Bibliography
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Found 91 entries in the Bibliography.
Showing entries from 1 through 50
| 2021 |
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The Link between Wedge-like and Nose-like Ion Spectral Structures in the Inner Magnetosphere AbstractThe wedge-like and nose-like ion spectral structures, named after their characteristic shapes in the energy-time spectrograms, appear to be distinctively different structures in the Earth s inner magnetosphere. Here we present a case study with conjugate observations from the Arase spacecraft and the twin Van Allen Probes on July 1 and 2, 2017, which displayed the characteristic signatures of the wedge-like and nose-like ion structures, respectively. When the spacecraft nearly intersected at L =2.8, the two structures overlapped with enhanced ion fluxes in the energy range of 1-10 keV. These observations suggest that the wedge-like and nose-like spectral signatures are merely the manifestations of one single structure along different spacecraft trajectories. This finding is further validated by the reproduction of both structures from a particle-tracing model, which also indicates their formation processes associated with the intermittent substorm injections in the nightside magnetosphere. Ren, Jie; Zhou, Xu-Zhi; Zong, Qiu-Gang; Yue, Chao; Fu, Sui-Yan; Miyoshi, Y.; Zhang, Xiao-Xin; Asamura, K.; Shinohara, I.; Published by: Geophysical Research Letters Published on: 06/2021 YEAR: 2021 DOI: https://doi.org/10.1029/2021GL093930 |
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AbstractThe spatial scales of whistler-mode waves, determined by their generation process, propagation, and damping, are important for assessing the scaling and efficiency of wave-particle interactions affecting the dynamics of the radiation belts. We use multi-point wave measurements in 2013-2019 by two identically equipped Van Allen Probes spacecraft covering all MLTs at L=2-6 near the geomagnetic equator to investigate the spatial extent of active regions of chorus and hiss waves, their wave amplitude distribution in the source/generation region, and the scales of chorus wave packets, employing a time-domain correlation technique to the spacecraft approaches closer than 1000 km, which happened every 70 days in 2012-2018 and every 35 days in 2018-2019. The correlation of chorus wave power dynamics using two spacecraft measurements is found to remain significant up to inter-spacecraft separations of 400 km to 750 km transverse to the background magnetic field direction, consistent with previous estimates of the chorus wave packet extent, but indicating the likely presence of two different scales of about 400 km and 750 km. Our results further suggest that the chorus source region can be slightly asymmetrical, more elongated in either the azimuthal or radial direction, which could also explain the aforementioned two different scales. An analysis of average chorus and hiss wave amplitudes at separate locations similarly reveals different radial and azimuthal extents of the corresponding wave active regions, complementing previous results based on THEMIS spacecraft statistics mainly at larger L>6. Both the chorus source region scale and the chorus active region size appear smaller inside the outer radiation belt (at L< 6) than at higher L-shells.This article is protected by copyright. All rights reserved. Agapitov, O.; Mourenas, D.; Artemyev, A.; Breneman, A.; Bonnell, J.W.; Hospodarsky, G.; Wygant, J.; Published by: Journal of Geophysical Research: Space Physics Published on: 06/2021 YEAR: 2021 DOI: https://doi.org/10.1029/2020JA028998 chorus waves; chorus genration; Radiation belts; Van Allen Probes |
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AbstractThe spatial scales of whistler-mode waves, determined by their generation process, propagation, and damping, are important for assessing the scaling and efficiency of wave-particle interactions affecting the dynamics of the radiation belts. We use multi-point wave measurements in 2013-2019 by two identically equipped Van Allen Probes spacecraft covering all MLTs at L=2-6 near the geomagnetic equator to investigate the spatial extent of active regions of chorus and hiss waves, their wave amplitude distribution in the source/generation region, and the scales of chorus wave packets, employing a time-domain correlation technique to the spacecraft approaches closer than 1000 km, which happened every 70 days in 2012-2018 and every 35 days in 2018-2019. The correlation of chorus wave power dynamics using two spacecraft measurements is found to remain significant up to inter-spacecraft separations of 400 km to 750 km transverse to the background magnetic field direction, consistent with previous estimates of the chorus wave packet extent, but indicating the likely presence of two different scales of about 400 km and 750 km. Our results further suggest that the chorus source region can be slightly asymmetrical, more elongated in either the azimuthal or radial direction, which could also explain the aforementioned two different scales. An analysis of average chorus and hiss wave amplitudes at separate locations similarly reveals different radial and azimuthal extents of the corresponding wave active regions, complementing previous results based on THEMIS spacecraft statistics mainly at larger L>6. Both the chorus source region scale and the chorus active region size appear smaller inside the outer radiation belt (at L< 6) than at higher L-shells.This article is protected by copyright. All rights reserved. Agapitov, O.; Mourenas, D.; Artemyev, A.; Breneman, A.; Bonnell, J.W.; Hospodarsky, G.; Wygant, J.; Published by: Journal of Geophysical Research: Space Physics Published on: 06/2021 YEAR: 2021 DOI: https://doi.org/10.1029/2020JA028998 chorus waves; chorus genration; Radiation belts; Van Allen Probes |
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Sustained oxygen spectral gaps and their dynamic evolution in the inner magnetosphere Abstract Van Allen Probes observations of ion spectra often show a sustained gap within a very narrow energy range throughout the full orbit. To understand their formation mechanism, we statistically investigate the characteristics of the narrow gaps for oxygen ions and find that they are most frequently observed near the noon sector with a peak occurrence rate of over 30\%. The magnetic moment (μ) of the oxygen ions in the gap shows a strong dependence on magnetic local time (MLT), with higher and lower μ in the morning and afternoon sectors, respectively. Moreover, we find through superposed epoch analysis that the gap formation also depends on geomagnetic conditions. Those gaps formed at lower magnetic moments (μ < 3000 keV/G) are associated with stable convection electric fields, which enable magnetospheric ions to follow a steady drift pattern that facilitates the gap formation by corotational drift resonance. On the other hand, gaps with higher μ values are statistically preceded by a gradual increase of geomagnetic activity. We suggest that ions within the gap were originally located inside the Alfven layer following closed drift paths, before they were transitioned into open drift paths as the convection electric field was enhanced. The sunward drift of these ions, with very low fluxes, forms a drainage void in the dayside magnetosphere manifested as the sustained gap in the oxygen spectrum. This scenario is supported by particle-tracing simulations, which reproduce most of the observed characteristics and therefore provide new insights into inner magnetospheric dynamics. This article is protected by copyright. All rights reserved. Yue, Chao; Zhou, Xu-Zhi; Bortnik, Jacob; Zong, Qiu-Gang; Li, Yuxuan; Ren, Jie; Reeves, Geoffrey; Spence, Harlan; Published by: Journal of Geophysical Research: Space Physics Published on: 04/2021 YEAR: 2021 DOI: https://doi.org/10.1029/2020JA029092 oxygen spectral gaps; corotational drift resonance; sustained gaps; drainage void; test particle simulations; Van Allen Probes |
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Abstract We investigate relativistic electron precipitation events detected by POES in low-Earth orbit in close conjunction with Van Allen Probe A observations of EMIC waves near the geomagnetic equator. We show that the occurrence rate of > 0.7 MeV electron precipitation recorded by POES during those times strongly increases, reaching statistically significant levels when the minimum electron energy for cyclotron resonance with hydrogen or helium band EMIC waves at the equator decreases below ≃ 1.0 − 2.5 MeV, as expected from quasi-linear theory. Both hydrogen and helium band EMIC waves can be effective in precipitating MeV electrons. However, > 0.7 MeV electron precipitation is more often observed (at statistically significant levels) when the minimum electron energy for cyclotron resonance with hydrogen band waves is low (Emin = 0.6 − 1.0 MeV), whereas it is more often observed when the minimum electron energy for cyclotron resonance with helium band waves is slightly larger (Emin = 1.0 − 2.5 MeV), indicative of warm plasma effects for waves approaching the He+ gyrofrequency. We further show that most precipitation events had energies > 0.7 − 1.0 MeV, consistent with the estimated minimum energy (Emin ∼ 0.6 − 2.5 MeV) of cyclotron resonance with the observed EMIC waves during the majority of these events. However, 4 out of the 12 detected precipitation events cannot be explained by electron quasi-linear scattering by the observed EMIC waves, and 12 out of 20 theoretically expected precipitation events were not detected by POES, suggesting the possibility of nonlinear effects likely present near the magnetic equator, or warm plasma effects, and/or narrowly localized bursts of EMIC waves. This article is protected by copyright. All rights reserved. Zhang, X.-J.; Mourenas, D.; Shen, X.-C.; Qin, M.; Artemyev, A.; Ma, Q.; Li, W.; Hudson, M.; Angelopoulos, V.; Published by: Journal of Geophysical Research: Space Physics Published on: 04/2021 YEAR: 2021 DOI: https://doi.org/10.1029/2021JA029193 EMIC waves; relativistic electron precipitation; minimum resonant energy; Van Allen Probes; POES; Radiation belts |
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Generation of realistic short chorus wave packets Abstract Most lower-band chorus waves observed in the inner magnetosphere propagate under the form of moderately intense short wave packets with fast frequency and phase variations. Therefore, understanding the formation mechanism of such short wave packets is crucial for accurately modelling electron nonlinear acceleration or precipitation into the atmosphere by these waves. We compare chorus wave statistics from the Van Allen Probes with predictions from a simple model of short wave packet generation by wave superposition with resonance non-overlap, as well as with results from Vlasov Hybrid Simulations of chorus wave generation in an inhomogeneous magnetic field in the presence of one or two simultaneous triggering waves. We show that the observed moderate amplitude short chorus wave packets can be formed by a superposition of two or more waves generated near the magnetic equator with a sufficiently large frequency difference. Nunn, D.; Zhang, X.-J.; Mourenas, D.; Artemyev, A.; Published by: Geophysical Research Letters Published on: 03/2021 YEAR: 2021 DOI: https://doi.org/10.1029/2020GL092178 chorus waves; Radiation belts; Wave-particle interaction; Van Allen Probes |
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Abstract The impact of radial diffusion in storm time radiation belt dynamics is well-debated. In this study we quantify the changes and variability in radial diffusion coefficients during geomagnetic storms. A statistical analysis of Van Allen Probes data (2012 − 2019) is conducted to obtain measurements of the magnetic and electric power spectral densities for Ultra Low Frequency (ULF) waves, and corresponding radial diffusion coefficients. The results show global wave power enhancements occur during the storm main phase, and continue into the recovery phase. Local time asymmetries show sources of wave power are both external solar wind driving and internal sources from coupling with ring current ions and substorms. Wave power enhancements are also observed at low L values (L < 4). The accessibility of wave power to low L is attributed to a depression of the Alfvén continuum. The increased wave power drives enhancements in both the magnetic and electric field diffusion coefficients by more than an order of magnitude. Significant variability in diffusion coefficients is observed, with values ranging over several orders of magnitude. A comparison to the Kp parameterised empirical model of Ozeke et al. (2014) is conducted and indicates important differences during storm times. Although the electric field diffusion coefficient is relatively well described by the empirical model, the magnetic field diffusion coefficient is approximately ∼ 10 times larger than predicted. We discuss how differences could be attributed to dataset limitations and assumptions. Alternative storm-time radial diffusion coefficients are provided as a function of L* and storm phase. Sandhu, J.; Rae, I.; Wygant, J.; Breneman, A.; Tian, S.; Watt, C.; Horne, R.; Ozeke, L.; Georgiou, M.; Walach, M.-T.; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2021 YEAR: 2021 DOI: https://doi.org/10.1029/2020JA029024 ULF waves; radial diffusion; outer radiation belt; Van Allen Probes; Geomagnetic storms |
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Abstract We present, for the first time, a plasmaspheric hiss event observed by the Van Allen probes in response to two successive interplanetary shocks occurring within an interval of ∼2 hours on December 19, 2015. The first shock arrived at 16:16 UT and caused disappearance of hiss for ∼30 minutes. Combined effect of plasmapause crossing, significant Landau damping by suprathermal electrons and their gradual removal by magnetospheric compression led to the disappearance of hiss. Calculation of electron phase space density and linear wave growth rates showed that the shock did not change the growth rate of whistler waves within the core frequency range of plasmaspheric hiss (0.1 - 0.5 kHz) during this interval making conditions unfavorable for the generation of hiss. The recovery began at ∼16:45 UT which is attributed to an enhancement in local plasma instability initiated by the first shock-induced substorm and additional possible contribution from chorus waves. This time, the wave growth rate peaked within the core frequency range ( ∼350 Hz). The second shock arrived at 18:02 UT and generated patchy hiss persisting up to ∼19:00 UT. It is shown that an enhanced growth rate and additional contribution from shock-induced poloidal Pc5 mode (periodicity ∼240 sec) ULF waves resulted in the excitation of hiss waves during this period. The hiss wave amplitudes were found to be additionally modulated by background plasma density and fluctuating plasmapause location. The investigation highlights the important roles of interplanetary shocks, substorms, ULF waves and background plasma density in the variability of plasmaspheric hiss. Chakraborty, S.; Chakrabarty, D.; Reeves, G.; Baker, D.; Claudepierre, S.; Breneman, A.; Hartley, D.; Larsen, B.; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2021 YEAR: 2021 DOI: https://doi.org/10.1029/2020JA028873 Plasmaspheric Hiss; Van Allen Probe; Interplanetary shocks; substorms; Whistlers; ULF waves; Van Allen Probes |
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AbstractIn-situ electron density profiles obtained from Arase in the night magnetic local time (MLT) sector and from RBSP-B covering all MLTs are used to study the small-scale density irregularities present in the plasmasphere and near the plasmapause. Electron density perturbations with amplitudes > 10\% from background density and with time-scales less than 30-min are investigated here as the small-scale density irregularities. The statistical survey of the density irregularities is carried out using nearly two years of density data obtained from RBSP-B and four months of data from Arase satellites. The results show that density irregularities are present globally at all MLT sectors and L-shells both inside and outside the plasmapause, with a higher occurrence at L > 4. The occurrence of density irregularities is found to be higher during disturbed geomagnetic and interplanetary conditions. The case studies presented here revealed: 1) The plasmaspheric density irregularities observed during both quiet and disturbed conditions are found to co-exist with the hot plasma sheet population. 2) During quiet periods, the plasma waves in the whistler-mode frequency range are found to be modulated by the small-scale density irregularities, with density depletions coinciding well with the decrease in whistler intensity. Our observations suggest that different source mechanisms are responsible for the generation of density structures at different MLTs and geomagnetic conditions.This article is protected by copyright. All rights reserved. Thomas, Neethal; Shiokawa, Kazuo; Miyoshi, Yoshizumi; Kasahara, Yoshiya; Shinohara, Iku; Kumamoto, Atsushi; Tsuchiya, Fuminori; Matsuoka, Ayako; Kasahara, Satoshi; Yokota, Shoichiro; Keika, Kunihiro; Hori, Tomo; Asamura, Kazushi; Wang, Shiang-Yu; Kazama, Yoichi; Tam, Sunny; Chang, Tzu-Fang; Wang, Bo-Jhou; Wygant, John; Breneman, Aaron; Reeves, Geoff; Published by: Journal of Geophysical Research: Space Physics Published on: 02/2021 YEAR: 2021 DOI: https://doi.org/10.1029/2020JA027917 Electron density; small-scale density irregularities; plasmasphere; inner magnetosphere; Van Allen Probes; Arase |
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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: https://doi.org/10.1029/2020JA028484 quasiperiodic emissions; electron precipitation; Radiation belt; chorus waves; Van Allen Probes; ULF wave |
| 2020 |
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Ring current decay during storm recovery phase may be affected by different loss processes. In this study, we have investigated the lifetimes of ring current ions (H+ and O+) of energies from 1 keV to several hundred keV at L shell from 3 to 6 during the storm recovery phase through a statistical survey. The observational data of 48 geomagnetic storms from March 2013 to May 2019 are collected based on Van Allen Probe observations. We find that (1) the observed lifetimes of H+ and O+ in general increase with L shell and (2) the lifetimes of H+ is short than that of O+ when E < ∼50 keV while the situation is reversed when E > ∼50 keV. In addition, we have made use of the charge exchange theory, combined with previous experimental results on the charge exchange cross section and two distribution models of neutral hydrogen atoms in the exosphere, so as to directly estimate the ring current ions decay caused by charge exchange mechanism only. Through the comparison between the model predictions of charge exchange lifetime and the observed lifetimes, we find that (3) the observed lifetimes are in general consistent with model results, which confirms that charge exchange is a dominant loss mechanism of ring current ions during storm recovery phase. Chen, Ao; Yue, Chao; Chen, HongFei; Zong, Qiugang; Fu, Suiyan; Wang, Yongfu; Ren, Jie; Published by: Journal of Geophysical Research: Space Physics Published on: 12/2020 YEAR: 2020 DOI: https://doi.org/10.1029/2020JA028500 charge exchange; lifetime; ring current decay; Van Allen Probes |
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We present new and previously unreported in situ observations of Hertz frequency multiharmonic mode field line resonances detected by the Electric Field and Waves instrument on-board the NASA Van Allen probes during low-L perigee passes. Spectral analysis of the spin-plane electric field data reveals the waves in numerous perigee passes, in sequential passes of probes A and B, and with harmonic frequency structures from ∼0.5 to 3.5 Hz which vary with L-shell, altitude, and from day-to-day. Comparing the observations to wave models using plasma mass density values along the field line given by empirical power laws and from the International Reference Ionosphere model, we conclude that the waves are standing Alfvén field line resonances and that only odd-mode harmonics are excited. The model eigenfrequencies are strongly controlled by the density close to the apex of the field line, suggesting a new diagnostic for equatorial ionospheric density dynamics. Lena, F.; Ozeke, L.; Wygant, J.; Tian, S.; Breneman, A.; Mann, I.; Published by: Geophysical Research Letters Published on: 12/2020 YEAR: 2020 DOI: https://doi.org/10.1029/2020GL090632 Field line resonance; Ionosphere; magneto-seismology; Magnetosphere; plasmasphere; standing Alfvén waves; Van Allen Probes |
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On the Formation of Wedge-Like Ion Spectral Structures in the Nightside Inner Magnetosphere Recent observations in the nightside inner magnetosphere have identified a series of wedge-like spectral structures in the energy-time spectrograms of oxygen, helium, and hydrogen ion fluxes. Although the shapes and distributions of these structures have been characterized by case and statistical studies, their formation mechanism remains unclear. Here we utilize a particle tracing model to reproduce the wedge-like structures successively observed by the twin Van Allen Probes. The model suggests that these structures originate from intermittent substorm injection, and it is the accessibility region of these injected ions that determines their shapes. This mechanism is similar to the formation of another kind of structures, the inner magnetospheric nose-like structures, except that the wedge-like structures are separated from the tail population by the discontinuation of ion injections. This scenario is also supported by the distribution statistics of wedge-like structures, which provides new insights into the dynamics of the magnetotail-inner magnetosphere coupled system. Zhou, Xu-Zhi; Ren, Jie; Yang, Fan; Yue, Chao; Zong, Qiu-Gang; Fu, Sui-Yan; Wang, Yongfu; Published by: Journal of Geophysical Research: Space Physics Published on: 11/2020 YEAR: 2020 DOI: https://doi.org/10.1029/2020JA028420 wedge-like structure; inner magnetosphere; substorm injection; magnetospheric convection; ring current; magnetotail; Van Allen Probes |
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Simultaneously Formed Wedge-Like Structures of Different Ion Species Deep in the Inner Magnetosphere In this study, ion data from the Helium, Oxygen, Proton, and Electron (HOPE) spectrometers onboard Van Allen Probes reveal the existence of wedge-like structures of O+, He+, and H+ ions deep in the inner magnetosphere. The behaviors of the wedge-like structures in terms of temporal evolution, spatial distribution, upper energy limit, as well as dependence on solar wind and different geomagnetic indices are investigated from both event studies of several consecutive orbits on 3 February 2013 and the subsequent statistical analyses using 4 years of data. Unlike the dominant distribution at –8 in the dayside observed by the polar orbit satellites in previous studies, the wedge-like structures deep in the equatorial plane of the inner magnetosphere are found mostly at the Mcllwain L shells of –5 and have a preferential location in the duskside and nightside. The O+ and He+ structures can extend to smaller L shells with higher upper energy limits than the H+ structures, while the upper energy limits of all these particle species show a similar variation tendency with respect to magnetic local time (MLT) and L. Observations indicate that these wedge-like structures are probably attributed to fresh substorm injections from the outer region. Ren, Jie; Zong, Q.; Yue, C.; Zhou, X.; Fu, S; Spence, H.; Funsten, H.; Published by: Journal of Geophysical Research: Space Physics Published on: 11/2020 YEAR: 2020 DOI: https://doi.org/10.1029/2020JA028192 wedge-like structures; Ring current ions; inner magnetosphere; Substorm Injections; Van Allen Probes |
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Whistler mode chorus waves have recently been established as the most likely candidate for scattering relativistic electrons to produce the electron microbursts observed by low altitude satellites and balloons. These waves would have to propagate from the equatorial source region to significantly higher magnetic latitude in order to scatter electrons of these relativistic energies. This theoretically proposed propagation has never been directly observed. We present the first direct observations of the same discrete rising tone chorus elements propagating from a near equatorial (Van Allen Probes) to an off-equatorial (Arase) satellite. The chorus is observed first on the more equatorial satellite and is found to be more oblique and significantly attenuated at the off-equatorial satellite. This is consistent with the prevailing theory of chorus propagation and with the idea that chorus must propagate from the equatorial source region to higher latitudes. Ray tracing of chorus at the observed frequencies confirms that these elements could be generated parallel to the field at the equator, and propagate through the medium unducted to Van Allen Probes A and then to Arase with the observed time delay, and have the observed obliquity and intensity at each satellite. Colpitts, Chris; Miyoshi, Yoshizumi; Kasahara, Yoshiya; Delzanno, Gian; Wygant, John; Cattell, Cynthia; Breneman, Aaron; Kletzing, Craig; Cunningham, Greg; Hikishima, Mitsuru; Matsuda, Shoya; Katoh, Yuto; Ripoll, Jean-Francois; Shinohara, Iku; Matsuoka, Ayako; Published by: Journal of Geophysical Research: Space Physics Published on: 10/2020 YEAR: 2020 DOI: https://doi.org/10.1029/2020JA028315 Chorus; wave; propagation; Simultaneous observations; Radiation belt; Van Allen Probes |
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The flux of energetic electrons in the outer radiation belt shows a high variability. The interactions of electrons with very low frequency (VLF) chorus waves play a significant role in controlling the flux variation of these particles. Quantifying the effects of these interactions is crucially important for accurately modeling the global dynamics of the outer radiation belt and to provide a comprehensive description of electron flux variations over a wide energy range (from the source population of 30 keV electrons up to the relativistic core population of the outer radiation belt). Here, we use a synthetic chorus wave model based on a combined database compiled from the Van Allen Probes and Cluster spacecraft VLF measurements to develop a comprehensive parametric model of electron lifetimes as a function of L-shell, electron energy, and geomagnetic activity. The wave model takes into account the wave amplitude dependence on geomagnetic latitude, wave normal angle distribution, and variations of wave frequency with latitude. We provide general analytical formulas to estimate electron lifetimes as a function of L-shell (for L = 3.0 to L = 6.5), electron energy (from 30 keV to 2 MeV), and geomagnetic activity parameterized by the AE index. The present model lifetimes are compared to previous studies and analytical results and also show a good agreement with measured lifetimes of 30 to 300 keV electrons at geosynchronous orbit. Aryan, Homayon; Agapitov, Oleksiy; Artemyev, Anton; Mourenas, Didier; Balikhin, Michael; Boynton, Richard; Bortnik, Jacob; Published by: Journal of Geophysical Research: Space Physics Published on: 07/2020 YEAR: 2020 DOI: https://doi.org/10.1029/2020JA028018 electron lifetimes; Van Allen radiation belts; chorus waves; pitch angle diffusion coefficients; Van Allen Probes; Cluster |
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Whistler-mode hiss waves generally determine MeV electron lifetimes inside the plasmasphere. We use Van Allen Probes measurements to provide the first comprehensive statistical survey of plasmaspheric hiss-driven quasi-linear pitch-angle diffusion rates and lifetimes of MeV electrons as a function of L*, local time, and AE index, taking into account hiss power, electron plasma frequency to gyrofrequency ratio ωpe/Ωce, hiss frequency at peak power ωm, and cross correlations of these parameters. We find that during geomagnetically active periods with hiss observations, ωpe/Ωce and ωm decrease, leading to faster electron loss. We demonstrate that spatiotemporal variations of ωm and ωpe/Ωce with AE, together with wave power changes, significantly affect MeV electron loss, potentially leading to short lifetimes of less than 1 day. A parametric model of MeV electron lifetime driven by AE for L > 2.5 up to the plasmapause is developed and validated using Magnetic Electron Ion Spectrometer (MagEIS) electron flux decay database. Agapitov, O.; Mourenas, D.; Artemyev, A.; Claudepierre, S.; Hospodarsky, G.; Bonnell, J.; Published by: Geophysical Research Letters Published on: 05/2020 YEAR: 2020 DOI: https://doi.org/10.1029/2020GL088052 electron lifetimes; plasmasphere; hiss waves; wave-particle interactions; Van Allen Probes |
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The Dynamics of the Inner Boundary of the Outer Radiation Belt During Geomagnetic Storms Abstract We investigate the shapes of the inner boundary of the outer radiation belt during different geomagnetic storm phases using energetic electron observations from Van Allen Probes. The case of two consecutive but isolated storms in April 2016 shows that (a) the inner boundary, as a function of L shell and energy, exhibits a “V-shaped” form with the energetic electrons showing a kappa-like energy spectrum (electron flux steeply falling with increasing energies), whereas it is in a “S-shaped” form as the energetic electrons show a reversed energy spectrum (electron flux going up with increasing energies from hundreds of keV to ∼1 MeV); (b) the boundary is abruptly transformed from S to V shape during the storm main phase and retains in V shape for several days until it evolves into S shape during the late recovery phase. The main statistical results from 37 isolated geomagnetic storms between 2013 and 2017 present that (a) the more SYM-H drops, the closest to Earth the transition from V to S shape starts, with a linear correlation coefficient of ∼0.7; (b) the minimum energy at which the transition starts is between 100 and 550 keV (typically, less than 250 keV); (c) the transition from V to S shape typically occurs in the plasmasphere. Shi, Xiaofei; Ren, Jie; Zong, Q.; Published by: Journal of Geophysical Research: Space Physics Published on: 04/2020 YEAR: 2020 DOI: 10.1029/2019JA027309 |
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The Dynamics of the Inner Boundary of the Outer Radiation Belt During Geomagnetic Storms We investigate the shapes of the inner boundary of the outer radiation belt during different geomagnetic storm phases using energetic electron observations from Van Allen Probes. The case of two consecutive but isolated storms in April 2016 shows that (a) the inner boundary, as a function of L shell and energy, exhibits a “V-shaped” form with the energetic electrons showing a kappa-like energy spectrum (electron flux steeply falling with increasing energies), whereas it is in a “S-shaped” form as the energetic electrons show a reversed energy spectrum (electron flux going up with increasing energies from hundreds of keV to ∼1 MeV); (b) the boundary is abruptly transformed from S to V shape during the storm main phase and retains in V shape for several days until it evolves into S shape during the late recovery phase. The main statistical results from 37 isolated geomagnetic storms between 2013 and 2017 present that (a) the more SYM-H drops, the closest to Earth the transition from V to S shape starts, with a linear correlation coefficient of ∼0.7; (b) the minimum energy at which the transition starts is between 100 and 550 keV (typically, less than 250 keV); (c) the transition from V to S shape typically occurs in the plasmasphere. Shi, Xiaofei; Ren, Jie; Zong, Q.; Published by: Journal of Geophysical Research: Space Physics Published on: 04/2020 YEAR: 2020 DOI: https://doi.org/10.1029/2019JA027309 |
| 2019 |
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Decay of Ultrarelativistic Remnant Belt Electrons Through Scattering by Plasmaspheric Hiss Ultrarelativistic electron remnant belts appear frequently following geomagnetic disturbances and are located in-between the inner radiation belt and a reforming outer belt. As remnant belts are relatively stable, here we explore the importance of hiss and electromagnetic ion cyclotron waves in controlling the observed decay rates of remnant belt ultrarelativistic electrons in a statistical way. Using measurements from the Van Allen Probes inside the plasmasphere for 25 remnant belt events that occurred between 2012 and 2017 and that are located in the region 2.9 Pinto, V.; Mourenas, D.; Bortnik, J.; Zhang, X.-J.; Artemyev, A.; Moya, P.; Lyons, L.; Published by: Journal of Geophysical Research: Space Physics Published on: Dec-07-2019 YEAR: 2019 DOI: 10.1029/2019JA026509 Decay rates; EMIC waves; MeV Electron Decay; Plasmaspheric Hiss; Radiation belts; Remnant Belt; Van Allen Probes |
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The magnetospheric driver of strong thermal emission velocity enhancement (STEVE) is investigated using conjugate observations when Van Allen Probes\textquoteright footprint directly crossed both STEVE and stable red aurora (SAR) arc. In the ionosphere, STEVE is associated with subauroral ion drift features, including electron temperature peak, density gradient, and westward ion flow. The SAR arc at lower latitudes corresponds to regions inside the plasmapause with isotropic plasma heating, which causes redline-only SAR emission via heat conduction. STEVE corresponds to the sharp plasmapause boundary containing quasi-static subauroral ion drift electric field and parallel-accelerated electrons by kinetic Alfv\ en waves. These parallel electrons could precipitate and be accelerated via auroral acceleration processes powered by Alfv\ en waves propagating along the magnetic field with the plasmapause as a waveguide. The electron precipitation, superimposed on the heat conduction, could explain multiwavelength continuous STEVE emission. The green picket-fence emissions are likely optical manifestations of electron precipitation associated with wave structures traveling along the plasmapause. Chu, Xiangning; Malaspina, David; Gallardo-Lacourt, Bea; Liang, Jun; Andersson, Laila; Ma, Qianli; Artemyev, Anton; Liu, Jiang; Ergun, Robert; Thaller, Scott; Akbari, Hassanali; Zhao, Hong; Larsen, Brian; Reeves, Geoffrey; Wygant, John; Breneman, Aaron; Tian, Sheng; Connors, Martin; Donovan, Eric; Archer, William; MacDonald, Elizabeth; Published by: Geophysical Research Letters Published on: 11/2019 YEAR: 2019 DOI: 10.1029/2019GL082789 aurora; kinetic Alfven wave; Plasmapause; STEVE; subauroral ion drift; table red auroral arc; Van Allen Probes |
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Six years of Van Allen Probes data are used to investigate cold plasmaspheric electrons affected by ultralow-frequency (ULF) waves in the inner magnetosphere (L<7) including spatial distributions, occurrence conditions, and resonant energy range. Events exhibit a global distribution within L= 4\textendash7 but preferentially occur at L\~5.5\textendash7 in the dayside, while there is higher occurrence rate in the duskside than dawnside. They can occur under different geomagnetic activities and solar wind velocities (VS), but the occurrence rates are increasing with larger AE, |SYMH|, and VS. These features are closely associated with the generation and propagation of ULF waves in Pc4 (45\textendash150 s) and Pc5 (150\textendash600 s) bands. Combined with electron observations from HOPE instrument, the resonant energies inferred from wave power indicate that cold electrons at ones to hundreds of electron volts can be affected by ULF waves. This study may shed new light on further investigations on the acceleration and transportation of cold plasmaspheric particles that would affect plasmaspheric material release to the Earth\textquoterights magnetosphere and instabilities for exciting various waves. Ren, Jie; Zong, Q.; Zhou, X.; Spence, H.; Funsten, H.; Wygant, J.; Rankin, R.; Published by: Journal of Geophysical Research: Space Physics Published on: 10/2019 YEAR: 2019 DOI: 10.1029/2019JA027009 Cold plasmaspheric electrons; drift-bounce resonance; ULF waves; Van Allen Probes; Wave-particle interaction |
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We present a statistical analysis with 100\% duty cycle and non-time-averaged amplitudes of the prevalence and distribution of high-amplitude >50-pT whistler mode waves in the outer radiation belt using 5 years of Van Allen Probes data. Whistler mode waves with high magnetic field amplitudes are most common above L=4.5 and between magnetic local time of 0\textendash14 where they are present approximately 1\textendash6\% of the time. During high geomagnetic activity, high-amplitude whistler mode wave occurrence rises above 25\% in some regions. The dayside population are more common during quiet or moderate geomagnetic activity and occur primarily >5\textdegree from the magnetic equator, while the night-to-dawn population are enhanced during active times and are primarily within 5\textdegree of the magnetic equator. These results are different from the distribution of electric field peaks discussed in our previous paper covering the same time period and spatial range. Our previous study found large-amplitude electric field peaks were common down to L=3.5 and were largely absent from afternoon and near noon. The different distribution of large electric and magnetic field amplitudes implies that the low-L component of whistler mode waves observed previously are primarily highly oblique, while the dayside and high-L populations are primarily field aligned. These results have important implications for modeling radiation belt particle interactions with chorus, as large-amplitude waves interact nonlinearly with electrons, resulting in rapid energization, de-energization, or pitch angle scattering. This also may provide clues regarding the mechanisms which can cause significant whistler mode wave growth up to more than 100 times the average wave amplitude. Tyler, E.; Breneman, A.; Cattell, C.; Wygant, J.; Thaller, S.; Malaspina, D.; Published by: Journal of Geophysical Research: Space Physics Published on: 07/2019 YEAR: 2019 DOI: 10.1029/2019JA026913 Magnetosphere; magnetospheric chorus; Radiation belts; Van Allen Probes; whistler wave |
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Nonlinear Electron Interaction With Intense Chorus Waves: Statistics of Occurrence Rates A comprehensive statistical analysis on 8 years of lower-band chorus wave packets measured by the Van Allen Probes and THEMIS spacecraft is performed to examine whether, when, and where these waves are above the theoretical threshold for nonlinear resonant wave-particle interaction. We find that \~5\textendash30\% of all chorus waves interact nonlinearly with \~30- to 300-keV electrons possessing equatorial pitch angles of >40\textdegree in the outer radiation belt, especially during disturbed (AE>500 nT) periods with energetic particles associated with injections from the plasma sheet. Such considerable occurrence rates of nonlinear interactions imply that the evolution of energetic electron fluxes should be dominated by nonlinear effects, rather than by quasi-linear diffusion as commonly assumed. We discuss the possible consequences of such a large amount of high-amplitude chorus waves and examine their characteristics that can influence the efficiency of nonlinear wave-particle interactions. Zhang, X.-J.; Mourenas, D.; Artemyev, A.; Angelopoulos, V.; Bortnik, J.; Thorne, R.; Kurth, W.; Kletzing, C.; Hospodarsky, G.; Published by: Geophysical Research Letters Published on: 06/2019 YEAR: 2019 DOI: 10.1029/2019GL083833 chorus waves; Electron acceleration; nonlinear wave particle interaction; THEMIS; Van Allen Probes; wave packet size |
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Electron scattering by chorus waves is an important mechanism that can lead to fast electron acceleration and loss in the outer radiation belt. Making use of Van Allen Probes measurements, we present the first statistical survey of megaelectron volt electron pitch angle and energy quasi-linear diffusion rates by chorus waves as a function of L-shell, local time, and AE index, taking into account the local electron plasma frequency to gyrofrequency ratio ωpe/Ωce, chorus wave frequency, and resonance wave amplitude. We demonstrate that during disturbed periods, ωpe/Ωce strongly decreases in the night sector, leading to a faster electron loss but also a much faster electron energization in two distinct regions just above the plasmapause and at L ~ 3.5\textendash5.5. Spatiotemporal variations of ωpe/Ωce with AE shape the evolution of electron energization in the outer belt, sometimes leading to very short time scales for quasi-linear megaelectron volt electron acceleration in agreement with Van Allen Probes observations. Agapitov, O.; Mourenas, D.; Artemyev, A.; Hospodarsky, G.; Bonnell, J.W.; Published by: Geophysical Research Letters Published on: 05/2019 YEAR: 2019 DOI: 10.1029/2019GL083446 magnetosphere plasma density; quasi-linear scattering and acceleration; Van Allen Probes; wave-particle interactions |
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EMIC Wave-Driven Bounce Resonance Scattering of Energetic Electrons in the Inner Magnetosphere While electromagnetic ion cyclotron (EMIC) waves have been long studied as a scattering mechanism for ultrarelativistic (megaelectron volt) electrons via cyclotron-resonant interactions, these waves are also of the right frequency to resonate with the bounce motion of lower-energy (approximately tens to hundreds of kiloelectron volts) electrons. Here we investigate the effectiveness of this bounce resonance interaction to better determine the effects of EMIC waves on subrelativistic electron populations in Earth\textquoterights inner magnetosphere. Using wave and plasma parameters directly measured by the Van Allen Probes, we estimate bounce resonance diffusion coefficients for four different events, illustrative of wave and plasma parameters to be encountered in the inner magnetosphere. The range of electron energies and pitch angles affected is examined to better assess the realistic effects of EMIC-driven bounce resonance on energetic electron populations based on actual, locally observed event-based parameters. Significant local diffusion coefficients (~ > 10-6 s-1) for 50- to 100-keV electrons are achieved for both H+ band wave events as well as He+ band, with diffusion coefficients peaking for near-90\textdegree pitch angles but remaining elevated for intermediate ones as well. Diffusion coefficients for higher-energy 200-keV electrons are typically multiple orders of magnitude lower (ranging from 10-11 to 10-6 s-1) and often peak at lower pitch angles (~20\textendash30\textdegree). These results suggest that both H+ and He+ band EMIC waves can play a role in shaping lower-energy electron dynamics via bounce-resonant interactions, in addition to their role in relativistic electron loss via cyclotron resonance. Blum, L.W.; Artemyev, A.; Agapitov, O.; Mourenas, D.; Boardsen, S.; Schiller, Q.; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2019 YEAR: 2019 DOI: 10.1029/2018JA026427 bounce resonance; EMIC wave; energetic electrons; Radiation belts; Van Allen Probes |
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This paper presents the first analysis of Van Allen Probes measurements of the cold plasma density and electric field in the inner magnetosphere to show that intervals of strong modulation at the solar rotation period occur in the locations of the outer plasmasphere and plasmapause (~0.7 RE peak-to-peak), in the large-scale electric field (~0.24 mV/m peak-to-peak), and in the cold plasma density (~250 cm-3 \textendash ~70 cm-3 peak-to-peak). Solar rotation modulation of the inner magnetosphere is more apparent in the declining phase of the solar cycle than near solar maximum. The periodicities in these parameters are compared to solar EUV irradiance, solar wind dawn-dusk electric field, and Kp. The variations in the plasmapause location at the solar rotation period anti-correlate with solar wind electric field, magnetospheric electric field, and Kp, but not with EUV irradiance, indicating that convective erosion is the dominant physical process controlling the plasmapause at these timescales. Thaller, S.; Wygant, J.; Cattell, C.; Breneman, A.; Tyler, E.; Tian, S.; Engel, A.; De Pascuale, S.; Kurth, W.; Kletzing, C.; Tears, J.; Malaspina, David; Published by: Journal of Geophysical Research: Space Physics Published on: 02/2019 YEAR: 2019 DOI: 10.1029/2018JA026365 convection electric field; inner magnetosphere; Plasmapause; plasmasphere; solar rotation; Van Allen Probes |
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We present the first statistical analysis with continuous data coverage and non-averaged amplitudes of the prevalence and distribution of high-amplitude (> 5 mV/m) whistler-mode waves in the outer radiation belt using 5 years of Van Allen Probes data. These waves are most common above L=3.5 and between MLT of 0-7 where they are present 1-4\% of the time. During high geomagnetic activity, high-amplitude whistler-mode wave occurrence rises above 30\% in some regions. During these active times the plasmasphere erodes to lower L and high-amplitude waves are observed at all L outside of it, with the highest occurrence at low L (3.5-4) in the pre-dawn sector. These results have important implications for modeling radiation belt particle interactions with chorus, as large-amplitude waves interact non-linearly with electrons. Results also may provide clues regarding the mechanisms which result in growth to large amplitudes. Tyler, E.; Breneman, A.; Cattell, C.; Wygant, J.; Thaller, S.; Malaspina, D.; Published by: Geophysical Research Letters Published on: 02/2019 YEAR: 2019 DOI: 10.1029/2019GL082292 Chorus; Radiation belt; Van Allen belt; Van Allen Probes; Whistler waves |
| 2018 |
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Electron flux measurements are an important diagnostic for interactions between ultralow-frequency (ULF) waves and relativistic (\~1 MeV) electrons. Since measurements are collected by particle detectors with finite energy channel width, they are affected by a phase mixing process that can obscure these interactions. We demonstrate that ultrahigh-resolution electron measurements from the Magnetic Electron Ion Spectrometer on the Van Allen Probes mission\textemdashobtained using a data product that improves the energy resolution by roughly an order of magnitude\textemdashare crucial for understanding ULF wave-particle interactions. In particular, the ultrahigh-resolution measurements reveal a range of complex dynamics that cannot be resolved by standard measurements. Furthermore, the standard measurements provide estimates for the ULF flux modulation amplitude, period, and phase that may not be representative of true flux modulations, potentially leading to ambiguous conclusions concerning electron dynamics. Hartinger, M.; Claudepierre, S.; Turner, D.; Reeves, G.; Breneman, A.; Mann, I.; Peek, T.; Chang, E.; Blake, J.; Fennell, J.; O\textquoterightBrien, T.; Looper, M.; Published by: Geophysical Research Letters Published on: 10/2018 YEAR: 2018 DOI: 10.1029/2018GL080291 drift resonance; particle detector; Pc5; Radiation belts; ULF wave; Van Allen Probes; Wave-particle interaction |
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Resonant electron interaction with whistler-mode chorus waves is recognized as one of the main drivers of radiation belt dynamics. For moderate wave intensity, this interaction is well described by quasi-linear theory. However, recent statistics of parallel propagating chorus waves have demonstrated that 5 - 20\% of the observed waves are sufficiently intense to interact nonlinearly with electrons. Such interactions include phase trapping and phase bunching (nonlinear scattering) effects not described by quasi-linear diffusion. For sufficiently long (large) wave-packets, these nonlinear effects can result in very rapid electron acceleration and scattering. In this paper we introduce a method to include trapping and nonlinear scattering into the kinetic equation describing the evolution of the electron distribution function. We use statistics of Van Allen Probes and Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations to determine the probability distribution of intense, long wave-packets as a function of power and frequency. Then we develop an analytical model of individual particle resonance with an intense chorus wave-packet and derive the main properties of this interaction: probability of electron trapping, energy change due to trapping and nonlinear scattering. These properties are combined in a nonlocal operator acting on the electron distribution function. When multiple waves are present, we average the obtained operator over the observed distributions of waves and examine solutions of the resultant kinetic equation. We also examine energy conservation and its implications in systems with nonlinear wave-particle interaction. Vainchtein, D.; Zhang, X.-J.; Artemyev, A.; Mourenas, D.; Angelopoulos, V.; Thorne, R.; Published by: Journal of Geophysical Research: Space Physics Published on: 09/2018 YEAR: 2018 DOI: 10.1029/2018JA025654 |
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By analyzing observations from Van Allen Probes in its inbound and outbound orbits, we present evidence of coherent enhancement of cold plasmaspheric electrons and ions due to drift-bounce resonance with ULF waves. From 18:00 UT on 28 May 2017 to 10:00 UT on 29 May 2017, newly formed poloidal mode standing ULF waves with significant electric field oscillations were observed in two consecutive orbits when Probe B was travelling inbound. In contrast to observations during outbound orbits, the cold (< 150 eV) electorns measured by the HOPE instrument were characterized by flux enhancements several times larger and bi-directional pitch angle distributions during inbound orbits. The electron number density inferred from upper hybrid waves is twice as larger as during inbound orbits, which were also confirmed by an increase of spacecraft potential. The observed ULF waves are identified as second harmonic modes that satisfy the drift-bounce resonant condition of N=1 with cold electrons. An enhancement of the plasmaspheric ion number density to restore charge neutrality of plasmas in inbound orbits is observed, which is associated with an increase of ULF wave periods. The observations suggest that the dynamics of plasmaspheric electrons is modified by ULF waves through drift-bounce resonance, and that plasmaspheric ions are indirectly impacted. Ren, Jie; Zong, Qiu-Gang; Miyoshi, Yoshizumi; Rankin, Robert; Spence, Harlan; Funsten, Herbert; Wygant, John; Kletzing, Craig; Published by: Journal of Geophysical Research: Space Physics Published on: 06/2018 YEAR: 2018 DOI: 10.1029/2018JA025255 Cold plasmaspheric electrons acceleration; Drfit-bounce resonance; Modification of electron and ion density profile; Substorm activities; ULF waves; Van Allen Probes |
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Determining solar wind and geomagnetic activity parameters most favorable to strong electron flux enhancements is an important step towards forecasting radiation belt dynamics. Using electron flux measurements from Global Positioning System satellites at L = 4.2 in 2009-2016, we seek statistical relationships between flux enhancements at different energies and solar wind dynamic pressure Pdyn, AE, and Kp, from hundreds of events inside and outside the plasmasphere. Most ⩾1 MeV electron flux enhancements occur during non-storm (or weak storm) times. Flux enhancements of 4 MeV electrons outside the plasmasphere occur during periods of low Pdyn and high AE. We perform superposed epoch analyses of GPS electron fluxes, along with solar wind and geomagnetic indices, 40 keV electron flux, ULF wave index from Geostationary Operational Environmental Satellite (GOES), and chorus wave intensity from the Van Allen Probes and Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission. We demonstrate that 4 MeV electron flux enhancements outside the plasmasphere start when the interplanetary magnetic field (Bz) reaches a minimum, and develop during periods of low Pdyn, high AE, low but increasing Dst, moderate ULF wave index, and intense chorus waves. Flux enhancements at 100 keV occur under conditions with higher Pdyn, higher ULF wave index, and elevated 40 keV electron flux at L = 6.6. Moreover, electron flux enhancements take much more time to develop at higher energies. This suggests that 100 keV flux enhancements are dominated by injections, while MeV electron energization is predominantly induced by chorus waves with further amplification by inward transport. Zhang, X.-J.; Mourenas, D.; Artemyev, A.; Angelopoulos, V.; Thorne, R.; Published by: Journal of Geophysical Research: Space Physics Published on: 06/2018 YEAR: 2018 DOI: 10.1029/2018JA025497 chorus waves; Electron energization; Electron flux enhancements; GPS satellites; Radiation belt; Solar wind and geomagnetic activities; Van Allen Probes |
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We use 3 years of Van Allen Probes observations of highly oblique lower-band chorus waves at low latitudes over L = 4\textendash6 to provide a comprehensive statistics of the distribution of their magnetic and electric powers and full energy density as a function of wave refractive index N, L shell, and geomagnetic activity AE. We use the refractive index calculated either in the cold plasma approximation or in the quasi-electrostatic (hot plasma) approximation and either observed wave electric fields or corrected wave electric fields accounting for the formation of a plasma sheath around antenna probes in a low-density plasma. Approximate fits to the maximum refractive index and to the magnetic wave power profile of highly oblique waves are provided as a function of AE and L. Such fits should be useful for simulations of quasi-linear electron diffusion induced by very oblique chorus waves. The magnetic wave power of these oblique waves remains elevated and roughly constant up to higher N values at lower L < 5 and during less disturbed periods AE*<200 nT, likely due to the corresponding lower temperature of hot electrons injected from the plasma sheet, which leads to weaker thermal effects and Landau damping of these very oblique waves. The average energy density of lower-band chorus waves is mainly distributed from N = 30\textendash50 up to N = 150\textendash300, mostly corresponding to highly oblique waves even at low magnetic latitudes. Shi, R.; Mourenas, D.; Artemyev, A.; Li, W.; Ma, Q.; Published by: Journal of Geophysical Research: Space Physics Published on: 06/2018 YEAR: 2018 DOI: 10.1029/2018JA025337 |
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Resonant interactions between electrons and chorus waves are responsible for a wide range of phenomena in near-Earth space (e.g., diffuse aurora, acceleration of MeV electrons, etc.). Although quasi-linear diffusion is believed to be the primary paradigm for describing such interactions, an increasing number of investigations suggest that nonlinear effects are also important in controlling the rapid dynamics of electrons. However, present models of nonlinear wave-particle interactions, which have been successfully used to describe individual short-term events, are not directly applicable for a statistical evaluation of nonlinear effects and the long-term dynamics of the outer radiation belt, because they lack information on the properties of intense (nonlinearly resonating with electrons) chorus waves. In this paper, we use the THEMIS and Van Allen Probes datasets of field-aligned chorus waveforms to study two key characteristics of these waves: effective amplitude w (nonlinear interaction can occur when w > 2) and wave-packet length β (the number of wave periods within it). While as many as 10 - 15\% of chorus wave-packets are sufficiently intense (w > 2 - 3) to interact nonlinearly with relativistic electrons, most of them are short (β < 10) reducing the efficacy of such interactions. Revised models of non-linear interactions are thus needed to account for the long-term effects of these common, intense but short chorus wave packets. We also discuss the dependence of w, β on location (MLT, L-shell) and on the properties of the suprathermal electron population. Zhang, X.-J.; Thorne, R.; Artemyev, A.; Mourenas, D.; Angelopoulos, V.; Bortnik, J.; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Published by: Journal of Geophysical Research: Space Physics Published on: 06/2018 YEAR: 2018 DOI: 10.1029/2018JA025390 chorus waves; Effective amplitude; nonlinear wave-particle interaction; spatial distribution; statistics; Van Allen Probes; Wave-packet length |
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Electron nonlinear resonant interaction with short and intense parallel chorus wave-packets One of the major drivers of radiation belt dynamics, electron resonant interaction with whistler-mode chorus waves, is traditionally described using the quasi-linear diffusion approximation. Such a description satisfactorily explains many observed phenomena, but its applicability can be justified only for sufficiently low intensity, long duration waves. Recent spacecraft observations of a large number of very intense lower band chorus waves (with magnetic field amplitudes sometimes reaching \~1\% of the background) therefore challenge this traditional description, and call for an alternative approach when addressing the global, long-term effects of the nonlinear interaction of these waves with radiation belt electrons. In this paper, we first use observations from the Van Allen Probes and Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft to show that the majority of intense parallel chorus waves consists of relatively short wave-packets. Then, we construct a kinetic equation describing the nonlinear resonant interaction of radiation belt electrons with such short and intense wave-packets. We demonstrate that this peculiar type of nonlinear interaction produces similar effects as quasi-linear diffusion, i.e., a flattening of the electron velocity distribution function within a certain energy/pitch-angle range. The main difference is the much faster evolution of the electron distribution when nonlinear interaction prevails. Mourenas, D.; Zhang, X.-J.; Artemyev, A.; Angelopoulos, V.; Thorne, R.; Bortnik, J.; Neishtadt, A.; Vasiliev, A.; Published by: Journal of Geophysical Research: Space Physics Published on: 05/2018 YEAR: 2018 DOI: 10.1029/2018JA025417 chorus waves; ; kinetic equation; nonlinear interaction; Radiation belts; short wave-packets; trapping; Van Allen Probes |
| 2017 |
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Synthetic empirical chorus wave model from combined Van Allen Probes and Cluster statistics Chorus waves are among the most important natural electromagnetic emissions in the magnetosphere as regards their potential effects on electron dynamics. They can efficiently accelerate or precipitate electrons trapped in the outer radiation belt, producing either fast increases of relativistic particle fluxes, or auroras at high latitudes. Accurately modeling their effects, however, requires detailed models of their wave power and obliquity distribution as a function of geomagnetic activity in a particularly wide spatial domain, rarely available based solely on the statistics obtained from only one satellite mission. Here, we seize the opportunity of synthesizing data from the Van Allen Probes and Cluster spacecraft to provide a new comprehensive chorus wave model in the outer radiation belt. The respective spatial coverages of these two missions are shown to be especially complementary and further allow a good cross-calibration in the overlap domain. We used 4 years (2012-2016) of Van Allen Probes VLF data in the chorus frequency range up to 12 kHz at latitudes lower than 20 degrees, combined with 10 years of Cluster VLF measurements up to 4 kHz in order to provide a full coverage of geomagnetic latitudes up to 45 degrees in the chorus frequency range 0.1fce-0.8fce. The resulting synthetic statistical model of chorus wave amplitude, obliquity, and frequency is presented in the form of analytical functions of latitude and Kp in three different MLT sectors and for two ranges of L-shells outside the plasmasphere. Such a synthetic and reliable chorus model is crucially important for accurately modeling global acceleration and loss of electrons over the long run in the outer radiation belt, allowing a comprehensive description of electron flux variations over a very wide energy range. Agapitov, O.; Mourenas, D.; Artemyev, A.; Mozer, F.; Hospodarsky, G.; Bonnell, J.; Krasnoselskikh, V.; Published by: Journal of Geophysical Research: Space Physics Published on: 12/2017 YEAR: 2017 DOI: 10.1002/2017JA024843 |
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Satellite observations of a significant population of very oblique chorus waves in the outer radiation belt have fueled considerable interest in the effects of these waves on energetic electron scattering and acceleration. However, corresponding diffusion rates are extremely sensitive to the refractive index N, controlled by hot plasma effects including Landau damping and wave dispersion modifications by suprathermal (15\textendash100 eV) electrons. A combined investigation of wave and electron distribution characteristics obtained from the Van Allen Probes shows that peculiarities of the measured electron distribution significantly reduce Landau damping, allowing wave propagation with high N \~ 100\textendash200. Further comparing measured refractive indexes with theoretical estimates incorporating hot plasma corrections to the wave dispersion, we provide the first experimental demonstration that suprathermal electrons indeed control the upper limit of the refractive index of highly oblique whistler mode waves. Such results further support the importance of incorporating very oblique waves into radiation belt models. Ma, Q.; Artemyev, A.; Mourenas, D.; Li, W.; Thorne, R.; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Reeves, G.; Spence, H.; Wygant, J.; Published by: Geophysical Research Letters Published on: 12/2017 YEAR: 2017 DOI: 10.1002/2017GL075892 Landau damping; maximum refractive index; oblique chorus waves; thermal electron effects; Van Allen Probes; Van Allen Probes observation |
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We present observations that provide the strongest evidence yet that discrete whistler mode chorus packets cause relativistic electron microbursts. On 20 January 2016 near 1944 UT the low Earth orbiting CubeSat Focused Investigations of Relativistic Electron Bursts: Intensity, Range, and Dynamics (FIREBIRD II) observed energetic microbursts (near L = 5.6 and MLT = 10.5) from its lower limit of 220 keV, to 1 MeV. In the outer radiation belt and magnetically conjugate, Van Allen Probe A observed rising-tone, lower band chorus waves with durations and cadences similar to the microbursts. No other waves were observed. This is the first time that chorus and microbursts have been simultaneously observed with a separation smaller than a chorus packet. A majority of the microbursts do not have the energy dispersion expected for trapped electrons bouncing between mirror points. This confirms that the electrons are rapidly (nonlinearly) scattered into the loss cone by a coherent interaction with the large amplitude (up to \~900 pT) chorus. Comparison of observed time-averaged microburst flux and estimated total electron drift shell content at L = 5.6 indicate that microbursts may represent a significant source of energetic electron loss in the outer radiation belt. Breneman, A.; Crew, A.; Sample, J.; Klumpar, D.; Johnson, A.; Agapitov, O.; Shumko, M.; Turner, D.; Santolik, O.; Wygant, J.; Cattell, C.; Thaller, S.; Blake, B.; Spence, H.; Kletzing, C.; Published by: Geophysical Research Letters Published on: 11/2017 YEAR: 2017 DOI: 10.1002/2017GL075001 |
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We analyse two ion scale dipolarization fronts associated with field-aligned currents detected by the Magnetospheric Multiscale mission during a large substorm on August 10, 2016. The first event corresponds to a fast dawnward flow with an anti-parallel current and could be generated by the wake of a previous fast earthward flow. It is associated with intense lower-hybrid drift waves detected at the front and propagating dawnward with a perpendicular phase speed close to the electric drift and the ion thermal velocity. The second event corresponds to a flow reversal: from southwward/dawnward to northward/duskward associated with a parallel current consistent with a brief expansion of the plasma sheet before the front crossing, and with a smaller lower-hybrid drift wave activity. Electromagnetic electron phase-space holes are detected near these low-frequency drift waves during both events. The drift waves could accelerate electrons parallel to the magnetic field and produce the parallel electron drift needed to generate the electron holes. Yet, we cannot rule out the possibility that the drift waves are produced by the anti-parallel current associated with the fast flows, leaving the source for the electron holes unexplained. Contel, O.; Nakamura, R.; Breuillard, H.; Argall, M.; Graham, D.; Fischer, D.; o, A.; Berthomier, M.; Pottelette, R.; Mirioni, L.; Chust, T.; Wilder, F.; Gershman, D.; Varsani, A.; Lindqvist, P.-A.; Khotyaintsev, Yu.; Norgren, C.; Ergun, R.; Goodrich, K.; Burch, J.; Torbert, R.; Needell, J.; Chutter, M.; Rau, D.; Dors, I.; Russell, C.; Magnes, W.; Strangeway, R.; Bromund, K.; Wei, H; Plaschke, F.; Anderson, B.; Le, G.; Moore, T.; Giles, B.; Paterson, W.; Pollock, C.; Dorelli, J.; Avanov, L.; Saito, Y.; Lavraud, B.; Fuselier, S.; Mauk, B.; Cohen, I.; Turner, D.; Fennell, J.; Leonard, T.; Jaynes, A.; Published by: Journal of Geophysical Research: Space Physics Published on: 10/2017 YEAR: 2017 DOI: 10.1002/2017JA024550 dipolarization front; electron hole; fast flow:Van allen Probes; Field-Aligned Current; lower-hybrid drift wave; substorm |
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We report observational evidence of cold plamsmaspheric electron (< 200 eV) acceleration by ultra-low-frequency (ULF) waves in the plasmaspheric boundary layer on 10 September 2015. Strongly enhanced cold electron fluxes in the energy spectrogram were observed along with second harmonic mode waves with a period of about 1 minute which lasted several hours during two consecutive Van Allen Probe B orbits. Cold electron (<200 eV) and energetic proton (10-20 keV) bi-directional pitch angle signatures observed during the event are suggestive of the drift-bounce resonance mechanism. The correlation between enhanced energy fluxes and ULF waves leads to the conclusions that plasmaspheric dynamics is strongly affected by ULF waves. Van Allen Probe A and B, GOES 13, GOES 15 and MMS 1 observations suggest ULF waves in the event were strongest on the dusk-side magnetosphere. Measurements from MMS 1 contain no evidence of an external wave source during the period when ULF waves and injected energetic protons with a bump-on-tail distribution were detected by Van Allen Probe B. This suggests that the observed ULF waves were probably excited by a localized drift-bounce resonant instability, with the free energy supplied by substorm-injected energetic protons. The observations by Van Allen Probe B suggest that energy transfer between particle species in different energy ranges can take place through the action of ULF waves, demonstrating the important role of these waves in the dynamical processes of the inner magnetosphere. Ren, Jie; Zong, Q.; Miyoshi, Y.; Zhou, X.; Wang, Y.; Rankin, R.; Yue, C.; Spence, H.; Funsten, H.; Wygant, J.; Kletzing, C.; Published by: Journal of Geophysical Research: Space Physics Published on: 08/2017 YEAR: 2017 DOI: 10.1002/2017JA024316 Cold plasmaspheric electrons; drift-bounce resonance; Plasma instability; Plasmaspheric boundary layer; Substorm-injected protons; ULF waves; 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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Contemporaneous EMIC and Whistler-Mode Waves: Observations and Consequences for MeV Electron Loss The high variability of relativistic (MeV) electron fluxes in the Earth\textquoterights radiation belts is partly controlled by loss processes involving resonant interactions with electromagnetic ion cyclotron (EMIC) and whistler-mode waves. But as previous statistical models were generated independently for each wave mode, whether simultaneous electron scattering by the two wave types has global importance remains an open question. Using >3 years of simultaneous Van Allen Probes and THEMIS measurements, we explore the contemporaneous presence of EMIC and whistler-mode waves in the same L-shell, albeit at different local times, determining the distribution of wave and plasma parameters as a function of L, Kp, and AE. We derive electron lifetimes from observations and provide the first statistics of combined effects of EMIC and whistler-mode wave scattering on MeV electrons as a function of L and geomagnetic activity. We show that MeV electron lifetimes are often strongly reduced by such combined scattering. Zhang, X.-J.; Mourenas, D.; Artemyev, A.; Angelopoulos, V.; Thorne, R.; Published by: Geophysical Research Letters Published on: 07/2017 YEAR: 2017 DOI: 10.1002/2017GL073886 electron lifetime; EMIC waves; Rediation belts; relativistic electron loss; Van Allen Probes; wave particle interaction; WHISTLER-MODE WAVES |
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Whistler-mode Very Low Frequency (VLF) waves from powerful ground-based transmitters can resonantly scatter energetic plasmaspheric electrons and precipitate them into the atmosphere. A comprehensive 4-year statistics of Van Allen Probes measurements is carried out to assess their consequences on the dynamics of the inner radiation belt and slot region. Statistical models of the measured wave electric field power and of the inferred full wave magnetic amplitude are provided as a function of L, magnetic local time, season, and Kp over L=1-3, revealing the localization of VLF wave intensity and its variation with geomagnetic activity over 2012-2016. Since this VLF wave model can be directly used together with existing hiss and lightning-generated wave models in radiation belt simulation codes, we perform numerical calculations of the corresponding quasilinear pitch angle diffusion rates, allowing us to demonstrate the crucial role played by VLF waves from transmitters in energetic electron loss at L<2.5. Ma, Qianli; Mourenas, Didier; Li, Wen; Artemyev, Anton; Thorne, Richard; Published by: Geophysical Research Letters Published on: 06/2017 YEAR: 2017 DOI: 10.1002/2017GL073885 Electron scattering; Statistical wave model; Van Allen Probes; Van Allen Probes observation; VLF waves |
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We investigate a quiet-time event of magnetospheric Pc5 ultra low frequency (ULF) waves and their likely external drivers using multiple spacecraft observations. Enhancements of electric and magnetic field perturbations in two narrow frequency bands, 1.5-2 mHz and 3.5-4 mHz, were observed over a large radial distance range from r ~5 to 11 RE. During the first half of this event, perturbations were mainly observed in the transverse components and only in the 3.5-4 mHz band. In comparison, enhancements were stronger during the second half in both transverse and compressional components and in both frequency bands. No indication of field line resonances was found for these magnetic field perturbations. Perturbations in these two bands were also observed in the magnetosheath, but not in the solar wind dynamic pressure perturbations. For the first interval, good correlations between the flow perturbations in the magnetosphere and magnetosheath and an indirect signature for Kelvin-Helmholtz (K-H) vortices suggest K-H surface waves as the driver. For the second interval, good correlations are found between the magnetosheath dynamic pressure perturbations, magnetopause deformation, and magnetospheric waves, all in good correspondence to IMF discontinuities. The characteristics of these perturbations can be explained by being driven by foreshock perturbations resulting from these IMF discontinuities. This event shows that even during quiet periods, KH-unstable magnetopause and ion foreshock perturbations can combine to create a highly dynamic magnetospheric ULF wave environment. Wang, Chih-Ping; Thorne, Richard; Liu, Terry; Hartinger, Michael; Nagai, Tsugunobu; Angelopoulos, Vassilis; Wygant, John; Breneman, Aaron; Kletzing, Craig; Reeves, Geoffrey; Claudepierre, Seth; Spence, Harlan; Published by: Journal of Geophysical Research: Space Physics Published on: 04/2017 YEAR: 2017 DOI: 10.1002/2016JA023610 IMF discontinuity; inner magnetosphere; Kelvin-Helmholtz vortices; magnetosheath; Pc5 waves; plasma sheet; Van Allen Probes |
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We present observations from the Van Allen Probes spacecraft that identify a region of intense whistler mode activity within a large density enhancement outside of the plasmasphere. We speculate that this density enhancement is part of a remnant plasmaspheric plume, with the observed wave being driven by a weakly anisotropic electron injection that drifted into the plume and became nonlinearly unstable to whistler emission. Particle measurements indicate that a significant fraction of thermal (<100 eV) electrons within the plume were subject to Landau acceleration by these waves, an effect that is naturally explained by whistler emission within a gradient and high-density ducting inside a density enhancement. Woodroffe, J.; Jordanova, V.; Funsten, H.; Streltsov, A.; Bengtson, M.; Kletzing, C.; Wygant, J.; Thaller, S.; Breneman, A.; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2017 YEAR: 2017 DOI: 10.1002/2015JA022219 Ducting; Van Allen Probes; wave-particle interactions; Whistlers |
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Comparing and contrasting dispersionless injections at geosynchronous orbit during a substorm event Particle injections in the magnetosphere transport electrons and ions from the magnetotail to the radiation belts. Here we consider generation mechanisms of \textquotedblleftdispersionless\textquotedblright injections, namely, those with simultaneous increase of the particle flux over a wide energy range. In this study we take advantage of multisatellite observations which simultaneously monitor Earth\textquoterights magnetospheric dynamics from the tail toward the radiation belts during a substorm event. Dispersionless injections are associated with instabilities in the plasma sheet during the growth phase of the substorm, with a dipolarization front at the onset and with magnetic flux pileup during the expansion phase. They show different spatial spread and propagation characteristics. Injection associated with the dipolarization front is the most penetrating. At geosynchronous orbit (6.6 RE), the electron distributions do not have a classic power law fit but instead a bump on tail centered on \~120 keV during dispersionless electron injections. However, electron distributions of injections associated with magnetic flux pileup in the magnetotail (13 RE) do not show such a signature. We surmise that an additional resonant acceleration occurs in between these locations. We relate the acceleration mechanism to the electron drift resonance with ultralow frequency waves localized in the inner magnetosphere. Kronberg, E.; Grigorenko, E.; Turner, D.; Daly, P.; Khotyaintsev, Y.; Kozak, L.; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2017 YEAR: 2017 DOI: 10.1002/2016JA023551 Acceleration; current wedge; Dipolarization; particle injections; substorm; ULF waves; Van Allen Probes |
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Transverse eV ion heating by random electric field fluctuations in the plasmasphere Charged particle acceleration in the Earth inner magnetosphere is believed to be mainly due to the local resonant wave-particle interaction or particle transport processes. However, the Van Allen Probes have recently provided interesting evidence of a relatively slow transverse heating of eV ions at distances about 2\textendash3 Earth radii during quiet times. Waves that are able to resonantly interact with such very cold ions are generally rare in this region of space, called the plasmasphere. Thus, non-resonant wave-particle interactions are expected to play an important role in the observed ion heating. We demonstrate that stochastic heating by random transverse electric field fluctuations of whistler (and possibly electromagnetic ion cyclotron) waves could explain this weak and slow transverse heating of H+ and O+ ions in the inner magnetosphere. The essential element of the proposed model of ion heating is the presence of trains of random whistler (hiss) wave packets, with significant amplitude modulations produced by strong wave damping, rapid wave growth, or a superposition of wave packets of different frequencies, phases, and amplitudes. Such characteristics correspond to measured characteristics of hiss waves in this region. Using test particle simulations with typical wave and plasma parameters, we demonstrate that the corresponding stochastic transverse ion heating reaches 0.07\textendash0.2 eV/h for protons and 0.007\textendash0.015 eV/h for O+ ions. This global temperature increase of the Maxwellian ion population from an initial Ti\~0.3Ti\~0.3 eV could potentially explain the observations. Artemyev, A.; Mourenas, D.; Agapitov, O.; Blum, L.; Published by: Physics of Plasmas Published on: 02/2017 YEAR: 2017 DOI: 10.1063/1.4976713 electric fields; Electrostatic Waves; protons; Van Allen Probes; Wave power; Whistler waves |
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We present observations from the Van Allen Probes spacecraft that identify an region of intense whistler-mode activity within a large density enhancement outside of the plasmasphere. We speculate that this density enhancement is part of a remnant plasmaspheric plume, with the observed wave being driven by a weakly anisotropic electron injection that drifted into the plume and became non-linearly unstable to whistler emission. Particle measurements indicate that a significant fraction of thermal (<100 eV) electrons within the plume were subject to Landau acceleration by these waves, an effect that is naturally explained by whistler emission within a gradient and high-density ducting inside a density enhancement. Woodroffe, J.; Jordanova, V.; Funsten, H.; Streltsov, A.; Bengtson, M.; Kletzing, C.; Wygant, J.; Thaller, S.; Breneman, A.; Published by: Journal of Geophysical Research: Space Physics Published on: 02/2017 YEAR: 2017 DOI: 10.1002/2015JA022219 Ducting; Van Allen Probes; wave-particle interactions; Whistlers |
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Utilizing simultaneous twin Van Allen Probes observations of whistler mode waves at variable separations, we are able to distinguish the temporal variations from spatial variations, determine the coherence spatial scale, and suggest the possible mechanism of wave modulation. The two probes observed coherently modulated whistler mode waves simultaneously at an unexpectedly large distance up to ~4.3 RE over 3 h during a relatively quiet period. The modulation of 150\textendash500 Hz plasmaspheric hiss was correlated with whistler mode waves measured outside the plasmasphere across 3 h in magnetic local time and 3 L shells, revealing that the modulation was temporal in nature. We suggest that the coherent modulation of whistler mode waves was associated with the coherent ULF waves measured over a large scale, which modulate the plasmaspheric density and result in the modulation of hiss waves via local amplification. In a later period, the 500\textendash1500 Hz periodic rising-tone whistler mode waves were strongly correlated when the two probes traversed large spatial regions and even across the plasmapause. These periodic rising-tone emissions recurred with roughly the same period as the ULF wave, but there was no one-to-one correspondence, and a cross-correlation analysis suggests that they possibly originated from large L shells although the actual cause needs further investigation. Li, Jinxing; Bortnik, Jacob; Li, Wen; Thorne, Richard; Ma, Qianli; Chu, Xiangning; Chen, Lunjin; Kletzing, Craig; Kurth, William; Hospodarsky, George; Wygant, John; Breneman, Aaron; Thaller, Scott; Published by: Journal of Geophysical Research: Space Physics Published on: 02/2017 YEAR: 2017 DOI: 10.1002/2016JA023706 coherent waves; multisatellite; periodic rising tone; Van Allen Probes; whistler mode |
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Utilizing simultaneous twin Van Allen Probes observations of whistler mode waves at variable separations, we are able to distinguish the temporal variations from spatial variations, determine the coherence spatial scale, and suggest the possible mechanism of wave modulation. The two probes observed coherently modulated whistler mode waves simultaneously at an unexpectedly large distance up to ~4.3 RE over 3 h during a relatively quiet period. The modulation of 150\textendash500 Hz plasmaspheric hiss was correlated with whistler mode waves measured outside the plasmasphere across 3 h in magnetic local time and 3 L shells, revealing that the modulation was temporal in nature. We suggest that the coherent modulation of whistler mode waves was associated with the coherent ULF waves measured over a large scale, which modulate the plasmaspheric density and result in the modulation of hiss waves via local amplification. In a later period, the 500\textendash1500 Hz periodic rising-tone whistler mode waves were strongly correlated when the two probes traversed large spatial regions and even across the plasmapause. These periodic rising-tone emissions recurred with roughly the same period as the ULF wave, but there was no one-to-one correspondence, and a cross-correlation analysis suggests that they possibly originated from large L shells although the actual cause needs further investigation. Li, Jinxing; Bortnik, Jacob; Li, Wen; Thorne, Richard; Ma, Qianli; Chu, Xiangning; Chen, Lunjin; Kletzing, Craig; Kurth, William; Hospodarsky, George; Wygant, John; Breneman, Aaron; Thaller, Scott; Published by: Journal of Geophysical Research: Space Physics Published on: 02/2017 YEAR: 2017 DOI: 10.1002/2016JA023706 coherent waves; multisatellite; periodic rising tone; Van Allen Probes; whistler mode |
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