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Found 153 entries in the Bibliography.
Showing entries from 101 through 150
| 2015 |
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Broadband low frequency electromagnetic waves in the inner magnetosphere A prominent yet largely unrecognized feature of the inner magnetosphere associated with particle injections, and more generally geomagnetic storms, is the occurrence of broadband electromagnetic field fluctuations over spacecraft frame frequencies (fsc) extending from effectively zero to fsc ≳ 100 Hz. Using observations from the Van Allen Probes we show that these waves most commonly occur pre-midnight but are observed over a range of local times extending into the dayside magnetosphere. We find that the variation of magnetic spectral energy density with fsc obeys inline image over several decades with a spectral break-point at fb ≈1 Hz. The values for α are log normally distributed with α = 1.9 \textpm 0.6 for fsc < fb andα = 2.9 \textpm 0.6 for fsc > fb. A is a function of geomagnetic activity with the largest values observed over intervals of decreasing Dst index during the main phase of geomagnetic storms. At these times these waves are nearly always present in the night-side inner magnetosphere and are commonly observed from L = 3 outward. The observed variation of the electric to magnetic field amplitude with fsc is well described by a dispersive Alfv\ en wave model under the assumption that fsc is primarily a consequence of the Doppler shift of plasma frame structures moving over the spacecraft. The robust anti-correlation between the time rate change of the Dst index and wave spectral energy density coupled with the ability of dispersive Alfv\ en waves to drive transverse ion acceleration suggests that these waves may boost ion energy density in the inner magnetosphere and intensify the ring current during storm times. Chaston, C.; Bonnell, J.; Kletzing, C.; Hospodarsky, G.; Wygant, J.; Smith, C.; Published by: Journal of Geophysical Research: Space Physics Published on: 09/2015 YEAR: 2015 DOI: 10.1002/2015JA021690 Alfven waves; Geomagnetic storms; ring current; turbulence; Van Allen Probes |
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Magnetohydrodynamic modeling of three Van Allen Probes storms in 2012 and 2013 Coronal mass ejection (CME)-shock compression of the dayside magnetopause has been observed to cause both prompt enhancement of radiation belt electron flux due to inward radial transport of electrons conserving their first adiabatic invariant and prompt losses which at times entirely eliminate the outer zone. Recent numerical studies suggest that enhanced ultra-low frequency (ULF) wave activity is necessary to explain electron losses deeper inside the magnetosphere than magnetopause incursion following CME-shock arrival. A combination of radial transport and magnetopause shadowing can account for losses observed at radial distances into L = 4.5, well within the computed magnetopause location. We compare ULF wave power from the Electric Field and Waves (EFW) electric field instrument on the Van Allen Probes for the 8 October 2013 storm with ULF wave power simulated using the Lyon\textendashFedder\textendashMobarry (LFM) global magnetohydrodynamic (MHD) magnetospheric simulation code coupled to the Rice Convection Model (RCM). Two other storms with strong magnetopause compression, 8\textendash9 October 2012 and 17\textendash18 March 2013, are also examined. We show that the global MHD model captures the azimuthal magnetosonic impulse propagation speed and amplitude observed by the Van Allen Probes which is responsible for prompt acceleration at MeV energies reported for the 8 October 2013 storm. The simulation also captures the ULF wave power in the azimuthal component of the electric field, responsible for acceleration and radial transport of electrons, at frequencies comparable to the electron drift period. This electric field impulse has been shown to explain observations in related studies (Foster et al., 2015) of electron acceleration and drift phase bunching by the Energetic Particle, Composition, and Thermal Plasma Suite (ECT) instrument on the Van Allen Probes. Paral, J.; Hudson, M.; Kress, B.; Wiltberger, M.; Wygant, J.; Singer, H.; Published by: Annales Geophysicae Published on: 08/2015 YEAR: 2015 DOI: 10.5194/angeo-33-1037-2015 |
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Low-harmonic magnetosonic waves observed by the Van Allen Probes Purely compressional electromagnetic waves (fast magnetosonic waves), generated at multiple harmonics of the local proton gyrofrequency, have been observed by various types of satellite instruments (fluxgate and search coil magnetometers and electric field sensors), but most recent studies have used data from search coil sensors, and many have been restricted to high harmonics. We report here on a survey of low-harmonic waves, based on electric and magnetic field data from the EFW double probe and EMFISIS fluxgate magnetometer instruments, respectively, on the Van Allen Probes spacecraft during its first full precession through all local times, from October 1, 2012 through July 13, 2014. These waves were observed both inside and outside the plasmapause (PP), at L shells from 2.4 to ~6 (the spacecraft apogee), and in regions with plasma number densities ranging from 10 to >1000 cm-3. Consistent with earlier studies, wave occurrence was sharply peaked near the magnetic equator. Waves appeared at all local times but were more common from noon to dusk, and often occurred within three hours after substorm injections. Outside the PP occurrence maximized broadly across noon, and inside the PP occurrence maximized in the dusk sector, in an extended plasmasphere. We confirm recent ray-tracing studies showing wave refraction and/or reflection at PP-like boundaries. Comparison with waveform receiver data indicates that in some cases these low-harmonic magnetosonic wave events occurred independently of higher-harmonic waves; this indicates the importance of including this population in future studies of radiation belt dynamics. Posch, J.; Engebretson, M.; Olson, C.; Thaller, S.; Breneman, A.; Wygant, J.; Boardsen, S.; Kletzing, C.; Smith, C.; Reeves, G.; Published by: Journal of Geophysical Research: Space Physics Published on: 07/2015 YEAR: 2015 DOI: 10.1002/2015JA021179 equatorial noise; inner magnetosphere; magnetosonic waves; Van Allen Probes; waves in plasmas |
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Substorms generally inject 10s-100s keV electrons, but intense substorm electric fields have been shown to inject MeV electrons as well. An intriguing question is whether such MeV electron injections can populate the outer radiation belt. Here we present observations of a substorm injection of MeV electrons into the inner magnetosphere. In the pre-midnight sector at L\~5.5, Van Allen Probes (RBSP)-A observed a large dipolarization electric field (50mV/m) over \~40s and a dispersionless injection of electrons up to \~3 MeV. Pitch angle observations indicated betatron acceleration of MeV electrons at the dipolarization front. Corresponding signals of MeV electron injection were observed at LANL-GEO, THEMIS-D, and GOES at geosynchronous altitude. Through a series of dipolarizations, the injections increased the MeV electron phase space density by one order of magnitude in less than 3 hours in the outer radiation belt (L>4.8). Our observations provide evidence that deep injections can supply significant MeV electrons. Dai, Lei; Wang, Chi; Duan, Suping; He, Zhaohai; Wygant, John; Cattell, Cynthia; Tao, Xin; Su, Zhenpeng; Kletzing, Craig; Baker, Daniel; Li, Xinlin; Malaspina, David; Blake, Bernard; Fennell, Joseph; Claudepierre, Seth; Turner, Drew; Reeves, Geoffrey; Funsten, Herbert; Spence, Harlan; Angelopoulos, Vassilis; Fruehauff, Dennis; Chen, Lunjin; Thaller, Scott; Breneman, Aaron; Tang, Xiangwei; Published by: Geophysical Research Letters Published on: 07/2015 YEAR: 2015 DOI: 10.1002/2015GL064955 electric fields; radiation belt electrons; substorm dipolarization; substorm injection; Van Allen Probes |
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Theory and observations have linked equatorial VLF waves with pulsating aurora for decades, invoking the process of pitch-angle scattering of 10\textquoterights keV electrons in the equatorial magnetosphere. Recently published satellite studies have strengthened this argument, by showing strong correlation between pulsating auroral patches and both lower-band chorus and 10\textquoterights keV electron modulation in the vicinity of geosynchronous orbit. Additionally, a previous link has been made between Pc4-5 compressional pulsations and modulation of whistler-mode chorus using THEMIS. In the current study, we present simultaneous in-situ observations of structured chorus waves and an apparent field line resonance (in the Pc4-5 range) as a result of a substorm injection, observed by Van Allen Probes, along with ground-based observations of pulsating aurora. We demonstrate the likely scenario being one of substorm-driven Pc4-5 ULF pulsations modulating chorus waves, and thus providing the driver for pulsating particle precipitation into the Earth\textquoterights atmosphere. Interestingly, the modulated chorus wave and ULF wave periods are well correlated, with chorus occurring at half the periodicity of the ULF waves. We also show, for the first time, a particular few-Hz modulation of individual chorus elements that coincides with the same modulation in a nearby pulsating aurora patch. Such modulation has been noticed as a high-frequency component in ground-based camera data of pulsating aurora for decades, and may be a result of nonlinear chorus wave interactions in the equatorial region. Jaynes, A.; Lessard, M.; Takahashi, K.; Ali, A.; Malaspina, D.; Michell, R.; Spanswick, E.; Baker, D.; Blake, J.; Cully, C.; Donovan, E.; Kletzing, C.; Reeves, G.; Samara, M.; Spence, H.; Wygant, J.; Published by: Journal of Geophysical Research: Space Physics Published on: 07/2015 YEAR: 2015 DOI: 10.1002/2015JA021380 aurora; precipitation; pulsating aurora; substorms; ULF waves; Van Allen Probes; VLF waves |
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Global-scale coherence modulation of radiation-belt electron loss from plasmaspheric hiss Over 40 years ago it was suggested that electron loss in the region of the radiation belts that overlaps with the region of high plasma density called the plasmasphere, within four to five Earth radii1, 2, arises largely from interaction with an electromagnetic plasma wave called plasmaspheric hiss3, 4, 5. This interaction strongly influences the evolution of the radiation belts during a geomagnetic storm, and over the course of many hours to days helps to return the radiation-belt structure to its \textquoteleftquiet\textquoteright pre-storm configuration. Observations have shown that the long-term electron-loss rate is consistent with this theory but the temporal and spatial dynamics of the loss process remain to be directly verified. Here we report simultaneous measurements of structured radiation-belt electron losses and the hiss phenomenon that causes the losses. Losses were observed in the form of bremsstrahlung X-rays generated by hiss-scattered electrons colliding with the Earth\textquoterights atmosphere after removal from the radiation belts. Our results show that changes of up to an order of magnitude in the dynamics of electron loss arising from hiss occur on timescales as short as one to twenty minutes, in association with modulations in plasma density and magnetic field. Furthermore, these loss dynamics are coherent with hiss dynamics on spatial scales comparable to the size of the plasmasphere. This nearly global-scale coherence was not predicted and may affect the short-term evolution of the radiation belts during active times. Breneman, A.; Halford, A.; Millan, R.; McCarthy, M.; Fennell, J.; Sample, J.; Woodger, L.; Hospodarsky, G.; Wygant, J.; Cattell, C.; Goldstein, J.; Malaspina, D.; Kletzing, C.; Published by: Nature Published on: 06/2015 YEAR: 2015 DOI: 10.1038/nature14515 |
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Although most studies of the effects of EMIC waves on Earth\textquoterights outer radiation belt have focused on events in the afternoon sector in the outer plasmasphere or plume region, strong magnetospheric compressions provide an additional stimulus for EMIC wave generation across a large range of local times and L shells. We present here observations of the effects of a wave event on February 23, 2014 that extended over 8 hours in UT and over 12 hours in local time, stimulated by a gradual 4-hour rise and subsequent sharp increases in solar wind pressure. Large-amplitude linearly polarized hydrogen band EMIC waves (up to 25 nT p-p) appeared for over 4 hours at both Van Allen Probes, from late morning through local noon, when these spacecraft were outside the plasmapause, with densities ~5-20 cm-3. Waves were also observed by ground-based induction magnetometers in Antarctica (near dawn), Finland (near local noon), Russia (in the afternoon), and in Canada (from dusk to midnight). Ten passes of NOAA-POES and METOP satellites near the northern footpoint of the Van Allen Probes observed 30-80 keV subauroral proton precipitation, often over extended L shell ranges; other passes identified a narrow L-shell region of precipitation over Canada. Observations of relativistic electrons by the Van Allen Probes showed that the fluxes of more field-aligned and more energetic radiation belt electrons were reduced in response to both the emission over Canada and the more spatially extended emission associated with the compression, confirming the effectiveness of EMIC-induced loss processes for this event. Engebretson, M.; Posch, J.; Wygant, J.; Kletzing, C.; Lessard, M.; Huang, C.-L.; Spence, H.; Smith, C.; Singer, H.; Omura, Y.; Horne, R.; Reeves, G.; Baker, D.; Gkioulidou, M.; Oksavik, K.; Mann, I.; Raita, T; Shiokawa, K.; Published by: Journal of Geophysical Research: Space Physics Published on: 06/2015 YEAR: 2015 DOI: 10.1002/2015JA021227 EMIC waves; magnetospheric compressions; Radiation belts; Van Allen Probes |
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n the dawn sector, L~ 5.5 and MLT~4-7, from 01:30 to 06:00 UT during the November 14th 2012 geomagnetic storm, both Van Allen Probes observed an alternating sequence of locally quiet and disturbed intervals with two strikingly different power fluctuation levels and magnetic field orientations: either small (~10-2 nT2) total power with strong GSM Bx and weak By, or large (~10 nT2) total power with weak Bx, and strong By and Bz components. During both kinds of intervals the fluctuations occur in the vicinity of the local ion gyro-frequencies (0.01-10 Hz) in the spacecraft frame, propagate oblique to the magnetic field, (θ ~ 60\textdegree) and have magnetic compressibility C = |δB|||/|δB⊥| \~ 1, where δB|| (δB⊥) are the average amplitudes of the fluctuations parallel (perpendicular) to the mean field. Electric field fluctuations are present whenever the magnetic field is disturbed, and large electric field fluctuations follow the same pattern for quiet and disturbed intervals. Magnetic frequency power spectra at both spacecraft correspond to steep power-laws \~ f \textendashα with 4 < α < 5 for f ≲ 2 Hz, and 1.1 < α < 1.7 for f ≲ 2 Hz, spectral profiles that are consistent with weak Kinetic Alfv\ en Waves (KAW) turbulence. Electric power is larger than magnetic power for all frequencies above 0.1 Hz, and the ratio increases with increasing frequency. Vlasov linear analysis is consistent with the presence of compressive KAW with k⊥ρi ≲ 1, right-handed polarization and positive magnetic helicity, in the plasma frame, considering a multi-ion plasma. All these results suggest the presence of weak KAW turbulence which dissipates the energy associated with the intermittent sudden changes in the magnetic field during the main phase of the storm. Moya, Pablo.; Pinto, V\; Vi\~nas, Adolfo; Sibeck, David; Kurth, William; Hospodarsky, George; Wygant, John; Published by: Journal of Geophysical Research: Space Physics Published on: 06/2015 YEAR: 2015 DOI: 10.1002/2014JA020281 Kinetic Alfven Waves; Magnetic Storms; Radiation belts; Van Allen Probes |
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The Van-Allen probes, low-altitude NOAA satellite, MetOp satellite and riometer are used to analyze variations of precipitating energetic electron fluxes and cosmic radio noise absorption (CNA) driven by plasmaspheric hiss with respect to geomagnetic activities. The hiss-driven energetic electron precipitations (at L~4.7-5.3, MLT~8-9) are observed during geomagnetic quiet condition and substorms, respectively. We find that the CNA detected by riometers increased very little in the hiss-driven event during quiet condition on September 06, 2012. The hiss-driven enhancement of riometer was still little during the first substorm on September 30, 2012. However, the absorption detected by the riometer largely increased while the energies of the injected electrons became higher during the second substorm on September 30, 2012. The enhancement of CNA (ΔCNA) observed by the riometer and calculated with precipitating energetic electrons are in agreement during the second substorm, implying that the precipitating energetic electrons increase CNA to an obviously detectable level of the riometer during the second substorm on September 30, 2012. The conclusion is consistent with Rodger et al. (2012), which suggests that the higher level of ΔCNA prefer to occur in the substorms, because substorms may produce more intense energetic electron precipitation associated with electron injection. Furthermore, the combination of the observations and theory calculations also suggests that higher-energy electron (>55 keV) precipitation contribute more to the ΔCNA than the lower-energy electron precipitation. In this paper, the higher-energy electron precipitation is related to lower-frequency hiss. Li, Haimeng; Yuan, Zhigang; Yu, Xiongdong; Huang, Shiyong; Wang, Dedong; Wang, Zhenzhen; Qiao, Zheng; Wygant, John; Published by: Journal of Geophysical Research: Space Physics Published on: 06/2015 YEAR: 2015 DOI: 10.1002/2015JA021113 cosmic radio noise absorption; energetic electron precipitation; hiss; substorm; Van Allen Probes |
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Simultaneous Pi2 observations by the Van Allen Probes inside and outside the plasmasphere Plasmaspheric virtual resonance (PVR) model has been proposed as one of source mechanisms for low-latitude Pi2 pulsations. Since PVR-associated Pi2 pulsations are not localized inside the plasmasphere, simultaneous multipoint observations inside and outside the plasmasphere require to test the PVR model. Until now, however, there are few studies using simultaneous multisatellite observations inside and outside the plasmasphere for understanding the radial structure of Pi2 pulsation. In this study, we focus on the Pi2 event observed at low-latitude Bohyun (BOH, L = 1.35) ground station in South Korea in the postmidnight sector (magnetic local time (MLT) = 3.0) for the interval from 1730 to 1900 UT on 12 March 2013. By using electron density derived from the frequency of the upper hybrid waves detected at Van Allen Probe-A (VAP-A) and Van Allen Probe-B (VAP-B), the plasmapause is identified. At the time of the Pi2 event, VAP-A was outside the plasmasphere near midnight (00:55 MLT and L = ~6), while VAP-B was inside the plasmasphere in the postmidnight sector (02:15 MLT and L = ~5). VAP-B observed oscillations in the compressional magnetic field component (Bz) and the dawn-to-dusk electric field component (Ey), having high coherence with the BOH Pi2 pulsation in the H component. The H - Bz and H - Ey cross phases at VAP-B inside the plasmasphere were near -180\textdegree and -90\textdegree, respectively.These phase relationships among Bz, Ey, and H are consistent with a radially standing oscillation of the fundamental mode reported in previous studies. At VAP-A outside the plasmasphere, Bz oscillations were highly correlated with BOH Pi2 pulsations with ~-180\textdegree phase delay, and the H-Ey cross phase is near -90\textdegree. From these two-satellite observations, we suggest that the fundamental PVR mode is directly detected by VAP-A and VAP-B. Ghamry, E.; Kim, K.-H.; Kwon, H.-J.; Lee, D.-H.; Park, J.-S.; Choi, J.; Hyun, K.; Kurth, W.; Kletzing, C.; Wygant, J.; Published by: Journal of Geophysical Research: Space Physics Published on: 05/2015 YEAR: 2015 DOI: 10.1002/2015JA021095 Pi2; plasmasphere; Plasmaspheric virtual resonance; Van Allen Probes |
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Statistical characteristic of EMIC waves: Van Allen Probe observations Utilizing the data from the magnetometer instrument which is a part of the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) instrument suite onboard the Van Allen Probe A from Sep. 2012 to Apr. 2014, when the apogee of the satellite has passed all the MLT sectors, we obtain the statistical distribution characteristic of EMIC waves in the inner magnetosphere over all local times from L=3 to L=6. Compared with the previous statistical results about EMIC waves, the occurrence rates of EMIC waves distribute relatively uniform in the MLT sectors in lower L-shells. On the other hand, in higher L-shells, there are indeed some peaks of the occurrence rate for the EMIC waves, especially in the noon, dusk and night sectors. EMIC waves appear at lower L-shells in the dawn sector than in other sectors. In the lower L-shells (L<4), the occurrence rates of EMIC waves are significant in the dawn sector. This phenomenon may result from the distribution characteristic of the plasmasphere. The location of the plasmapause is usually lower in the dawn sector than that in other sectors, and the plasmapause is considered to be the favored region for the generation of EMIC waves. In higher L-shells (L>4) the occurrence rates of EMIC waves are most significant in the dusk sector, implying the important role of the plasmapause or plasmaspheric plume in generating EMIC waves. We have also investigated the distribution characteristics of the hydrogen band and the helium band EMIC waves. Surprisingly, in the inner magnetosphere, the hydrogen band EMIC waves occur more frequently than the helium band EMIC waves. Both them have peaks of occurrence rate in noon, dusk and night sectors, and the hydrogen band EMIC waves have more obvious peaks than the helium band EMIC waves in the night sector, while the helium band EMIC waves are more concentrated than the hydrogen band EMIC waves in the dusk sector. Both them occur significantly in the noon sector, which implies the important role of the solar wind dynamic pressure. Wang, Dedong; Yuan, Zhigang; Yu, Xiongdong; Deng, Xiaohua; Zhou, Meng; Huang, Shiyong; Li, Haimeng; Wang, Zhenzhen; Qiao, Zheng; Kletzing, C.; Wygant, J.; Published by: Journal of Geophysical Research: Space Physics Published on: 05/2015 YEAR: 2015 DOI: 10.1002/2015JA021089 |
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Poloidal ULF waves are capable of efficiently interacting with energetic particles in the ring current and the radiation belt. Using Van Allen Probes (RBSP) data from October 2012 to July 2014, we investigate the spatial distribution and storm-time occurrence of Pc4 (7-25 mHz) poloidal waves in the inner magnetosphere. Pc4 poloidal waves are sorted into two categories: waves with and without significant magnetic compressional components. Two types of poloidal waves have comparable occurrence rates, both of which are much higher during geomagnetic storms. The non-compressional poloidal waves mostly occur in the late recovery phase associated with an increase of Dst toward 0, suggesting that the decay of the ring current provides their free energy source. The occurrence of dayside compressional Pc4 poloidal waves is found correlated with the variation of the solar wind dynamic pressure, indicating their origin in the solar wind. Both compressional and non-compressional waves preferentially occur on the dayside near noon at L~5-6. In addition, compressional poloidal waves are observed at MLT 18-24 on the nightside. The location of the Pc4 poloidal waves relative to the plasmapause is investigated. The RBSP statistical results may shed light on the in-depth investigations of the generation and propagation of Pc4 poloidal waves. Dai, Lei; Takahashi, Kazue; Lysak, Robert; Wang, Chi; Wygant, John; Kletzing, Craig; Bonnell, John; Cattell, Cynthia; Smith, Charles; MacDowall, Robert; Thaller, Scott; Breneman, Aaron; Tang, Xiangwei; Tao, Xin; Chen, Lunjin; Published by: Journal of Geophysical Research: Space Physics Published on: 05/2015 YEAR: 2015 DOI: 10.1002/2015JA021134 Geomagnetic storm; Pc4 ULF waves; poloidal waves; ring current; solar wind dynamic pressure; Van Allen Probes |
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Using the Van Allen Probes we investigate the enhancement in the large scale duskward convection electric field during the geomagnetic storm (Dst ~ -120 nT) on June 1, 2013 and its role in ring current ion transport and energization, and plasmasphere erosion. During this storm, enhancements of ~1-2 mV/m in the duskward electric field in the co-rotating frame are observed down to L shells as low as ~2.3. A simple model consisting of a dipole magnetic field and constant, azimuthally westward, electric field is used to calculate the earthward and westward drift of 90\textdegree pitch angle ions. This model is applied to determine how far earthward ions can drift while remaining on Earth\textquoterights night side, given the strength and duration of the convection electric field. The calculation based on this simple model indicates that the enhanced duskward electric field is of sufficient intensity and duration to transport ions from a range of initial locations and initial energies characteristic of (though not observed by the Van Allen Probes) the earthward edge of the plasma sheet during active times ( L ~ 6\textendash10 and ~1-20 keV) to the observed location of the 58\textendash267 keV ion population, chosen as representative of the ring current (L ~3.5 \textendash 5.8). According to the model calculation, this transportation should be concurrent with an energization to the range observed, ~58-267 keV. Clear coincidence between the electric field enhancement and both plasmasphere erosion and ring current ion (58\textendash267 keV) pressure enhancements are presented. We show for the first time, nearly simultaneous enhancements in the duskward convection electric field, plasmasphere erosion, and increased pressure of 58\textendash267 keV ring current ions. These 58\textendash267 keV ions have energies that are consistent with what they are expected to pick up by gradient B drifting across the electric field. These observations strongly suggest that we are observing the electric field that energizes the ions and produces the erosion of the plasmasphere. Thaller, S.; Wygant, J.; Dai, L.; Breneman, A.W.; Kersten, K.; Cattell, C.A.; Bonnell, J.W.; Fennell, J.F.; Gkioulidou, Matina; Kletzing, C.A.; De Pascuale, S.; Hospodarsky, G.B.; Bounds, S.; Published by: Journal of Geophysical Research: Space Physics Published on: 05/2015 YEAR: 2015 DOI: 10.1002/2014JA020875 electric field; inner magnetosphere; plasma convection; plasmasphere; ring current; Van Allen Probes |
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Electric field structures and waves at plasma boundaries in the inner magnetosphere Recent observations by the Van Allen Probes spacecraft have demonstrated that a variety of electric field structures and nonlinear waves frequently occur in the inner terrestrial magnetosphere, including phase space holes, kinetic field line resonances, nonlinear whistler mode waves, and several types of double layer. However, it is unclear whether such structures and waves have a significant impact on the dynamics of the inner magnetosphere, including the radiation belts and ring current. To make progress toward quantifying their importance, this study statistically evaluates the correlation of such structures and waves with plasma boundaries. A strong correlation is found. These statistical results, combined with observations of electric field activity at propagating plasma boundaries, are consistent with the scenario that the sources of the free energy for the structures and waves of interest are localized near and comove with these boundaries. Therefore, the ability of these structures and waves to influence plasma in the inner magnetosphere is governed in part by the spatial extent and dynamics of macroscopic plasma boundaries in that region. Malaspina, David; Wygant, John; Ergun, Robert; Reeves, Geoff; Skoug, Ruth; Larsen, Brian; Published by: Journal of Geophysical Research: Space Physics Published on: 05/2015 YEAR: 2015 DOI: 10.1002/2015JA021137 injection; inner magnetosphere; nonlinear electric field structures; plasma boundary; plasma sheet; Van Allen Probes |
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Postmidnight depletion of the high-energy tail of the quiet plasmasphere The Van Allen Probes Helium Oxygen Proton Electron (HOPE) instrument measures the high-energy tail of the thermal plasmasphere allowing study of topside ionosphere and inner magnetosphere coupling. We statistically analyze a 22 month period of HOPE data, looking at quiet times with a Kp index of less than 3. We investigate the high-energy range of the plasmasphere, which consists of ions at energies between 1 and 10 eV and contains approximately 5\% of total plasmaspheric density. Both the fluxes and partial plasma densities over this energy range show H+ is depleted the most in the postmidnight sector (1\textendash4 magnetic local time), followed by O+ and then He+. The relative depletion of each species across the postmidnight sector is not ordered by mass, which reveals ionospheric influence. We compare our results with keV energy electron data from HOPE and the Van Allen Probes Electric Fields and Waves instrument spacecraft potential to rule out spacecraft charging. Our conclusion is that the postmidnight ion disappearance is due to diurnal ionospheric temperature variation and charge exchange processes. Sarno-Smith, Lois; Liemohn, Michael; Katus, Roxanne; Skoug, Ruth; Larsen, Brian; Thomsen, Michelle; Wygant, John; Moldwin, Mark; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2015 YEAR: 2015 DOI: 10.1002/2014JA020682 ion composition; Ionosphere; plasmasphere; postmidnight; quiet time magnetosphere; Van Allen Probes |
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Study of EMIC wave excitation using direct ion measurements With data from Van Allen Probes, we investigate EMIC wave excitation using simultaneously observed ion distributions. Strong He-band waves occurred while the spacecraft was moving through an enhanced density region. We extract from Helium, Oxygen, Proton, and Electron (HOPE) Mass Spectrometer measurement the velocity distributions of warm heavy ions as well as anisotropic energetic protons that drive wave growth through the ion cyclotron instability. Fitting the measured ion fluxes to multiple sinm-type distribution functions, we find that the observed ions make up about 15\% of the total ions, but about 85\% of them are still missing. By making legitimate estimates of the unseen cold (below ~2 eV) ion composition from cutoff frequencies suggested by the observed wave spectrum, a series of linear instability analyses and hybrid simulations are carried out. The simulated waves generally vary as predicted by linear theory. They are more sensitive to the cold O+ concentration than the cold He+ concentration. Increasing the cold O+ concentration weakens the He-band waves but enhances the O-band waves. Finally, the exact cold ion composition is suggested to be in a range when the simulated wave spectrum best matches the observed one. Min, Kyungguk; Liu, Kaijun; Bonnell, John; Breneman, Aaron; Denton, Richard; Funsten, Herbert; Jahn, öerg-Micha; Kletzing, Craig; Kurth, William; Larsen, Brian; Reeves, Geoffrey; Spence, Harlan; Wygant, John; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2015 YEAR: 2015 DOI: 10.1002/2014JA020717 EMIC wave excitation; observation; linear theory and hybrid simulation; Van Allen Probes |
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Disappearance of plasmaspheric hiss following interplanetary shock Plasmaspheric hiss is one of the important plasma waves controlling radiation belt dynamics. Its spatiotemporal distribution and generation mechanism are presently the object of active research. We here give the first report on the shock-induced disappearance of plasmaspheric hiss observed by the Van Allen Probes on 8 October 2013. This special event exhibits the dramatic variability of plasmaspheric hiss and provides a good opportunity to test its generation mechanisms. The origination of plasmaspheric hiss from plasmatrough chorus is suggested to be an appropriate prerequisite to explain this event. The shock increased the suprathermal electron fluxes, and then the enhanced Landau damping promptly prevented chorus waves from entering the plasmasphere. Subsequently, the shrinking magnetopause removed the source electrons for chorus, contributing significantly to the several-hours-long disappearance of plasmaspheric hiss. Su, Zhenpeng; Zhu, Hui; Xiao, Fuliang; Zheng, Huinan; Wang, Yuming; Shen, Chao; Zhang, Min; Wang, Shui; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Spence, H.; Reeves, G.; Funsten, H.; Blake, J.; Baker, D.; Wygant, J.; Published by: Geophysical Research Letters Published on: 03/2015 YEAR: 2015 DOI: 10.1002/2015GL063906 Cyclotron instability; Cyclotron resonance; interplanetary shock; Landau damping; Plasmaspheric Hiss; Radiation belt; Van Allen Probes |
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In situ observations of EMIC waves in O + band by the Van Allen Probe A Through polarization and spectra analysis of the magnetic field observed by the Van Allen Probe A, we present two typical cases of O+ band EMIC waves in the outer plasmasphere or plasma trough. Although such O+ band EMIC waves are rarely observed, 18 different events of O+ band EMIC waves (16 events in the outer plasmasphere and 2 events in the plasma trough) are found from September 2012 to August 2014 with observations of the Van Allen Probe A. We find that the preferred region for the occurrence of O+ band EMIC waves is in L = 2-5 and MLT = 03-13, 19-20, which is in accordance with the occurrence region of O+ ion torus. Therefore, our result suggests that the O+ ion torus in the outer plasmasphere during geomagnetic activities should play an important role in the generation of EMIC waves in O+ band. Yu, Xiongdong; Yuan, Zhigang; Wang, Dedong; Li, Haimeng; Huang, Shiyong; Wang, Zhenzhen; Zheng, Qiao; Zhou, Mingxia; Kletzing, C.; Wygant, J.; Published by: Geophysical Research Letters Published on: 02/2015 YEAR: 2015 DOI: 10.1002/2015GL063250 |
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We report, for the first time, an auroral undulation event on 1 May 2013 observed by an all-sky imager (ASI) at Athabasca (L = 4.6), Canada, for which in situ field and particle measurements in the conjugate magnetosphere were available from a Van Allen Probes spacecraft. The ASI observed a train of auroral undulation structures emerging spontaneously in the pre-midnight subauroral ionosphere, during the growth phase of a substorm. The undulations had an azimuthal wavelength of ~180 km and propagated westward at a speed of 3\textendash4 km s-1. The successive passage over an observing point yielded quasi-periodic oscillations in diffuse auroral emissions with a period of ~40 s. The azimuthal wave number m of the auroral luminosity oscillations was found to be m ~ -103. During the event the spacecraft \textendash being on tailward stretched field lines ~0.5 RE outside the plasmapause that mapped into the ionosphere conjugate to the auroral undulations \textendash encountered intense poloidal ULF oscillations in the magnetic and electric fields. We identify the field oscillations to be the second harmonic mode along the magnetic field line through comparisons of the observed wave properties with theoretical predictions. The field oscillations were accompanied by oscillations in proton and electron fluxes. Most interestingly, both field and particle oscillations at the spacecraft had one-to-one association with the auroral luminosity oscillations around its footprint. Our findings strongly suggest that this auroral undulation event is closely linked to the generation of second harmonic poloidal waves Motoba, T.; Takahashi, K.; Ukhorskiy, A.; Gkioulidou, M.; Mitchell, D.; Lanzerotti, L.; Korotova, G.; Donovan, E.; Wygant, J.; Kletzing, C.; Kurth, W.; Blake, J.; Published by: Journal of Geophysical Research: Space Physics Published on: 02/2015 YEAR: 2015 DOI: 10.1002/2014JA020863 |
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Modeling sub-auroral polarization streams (SAPS) during the March 17, 2013 storm The sub-auroral polarization streams (SAPS) are one of the most important features in representing magnetosphere-ionosphere coupling processes. In this study, we use a state-of-the-art modeling framework that couples an inner magnetospheric ring current model RAM-SCB with a global MHD model BATS-R-US and an ionospheric potential solver to study the SAPS that occurred during the March 17, 2013 storm event as well as to assess the modeling capability. Both ionospheric and magnetospheric signatures associated with SAPS are analyzed to understand the spatial and temporal evolution of the electrodynamics in the mid-latitude regions. Results show that the model captures the SAPS at sub-auroral latitudes, where Region-2 field-aligned currents (FACs) flow down to the ionosphere and the conductance is lower than in the higher-latitude auroral zone. Comparisons to observations such as FACs observed by AMPERE, cross-track ion drift from DMSP, and in-situ electric field observations from the Van Allen Probes indicate that the model generally reproduces the global dynamics of the Region-2 FACs, the position of SAPS along the DMSP, and the location of the SAPS electric field around L of 3.0 in the inner magnetosphere near the equator. While the model demonstrates double westward flow channels in the dusk sector (the higher-latitude auroral convection and the sub-auroral SAPS) and captures the mechanism of the SAPS, the comparison with ion drifts along DMSP trajectories shows an underestimate of the magnitude of the SAPS and the sensitivity to the specific location and time. The comparison of the SAPS electric field with that measured from the Van Allen Probes shows that the simulated SAPS electric field penetrates deeper than in reality, implying that the shielding from the Region-2 FACs in the model is not well represented. Possible solutions in future studies to improve the modeling capability include implementing a self-consistent ionospheric conductivity module from particle precipitation, coupling with the thermosphere-ionosphere chemical processes, and connecting the ionosphere with the inner magnetosphere by the stronger Region-2 FACs calculated in the inner magnetosphere model. Yu, Yiqun; Jordanova, Vania; Zou, Shasha; Heelis, Roderick; Ruohoniemi, Mike; Wygant, John; Published by: Journal of Geophysical Research: Space Physics Published on: 02/2015 YEAR: 2015 DOI: 10.1002/2014JA020371 |
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Energetic electron injections deep into the inner magnetosphere associated with substorm activity From a survey of the first nightside season of NASA\textquoterights Van Allen Probes mission (Dec/2012 \textendash Sep/2013), 47 energetic (10s to 100s of keV) electron injection events were found at L-shells <= 4, all of which are deeper than any previously reported substorm-related injections. Preliminary details from these events are presented, including how: all occurred shortly after dipolarization signatures and injections were observed at higher L-shells; the deepest observed injection was at L~2.5; and, surprisingly, L<=4 injections are limited in energy to <=250 keV. We present a detailed case study of one example event revealing that the injection of electrons down to L~3.5 was different from injections observed at higher L and likely resulted from drift resonance with a fast magnetosonic wave in the Pi 2 frequency range inside the plasmasphere. These observations demonstrate that injections occur at very low L-shells and may play an important role for inner zone electrons. Turner, D.; Claudepierre, S.; Fennell, J.; O\textquoterightBrien, T.; Blake, J.; Lemon, C.; Gkioulidou, M.; Takahashi, K.; Reeves, G.; Thaller, S.; Breneman, A.; Wygant, J.; Li, W.; Runov, A.; Angelopoulos, V.; Published by: Geophysical Research Letters Published on: 02/2015 YEAR: 2015 DOI: 10.1002/2015GL063225 energetic particle injections; inner magnetosphere; Radiation belts; substorms; THEMIS; Van Allen Probes |
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Modeling CME-shock driven storms in 2012 - 2013: MHD-test particle simulations The Van Allen Probes spacecraft have provided detailed observations of the energetic particles and fields environment for CME-shock driven storms in 2012 to 2013 which have now been modeled with MHD-test particle simulations. The Van Allen Probes orbital plane longitude moved from the dawn sector in 2012 to near midnight and pre-noon for equinoctial storms of 2013, providing particularly good measurements of the inductive electric field response to magnetopause compression for the 8 October 2013 CME-shock driven storm. An abrupt decrease in the outer boundary of outer zone electrons coincided with inward motion of the magnetopause for both 17 March and 8 October 2013 storms, as was the case for storms shortly after launch (Hudson et al., 2014). Modeling magnetopause dropout events in 2013 with electric field diagnostics that were not available for storms immediately following launch has improved our understanding of the complex role that ULF waves play in radial transport during such events. Hudson, M.; Paral, J.; Kress, B.; Wiltberger, M.; Baker, D.; Foster, J.; Turner, D.; Wygant, J.; Published by: Journal of Geophysical Research: Space Physics Published on: 01/2015 YEAR: 2015 DOI: 10.1002/2014JA020833 |
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Shock-Induced Prompt Relativistic Electron Acceleration In the Inner Magnetosphere We present twin Van Allen Probes spacecraft observations of the effects of a solar wind shock impacting the magnetosphere on 8 October 2013. The event provides details both of the accelerating electric fields associated with the shock and the response of inner magnetosphere electron populations across a broad range of energies. During this period the two Van Allen Probes observed shock effects from the vantage point of the dayside magnetosphere at radial positions of L=3 and L=5, at the location where shock-induced acceleration of relativistic electrons occurs. The extended (~1 min) duration of the accelerating electric field across a broad extent of the dayside magnetosphere, coupled with energy dependent relativistic electron gradient drift velocities, selects a preferred range of energies (3 \textendash 4 MeV) for the initial enhancement. Those electrons—whose drift velocity closely matches the azimuthal phase velocity of the shock-induced pulse— stayed in the accelerating wave as it propagated tailward and received the largest increase in energy. Drift resonance with subsequent strong ULF waves further accentuated this range of electron energies. Phase space density and positional considerations permit identification of the source population of the energized electrons. Observations detail the promptness (<20 min), energy range (1.5-4.5 MeV), energy increase (~500 keV), and spatial extent (L*~3.5-4.0) of the enhancement of the relativistic electrons. Prompt acceleration by impulsive shock-induced electric fields and subsequent ULF wave processes therefore comprises a significant mechanism for the acceleration of highly relativistic electrons deep inside the outer radiation belt as shown clearly by this event. Foster, J.; Wygant, J.; Hudson, M.; Boyd, A.; Baker, D.; Erickson, P.; Spence, H.; Published by: Journal of Geophysical Research: Space Physics Published on: 01/2015 YEAR: 2015 DOI: 10.1002/2014JA020642 |
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We present Van Allen Probe B observations of azimuthally limited, antisymmetric, poloidal Pc 4 electric and magnetic field pulsations in the pre-midnight sector of the magnetosphere from 05:40 to 06:00 UT on 1 May 2013. Oscillation periods were similar for the magnetic and electric fields and proton fluxes. The flux of energetic protons exhibited an energy-dependent response to the pulsations. Energetic proton variations were anticorrelated at medium and low energies. Although we attribute the pulsations to a drift-bounce resonance, we demonstrate that the energy-dependent response of the ion fluxes results from pulsation-associated velocities sweeping energy-dependent radial ion flux gradients back and forth past the spacecraft. Korotova, G.; Sibeck, D.; Tahakashi, K.; Dai, L.; Spence, H.; Kletzing, C.; Wygant, J.; Manweiler, J.; Moya, P.; Hwang, K.-J.; Redmon, R.; Published by: Annales Geophysicae Published on: 01/2015 YEAR: 2015 DOI: 10.5194/angeo-33-955-2015 |
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During 18 February to 2 March 2014, the Van Allen Probes encountered multiple geomagnetic storms and simultaneously observed intensified chorus and hiss waves. During this period, there were substantial enhancements in fluxes of energetic (53.8 - 108.3 keV) and relativistic (2 - 3.6 MeV) electrons. Chorus waves were excited at locations L = 4 - 6.2 after the fluxes of energetic were greatly enhanced, with a lower frequency band and wave amplitudes \~ 20 - 100 pT. Strong hiss waves occurred primarily in the main phases or below the location L = 4 in the recovery phases. Relativistic electron fluxes decreased in the main phases due to the adiabatic (e.g., the magnetopause shadowing) or non-adiabatic (hiss-induced scattering) processes. In the recovery phases, relativistic electron fluxes either increased in the presence of enhanced chorus, or remained unchanged in the absence of strong chorus or hiss. The observed relativistic electron phase space density peaked around L* = 4.5, characteristic of local acceleration. This multiple-storm period reveals a typical picture that chorus waves are excited by the energetic electrons at first and then produce efficient acceleration of relativistic electrons. This further demonstrates that the interplay between both competing mechanisms of chorus-driven acceleration and hiss-driven scattering often occurs in the outer radiation belts. Liu, Si; Xiao, Fuliang; Yang, Chang; He, Yihua; Zhou, Qinghua; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Spence, H.; Reeves, G.; Funsten, H.; Blake, J.; Baker, D.; Wygant, J.; Published by: Journal of Geophysical Research: Space Physics Published on: 01/2015 YEAR: 2015 DOI: 10.1002/2014JA020781 |
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The twin Van Allen Probe spacecraft, launched in August 2012, carry identical scientific payloads. The Electric and Magnetic Fields Instrument Suite and Integrated Science (EMFISIS) suite includes a plasma wave instrument (Waves) that measures three magnetic and three electric components of plasma waves in the frequency range of 10 Hz to 12 kHz using triaxial search coils and the Electric Fields and Waves (EFW) triaxial electric field sensors. The Waves instrument also measures a single electric field component of waves in the frequency range of 10 to 500 kHz. A primary objective of the higher frequency measurements is the determination of the electron density ne at the spacecraft, primarily inferred from the upper hybrid resonance frequency fuh. Considerable work has gone into developing a process and tools for identifying and digitizing the upper hybrid resonance frequency in order to infer the electron density as an essential parameter for interpreting not only the plasma wave data from the mission, but also as input to various magnetospheric models. Good progress has been made in developing algorithms to identify fuh and create a data set of electron densities. However, it is often difficult to interpret the plasma wave spectra during active times to identify fuh and accurately determine ne. In some cases there is not a clear signature of the upper hybrid band and the low-frequency cutoff of the continuum radiation is used. We describe the expected accuracy of ne and issues in the interpretation of the electrostatic wave spectrum. Kurth, W.; De Pascuale, S.; Faden, J.; Kletzing, C.; Hospodarsky, G.; Thaller, S.; Wygant, J.; Published by: Journal of Geophysical Research: Space Physics Published on: 01/2015 YEAR: 2015 DOI: 10.1002/2014JA020857 |
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Van Allen Probe Observations of Periodic Rising Frequencies of the Fast Magnetosonic Mode Near simultaneous periodic dispersive features of fast magnetosonic mode emissions are observed by both Van Allen Probes spacecraft while separated in magnetic local time by ~5 hours: Probe A at 15 and Probe B at 9\textendash11 hours. Both spacecraft see similar frequency features, characterized by a periodic repetition at ~180 s. Each repetition is characterized by a rising frequency. Since no modulation is observed in the proton shell distribution, the plasma density, or in the background magnetic field at either spacecraft we conclude that these waves are not generated near the spacecraft but external to both spacecraft locations. Probe A while outside the plasmapause sees the start of each repetition ~40 s before probe B while deep inside the plasmasphere. We can qualitatively reproduce the dispersive features, but not the quantitative details. The cause for this phenomena remains to be identified. Boardsen, S.; Hospodarsky, G.; Kletzing, C.; Pfaff, R.; Kurth, W.; Wygant, J.; MacDonald, E.; Published by: Geophysical Research Letters Published on: 12/2014 YEAR: 2014 DOI: 10.1002/2014GL062020 Fast Magnetosonic Waves; Inner Dayside Magnetosphere; Periodic-Dispersive Features; Van Allen Probes |
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Evolution of relativistic outer belt electrons during an extended quiescent period To effectively study steady loss due to hiss-driven precipitation of relativistic electrons in the outer radiation belt, it is useful to isolate this loss by studying a time of relatively quiet geomagnetic activity. We present a case of initial enhancement and slow, steady decay of 700 keV - 2 MeV electron populations in the outer radiation belt during an extended quiescent period from ~15 December 2012 - 13 January 2013. We incorporate particle measurements from a constellation of satellites, including the Colorado Student Space Weather Experiment (CSSWE) CubeSat, the Van Allen Probes twin spacecraft, and THEMIS, to understand the evolution of the electron populations across pitch angle and energy. Additional data from calculated phase space density (PSD), as well as hiss and chorus wave data from Van Allen Probes, helps complete the picture of the slow precipitation loss of relativistic electrons during a quiet time. Electron loss to the atmosphere during this event is quantified through use of the Loss Index Method, utilizing CSSWE measurements at LEO. By comparing these results against equatorial Van Allen Probes electron flux data, we conclude the net precipitation loss of the outer radiation belt content to be greater than 92\%, suggesting no significant acceleration during this period, and resulting in faster electron loss rates than have previously been reported. Jaynes, A.; Li, X.; Schiller, Q.; Blum, L.; Tu, W.; Turner, D.; Ni, B.; Bortnik, J.; Baker, D.; Kanekal, S.; Blake, J.; Wygant, J.; Published by: Journal of Geophysical Research: Space Physics Published on: 12/2014 YEAR: 2014 DOI: 10.1002/2014JA020125 electron lifetime; hiss waves; pitch angle scattering; precipitation loss; Radiation belts; Van Allen Probes |
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Externally driven plasmaspheric ULF waves observed by the Van Allen Probes We analyze data acquired by the Van Allen Probes on 8 November 2012, during a period of extended low geomagnetic activity, to gain new insight into plasmaspheric ultra-low-frequency (ULF) waves. The waves exhibited strong spectral power in the 5\textendash40 mHzband and included multiharmonic toroidal waves visible up to the 11th harmonic, unprecedented in the plasmasphere. During this wave activity, the interplanetary magnetic field cone angle was small, suggesting that the waves were driven by broadband compressional ULF waves originating in the foreshock region. This source mechanism is supported by the tailward propagation of the compressional magnetic field perturbations at a phase velocity of a few hundred kilometers per second that is determined bythe cross phase analysis of data from the two spacecraft. We also find that the coherence and phase delay of the azimuthal components of the magnetic field from the two spacecraft strongly depend on the radial separation of the spacecraft and attribute this feature to field line resonance effects. Finally, using the observed toroidal wave frequencies, we estimate the plasma mass density for L = 2.6\textendash5.8. By comparing the mass density with the electron number density that is estimated from the spectrum of plasma waves, we infer that the plasma was dominated by H+ ions and was distributed uniformly along the magnetic field lines. The electron density is higher than the prediction of saturated plasmasphere models, and this \textquotedblleftsuper saturated\textquotedblright plasmasphere and the uniform ion distribution are consistent with the low geomagnetic activity that prevailed. Takahashi, Kazue; Denton, Richard; Kurth, William; Kletzing, Craig; Wygant, John; Bonnell, John; Dai, Lei; Min, Kyungguk; Smith, Charles; MacDowall, Robert; Published by: Journal of Geophysical Research: Space Physics Published on: 12/2014 YEAR: 2014 DOI: 10.1002/2014JA020373 multispacecraft observation; Van Allen Probes; plasmasphere; ULF waves |
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Excitation of nightside magnetosonic waves observed by Van Allen Probes During the recovery phase of the geomagnetic storm on 30-31 March 2013, Van Allen Probe A detected enhanced magnetosonic (MS) waves in a broad range of L =1.8-4.7 and MLT =17-22 h, with a frequency range ~10-100 Hz. In the meanwhile, distinct proton ring distributions with peaks at energies of ~10 keV, were also observed in L =3.2-4.6 and L =5.0-5.6. Using a subtracted bi-Maxwellian distribution to model the observed proton ring distribution, we perform three dimensional ray tracing to investigate the instability, propagation and spatial distribution of MS waves. Numerical results show that nightside MS waves are produced by proton ring distribution and grow rapidly from the source location L =5.6 to the location L =5.0, but remain nearly stable at locations L <5.0 Moreover, waves launched toward lower L-shells with different initial azimuthal angles propagate across different MLT regions with divergent paths at first, then gradually turn back toward higher L-shells and propagate across different MLT regions with convergent paths. The current results further reveal that MS waves are generated by a ring distribution of ~10 keV proton and proton ring in one region can contribute to the MS wave power in another region. Zhou, Qinghua; Xiao, Fuliang; Yang, Chang; Liu, Si; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Spence, H.; Reeves, G.; Funsten, H.; Blake, J.; Baker, D.; Wygant, J.; Published by: Journal of Geophysical Research: Space Physics Published on: 11/2014 YEAR: 2014 DOI: 10.1002/2014JA020481 magnetosonic wave; RBSP results; Van Allen Probes; Wave-particle interaction |
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An impenetrable barrier to ultrarelativistic electrons in the Van Allen radiation belts Early observations1, 2 indicated that the Earth\textquoterights Van Allen radiation belts could be separated into an inner zone dominated by high-energy protons and an outer zone dominated by high-energy electrons. Subsequent studies3, 4 showed that electrons of moderate energy (less than about one megaelectronvolt) often populate both zones, with a deep \textquoteleftslot\textquoteright region largely devoid of particles between them. There is a region of dense cold plasma around the Earth known as the plasmasphere, the outer boundary of which is called the plasmapause. The two-belt radiation structure was explained as arising from strong electron interactions with plasmaspheric hiss just inside the plasmapause boundary5, with the inner edge of the outer radiation zone corresponding to the minimum plasmapause location6. Recent observations have revealed unexpected radiation belt morphology7, 8, especially at ultrarelativistic kinetic energies9, 10 (more than five megaelectronvolts). Here we analyse an extended data set that reveals an exceedingly sharp inner boundary for the ultrarelativistic electrons. Additional, concurrently measured data11 reveal that this barrier to inward electron radial transport does not arise because of a physical boundary within the Earth\textquoterights intrinsic magnetic field, and that inward radial diffusion is unlikely to be inhibited by scattering by electromagnetic transmitter wave fields. Rather, we suggest that exceptionally slow natural inward radial diffusion combined with weak, but persistent, wave\textendashparticle pitch angle scattering deep inside the Earth\textquoterights plasmasphere can combine to create an almost impenetrable barrier through which the most energetic Van Allen belt electrons cannot migrate. Baker, D.; Jaynes, A.; Hoxie, V.; Thorne, R.; Foster, J.; Li, X.; Fennell, J.; Wygant, J.; Kanekal, S.; Erickson, P.; Kurth, W.; Li, W.; Ma, Q.; Schiller, Q.; Blum, L.; Malaspina, D.; Gerrard, A.; Lanzerotti, L.; Published by: Nature Published on: 11/2014 YEAR: 2014 DOI: 10.1038/nature13956 Magnetospheric physics; ultrarelativistic electrons; Van Allen Belts; Van Allen Probes |
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We present in situ observations of a shock-induced substorm-like event on 13 April 2013 observed by the newly launched Van Allen twin probes. Substorm-like electron injections with energy of 30\textendash500 keV were observed in the region from L\~5.2 to 5.5 immediately after the shock arrival (followed by energetic electron drift echoes). Meanwhile, the electron flux was clearly and strongly varying on the ULF wave time scale. It is found that both toroidal and poloidal mode ULF waves with a period of 150 s emerged following the magnetotail magnetic field reconfiguration after the interplanetary (IP) shock passage. The poloidal mode is more intense than the toroidal mode. The 90\textdegree phase shift between the poloidal mode Br and Ea suggests the standing poloidal waves in the Northern Hemisphere. Furthermore, the energetic electron flux modulations indicate that the azimuthal wave number is \~14. Direct evidence of drift resonance between the injected electrons and the excited poloidal ULF wave has been obtained. The resonant energy is estimated to be between 150 keV and 230 keV. Two possible scenaria on ULF wave triggering are discussed: vortex-like flow structure-driven field line resonance and ULF wave growth through drift resonance. It is found that the IP shock may trigger intense ULF wave and energetic electron behavior at L\~3 to 6 on the nightside, while the time profile of the wave is different from dayside cases. Hao, Y.; Zong, Q.-G.; Wang, Y.; Zhou, X.-Z.; Zhang, Hui; Fu, S; Pu, Z; Spence, H.; Blake, J.; Bonnell, J.; Wygant, J.; Kletzing, C.; Published by: Journal of Geophysical Research: Space Physics Published on: 10/2014 YEAR: 2014 DOI: 10.1002/2014JA020023 energetic particles; interplanetary shock; magnetotail ULF wave; poloidal and toroidal mode; Van Allen Probes; wave-particle interactions |
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March 2013 provided the first equinoctial period when all of the instruments on the Van Allen Probes spacecraft were fully operational. This interval was characterized by disturbances of outer zone electrons with two timescales of variation, diffusive and rapid dropout and restoration [Baker et al., 2014]. A radial diffusion model was applied to the month-long interval to confirm that electron phase space density is well described by radial diffusion for the whole month at low first invariant <=400 MeV/G, but peaks in phase space density observed by the ECT instrument suite at higher first invariant are not reproduced by radial transport from a source at higher L. The model does well for much of the month-long interval, capturing three of four enhancements in phase space density which emerge from the outer boundary, while the strong enhancement following dropout on 17-18 March requires local acceleration at higher first invariant (M = 1000 MeV/G vs. 200 MeV/G) not included in our model. We have incorporated phase space density from ECT measurement at the outer boundary and plasmapause determination from the EFW instrument to separate hiss and chorus loss models. Li, Zhao; Hudson, Mary; Jaynes, Allison; Boyd, Alexander; Malaspina, David; Thaller, Scott; Wygant, John; Henderson, Michael; Published by: Journal of Geophysical Research: Space Physics Published on: 10/2014 YEAR: 2014 DOI: 10.1002/2014JA020359 |
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THEMIS measurements of quasi-static electric fields in the inner magnetosphere We use four years of THEMIS double-probe measurements to offer, for the first time, a complete picture of the dawn-dusk electric field covering all local times and radial distances in the inner magnetosphere based on in situ equatorial observations. This study is motivated by the results from the CRRES mission, which revealed a local maximum in the electric field developing near Earth during storm times, rather than the expected enhancement at higher L shells that is shielded near Earth as suggested by the Volland-Stern model. The CRRES observations were limited to the dusk side, while THEMIS provides complete local time coverage. We show strong agreement with the CRRES results on the dusk side, with a local maximum near L =4 for moderate levels of geomagnetic activity and evidence of strong electric fields inside L =3 during the most active times. The extensive dataset from THEMIS also confirms the day/night asymmetry on the dusk side, where the enhancement is closest to Earth in the dusk-midnight sector, and is farther away closer to noon. A similar, but smaller in magnitude, local maximum is observed on the dawn side near L =4. The noon sector shows the smallest average electric fields, and for more active times, the enhancement develops near L =7 rather than L =4. We also investigate the impact of the uncertain boom-shorting factor on the results, and show that while the absolute magnitude of the electric field may be underestimated, the trends with geomagnetic activity remain intact. Califf, S.; Li, X.; Blum, L.; Jaynes, A.; Schiller, Q.; Zhao, H.; Malaspina, D.; Hartinger, M.; Wolf, R.; Rowland, D.; Wygant, J.; Bonnell, J.; Published by: Journal of Geophysical Research: Space Physics Published on: 10/2014 YEAR: 2014 DOI: 10.1002/2014JA020360 convection; double probe; electric field; inner magnetosphere |
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Nonlinear Electric Field Structures in the Inner Magnetosphere Van Allen Probes observations are presented which demonstrate the presence of nonlinear electric field structures in the inner terrestrial magnetosphere (< 6 RE). A range of structures are observed, including phase space holes and double layers.These structures are observed over several Earth radii in radial distance and over a wide range of magnetic local times. They are observed in the dusk, midnight, and dawn sectors, with the highest concentration pre-midnight. Some nonlinear electric field structures are observed to coincide with dipolarizations of the magnetic field and increases in electron energy flux for energies between 1 keV and 30 keV. Nonlinear electric field structures possess isolated impulsive electric fields, often with a significant component parallel to the ambient magnetic field, providing a mechanism for non-adiabatic wave-particle interactions in the inner magnetosphere. Malaspina, D.; Andersson, L.; Ergun, R.; Wygant, J.; Bonnell, J; Kletzing, C.; Reeves, G.; Skoug, R.; Larsen, B.; Published by: Geophysical Research Letters Published on: 08/2014 YEAR: 2014 DOI: 10.1002/2014GL061109 |
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Local acceleration driven by whistler mode chorus waves largely accounts for the enhancement of radiation belt relativistic electron fluxes, whose favored region is usually considered to be the plasmatrough with magnetic local time approximately from midnight through dawn to noon. On 2 October 2013, the Van Allen Probes recorded a rarely reported event of intense duskside lower band chorus waves (with power spectral density up to 10-3nT2/Hz) in the low-latitude region outside of L=5. Such chorus waves are found to be generated by the substorm-injected anisotropic suprathermal electrons and have a potentially strong acceleration effect on the radiation belt energetic electrons. This event study demonstrates the possibility of broader spatial regions with effective electron acceleration by chorus waves than previously expected. For such intense duskside chorus waves, the occurrence probability, the preferential excitation conditions, the time duration, and the accurate contribution to the long-term evolution of radiation belt electron fluxes may need further investigations in future. Su, Zhenpeng; Zhu, Hui; Xiao, Fuliang; Zheng, Huinan; Wang, Yuming; He, Zhaoguo; Shen, Chao; Shen, Chenglong; Wang, C.; Liu, Rui; Zhang, Min; Wang, Shui; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Spence, H.; Reeves, G.; Funsten, H.; Blake, J.; Baker, D.; Wygant, J.; Published by: Journal of Geophysical Research: Space Physics Published on: 06/2014 YEAR: 2014 DOI: 10.1002/jgra.v119.610.1002/2014JA019919 |
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Chorus acceleration of radiation belt relativistic electrons during March 2013 geomagnetic storm The recent launching of Van Allen probes provides an unprecedent opportunity to investigate variations of the radiation belt relativistic electrons. During the 17\textendash19 March 2013 storm, the Van Allen probes simultaneously detected strong chorus waves and substantial increases in fluxes of relativistic (2 - 4.5 MeV) electrons around L = 4.5. Chorus waves occurred within the lower band 0.1\textendash0.5fce (the electron equatorial gyrofrequency), with a peak spectral density \~10-4 nT2/Hz. Correspondingly, relativistic electron fluxes increased by a factor of 102\textendash103 during the recovery phase compared to the main phase levels. By means of a Gaussian fit to the observed chorus spectra, the drift and bounce-averaged diffusion coefficients are calculated and then used to solve a 2-D Fokker-Planck diffusion equation. Numerical simulations demonstrate that the lower-band chorus waves indeed produce such huge enhancements in relativistic electron fluxes within 15 h, fitting well with the observation. Xiao, Fuliang; Yang, Chang; He, Zhaoguo; Su, Zhenpeng; Zhou, Qinghua; He, Yihua; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Spence, H.; Reeves, G.; Funsten, H.; Blake, J.; Baker, D.; Wygant, J.; Published by: Journal of Geophysical Research: Space Physics Published on: 05/2014 YEAR: 2014 DOI: 10.1002/2014JA019822 |
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Van Allen Probes observations of direct wave-particle interactions Quasiperiodic increases, or \textquotedblleftbursts,\textquotedblright of 17\textendash26 keV electron fluxes in conjunction with chorus wave bursts were observed following a plasma injection on 13 January 2013. The pitch angle distributions changed during the burst events, evolving from sinN(α) to distributions that formed maxima at α = 75\textendash80\textdegree, while fluxes at 90\textdegree and <60\textdegree remained nearly unchanged. The observations occurred outside of the plasmasphere in the postmidnight region and were observed by both Van Allen Probes. Density, cyclotron frequency, and pitch angle of the peak flux were used to estimate resonant electron energy. The result of ~15\textendash35 keV is consistent with the energies of the electrons showing the flux enhancements and corresponds to electrons in and above the steep flux gradient that signals the presence of an Alfv\ en boundary in the plasma. The cause of the quasiperiodic nature (on the order of a few minutes) of the bursts is not understood at this time. Fennell, J.; Roeder, J.; Kurth, W.; Henderson, M.; Larsen, B.; Hospodarsky, G.; Wygant, J.; Claudepierre, J.; Blake, J.; Spence, H.; Clemmons, J.; Funsten, H.; Kletzing, C.; Reeves, G.; Published by: Geophysical Research Letters Published on: 03/2014 YEAR: 2014 DOI: 10.1002/2013GL059165 |
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We study the effect of electromagnetic ion cyclotron (EMIC) waves on the loss and pitch angle scattering of relativistic and ultrarelativistic electrons during the recovery phase of a moderate geomagnetic storm on 11 October 2012. The EMIC wave activity was observed in situ on the Van Allen Probes and conjugately on the ground across the Canadian Array for Real-time Investigations of Magnetic Activity throughout an extended 18 h interval. However, neither enhanced precipitation of >0.7 MeV electrons nor reductions in Van Allen Probe 90\textdegree pitch angle ultrarelativistic electron flux were observed. Computed radiation belt electron pitch angle diffusion rates demonstrate that rapid pitch angle diffusion is confined to low pitch angles and cannot reach 90\textdegree. For the first time, from both observational and modeling perspectives, we show evidence of EMIC waves triggering ultrarelativistic (~2\textendash8 MeV) electron loss but which is confined to pitch angles below around 45\textdegree and not affecting the core distribution. Usanova, M.; Drozdov, A.; Orlova, K.; Mann, I.; Shprits, Y.; Robertson, M.; Turner, D.; Milling, D.; Kale, A.; Baker, D.; Thaller, S.; Reeves, G.; Spence, H.; Kletzing, C.; Wygant, J.; Published by: Geophysical Research Letters Published on: 03/2014 YEAR: 2014 DOI: 10.1002/2013GL059024 |
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The dual-spacecraft Van Allen Probes mission has provided a new window into mega electron volt (MeV) particle dynamics in the Earth\textquoterights radiation belts. Observations (up to E ~10 MeV) show clearly the behavior of the outer electron radiation belt at different timescales: months-long periods of gradual inward radial diffusive transport and weak loss being punctuated by dramatic flux changes driven by strong solar wind transient events. We present analysis of multi-MeV electron flux and phase space density (PSD) changes during March 2013 in the context of the first year of Van Allen Probes operation. This March period demonstrates the classic signatures both of inward radial diffusive energization and abrupt localized acceleration deep within the outer Van Allen zone (L ~4.0 \textpm 0.5). This reveals graphically that both \textquotedblleftcompeting\textquotedblright mechanisms of multi-MeV electron energization are at play in the radiation belts, often acting almost concurrently or at least in rapid succession. Baker, D.; Jaynes, A.; Li, X.; Henderson, M.; Kanekal, S.; Reeves, G.; Spence, H.; Claudepierre, S.; Fennell, J.; Hudson, M.; Thorne, R.; Foster, J.; Erickson, P.; Malaspina, D.; Wygant, J.; Boyd, A.; Kletzing, C.; Drozdov, A.; Shprits, Y; Published by: Geophysical Research Letters Published on: 03/2014 YEAR: 2014 DOI: 10.1002/2013GL058942 |
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Observation and model results accumulated in the last decade indicate that substorms can promptly inject relativistic \textquoteleftkiller\textquoteright electrons (>=MeV) in addition to 10\textendash100 keV subrelativistic populations. Using measurements from Cluster, Polar, LANL, and GOES satellites near the midnight sector, we show in two events that intense electric fields, as large as 20 mV/m, associated with substorm dipolarization are associated with injections of relativistic electrons into the outer radiation belt. Enhancements of hundreds of keV electrons at dipolarization in the magnetotail can account for the injected MeV electrons through earthward transport. These observations provide evidence that substorm electric fields inject relativistic electrons by transporting magnetotail electrons into the outer radiation belt. In these two events, injected relativistic electrons dominated the substorm timescale enhancement of MeV electrons as observed at geosynchronous orbit. Dai, Lei; Wygant, John; Cattell, Cynthia; Thaller, Scott; Kersten, Kris; Breneman, Aaron; Tang, Xiangwei; Friedel, Reiner; Claudepierre, Seth; Tao, Xin; Published by: Geophysical Research Letters Published on: 02/2014 YEAR: 2014 DOI: 10.1002/2014GL059228 radiation belt relativistic electrons; substorm dipolarization; substorm electric fields; substorm injection |
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Spatial localization and ducting of EMIC waves: Van Allen Probes and ground-based observations On 11 October 2012, during the recovery phase of a moderate geomagnetic storm, an extended interval (> 18 h) of continuous electromagnetic ion cyclotron (EMIC) waves was observed by Canadian Array for Real-time Investigations of Magnetic Activity and Solar-Terrestrial Environment Program induction coil magnetometers in North America. At around 14:15 UT, both Van Allen Probes B and A (65\textdegree magnetic longitude apart) in conjunction with the ground array observed very narrow (ΔL ~ 0.1\textendash0.4) left-hand polarized EMIC emission confined to regions of mass density gradients at the outer edge of the plasmasphere at L ~ 4. EMIC waves were seen with complex polarization patterns on the ground, in good agreement with model results from Woodroffe and Lysak (2012) and consistent with Earth\textquoterights rotation sweeping magnetometer stations across multiple polarization reversals in the fields in the Earth-ionosphere duct. The narrow L-widths explain the relative rarity of space-based EMIC occurrence, ground-based measurements providing better estimates of global EMIC wave occurrence for input into radiation belt dynamical models. Mann, I.; Usanova, M.; Murphy, K.; Robertson, M.; Milling, D.; Kale, A.; Kletzing, C.; Wygant, J.; Thaller, S.; Raita, T.; Published by: Geophysical Research Letters Published on: 02/2014 YEAR: 2014 DOI: 10.1002/2013GL058581 |
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Storm time observations of plasmasphere erosion flux in the magnetosphere and ionosphere Plasmasphere erosion carries cold dense plasma of ionospheric origin in a storm-enhanced density plume extending from dusk toward and through the noontime cusp and dayside magnetopause and back across polar latitudes in a polar tongue of ionization. We examine dusk sector (20 MLT) plasmasphere erosion during the 17 March 2013 storm (Dst ~ -130 nT) using simultaneous, magnetically aligned direct sunward ion flux observations at high altitude by Van Allen Probes RBSP-A (at ~3.0 Re) and at ionospheric heights (~840 km) by DMSP F-18. Plasma erosion occurs at both high and low altitudes where the subauroral polarization stream flow overlaps the outer plasmasphere. At ~20 UT, RBSP-A observed ~1.2E12 m-2 s-1 erosion flux, while DMSP F-18 observed ~2E13 m-2 s-1 sunward flux. We find close similarities at high and low altitudes between the erosion plume in both invariant latitude spatial extent and plasma characteristics. Foster, J.; Erickson, P.; Coster, A.; Thaller, S.; Tao, J.; Wygant, J.; Bonnell, J; Published by: Geophysical Research Letters Published on: 02/2014 YEAR: 2014 DOI: 10.1002/2013GL059124 |
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Chorus waves and spacecraft potential fluctuations: Evidence for wave-enhanced photoelectron escape Chorus waves are important for electron energization and loss in Earth\textquoterights radiation belts and inner magnetosphere. Because the amplitude and spatial distribution of chorus waves can be strongly influenced by plasma density fluctuations and spacecraft floating potential can be a diagnostic of plasma density, the relationship between measured potential and chorus waves is examined using Van Allen Probes data. While measured potential and chorus wave electric fields correlate strongly, potential fluctuation properties are found not to be consistent with plasma density fluctuations on the timescales of individual chorus wave packets. Instead, potential fluctuations are consistent with enhanced photoelectron escape driven by chorus wave electric fields. Enhanced photoelectron escape may result in potential fluctuations of the spacecraft body, the electric field probes, or both, depending on the ambient plasma and magnetic field environment. These results differ significantly from prior interpretations of the correspondence between measured potential and wave electric fields. Malaspina, D.; Ergun, R.; Sturner, A.; Wygant, J.; Bonnell, J; Breneman, A.; Kersten, K.; Published by: Geophysical Research Letters Published on: 01/2014 YEAR: 2014 DOI: 10.1002/2013GL058769 |
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Observations of kinetic scale field line resonances We identify electromagnetic field variations from the Van Allen Probes which have the properties of Doppler shifted kinetic scale Alfv\ enic field line resonances. These variations are observed during injections of energetic plasmas into the inner magnetosphere. These waves have scale sizes perpendicular to the magnetic field which are determined to be of the order of an ion gyro-radius (ρi) and less. Cross-spectral analysis of the electric and magnetic fields reveals phase transitions at frequencies correlated with enhancements and depressions in the ratio of the electric and magnetic fields. Modeling shows that these observations are consistent with the excitation of field-line resonances over a broad range of wave numbers perpendicular to the magnetic field (k⊥) extending to k⊥ρi >> 1. The amplitude of these waves is such that E/Bo ≳ Ωi/k⊥ (E, Bo, and Ωi are the wave amplitude, background field strength, and ion gyro-frequency, respectively) leading to ion demagnetization and acceleration for multiple transitions through the wave potential. Chaston, Christopher; Bonnell, J; Wygant, John; Mozer, Forrest; Bale, Stuart; Kersten, Kris; Breneman, Aaron; Kletzing, Craig; Kurth, William; Hospodarsky, George; Smith, Charles; MacDonald, Elizabeth; Published by: Geophysical Research Letters Published on: 01/2014 YEAR: 2014 DOI: 10.1002/2013GL058507 |
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On 17 March 2013, a large magnetic storm significantly depleted the multi-MeV radiation belt. We present multi-instrument observations from the Van Allen Probes spacecraft Radiation Belt Storm Probe A and Radiation Belt Storm Probe B at ~6 Re in the midnight sector magnetosphere and from ground-based ionospheric sensors during a substorm dipolarization followed by rapid reenergization of multi-MeV electrons. A 50\% increase in magnetic field magnitude occurred simultaneously with dramatic increases in 100 keV electron fluxes and a 100 times increase in VLF wave intensity. The 100 keV electrons and intense VLF waves provide a seed population and energy source for subsequent radiation belt enhancements. Highly relativistic (>2 MeV) electron fluxes increased immediately at L* ~ 4.5 and 4.5 MeV flux increased >90 times at L* = 4 over 5 h. Although plasmasphere expansion brings the enhanced radiation belt multi-MeV fluxes inside the plasmasphere several hours postsubstorm, we localize their prompt reenergization during the event to regions outside the plasmasphere. Foster, J.; Erickson, P.; Baker, D.; Claudepierre, S.; Kletzing, C.; Kurth, W.; Reeves, G.; Thaller, S.; Spence, H.; Shprits, Y; Wygant, J.; Published by: Geophysical Research Letters Published on: 01/2014 YEAR: 2014 DOI: 10.1002/2013GL058438 |
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Huge numbers of double layers carrying electric fields parallel to the local magnetic field line have been observed on the Van Allen probes in connection with in situ relativistic electron acceleration in the Earth\textquoterights outer radiation belt. For one case with adequate high time resolution data, 7000 double layers were observed in an interval of 1 min to produce a 230 000 V net parallel potential drop crossing the spacecraft. Lower resolution data show that this event lasted for 6 min and that more than 1 000 000 volts of net parallel potential crossed the spacecraft during this time. A double layer traverses the length of a magnetic field line in about 15 s and the orbital motion of the spacecraft perpendicular to the magnetic field was about 700 km during this 6 min interval. Thus, the instantaneous parallel potential along a single magnetic field line was the order of tens of kilovolts. Electrons on the field line might experience many such potential steps in their lifetimes to accelerate them to energies where they serve as the seed population for relativistic acceleration by coherent, large amplitude whistler mode waves. Because the double-layer speed of 3100 km/s is the order of the electron acoustic speed (and not the ion acoustic speed) of a 25 eV plasma, the double layers may result from a new electron acoustic mode. Acceleration mechanisms involving double layers may also be important in planetary radiation belts such as Jupiter, Saturn, Uranus, and Neptune, in the solar corona during flares, and in astrophysical objects. Mozer, F.; Bale, S.; Bonnell, J; Chaston, C.; Roth, I.; Wygant, J.; Published by: Physical Review Letters Published on: 12/2013 YEAR: 2013 DOI: 10.1103/PhysRevLett.111.235002 |
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The Electric Field and Waves (EFW) Instruments on the Radiation Belt Storm Probes Mission The Electric Fields and Waves (EFW) Instruments on the two Radiation Belt Storm Probe (RBSP) spacecraft (recently renamed the Van Allen Probes) are designed to measure three dimensional quasi-static and low frequency electric fields and waves associated with the major mechanisms responsible for the acceleration of energetic charged particles in the inner magnetosphere of the Earth. For this measurement, the instrument uses two pairs of spherical double probe sensors at the ends of orthogonal centripetally deployed booms in the spin plane with tip-to-tip separations of 100 meters. The third component of the electric field is measured by two spherical sensors separated by \~15 m, deployed at the ends of two stacer booms oppositely directed along the spin axis of the spacecraft. The instrument provides a continuous stream of measurements over the entire orbit of the low frequency electric field vector at 32 samples/s in a survey mode. This survey mode also includes measurements of spacecraft potential to provide information on thermal electron plasma variations and structure. Survey mode spectral information allows the continuous evaluation of the peak value and spectral power in electric, magnetic and density fluctuations from several Hz to 6.5 kHz. On-board cross-spectral data allows the calculation of field-aligned wave Poynting flux along the magnetic field. For higher frequency waveform information, two different programmable burst memories are used with nominal sampling rates of 512 samples/s and 16 k samples/s. The EFW burst modes provide targeted measurements over brief time intervals of 3-d electric fields, 3-d wave magnetic fields (from the EMFISIS magnetic search coil sensors), and spacecraft potential. In the burst modes all six sensor-spacecraft potential measurements are telemetered enabling interferometric timing of small-scale plasma structures. In the first burst mode, the instrument stores all or a substantial fraction of the high frequency measurements in a 32 gigabyte burst memory. The sub-intervals to be downloaded are uplinked by ground command after inspection of instrument survey data and other information available on the ground. The second burst mode involves autonomous storing and playback of data controlled by flight software algorithms, which assess the \textquotedbllefthighest quality\textquotedblright events on the basis of instrument measurements and information from other instruments available on orbit. The EFW instrument provides 3-d wave electric field signals with a frequency response up to 400 kHz to the EMFISIS instrument for analysis and telemetry (Kletzing et al. Space Sci. Rev. 2013). Wygant, J.; Bonnell, J; Goetz, K.; Ergun, R.E.; Mozer, F.; Bale, S.D.; Ludlam, M.; Turin, P.; Harvey, P.R.; Hochmann, R.; Harps, K.; Dalton, G.; McCauley, J.; Rachelson, W.; Gordon, D.; Donakowski, B.; Shultz, C.; Smith, C.; Diaz-Aguado, M.; Fischer, J.; Heavner, S.; Berg, P.; Malaspina, D.; Bolton, M.; Hudson, M.; Strangeway, R.; Baker, D.; Li, X.; Albert, J.; Foster, J.C.; Chaston, C.C.; Mann, I.; Donovan, E.; Cully, C.M.; Cattell, C.; Krasnoselskikh, V.; Kersten, K.; Brenneman, A; Tao, J.; Published by: Space Science Reviews Published on: 11/2013 YEAR: 2013 DOI: 10.1007/s11214-013-0013-7 |
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[1] We present NASA Van Allen Probes observations of wave-particle interactions between magnetospheric ultra-low frequency (ULF) waves and energetic electrons (20\textendash500 keV) on 31 October 2012. The ULF waves are identified as the fundamental poloidal mode oscillation and are excited following an interplanetary shock impact on the magnetosphere. Large amplitude modulations in energetic electron flux are observed at the same period (≈ 3 min) as the ULF waves and are consistent with a drift-resonant interaction. The azimuthal mode number of the interacting wave is estimated from the electron measurements to be ~40, based on an assumed symmetric drift resonance. The drift-resonant interaction is observed to be localized and occur over 5\textendash6 wave cycles, demonstrating peak electron flux modulations at energies ~60 keV. Our observation clearly shows electron drift resonance with the fundamental poloidal mode, the energy dependence of the amplitude and phase of the electron flux modulations providing strong evidence for such an interaction. Significantly, the observation highlights the importance of localized wave-particle interactions for understanding energetic particle dynamics in the inner magnetosphere, through the intermediary of ULF waves. Claudepierre, S.; Mann, I.R.; Takahashi, K; Fennell, J.; Hudson, M.; Blake, J.; Roeder, J.; Clemmons, J.; Spence, H.; Reeves, G.; Baker, D.; Funsten, H.; Friedel, R.; Henderson, M.; Kletzing, C.; Kurth, W.; Wygant, J.; Published by: Geophysical Research Letters Published on: 09/2013 YEAR: 2013 DOI: 10.1002/grl.50901 |
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Excitation of Poloidal standing Alfven waves through the drift resonance wave-particle interaction Drift-resonance wave-particle interaction is a fundamental collisionless plasma process studied extensively in theory. Using cross-spectral analysis of electric field, magnetic field, and ion flux data from the Van Allen Probe (Radiation Belt Storm Probes) spacecraft, we present direct evidence identifying the generation of a fundamental mode standing poloidal wave through drift-resonance interactions in the inner magnetosphere. Intense azimuthal electric field (Eφ) oscillations as large as 10mV/m are observed, associated with radial magnetic field (Br) oscillations in the dawn-noon sector near but south of the magnetic equator at L\~5. The observed wave period, Eφ/Br ratio and the 90\textdegree phase lag between Br and Eφ are all consistent with fundamental mode standing Poloidal waves. Phase shifts between particle fluxes and wave electric fields clearly demonstrate a drift resonance with \~90 keV ring current ions. The estimated earthward gradient of ion phase space density provides a free energy source for wave generation through the drift-resonance instability. A similar drift-resonance process should occur ubiquitously in collisionless plasma systems. One specific example is the \textquotedblleftfishbone\textquotedblright instability in fusion plasma devices. In addition, our observations have important implications for the long-standing mysterious origin of Giant Pulsations. Dai, L.; Takahashi, K; Wygant, J.; Chen, L.; Bonnell, J; Cattell, C.; Thaller, S.; Kletzing, C.; Smith, C.; MacDowall, R.; Baker, D.; Blake, J.; Fennell, J.; Claudepierre, S.; Funsten, H.; Reeves, G.; Spence, H.; Published by: Geophysical Research Letters Published on: 08/2013 YEAR: 2013 DOI: 10.1002/grl.50800 |