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Found 14 entries in the Bibliography.

Showing entries from 1 through 14


Multi-Point Observations of Quasiperiodic Emission Intensification and Effects on Energetic Electron Precipitation

AbstractThe two Van Allen Probes simultaneously recorded a coherently modulated quasiperiodic (QP) emission that persisted for 3 hours. The magnetic field pulsation at the locations of the two satellites showed a substantial difference, and their frequencies were close to but did not exactly match the repetition frequency of QP emissions for most of the time, suggesting that those coherent QP emissions probably originated from a common source, which then propagated over a broad area in the magnetosphere. The QP emissions were amplified by local anisotropic electron distributions, and their large-scale amplitudes were modulated by the plasma density. A novel observation of this event is that chorus waves at frequencies above QP emissions exhibit a strong correlation with QP emissions. Those chorus waves intensified when the QP emissions reach their peak frequency. This indicates that embryonic QP emissions may be critical for its own intensification as well as chorus waves under certain circumstances. The low-earth-orbit POES satellite observed enhanced energetic electron precipitation in conjunction with the Van Allen Probes, providing direct evidence that QP emissions precipitate energetic electrons into the atmosphere. This scenario is quantitatively confirmed by our quasilinear diffusion simulation results.

Li, Jinxing; Bortnik, Jacob; Ma, Qianli; Li, Wen; Shen, Xiaochen; Nishimura, Yukitoshi; An, Xin; Thaller, Scott; Breneman, Aaron; Wygant, John; Kurth, William; Hospodarsky, George; Hartley, David; Reeves, Geoffrey; Funsten, Herbert; Blake, Bernard; Spence, Harlan; Baker, Daniel;

Published by: Journal of Geophysical Research: Space Physics      Published on: 01/2021

YEAR: 2021     DOI:

quasiperiodic emissions; electron precipitation; Radiation belt; chorus waves; Van Allen Probes; ULF wave


Global Propagation of Magnetospheric Pc5 ULF Waves Driven by Foreshock Transients

Pc5 (2–7 mHz) ultralow frequency (ULF) waves play a significant role in resonating with particles and transferring energy in the coupled magnetospheric and ionospheric system. Recent studies found that Pc5 ULF waves can be triggered by foreshock transients which can perturb the magnetopause through dynamic pressure variation. However, whether foreshock transient-driven Pc5 ULF waves are geoeffective and can propagate globally is still poorly understood. In this study, we take advantage of the conjunction between in situ (by the THEMIS probes, Geotail satellite, GOES satellites, and Van Allen probes) and ground-based (by the all-sky imager at South Pole and ground-based magnetometers) observations to simultaneously analyze the waves from the foreshock region to the dayside and nightside magnetosphere. Both of our two events show that the Pc5 ULF waves are generated by foreshock transients in the dayside magnetosphere. The in situ observations by THEMIS A and D and the 2-D auroral signatures show that the compressional mode waves are likely broadband and coupled to the FLRs with different frequencies and different azimuthal phase speeds. This is the first report that foreshock transients can drive both low- and high-m FLRs, with the azimuthal wave numbers varying from ~5 to ~23. Moreover, the Pc5 ULF waves propagated antisunward to midnight, this can potentially modulate magnetospheric and ionospheric dynamics globally.

Wang, Boyi; Liu, Terry; Nishimura, Yukitoshi; Zhang, Hui; Hartinger, Michael; Shi, Xueling; Ma, Qianli; Angelopoulos, Vassilis; Frey, Harald;

Published by: Journal of Geophysical Research: Space Physics      Published on: 11/2020

YEAR: 2020     DOI:

ULF wave; Field line resonance; wave number; global; THEMIS; aurora; Van Allen Probes

Episodic Occurrence of Field-Aligned Energetic Ions on the Dayside

The tens of kiloelectron volt ions observed in the ring current region at L ~ 3\textendash7 generally have pancake pitch angle distributions, that is, peaked at 90\textdegree. However, in this study, by using the Van Allen Probe observations on the dayside, unexpectedly, we have found that about 5\% time, protons with energies of ~30 to 50 keV show two distinct populations, having an additional field-aligned population overlapping with the original pancake population. The newly appearing field-aligned populations have higher occurrence rates at ~12\textendash16 magnetic local time during geomagnetically active times. In particular, we have studied eight such events in detail and found that the source regions are located around 12 to 18 magnetic local time which coincides with our statistical result. Based on the ionospheric and geosynchronous observations, it is suggested that these energetic ions with field-aligned pitch angle distributions probably are accelerated near postnoon in association with ionospheric disturbances that are triggered by tail injections.

Yue, Chao; Bortnik, Jacob; Zou, Shasha; Nishimura, Yukitoshi; Foster, John; Coppeans, Thomas; Ma, Qianli; Zong, Qiugang; Hull, A.; Henderson, Mike; Reeves, Geoffrey; Spence, Harlan;

Published by: Geophysical Research Letters      Published on: 01/2020

YEAR: 2020     DOI: 10.1029/2019GL086384

Van Allen Probes


Modeling the Electron Flux Enhancement and Butterfly Pitch Angle Distributions on L Shells <2.5

We analyze an energetic electron flux enhancement event in the inner radiation belt observed by Van Allen Probes during an intense geomagnetic storm. The energetic electron flux at L~1.5 increased by a factor of 3 with pronounced butterfly pitch angle distributions (PADs). Using a three-dimensional radiation belt model, we simulate the electron evolution under the impact of radial diffusion, local wave-particle interactions including hiss, very low frequency transmitters, and magnetosonic waves, as well as Coulomb scattering. Consistency between observation and simulation suggests that inward radial diffusion plays a dominant role in accelerating electrons up to 900 keV and transporting the butterfly PADs from higher L shells to form the butterfly PADs at L~1.5. However, local wave-particle interactions also contribute to drive butterfly PADs at L ≳ 1.9. Our study provides a feasible mechanism to explain the electron flux enhancement in the inner belt and the persistent presence of the butterfly PADs at L~1.5.

Hua, Man; Li, Wen; Ma, Qianli; Ni, Binbin; Nishimura, Yukitoshi; Shen, Xiao-Chen; Li, Haimeng;

Published by: Geophysical Research Letters      Published on: 09/2019

YEAR: 2019     DOI: 10.1029/2019GL084822

3-D radial belt modeling; Butterfly pitch angle distribution; Electron flux enhancement; inner belt and slot region; Inward radial diffusion; local wave-particle interactions; Van Allen Probes

Statistical Analysis of Transverse Size of Lower Band Chorus Waves Using Simultaneous Multisatellite Observations

Chorus waves are known to accelerate or scatter energetic electrons via quasi-linear or nonlinear wave-particle interactions in the Earth\textquoterights magnetosphere. In this letter, by taking advantage of simultaneous observations of chorus waveforms from at least a pair of probes among Van Allen Probes and/or Time History of Events and Macroscale Interactions during Substorms (THEMIS) missions, we statistically calculate the transverse size of lower band chorus wave elements. The average size of lower band chorus wave element is found to be ~315\textpm32 km over L shells of ~5\textendash6. Furthermore, our results suggest that the scale size of lower band chorus tends to be (1) larger at higher L shells; (2) larger at higher magnetic latitudes, especially on the dayside; and (3) larger in the azimuthal direction than in the radial direction. Our findings are crucial to quantify wave-particle interaction process, particularly the nonlinear interactions between chorus and energetic electrons.

Shen, Xiao-Chen; Li, Wen; Ma, Qianli; Agapitov, Oleksiy; Nishimura, Yukitoshi;

Published by: Geophysical Research Letters      Published on: 05/2019

YEAR: 2019     DOI: 10.1029/2019GL083118

Chorus wave; Magnetosphere; Scale size; Van Allen Probes

A Statistical Study of EMIC Waves Associated With and Without Energetic Particle Injection From the Magnetotail

To understand the relationship between generation of electromagnetic ion cyclotron (EMIC) waves and energetic particle injections, we performed a statistical study of EMIC waves associated with and without injections based on the Van Allen Probes (Radiation Belt Storm Probes) and Geostationary Operational Environmental Satellite (GOES; GOES-13 and GOES-15) observations. Using 47 months of observations, we identified wave events seen by the Van Allen Probes relative to the plasmapause and to energetic particle injections seen by GOES-13 and GOES-15 on the nightside. We separated the events into four categories: EMIC waves with (without) injections inside (outside) the plasmasphere. We found that He+ EMIC waves have higher occurrence rate inside the plasmasphere, while H+ EMIC waves predominantly occur outside the plasmasphere. Meanwhile, the time duration and peak occurrence rate of EMIC waves associated with injections are shorter and limited to a narrower magnetic local time region than those without injections, indicating that these waves have localized source regions. He+ EMIC waves inside the plasmasphere associated with injection are usually accompanied by an increase in H+ flux within energies of 1\textendash50 keV through all magnetic local time regions, while most wave events outside the plasmasphere show less relationship with H+ flux increase. From these observations, we suggest that injected hot ions are the major driver of He+ EMIC waves inside the plasmasphere during active time. Expanding plasmasphere during quiet times can provide broad wave source regions for He+ EMIC waves on the dayside. However, H+ EMIC waves outside the plasmasphere show different characteristics, suggesting that these waves are generated by other processes.

Jun, C.-W.; Yue, C.; Bortnik, J.; Lyons, L.; Nishimura, Y.; Kletzing, C.; Wygant, J.; Spence, H.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 01/2019

YEAR: 2019     DOI: 10.1029/2018JA025886

EMIC waves associated with and without injections; Relationship between EMIC wave activity and energetic H+ flux variation; Simultaneous observations using the Van Allen Probes and GOES satellites; Spatial occurrence distributions of EMIC waves; Van Allen Probes


Physical mechanism causing rapid changes in ultrarelativistic electron pitch angle distributions right after a shock arrival: Evaluation of an electron dropout event

Three mechanisms have been proposed to explain relativistic electron flux depletions (dropouts) in the Earth\textquoterights outer radiation belt during storm times: adiabatic expansion of electron drift shells due to a decrease in magnetic field strength, magnetopause shadowing and subsequent outward radial diffusion, and precipitation into the atmosphere (driven by EMIC wave scattering). Which mechanism predominates in causing electron dropouts commonly observed in the outer radiation belt is still debatable. In the present study, we evaluate the physical mechanism that may be primarily responsible for causing the sudden change in relativistic electron pitch angle distributions during a dropout event observed by Van Allen Probes during the main phase of the 27 February 2014 storm. During this event, the phase space density of ultrarelativistic (>1 MeV) electrons was depleted by more than 1 order of magnitude over the entire radial extent of the outer radiation belt (3 < L* < 5) in less than 6 h after the passage of an interplanetary shock. We model the electron pitch angle distribution under a compressed magnetic field topology based on actual solar wind conditions. Although these ultrarelativistic electrons exhibit highly anisotropic (peaked in 90\textdegree), energy-dependent pitch angle distributions, which appear to be associated with the typical EMIC wave scattering, comparison of the modeled electron distribution to electron measurements indicates that drift shell splitting is responsible for this rapid change in electron pitch angle distributions. This further indicates that magnetopause loss is the predominant cause of the electron dropout right after the shock arrival.

Zhang, X.-J.; Li, W.; Thorne, R.; Angelopoulos, V.; Ma, Q.; Li, J.; Bortnik, J.; Nishimura, Y.; Chen, L.; Baker, D.; Reeves, G.; Spence, H.; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Blake, J.; Fennell, J.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 09/2016

YEAR: 2016     DOI: 10.1002/2016JA022517

Drift shell splitting; dropouts; magnetic storm; magnetopause shadowing; outer radiation belt; relativistic electron loss; Van Allen Probes

Rapid enhancement of low-energy (<100 eV) ion flux in response to interplanetary shocks based on two Van Allen Probes case studies: Implications for source regions and heating mechanisms

Interactions between interplanetary (IP) shocks and the Earth\textquoterights magnetosphere manifest many important space physics phenomena including low-energy ion flux enhancements and particle acceleration. In order to investigate the mechanisms driving shock-induced enhancement of low-energy ion flux, we have examined two IP shock events that occurred when the Van Allen Probes were located near the equator while ionospheric and ground observations were available around the spacecraft footprints. We have found that, associated with the shock arrival, electromagnetic fields intensified, and low-energy ion fluxes, including H+, He+, and O+, were enhanced dramatically in both the parallel and perpendicular directions. During the 2 October 2013 shock event, both parallel and perpendicular flux enhancements lasted more than 20 min with larger fluxes observed in the perpendicular direction. In contrast, for the 15 March 2013 shock event, the low-energy perpendicular ion fluxes increased only in the first 5 min during an impulse of electric field, while the parallel flux enhancement lasted more than 30 min. In addition, ionospheric outflows were observed after shock arrivals. From a simple particle motion calculation, we found that the rapid response of low-energy ions is due to drifts of plasmaspheric population by the enhanced electric field. However, the fast acceleration in the perpendicular direction cannot solely be explained by E \texttimes B drift but betatron acceleration also plays a role. Adiabatic acceleration may also explain the fast response of the enhanced parallel ion fluxes, while ion outflows may contribute to the enhanced parallel fluxes that last longer than the perpendicular fluxes.

Yue, Chao; Li, Wen; Nishimura, Yukitoshi; Zong, Qiugang; Ma, Qianli; Bortnik, Jacob; Thorne, Richard; Reeves, Geoffrey; Spence, Harlan; Kletzing, Craig; Wygant, John; Nicolls, Michael;

Published by: Journal of Geophysical Research: Space Physics      Published on: 06/2016

YEAR: 2016     DOI: 10.1002/2016JA022808

adiabatic accelerations; enhancement of low-energy ion flux; ionospheric ion outflows; response to IP shocks; Van Allen Probes

Simulation of energy-dependent electron diffusion processes in the Earth\textquoterights outer radiation belt

The radial and local diffusion processes induced by various plasma waves govern the highly energetic electron dynamics in the Earth\textquoterights radiation belts, causing distinct characteristics in electron distributions at various energies. In this study, we present our simulation results of the energetic electron evolution during a geomagnetic storm using the University of California, Los Angeles 3-D diffusion code. Following the plasma sheet electron injections, the electrons at different energy bands detected by the Magnetic Electron Ion Spectrometer (MagEIS) and Relativistic Electron Proton Telescope (REPT) instruments on board the Van Allen Probes exhibit a rapid enhancement followed by a slow diffusive movement in differential energy fluxes, and the radial extent to which electrons can penetrate into depends on energy with closer penetration toward the Earth at lower energies than higher energies. We incorporate radial diffusion, local acceleration, and loss processes due to whistler mode wave observations to perform a 3-D diffusion simulation. Our simulation results demonstrate that chorus waves cause electron flux increase by more than 1 order of magnitude during the first 18 h, and the subsequent radial extents of the energetic electrons during the storm recovery phase are determined by the coupled radial diffusion and the pitch angle scattering by EMIC waves and plasmaspheric hiss. The radial diffusion caused by ULF waves and local plasma wave scattering are energy dependent, which lead to the observed electron flux variations with energy dependences. This study suggests that plasma wave distributions in the inner magnetosphere are crucial for the energy-dependent intrusions of several hundred keV to several MeV electrons.

Ma, Q.; Li, W.; Thorne, R.; Nishimura, Y.; Zhang, X.-J.; Reeves, G.; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Henderson, M.; Spence, H.; Baker, D.; Blake, J.; Fennell, J.; Angelopoulos, V.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 05/2016

YEAR: 2016     DOI: 10.1002/2016JA022507

electron acceleration and loss; energy-dependent diffusion; radial diffusion; radiation belt simulation; Van Allen Probes


Statistical properties of plasmaspheric hiss derived from Van Allen Probes data and their Effects on radiation belt electron dynamics

Plasmaspheric hiss is known to play an important role in controlling the overall structure and dynamics of radiation belt electrons inside the plasmasphere. Using newly available Van Allen Probes wave data, which provide excellent coverage in the entire inner magnetosphere, we evaluate the global distribution of the hiss wave frequency spectrum and wave intensity for different levels of substorm activity. Our statistical results show that observed hiss peak frequencies are generally lower than the commonly adopted value (~550 Hz), which was in frequent use, and that the hiss wave power frequently extends below 100 Hz, particularly at larger L shells (> ~3) on the dayside during enhanced levels of substorm activity. We also compare electron pitch angle scattering rates caused by hiss using the new statistical frequency spectrum and the previously adopted Gaussian spectrum and find that the differences are up to a factor of ~5 and are dependent on energy and L shell. Moreover, the new statistical hiss wave frequency spectrum including wave power below 100 Hz leads to increased pitch angle scattering rates by a factor of ~1.5 for electrons above ~100 keV at L~5, although their effect is negligible at L <= 3. Consequently, we suggest that the new realistic hiss wave frequency spectrum should be incorporated into future modeling of radiation belt electron dynamics.

Li, W.; Ma, Q.; Thorne, R.; Bortnik, J.; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Nishimura, Y.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 05/2015

YEAR: 2015     DOI: 10.1002/2015JA021048

hiss diffusion coefficient; hiss frequency spectrum; Plasmaspheric Hiss; Van Allen Probes

Azimuthal flow bursts in the Inner Plasma Sheet and Possible Connection with SAPS and Plasma Sheet Earthward Flow Bursts

We have combined radar observations and auroral images obtained during the PFISR Ion Neutral Observations in the Thermosphere campaign to show the common occurrence of westward moving, localized auroral brightenings near the auroral equatorward boundary and to show their association with azimuthally moving flow bursts near or within the SAPS region. These results indicate that the SAPS region, rather than consisting of relatively stable proton precipitation and westward flows, can have rapidly varying flows, with speeds varying from ~100 m/s to ~1 km/s in just a few minutes. The auroral brightenings are associated with bursts of weak electron precipitation that move westward with the westward flow bursts and extend into the SAPS region. Additionally, our observations show evidence that the azimuthally moving flow bursts often connect to earthward (equatorward in the ionosphere) plasma sheet flow bursts. This indicates that rather than stopping or bouncing, some flow bursts turn azimuthally after reaching the inner plasma sheet and lead to the bursts of strong azimuthal flow. Evidence is also seen for a general guiding of the flow bursts by the large-scale convection pattern, flow bursts within the duskside convection being azimuthally turned to the west and those within the dawn cell being turned toward the east. The possibility that the SAPS-region flow structures considered here may be connected to localized flow enhancements from the polar cap that cross the nightside auroral poleward boundary and lead to flow bursts within the plasma sheet warrants further consideration.

Lyons, L.; Nishimura, Y.; Gallardo-Lacourt, B.; Nicolls, M.; Chen, S.; Hampton, D.; Bristow, W.; Ruohoniemi, J.; Nishitani, N.; Donovan, E.; Angelopoulos, V.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 05/2015

YEAR: 2015     DOI: 10.1002/2015JA021023

aurora; convection; Flow bursts; plasma sheet; SAPS; streamers

The global context of the 14 November, 2012 storm event

From 2 to 5 UT on 14 November, 2012, the Van Allen Probes observed repeated particle flux dropouts during the main phase of a geomagnetic storm as the satellites traversed the post-midnight to dawnside inner magnetosphere. Each flux dropout corresponded to an abrupt change in the magnetic topology, i.e., from a more dipolar configuration to a configuration with magnetic field lines stretched in the dawn-dusk direction. Geosynchronous GOES spacecraft located in the dusk and near-midnight sectors and the LANL constellation with wide local time coverage also observed repeated flux dropouts and stretched field lines with similar occurrence patterns to those of the Van Allen Probe events. THEMIS recorded multiple transient abrupt expansions of the evening-side magnetopause ~20\textendash30 min prior to the sequential Van Allen Probes observations. Ground-based magnetograms and all sky images demonstrate repeatable features in conjunction with the dropouts. We combine the various in-situ and ground-based measurements to define and understand the global spatiotemporal features associated with the dropouts observed by the Van Allen Probes. We discuss various proposed hypotheses for the mechanism that plausibly caused this storm-time dropout event as well as formulate a new hypothesis that explains the combined in-situ and ground-based observations: the earthward motion of magnetic flux ropes containing lobe plasmas that form along an extended magnetotail reconnection line in the near-Earth plasma sheet.

Hwang, K.-J.; Sibeck, D.; Fok, M.-C.; Zheng, Y.; Nishimura, Y.; Lee, J.-J.; Glocer, A.; Partamies, N.; Singer, H.; Reeves, G.; Mitchell, D.; Kletzing, C.; Onsager, T.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 02/2015

YEAR: 2015     DOI: 10.1002/2014JA020826

Van Allen Probes


Evolution of nightside subauroral proton aurora caused by transient plasma sheet flows

While nightside subauroral proton aurora shows rapid temporal variations, the cause of this variability has rarely been investigated. Using well-coordinated observations by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) all-sky imagers, THEMIS satellites in the equatorial magnetosphere, and the low-altitude NOAA 17 satellite, we examined the rapid temporal evolution of subauroral proton aurora in the premidnight sector. An isolated proton aurora occurred soon after an auroral poleward boundary intensification that was followed by an auroral streamer reaching the equatorward boundary of the auroral oval. Three THEMIS satellites in the magnetotail detected flow bursts and one of the THEMIS satellites in the outer plasmasphere observed a ring current injection together with electromagnetic ion cyclotron wave intensifications. Proton auroral brightenings occurred multiple times throughout the storm main phase and a majority of those were correlated with auroral streamers reaching the auroral equatorward boundary. This sequence highlights the important role of transient flow bursts and particle injections for plasma transport into the inner magnetosphere and thus reflects a tail-inner magnetospheric interaction process in which transient flow bursts play an important role in injecting ring current ions into the plasmasphere, causing rapid modulation of precipitation and the resultant subauroral proton aurora.

Nishimura, Y.; Bortnik, J.; Li, W.; Lyons, L.; Donovan, E.; Angelopoulos, V.; Mende, S.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 07/2014

YEAR: 2014     DOI: 10.1002/2014JA020029

EMIC waves; plasma sheet flow burst; plasmasphere; proton aurora; THEMIS ASI; THEMIS satellite

Quantifying hiss-driven energetic electron precipitation: A detailed conjunction event analysis

We analyze a conjunction event between the Van Allen Probes and the low-altitude Polar Orbiting Environmental Satellite (POES) to quantify hiss-driven energetic electron precipitation. A physics-based technique based on quasi-linear diffusion theory is used to estimate the ratio of precipitated and trapped electron fluxes (R), which could be measured by the two-directional POES particle detectors, using wave and plasma parameters observed by the Van Allen Probes. The remarkable agreement between modeling and observations suggests that this technique is applicable for quantifying hiss-driven electron scattering near the bounce loss cone. More importantly, R in the 100\textendash300 keV energy channel measured by multiple POES satellites over a broad L magnetic local time region can potentially provide the spatiotemporal evolution of global hiss wave intensity, which is essential in evaluating radiation belt electron dynamics, but cannot be obtained by in situ equatorial satellites alone.

Li, W.; Ni, B.; Thorne, R.; Bortnik, J.; Nishimura, Y.; Green, J.; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Spence, H.; Reeves, G.; Blake, J.; Fennell, J.; Claudepierre, S.; Gu, X.;

Published by: Geophysical Research Letters      Published on: 02/2014

YEAR: 2014     DOI: 10.1002/2013GL059132

Van Allen Probes