary wave frequency to accommodate the growth and damping of the waves in the drift-resonance theory, so that the wave-particle interactions during the entire wave lifespan can be studied. We then predict from the generalized theory particle signatures during different stages of the wave evolution, which are consistent with observations from Van Allen Probes. The more generalized theory, therefore, provides new insights into ULF wave evolution and wave-particle interactions in the magnetosphere. Zhou, Xu-Zhi; Wang, Zi-Han; Zong, Qiu-Gang; Rankin, Robert; Kivelson, Margaret; Chen, Xing-Ran; Blake, Bernard; Wygant, John; Kletzing, Craig; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2016 YEAR: 2016 DOI: 10.1002/2016JA022447 drift resonance; Radiation belt; ULF waves; Van Allen Probes; wave growth and damping; Wave-particle interaction |
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Inward diffusion and loss of radiation belt protons 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, shows that the observed average rate of increase near L = 2 is predicted by the same model diffusion coefficient that is required to form the entire proton radiation belt, down to low L, over an extended (\~103 year) interval. A slower intensity decrease for lower energies near L = 1.5 may also be caused by inward diffusion, though it is faster than predicted by the model. Higher-energy (≳40 MeV) protons near the L = 1.5 intensity maximum are from cosmic ray albedo neutron decay. Their observed intensity is lower than expected by a factor \~2, but the discrepancy is resolved by adding an unspecified loss process to the model with a mean lifetime \~120 years. Selesnick, R.; Baker, D.; Jaynes, A.; Li, X.; Kanekal, S.; Hudson, M.; Kress, B.; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2016 YEAR: 2016 DOI: 10.1002/2015JA022154 |
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Frequency distribution is a vital factor in determining the contribution of whistler-mode chorus to radiation belt electron dynamics. Chorus is usually considered to occur in the frequency range 0.1\textendash0.8 inline image (with the equatorial electron gyrofrequency inline image). We here report an event of intense low-frequency chorus with nearly half of wave power distributed below 0.1 inline image observed by Van Allen Probe A on 27 August 2014. This emission propagated quasi-parallel to the magnetic field and exhibited hiss-like signatures most of the time. The low-frequency chorus can produce the rapid loss of low-energy (\~0.1 MeV) electrons, different from the normal chorus. For high-energy (>=0.5 MeV) electrons, the low-frequency chorus can yield comparable momentum diffusion to that of the normal chorus, but much stronger (up to 2 orders of magnitude) pitch-angle diffusion near the loss cone. Gao, Zhonglei; Su, Zhenpeng; Zhu, Hui; Xiao, Fuliang; Zheng, Huinan; Wang, Yuming; Shen, Chao; Wang, Shui; Published by: Geophysical Research Letters Published on: 02/2016 YEAR: 2016 DOI: 10.1002/2016GL067687 Cyclotron resonance; Hiss-like band; Low-frequency chorus; Radiation belt; Van Allen Probes; Rising tones; Van Allen Probes |
| 2015 |
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Measurement of inner radiation belt electrons with kinetic energy above 1~MeV Data from the Proton-Electron Telescope on the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) satellite, taken during 1992\textendash2009, are analyzed for evidence of inner radiation belt electrons with kinetic energy E > 1 MeV. It is found that most of the data from a detector combination with a nominal energy threshold of 1 MeV were, in fact, caused by a chance coincidence response to lower energy electrons or high-energy protons. In particular, there was no detection of inner belt or slot region electrons above 1 MeV following the 2003 Halloween storm injection, though they may have been present. However, by restricting data to a less-stable, low-altitude trapping region, a persistent presence of inner belt electrons in the energy range 1 to 1.6 MeV is demonstrated. Their soft, exponential energy spectra are consistent with extrapolation of lower energy measurements. Published by: Journal of Geophysical Research: Space Physics Published on: 10/2015 YEAR: 2015 DOI: 10.1002/2015JA021387 |
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Imprints of impulse-excited hydromagnetic waves on electrons in the Van Allen radiation belts Ultralow frequency electromagnetic oscillations, interpreted as standing hydromagnetic waves in the magnetosphere, are a major energy source that accelerates electrons to relativistic energies in the Van Allen radiation belt. Electrons can rapidly gain energy from the waves when they resonate via a process called drift resonance, which is observationally characterized by energy-dependent phase differences between electron flux and electromagnetic oscillations. Such dependence has been recently observed and interpreted as spacecraft identifications of drift resonance electron acceleration. Here we show that in the initial wave cycles, the observed phase relationship differs from that characteristic of well-developed drift resonance. We further examine the differences and find that they are imprints of impulse-excited, coupled fast-Alfv\ en waves before they transform into more typical standing waves. Our identification of such imprints provides a new understanding of how energy couples in the inner magnetosphere, and a new diagnostic for the generation and growth of magnetospheric hydromagnetic pulsations. Zhou, Xu-Zhi; Wang, Zi-Han; Zong, Qiu-Gang; Claudepierre, Seth; Mann, Ian; Kivelson, Margaret; Angelopoulos, Vassilis; Hao, Yi-Xin; Wang, Yong-Fu; Pu, Zu-Yin; Published by: Geophysical Research Letters Published on: 08/2015 YEAR: 2015 DOI: 10.1002/grl.v42.1510.1002/2015GL064988 drift resonance; Radiation belt; ULF waves; Van Allen Probes; wave growth; Wave-particle interaction |
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Nonlinear Bounce Resonances between Magnetosonic Waves and Equatorially Mirroring Electrons Equatorially mirroring energetic electrons pose an interesting scientific problem, since they generally cannot resonate with any known plasma waves and hence cannot be scattered down to lower pitch angles. Observationally it is well known that the fluxof these equatorial particles does not simply continue to build up indefinitely, and so a mechanism must necessarily exist that transports these particles from a equatorial pitch angle of 90 degrees down to lower values. However this mechanism has not been uniquely identified yet. Here, we investigate the mechanism of bounce resonance with equatorial noise (or fast magnetosonic waves). A test particle simulation is used to examine the effects of monochromatic magnetosonic waves on the equatorially mirroring energetic electrons, with a special interest in characterizing the effectiveness of bounce resonances. Our analysis shows that bounce resonances can occur at the first three harmonics of the bounce frequency (nωb, n = 1 , 2, and 3 ) and can effectively reduce the equatorial pitch angle to values where resonant scattering by whistler-mode waves becomes possible. We demonstrate that the nature of bounce resonance is nonlinear and we propose a nonlinear oscillation model for characterizing bounce resonances using two key parameters, effective wave amplitude \~A and normalized wave number inline image. The threshold for higher harmonic resonance is more strict, favoring higher \~A and inline image and the change in equatorial pitch angle is strongly controlled by inline image. We also investigate the dependence of bounce resonance effects on various physical parameters, including wave amplitude, frequency, wave normal angle and initial phase, plasmadensity, and electron energy. It is found that the effect of bounce resonance is sensitive to the wave normal angle. We suggest that the bounce resonant interaction might lead to an observed pitch angle distribution with a minimum at 90o. Chen, Lunjin; Maldonado, Armando; Bortnik, Jacob; Thorne, Richard; Li, Jinxing; Dai, Lei; Zhan, Xiaoya; Published by: Journal of Geophysical Research: Space Physics Published on: 06/2015 YEAR: 2015 DOI: 10.1002/2015JA021174 bounce resonance; equatorioal noise; magnetosonic waves; nonlinear; Radiation belt; wave particle interaction |
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3D test-particle simulation of the 17-18 March, 2013 CME-shock driven storm D test-particle simulation of energetic electrons (hundreds of keV to MeV), including both an initially trapped population and continuously injected population, driven by the Lyon-Fedder-Mobarry (LFM) global MHD model coupled with Magnetosphere-Ionosphere Coupler/Solver (MIX) boundary conditions, is performed for the March 17, 2013 storm. The electron trajectories are calculated and weighted using the ESA model for electron flux vs. energy and L. The simulation captures the flux dropout at both GOES-13 and GOES-15 locations after a strong CME-shock arrival which produced a Dst=-132 nT storm, and recovery to the pre-storm value later, consistent with GOES satellite measurements. This study provides the first 3D test-particle simulation combining the trapped and injected populations. The result demonstrates that including both populations in the simulation is essential to study the dynamics of the outer radiation belt over the typical day-long timescale of ring current development, main phase and early recovery phase. Li, Zhao; Hudson, Mary; Kress, Brian; Paral, Jan; Published by: Geophysical Research Letters Published on: 06/2015 YEAR: 2015 DOI: 10.1002/2015GL064627 17 March; 2013; CME-shock driven storm; Radiation belt; test-particle simulation |
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A background correction algorithm for Van Allen Probes MagEIS electron flux measurements We describe an automated computer algorithm designed to remove background contamination from the Van Allen Probes MagEIS electron flux measurements. We provide a detailed description of the algorithm with illustrative examples from on-orbit data. We find two primary sources of background contamination in the MagEIS electron data: inner zone protons and bremsstrahlung X-rays generated by energetic electrons interacting with the spacecraft material. Bremsstrahlung X-rays primarily produce contamination in the lower energy MagEIS electron channels (~30-500 keV) and in regions of geospace where multi-MeV electrons are present. Inner zone protons produce contamination in all MagEIS energy channels at roughly L < 2.5. The background corrected MagEIS electron data produce a more accurate measurement of the electron radiation belts, as most earlier measurements suffer from unquantifiable and uncorrectable contamination in this harsh region of the near-Earth space environment. These background-corrected data will also be useful for spacecraft engineering purposes, providing ground truth for the near-Earth electron environment and informing the next generation of spacecraft design models (e.g., AE9). Claudepierre, S.; O\textquoterightBrien, T.; Blake, J.; Fennell, J.; Roeder, J.; Clemmons, J.; Looper, M.; Mazur, J.; Mulligan, T.; Spence, H.; Reeves, G.; Friedel, R.; Henderson, M.; Larsen, B.; Published by: Journal of Geophysical Research: Space Physics Published on: 06/2015 YEAR: 2015 DOI: 10.1002/2015JA021171 Background contamination; Inner radiation belt; outer radiation belt; Particle measurements; Radiation belt; Spacecraft engineering; Van Allen Probes |
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We investigate an electron flux dropout during a weak storm on 7\textendash8 November 2008, with Dst minimum value being -37 nT. During this period, two clear dropouts were observed on GOES 11 > 2 MeV electrons. We also find a simultaneous dropout in the subrelativistic electrons recorded by Time History of Events and Macroscale Interactions during Substorms probes in the outer radiation belt. Using the Radiation Belt Environment model, we try to reproduce the observed dropout features in both relativistic and subrelativistic electrons. We found that there are local time dependences in the dropout for both observation and simulation in subrelativistic electrons: (1) particle loss begins from nightside and propagates into dayside and (2) resupply starts from near dawn magnetic local time and propagates into the dayside following electron drift direction. That resupply of the particles might be caused by substorm injections due to enhanced convection. We found a significant precipitation in hundreds keV electrons during the dropout. We observe electromagnetic ion cyclotron and chorus waves both on the ground and in space. We find the drift shells are opened near the beginning of the first dropout. The dropout in MeV electrons at GEO might therefore be initiated due to the magnetopause shadowing, and the followed dropout in hundreds keV electrons might be the result of the combination of magnetopause shadowing and precipitation loss into the Earth\textquoterights atmosphere. Hwang, J.; Choi, E.-J.; Park, J.-S.; Fok, M.-C.; Lee, D.-Y.; Kim, K.-C.; Shin, D.-K.; Usanova, M.; Reeves, G.; Published by: Journal of Geophysical Research: Space Physics Published on: 05/2015 YEAR: 2015 DOI: 10.1002/2015JA021085 atmospheric precipitation; flux dropout; Geomagnetic storm; magneopause shadowing; Radiation belt; RBE model |
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Whistler mode chorus waves are considered to play a central role in accelerating and scattering electrons in the outer radiation belt. While in situ measurements are usually limited to the trajectories of a small number of satellites, rigorous theoretical modeling requires a global distribution of chorus wave characteristics. In the present work, by using a large database of chorus wave observations made on the Time History of Events and Macroscale Interactions during Substorms satellites for about 5 years, we develop prediction models for a global distribution of chorus amplitudes. The development is based on two main components: (a) the temporal dependence of average chorus amplitudes determined by correlating with the preceding solar wind and geomagnetic conditions as represented by the interplanetary magnetic field (IMF) Bz and AE index and (b) the determination of spatial distribution pattern of chorus amplitudes, specifically, the profiles in L in all 2 h magnetic local time zones, which are categorized by activity levels of either the IMF Bz or AE index. Two separate models are developed: one based only on the IMF Bz and the other based only on AE. Both models predict chorus amplitudes for two different latitudinal zones separately: |magnetic latitude (MLAT)| < 10\textdegree, and |MLAT| = 10\textdegree\textendash25\textdegree. The model performance is measured by the coefficient of determination R2 and the rank-order correlation coefficient (ROCC) between the observations and model prediction results. When tested for a new data interval of ~1.5 years, the AE-based model works slightly better than the IMF Bz-based model: for the AE-based model, the mean R2 and ROCC values are ~0.46 and ~0.78 for |MLAT| < 10\textdegree, respectively, and ~0.4 and ~0.74 for |MLAT| = 10\textdegree\textendash25\textdegree, respectively; for the IMF Bz-based model, the mean R2 and ROCC values are ~0.39 and ~0.74 for |MLAT| < 10\textdegree, respectively, and ~0.33 and ~0.70 for |MLAT| = 10\textdegree\textendash25\textdegree, respectively. We provide all of the model information in the text and supporting information so that the developed chorus models can be used for the existing outer radiation belt electron models. Kim, Jin-Hee; Lee, Dae-Young; Cho, Jung-Hee; Shin, Dae-Kyu; Kim, Kyung-Chan; Li, Wen; Kim, Thomas; Published by: Journal of Geophysical Research: Space Physics Published on: 04/2015 YEAR: 2015 DOI: 10.1002/2014JA020900 |
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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. Zhao, Lei; Yu, Yiqun; Delzanno, Gian; Jordanova, Vania; Published by: Journal of Geophysical Research: Space Physics Published on: 04/2015 YEAR: 2015 DOI: 10.1002/2014JA020858 |
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Global Storm-Time Depletion of the Outer Electron Belt The outer radiation belt consists of relativistic (>0.5 MeV) electrons trapped on closed trajectories around Earth where the magnetic field is nearly dipolar. During increased geomagnetic activity, electron intensities in the belt can vary by ordersof magnitude at different spatial and temporal scale. The main phase of geomagnetic storms often produces deep depletions of electron intensities over broad regions of the outer belt. Previous studies identified three possible processes that can contribute to the main-phase depletions: adiabatic inflation of electron drift orbits caused by the ring current growth, electron loss into the atmosphere, and electron escape through the magnetopause boundary. In this paper we investigate the relative importance of the adiabatic effect and magnetopause loss to the rapid depletion of the outer belt observed at the Van Allen Probes spacecraft during the main phase of March 17, 2013 storm. The intensities of >1 MeV electrons were depleted by more than an order of magnitude over the entire radial extent of the belt in less than 6 hours after the sudden storm commencement. For the analysis we used three-dimensional test-particle simulations of global evolution of the outer belt in the Tsyganenko-Sitnov (TS07D) magnetic field model with an inductive electric field. Comparison of the simulation results with electron measurements from the MagEIS experiment shows that magnetopause loss accounts for most of the observed depletion at L>5, while at lower L shells the depletion is adiabatic. Both magnetopause loss and the adiabatic effect are controlled by the change in global configuration of the magnetic field due to storm-time development of the ring current; a simulation of electron evolution without a ring current produces a much weaker depletion. Ukhorskiy, A; Sitnov, M.; Millan, R.; Kress, B.; Fennell, J.; Claudepierre, S.; Barnes, R.; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2015 YEAR: 2015 DOI: 10.1002/2014JA020645 dropout; Geomagnetic storms; magnetopause loss; Radial Transport; Radiation belt; ring current; Van Allen Probes |
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Disappearance of plasmaspheric hiss following interplanetary shock Plasmaspheric hiss is one of the important plasma waves controlling radiation belt dynamics. Its spatiotemporal distribution and generation mechanism are presently the object of active research. We here give the first report on the shock-induced disappearance of plasmaspheric hiss observed by the Van Allen Probes on 8 October 2013. This special event exhibits the dramatic variability of plasmaspheric hiss and provides a good opportunity to test its generation mechanisms. The origination of plasmaspheric hiss from plasmatrough chorus is suggested to be an appropriate prerequisite to explain this event. The shock increased the suprathermal electron fluxes, and then the enhanced Landau damping promptly prevented chorus waves from entering the plasmasphere. Subsequently, the shrinking magnetopause removed the source electrons for chorus, contributing significantly to the several-hours-long disappearance of plasmaspheric hiss. Su, Zhenpeng; Zhu, Hui; Xiao, Fuliang; Zheng, Huinan; Wang, Yuming; Shen, Chao; Zhang, Min; Wang, Shui; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Spence, H.; Reeves, G.; Funsten, H.; Blake, J.; Baker, D.; Wygant, J.; Published by: Geophysical Research Letters Published on: 03/2015 YEAR: 2015 DOI: 10.1002/2015GL063906 Cyclotron instability; Cyclotron resonance; interplanetary shock; Landau damping; Plasmaspheric Hiss; Radiation belt; Van Allen Probes |
| 2014 |
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Asymmetries in the magnetospheric magnetic field produce drift shell splitting, which causes the radial (drift shell) invariant to sometimes depend on pitch angle. Where drift shell splitting is significant, pitch angle scattering leads to diffusion in all three invariants of the particle\textquoterights motion, including cross diffusion. We examine the magnitude of drift shell splitting-related anomalous diffusion for outer zone electrons compared to conventional diffusion in the absence of drift shell splitting. We assume the primary local scattering process is wave-particle interactions with chorus. We find that anomalous radial diffusion can exceed that of conventional drift resonant radial diffusion for particles with energies near 0.1 MeV at all radial distances outside the plasmasphere during quiet to moderate geomagnetic activity and, it is significant at 0.5 MeV. Cross diffusion involving the radial invariant can exceed the geometric mean of the corresponding pure diffusion coefficients at 0.1 MeV, and that such cross diffusion is significant even at 0.5-1 MeV. At 1 MeV, cross diffusion is often significant. The highest radial distances and magnetic activity levels in our study do not always exhibit as much significant anomalous diffusion as moderate radial distances and activity levels. This can be explained by (a) stronger dependence of conventional diffusion on magnetic activity and radius, and (b) strongest drift shell splitting at moderate magnetic activity. Simulation codes that neglect the possibility for cross terms will likely systematically underperform, especially for 0.1-0.5 MeV electrons, for much of the outer zone for quiet to moderate levels of magnetic activity. Published by: Journal of Geophysical Research: Space Physics Published on: 12/2014 YEAR: 2014 DOI: 10.1002/2014JA020422 |
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We study the rapid outward extension of the electron radiation belt on a timescale of several hours during three events observed by RBSP and THEMIS satellites, and particularly quantify the contributions of substorm injections and chorus waves to the electron flux enhancement near the outer boundary of radiation belt. A comprehensive analysis including both observations and simulations is performed for the first event on 26 May 2013. The outer boundary of electron radiation belt moved from L = 5.5 to L > 6.07 over about 6 hours, with up to four orders of magnitude enhancement in the 30 keV-5 MeV electron fluxes at L = 6. The observations show that the substorm injection can cause 100\% and 20\% of the total subrelativistic (~0.1 MeV) and relativistic (2-5 MeV) electron flux enhancements within a few minutes. The data-driven simulation supports that the strong chorus waves can yield 60\%-80\% of the total energetic (0.2-5.0 MeV) electron flux enhancement within about 6 hours. Some simple analyses are further given for the other two events on 2 and 29 June 2013, in which the contributions of substorm injections and chorus waves are shown to be qualitatively comparable to those for the first event. These results clearly illustrate the respective importance of substorm injections and chorus waves for the evolution of radiation belt electrons at different energies on a relatively short timescale. Su, Zhenpeng; Zhu, Hui; Xiao, Fuliang; Zheng, Huinan; Wang, Yuming; Zong, Q.-G.; He, Zhaoguo; Shen, Chao; Zhang, Min; Wang, Shui; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Spence, H.; Reeves, G.; Funsten, H.; Blake, J.; Baker, D.; Published by: Journal of Geophysical Research: Space Physics Published on: 12/2014 YEAR: 2014 DOI: 10.1002/2014JA020709 Chorus wave; Electron acceleration; Radiation belt; substorm injection; Van Allen Probes; Wave-particle interaction |
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The distribution of trapped energetic electrons inside the Earth\textquoterights radiation belts is the focus of intense studies aiming at better describing the evolution of the space environment in the presence of various disturbances induced by the solar wind or by an enhanced lightning activity. Such studies are usually performed by means of comparisons with full numerical simulations solving the Fokker-Planck quasi-linear diffusion equation for the particle distribution function. Here, we present for the first time approximate but realistic analytical solutions for the electron distribution, which are shown to be in good agreement with exact numerical solutions in situations where resonant scattering of energetic electrons by whistler-mode hiss, lightning-generated or chorus waves, is the dominant process. Quiet-time distributions are well-recovered, as well as the evolution of energized relativistic electron distributions during disturbed geomagnetic conditions. It is further shown that careful comparisons between the analytical solutions and measured distributions may allow to infer important bounce and drift averaged wave characteristics (such as wave amplitude). It could also help to improve the global understanding of underlying physical phenomena. Mourenas, D.; Artemyev, A.; Agapitov, O.V.; Krasnoselskikh, V.; Li, W.; Published by: Journal of Geophysical Research: Space Physics Published on: 11/2014 YEAR: 2014 DOI: 10.1002/2014JA020443 electron distribution; pitch-angle distribution; Radiation belt |
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We present simulations of the outer electron radiation belt using a new ULF wave-driven radial diffusion model, including empirical representations of loss due to chorus and plasmaspheric hiss. With an outer boundary condition constrained by in situ electron flux observations, we focus on the impacts of magnetopause shadowing and outward radial diffusion in the heart of the radiation belt. Third invariant conserving solutions are combined to simulate the L shell and time dependence of the differential flux at a fixed energy. Results for the geomagnetically quiet year of 2008 demonstrate not only remarkable cross L shell impacts from magnetopause shadowing but also excellent agreement with the in situ observations even though no internal acceleration source is included in the model. Our model demonstrates powerful utility for capturing the cross-L impacts of magnetopause shadowing with significant prospects for improved space weather forecasting. The potential role of the plasmasphere in creating a third belt is also discussed. Ozeke, Louis; Mann, Ian; Turner, Drew; Murphy, Kyle; Degeling, Alex; Rae, Jonathan; Milling, David; Published by: Geophysical Research Letters Published on: 10/2014 YEAR: 2014 DOI: 10.1002/2014GL060787 magnetopause shadowing; Radiation belt; ULF wave radial diffusion |
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In the Earth\textquoterights radiation belts the flux of relativistic electrons is highly variable, sometimes changing by orders of magnitude within a few hours. Since energetic electrons can damage satellites it is important to understand the processes driving these changes and, ultimately, to develop forecasts of the energetic electron population. One approach is to use three-dimensional diffusion models, based on a Fokker-Planck equation. Here we describe a model where the phase-space density is set to zero at the outer L* boundary, simulating losses to the magnetopause, using recently published chorus diffusion coefficients for 1.5<=L*<=10. The value of the phase-space density on the minimum-energy boundary is determined from a recently published, solar wind-dependent, statistical model. Our simulations show that an outer radiation belt can be created by local acceleration of electrons from a very soft energy spectrum without the need for a source of electrons from inward radial transport. The location in L* of the peaks in flux for these steady state simulations is energy dependent and moves earthward with increasing energy. Comparisons between the model and data from the CRRES spacecraft are shown; flux dropouts are reproduced in the model by the increased outward radial diffusion that occurs during storms. Including the inward movement of the magnetopause in the model has little additional effect on the results. Finally, the location of the low-energy boundary is shown to be important for accurate modeling of observations. Glauert, Sarah; Horne, Richard; Meredith, Nigel; Published by: Journal of Geophysical Research: Space Physics Published on: 09/2014 YEAR: 2014 DOI: 10.1002/jgra.v119.910.1002/2014JA020092 |
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Three-dimensional stochastic modeling of radiation belts in adiabatic invariant coordinates A 3-D model for solving the radiation belt diffusion equation in adiabatic invariant coordinates has been developed and tested. The model, named Radbelt Electron Model, obtains a probabilistic solution by solving a set of It\^o stochastic differential equations that are mathematically equivalent to the diffusion equation. This method is capable of solving diffusion equations with a full 3-D diffusion tensor, including the radial-local cross diffusion components. The correct form of the boundary condition at equatorial pitch angle α0=90\textdegree is also derived. The model is applied to a simulation of the October 2002 storm event. At α0 near 90\textdegree, our results are quantitatively consistent with GPS observations of phase space density (PSD) increases, suggesting dominance of radial diffusion; at smaller α0, the observed PSD increases are overestimated by the model, possibly due to the α0-independent radial diffusion coefficients, or to insufficient electron loss in the model, or both. Statistical analysis of the stochastic processes provides further insights into the diffusion processes, showing distinctive electron source distributions with and without local acceleration. Zheng, Liheng; Chan, Anthony; Albert, Jay; Elkington, Scot; Koller, Josef; Horne, Richard; Glauert, Sarah; Meredith, Nigel; Published by: Journal of Geophysical Research: Space Physics Published on: 09/2014 YEAR: 2014 DOI: 10.1002/jgra.v119.910.1002/2014JA020127 adiabatic invariant coordinates; diffusion equation; fully 3-D model; Radiation belt; stochastic differential equation |
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Analytic expressions for ULF wave radiation belt radial diffusion coefficients We present analytic expressions for ULF wave-derived radiation belt radial diffusion coefficients, as a function of L and Kp, which can easily be incorporated into global radiation belt transport models. The diffusion coefficients are derived from statistical representations of ULF wave power, electric field power mapped from ground magnetometer data, and compressional magnetic field power from in situ measurements. We show that the overall electric and magnetic diffusion coefficients are to a good approximation both independent of energy. We present example 1-D radial diffusion results from simulations driven by CRRES-observed time-dependent energy spectra at the outer boundary, under the action of radial diffusion driven by the new ULF wave radial diffusion coefficients and with empirical chorus wave loss terms (as a function of energy, Kp and L). There is excellent agreement between the differential flux produced by the 1-D, Kp-driven, radial diffusion model and CRRES observations of differential electron flux at 0.976 MeV\textemdasheven though the model does not include the effects of local internal acceleration sources. Our results highlight not only the importance of correct specification of radial diffusion coefficients for developing accurate models but also show significant promise for belt specification based on relatively simple models driven by solar wind parameters such as solar wind speed or geomagnetic indices such as Kp. Ozeke, Louis; Mann, Ian; Murphy, Kyle; Rae, Jonathan; Milling, David; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2014 YEAR: 2014 DOI: 10.1002/2013JA019204 |
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Radial diffusion comparing a THEMIS statistical model with geosynchronous measurements as input The outer boundary energetic electron flux is used as a driver in radial diffusion calculations, and its precise determination is critical to the solution. A new model was proposed recently based on Time History of Events and Macroscale Interactions during Substorms (THEMIS) measurements to express the boundary flux as three fit functions of solar wind parameters in a response window that depend on energy and which solar wind parameter is used: speed, density, or both. The Dartmouth radial diffusion model has been run using Los Alamos National Laboratory (LANL) geosynchronous satellite measurements as the constraint for a one-month interval in July to August 2004, and the calculated phase space density (PSD) is compared with GPS measurements, at magnetic equatorial plane crossings, as a test of the model. We also used the PSD generated from the Shin and Lee model as constraint and examined it by computing the error relative to the LANL geosynchronous spacecraft data-driven run. The calculation shows that there is overestimation and underestimation at different times; however, the direct insertion of the statistical model can be used to drive the radial diffusion model generally, producing the phase space density dropout and increase during a storm. Having this model based on a solar wind parameterized data set, we can run the radial diffusion model for storms when particle measurements are not available as input. We chose the Whole Heliosphere Interval as an example and compared the result with MHD/test-particle simulations, obtaining better agreement with GPS measurement using the diffusion model, which incorporates atmospheric losses and an initial equilibrium radial profile. Li, Zhao; Hudson, Mary; Chen, Yue; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2014 YEAR: 2014 DOI: 10.1002/jgra.v119.310.1002/2013JA019320 outer boundary; radial diffusion; Radiation belt; Van Allen Probes |
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