Bibliography
|
Notice:
|
Found 100 entries in the Bibliography.
Showing entries from 51 through 100
| 2016 |
|
We present multipoint observations of earthward moving dipolarization fronts and energetic particle injections from NASA\textquoterights Magnetospheric Multiscale mission with a focus on electron acceleration. From a case study during a substorm on 02 August 2015, we find that electrons are only accelerated over a finite energy range, from a lower energy threshold at ~7\textendash9 keV up to an upper energy cutoff in the hundreds of keV range. At energies lower than the threshold energy, electron fluxes decrease, potentially due to precipitation by strong parallel electrostatic wavefields or initial sources in the lobes. Electrons at energies higher than the threshold are accelerated cumulatively by a series of impulsive magnetic dipolarization events. This case demonstrates how the upper energy cutoff increases, in this case from ~130 keV to >500 keV, with each dipolarization/injection during sustained activity. We also present a simple model accounting for these energy limits that reveals that electron energization is dominated by betatron acceleration. Turner, D.; Fennell, J.; Blake, J.; Clemmons, J.; Mauk, B.; Cohen, I.; Jaynes, A.; Craft, J.; Wilder, F.; Baker, D.; Reeves, G.; Gershman, D.; Avanov, L.; Dorelli, J.; Giles, B.; Pollock, C.; Schmid, D.; Nakamura, R.; Strangeway, R.; Russell, C.; Artemyev, A.; Runov, A.; Angelopoulos, V.; Spence, H.; Torbert, R.; Burch, J.; Published by: Geophysical Research Letters Published on: 08/2016 YEAR: 2016 DOI: 10.1002/2016GL069691 energetic particle injections; magnetotail; Particle acceleration; plasma sheet; reconnection; substorm; Van Allen Probes |
|
Trapped electrons in Earth\textquoterights outer Van Allen radiation belt are influenced profoundly by solar phenomena such as high-speed solar wind streams, coronal mass ejections (CME), and interplanetary (IP) shocks. In particular, strong IP shocks compress the magnetosphere suddenly and result in rapid energization of electrons within minutes. It is believed that the electric fields induced by the rapid change in the geomagnetic field are responsible for the energization. During the latter part of March 2015, a CME impact led to the most powerful geomagnetic storm (minimum Dst = -223 nT at 17 March, 23 UT) observed not only during the Van Allen Probe era but also the entire preceding decade. Magnetospheric response in the outer radiation belt eventually resulted in elevated levels of energized electrons. The CME itself was preceded by a strong IP shock whose immediate effects vis-a-vis electron energization were observed by sensors on board the Van Allen Probes. The comprehensive and high-quality data from the Van Allen Probes enable the determination of the location of the electron injection, timescales, and spectral aspects of the energized electrons. The observations clearly show that ultrarelativistic electrons with energies E > 6 MeV were injected deep into the magnetosphere at L ≈ 3 within about 2 min of the shock impact. However, electrons in the energy range of ≈250 keV to ≈900 keV showed no immediate response to the IP shock. Electric and magnetic fields resulting from the shock-driven compression complete the comprehensive set of observations that provide a full description of the near-instantaneous electron energization. Kanekal, S.; Baker, D.; Fennell, J.; Jones, A.; Schiller, Q.; Richardson, I.; Li, X.; Turner, D.; Califf, S.; Claudepierre, S.; Wilson, L.; Jaynes, A.; Blake, J.; Reeves, G.; Spence, H.; Kletzing, C.; Wygant, J.; Published by: Journal of Geophysical Research: Space Physics Published on: 08/2016 YEAR: 2016 DOI: 10.1002/2016JA022596 electron; energizaiton; IP shock; ultrarelativsti; Van Allen Probes |
|
We examine enhancements of energetic (>50 keV) oxygen ions observed by the Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) instrument on board the Van Allen Probes spacecraft in the inner magnetosphere (L ~ 6) at 22\textendash23 h magnetic local time (MLT) during an injection event of the 6 June 2013 storm. Simultaneous observations by two Van Allen Probes spacecraft located close together (~0.5 RE) indicate that particle injections occurred in the premidnight sector (< ~24 h MLT). We also examine the evolution of the proton and oxygen energy spectra at L ~ 6 during the injection event. The spectral slope did not significantly change during the storm. The oxygen phase space density (PSD) was shifted toward higher PSD in a wide range of the first adiabatic invariant. The spectral evolution manifests the characteristics of adiabatic acceleration and density increase of oxygen ions. Warm (0.1\textendash10 keV) oxygen measured by the Helium, Oxygen, Proton, and Electron (HOPE) instrument was enhanced prior to the storm mostly in magnetic field-aligned directions. The most reasonable scenario of this event is that warm oxygen ions that preexisted in the inner magnetosphere were picked up and adiabatically transported and accelerated by spatially localized, temporarily impulsive electric fields. Keika, Kunihiro; Seki, Kanako; e, Masahito; Machida, Shinobu; Miyoshi, Yoshizumi; Lanzerotti, Louis; Mitchell, Donald; Gkioulidou, Matina; Turner, Drew; Spence, Harlan; Larsen, Brian; Published by: Journal of Geophysical Research: Space Physics Published on: 08/2016 YEAR: 2016 DOI: 10.1002/2016JA022384 adiabatic transport from the plasma sheet; oxygen ions of ionospheric origin; preconditions of magnetic storms; preexisting oxygen ions trapped in the inner magnetosphere; Van Allen Probes; Van Allen Probes RBSPICE observations |
|
The 17\textendash18 March 2015 storm is the largest geomagnetic storm in the Van Allen Probes era to date. The Lyon-Fedder-Mobarry global MHD model has been run for this event using ARTEMIS data as solar wind input. The ULF wave power spectral density of the azimuthal electric field and compressional magnetic field is analyzed in the 0.5\textendash8.3 mHz range. The lowest three azimuthal modes account for 70\% of the total power during quiet times. However, during high activity, they are not exclusively dominant. The calculation of the radial diffusion coefficient is presented. We conclude that the electric field radial diffusion coefficient is dominant over the magnetic field coefficient by one to two orders of magnitude. This result contrasts with the dominant magnetic field diffusion coefficient used in most 3-D diffusion models. Li, Zhao; Hudson, Mary; Paral, Jan; Wiltberger, Michael; Turner, Drew; Published by: Journal of Geophysical Research: Space Physics Published on: 07/2016 YEAR: 2016 DOI: 10.1002/2016JA022508 March 2015; radial diffusion; radial diffusion coefficient; Radiation belt; ULF waves; Van Allen Probes |
|
Inner zone and slot electron radial diffusion revisited Using recent data from NASA\textquoterights Van Allen Probes, we estimate the quiet time radial diffusion coefficients for electrons in the inner radiation belt (L < 3) with energies from ~50 to 750 keV. The observations are consistent with dynamics dominated by pitch angle scattering and radial diffusion. We use a coordinate system in which these two modes of diffusion are separable. Then we integrate phase space density over pitch angle to obtain a \textquotedblleftbundle content\textquotedblright that is invariant to pitch angle scattering, except for atmospheric loss. We estimate the effective radial diffusion coefficient from the temporal and radial variation of the bundle content. We show that our diffusion coefficients agree well with previously determined values obtained in the 1960s and 1970s and follow the form one expects for radial diffusion caused by exponentially decaying impulses in the large-scale electrostatic potential. O\textquoterightBrien, T.; Claudepierre, S.; Guild, T.; Fennell, J.; Turner, D.; Blake, J.; Clemmons, J.; Roeder, J.; Published by: Geophysical Research Letters Published on: 07/2016 YEAR: 2016 DOI: 10.1002/2016GL069749 Inner zone; radial diffusion; Radiation belt; Van Allen Probes |
|
Local time variations of high-energy plasmaspheric ion pitch angle distributions Recent observations from the Van Allen Probes Helium Oxygen Proton Electron (HOPE) instrument revealed a persistent depletion in the 1\textendash10 eV ion population in the postmidnight sector during quiet times in the 2 < L < 3 region. This study explores the source of this ion depletion by developing an algorithm to classify 26 months of pitch angle distributions measured by the HOPE instrument. We correct the HOPE low energy fluxes for spacecraft potential using measurements from the Electric Field and Waves (EFW) instrument. A high percentage of low count pitch angle distributions is found in the postmidnight sector coupled with a low percentage of ion distributions peaked perpendicular to the field line. A peak in loss cone distributions in the dusk sector is also observed. These results characterize the nature of the dearth of the near 90\textdegree pitch angle 1\textendash10 eV ion population in the near-Earth postmidnight sector. This study also shows, for the first time, low-energy HOPE differential number fluxes corrected for spacecraft potential and 1\textendash10 eV H+ fluxes at different levels of geomagnetic activity. Sarno-Smith, Lois; Liemohn, Michael; Skoug, Ruth; Larsen, Brian; Moldwin, Mark; Katus, Roxanne; Wygant, John; Published by: Journal of Geophysical Research: Space Physics Published on: 07/2016 YEAR: 2016 DOI: 10.1002/2015JA022301 algorithm; Magnetosphere; pitch angles; plasmasphere; spacecraft potential corrections; Van Allen Probes |
|
Statistical Properties of the Radiation Belt Seed Population We present a statistical analysis of phase space density data from the first 26 months of the Van Allen Probes mission. In particular we investigate the relationship between the 10s-100s keV seed electrons and >1 MeV core radiation belt electron population. Using a cross correlation analysis, we find that the seed and core populations are well correlated with a coefficient of ≈ 0.73 with a time lag of 10-15 hours. We present evidence of a seed population threshold that is necessary for subsequent acceleration. The depth of penetration of the seed population determines the inner boundary of the acceleration process. However, we show that an enhanced seed population alone is not enough to produce acceleration in the higher energies, implying that the seed population of 100s of keV electrons is only one of several conditions required for MeV electron radiation belt acceleration. Boyd, A.J.; Spence, H.E.; Huang, C.-L.; Reeves, G.; Baker, D.; Turner, D.L.; Claudepierre, S.; Fennell, J.; Blake, J.; Shprits, Y.Y.; Published by: Journal of Geophysical Research: Space Physics Published on: 07/2016 YEAR: 2016 DOI: 10.1002/2016JA022652 Phase space density; Radiation belt; seed population; Van Allen Probes |
|
Explaining the dynamics of the ultra-relativistic third Van Allen radiation belt Since the discovery of the Van Allen radiation belts over 50 years ago, an explanation for their complete dynamics has remained elusive. Especially challenging is understanding the recently discovered ultra-relativistic third electron radiation belt. Current theory asserts that loss in the heart of the outer belt, essential to the formation of the third belt, must be controlled by high-frequency plasma wave\textendashparticle scattering into the atmosphere, via whistler mode chorus, plasmaspheric hiss, or electromagnetic ion cyclotron waves. However, this has failed to accurately reproduce the third belt. Using a datadriven, time-dependent specification of ultra-low-frequency (ULF) waves we show for the first time how the third radiation belt is established as a simple, elegant consequence of storm-time extremely fast outward ULF wave transport. High-frequency wave\textendashparticle scattering loss into the atmosphere is not needed in this case. When rapid ULF wave transport coupled to a dynamic boundary is accurately specified, the sensitive dynamics controlling the enigmatic ultra-relativistic third radiation belt are naturally explained. Mann, I.; Ozeke, L.; Murphy, K.; Claudepierre, S.; Turner, D.; Baker, D.; Rae, I.; Kale, A.; Milling, D.; Boyd, A.; Spence, H.; Reeves, G.; Singer, H.; Dimitrakoudis, S.; Daglis, I.; Honary, F.; Published by: Nature Physics Published on: 06/2016 YEAR: 2016 DOI: 10.1038/nphys3799 Astrophysical plasmas; Magnetospheric physics; Van Allen Probes |
|
An active storm period in June 2015 showed that particle injection events seen sequentially by the four (Magnetospheric Multiscale) MMS spacecraft subsequently fed the enhancement of the outer radiation belt observed by Van Allen Probes mission sensors. Several episodes of significant southward interplanetary magnetic field along with a period of high solar wind speed (Vsw ≳ 500 km/s) on 22 June occurred following strong interplanetary shock wave impacts on the magnetosphere. Key events on 22 June 2015 show that the magnetosphere progressed through a sequence of energy-loading and stress-developing states until the entire system suddenly reconfigured at 19:32 UT. Energetic electrons, plasma, and magnetic fields measured by the four MMS spacecraft revealed clear dipolarization front characteristics. It was seen that magnetospheric substorm activity provided a \textquotedblleftseed\textquotedblright electron population as observed by MMS particle sensors as multiple injections and related enhancements in electron flux. Baker, D.; Jaynes, A.; Turner, D.; Nakamura, R.; Schmid, D.; Mauk, B.; Cohen, I.; Fennell, J.; Blake, J.; Strangeway, R.; Russell, C.; Torbert, R.; Dorelli, J.; Gershman, D.; Giles, B.; Burch, J.; Published by: Geophysical Research Letters Published on: 06/2016 YEAR: 2016 DOI: 10.1002/grl.v43.1210.1002/2016GL069643 Magnetic reconnection; magnetospheres; Radiation belts; substorms; Van Allen Probes |
|
We present dynamic simulations of energy-dependent losses in the radiation belt " slot region" and the formation of the two-belt structure for the quiet days after the March 1st storm. The simulations combine radial diffusion with a realistic scattering model, based data-driven spatially and temporally-resolved whistler mode hiss wave observations from the Van Allen Probes satellites. The simulations reproduce Van Allen Probes observations for all energies and L-shells (2 to 6) including (a) the strong energy-dependence to the radiation belt dynamics (b) an energy-dependent outer boundary to the inner zone that extends to higher L-shells at lower energies and (c) an " S-shaped" energy-dependent inner boundary to the outer zone that results from the competition between diffusive radial transport and losses. We find that the characteristic energy-dependent structure of the radiation belts and slot region is dynamic and can be formed gradually in ~15 days, although the " S-shape" can also be reproduced by assuming equilibrium conditions. The highest energy electrons (E > 300 keV) of the inner region of the outer belt (L ~ 4-5) also constantly decay, demonstrating that hiss wave scattering affects the outer belt during times of extended plasmasphere. Through these simulations, we explain the full structure in energy and L-shell of the belts and the slot formation by hiss scattering during storm recovery. We show the power and complexity of looking dynamically at the effects over all energies and L-shells and the need for using data-driven and event-specific conditions. Ripoll, J.; Reeves, G.; Cunningham, G.; Loridan, V.; Denton, M.; ik, O.; Kurth, W.; Kletzing, C.; Turner, D.; Henderson, M.; Ukhorskiy, A; Published by: Geophysical Research Letters Published on: 05/2016 YEAR: 2016 DOI: 10.1002/2016GL068869 electron lifetimes; electron losses; hiss waves; Radiation belts; Slot region; Van Allen Probes; wave particle interactions |
|
In this paper we review recent spacecraft observations of oblique whistler-mode waves in the Earth\textquoterights inner magnetosphere as well as the various consequences of the presence of such waves for electron scattering and acceleration. In particular, we survey the statistics of occurrences and intensity of oblique chorus waves in the region of the outer radiation belt, comprised between the plasmapause and geostationary orbit, and discuss how their actual distribution may be explained by a combination of linear and non-linear generation, propagation, and damping processes. We further examine how such oblique wave populations can be included into both quasi-linear diffusion models and fully nonlinear models of wave-particle interaction. On this basis, we demonstrate that varying amounts of oblique waves can significantly change the rates of particle scattering, acceleration, and precipitation into the atmosphere during quiet times as well as in the course of a storm. Finally, we discuss possible generation mechanisms for such oblique waves in the radiation belts. We demonstrate that oblique whistler-mode chorus waves can be considered as an important ingredient of the radiation belt system and can play a key role in many aspects of wave-particle resonant interactions. Artemyev, Anton; Agapitov, Oleksiy; Mourenas, Didier; Krasnoselskikh, Vladimir; Shastun, Vitalii; Mozer, Forrest; Published by: Space Science Reviews Published on: 04/2016 YEAR: 2016 DOI: 10.1007/s11214-016-0252-5 Earth radiation belts; Van Allen Probes; Wave-particle interaction; Whistler waves |
|
Dipolarizing flux bundles (DFBs) are small flux tubes (typically < 3 RE in XGSM and YGSM) in the nightside magnetosphere that have magnetic field more dipolar than the background. Although DFBs are known to accelerate particles, creating energetic particle injections outside geosynchronous orbit (trans-GEO), the nature of the acceleration mechanism and the importance of DFBs in generating injections inside geosynchronous orbit (cis-GEO) are unclear. Our statistical study of cis-GEO DFBs using data from the Van Allen Probes reveals that just like trans-GEO DFBs, cis-GEO DFBs occur most often in the pre-midnight sector, but their occurrence rate is ~1/3 that of trans-GEO DFBs. Half the cis-GEO DFBs are accompanied by an energetic particle injection and have an electric field three times stronger than that of the injectionless half. All DFB injections are dispersionless within the temporal resolution considered (11 seconds). Our findings suggest that these injections are ushered or produced locally by the DFB, and the DFB\textquoterights strong electric field is an important aspect of the injection generation mechanism. Liu, Jiang; Angelopoulos, V.; Zhang, Xiao-Jia; Turner, D.; Gabrielse, C.; Runov, A.; Li, Jinxing; Funsten, H.; Spence, H.; Published by: Journal of Geophysical Research: Space Physics Published on: 01/2016 YEAR: 2016 DOI: 10.1002/2015JA021691 dipolarization front; dipolarizing flux bundle; energetic particle injection; geosynchronous orbit; magnetic storm; Particle acceleration |
| 2015 |
|
Nonlinear Generation of Electromagnetic Waves through Induced Scattering by Thermal Plasma We demonstrate the conversion of electrostatic pump waves into electromagnetic waves through nonlinear induced scattering by thermal particles in a laboratory plasma. Electrostatic waves in the whistler branch are launched that propagate near the resonance cone. When the amplitude exceeds a threshold ~5 \texttimes 10-6 times the background magnetic field, wave power is scattered below the pump frequency with wave normal angles (~59\textdegree), where the scattered wavelength reaches the limits of the plasma column. The scattered wave has a perpendicular wavelength that is an order of magnitude larger than the pump wave and longer than the electron skin depth. The amplitude threshold, scattered frequency spectrum, and scattered wave normal angles are in good agreement with theory. The results may affect the analysis and interpretation of space observations and lead to a comprehensive understanding of the nature of the Earth\textquoterights plasma environment. Tejero, E.; Crabtree, C.; Blackwell, D.; Amatucci, W.; Mithaiwala, M.; Ganguli, G.; Rudakov, L.; Published by: Scientific Reports Published on: 12/2015 YEAR: 2015 DOI: 10.1038/srep17852 |
|
Energy dependent dynamics of keV to MeV electrons in the inner zone, outer zone, and slot regions. We present observations of the radiation belts from the HOPE and MagEIS particle detectors on the Van Allen Probes satellites that illustrate the energy-dependence and L-shell dependence of radiation belt enhancements and decays. We survey events in 2013 and analyze an event on March 1 in more detail. The observations show: (a) At all L-shells, lower-energy electrons are enhanced more often than higher energies; (b) Events that fill the slot region are more common at lower energies; (c) Enhancements of electrons in the inner zone are more common at lower energies; and (d) Even when events do not fully fill the slot region, enhancements at lower-energies tend to extend to lower L-shells than higher energies. During enhancement events the outer zone extends to lower L-shells at lower energies while being confined to higher L-shells at higher energies. The inner zone shows the opposite with an outer boundary at higher L-shells for lower energies. Both boundaries are nearly straight in log(energy) vs. L-shell space. At energies below a few hundred keV radiation belt electron penetration through the slot region into the inner zone is commonplace but the number and frequency of \textquotedblleftslot filling\textquotedblright events decreases with increasing energy. The inner zone is enhanced only at energies that penetrate through the slot. Energy- and L-shell dependent losses (that are consistent with whistler hiss interactions) return the belts to more quiescent conditions. Reeves, Geoffrey; Friedel, Reiner; Larsen, Brian; Skoug, Ruth; Funsten, Herbert; Claudepierre, Seth; Fennell, Joseph; Turner, Drew; Denton, Mick; Spence, H.; Blake, Bernard; Baker, D.; Published by: Journal of Geophysical Research: Space Physics Published on: 12/2015 YEAR: 2015 DOI: 10.1002/2015JA021569 Acceleration; energetic particles; Inner zone; Outer Zone; Radiation belts; Slot region; Van Allen Probes |
|
Laboratory studies of nonlinear whistler wave processes in the Van Allen radiation belts Important nonlinear wave-wave and wave-particle interactions that occur in the Earth\textquoterights Van Allen radiation belts are investigated in a laboratory experiment. Predominantly electrostatic waves in the whistler branch are launched that propagate near the resonance cone with measured wave normal angle greater than 85\textdegree . When the pump amplitude exceeds a threshold \~5\texttimes10-6 times the background magnetic field, wave power at frequencies below the pump frequency is observed at wave normal angles (\~55\textdegree) . The scattered wave has a perpendicular wavelength that is nearly an order of magnitude larger than that of the pump wave. Occasionally, the parametric decay of a lower hybrid wave into a magnetosonic wave and a whistler wave is simultaneously observed with a threshold of δB/B0\~7\texttimes10-7 . Tejero, E.; Crabtree, C.; Blackwell, D.; Amatucci, W.; Mithaiwala, M.; Ganguli, G.; Rudakov, L.; Published by: Physics of Plasmas Published on: 09/2015 YEAR: 2015 DOI: 10.1063/1.4928944 |
|
Combined effects of concurrent Pc5 and chorus waves on relativistic electron dynamics We present electron phase space density (PSD) calculations as well as concurrent Pc5 and chorus wave activity observations during two intense geomagnetic storms caused by interplanetary coronal mass ejections (ICMEs) resulting in contradicting net effect. We show that, during the 17 March 2013 storm, the coincident observation of chorus and relativistic electron enhancements suggests that the prolonged chorus wave activity seems to be responsible for the enhancement of the electron population in the outer radiation belt even in the presence of pronounced outward diffusion. On the other hand, the significant depletion of electrons, during the 12 September 2014 storm, coincides with long-lasting outward diffusion driven by the continuous enhanced Pc5 activity since chorus wave activity was limited both in space and time. Katsavrias, C.; Daglis, I.; Li, W.; Dimitrakoudis, S.; Georgiou, M.; Turner, D.; Papadimitriou, C.; Published by: Annales Geophysicae Published on: 09/2015 YEAR: 2015 DOI: 10.5194/angeo-33-1173-2015 |
|
Laboratory studies of nonlinear whistler wave processes in the Van Allen radiation belts Important nonlinear wave-wave and wave-particle interactions that occur in the Earth\textquoterights Van Allen radiation belts are investigated in a laboratory experiment. Predominantly electrostatic waves in the whistler branch are launched that propagate near the resonance cone with measured wave normal angle greater than 85\textordmasculine. When the pump amplitude exceeds a threshold ~5 x10^6 times the back- ground magnetic field, wave power at frequencies below the pump frequency is observed at wave normal angles (~55\textordmasculine). The scattered wave has a perpendicular wavelength that is nearly an order of magnitude larger than that of the pump wave. Occasionally, the parametric decay of a lower hybrid wave into a magnetosonic wave and a whistler wave is simultaneously observed with a Tejero, E.; Crabtree, C.; Blackwell, D.; Amatucci, W.; Mithaiwala, M.; Ganguli, G.; Rudakov, L.; Published by: Physics of Plasmas Published on: 08/2015 YEAR: 2015 DOI: 10.1063/1.4928944 Electrostatic Waves; magnetic fields; Nonlinear scattering; Plasma electromagnetic waves; Whistler waves |
|
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 |
|
The effects of geomagnetic storms on electrons in Earth\textquoterights radiation belts We use Van Allen Probes data to investigate the responses of 10s of keV to 2 MeV electrons throughout a broad range of the radiation belts (2.5 <= L <= 6.0) during 52 geomagnetic storms from the most recent solar maximum. Electron storm-time responses are highly dependent on both electron energy and L-shell. 10s of keV electrons typically have peak fluxes in the inner belt or near-Earth plasma sheet and fill the inner magnetosphere during storm main phases. ~100 to ~600 keV electrons are enhanced in up to 87\% of cases around L~3.7, and their peak flux location moves to lower L-shells during storm recovery phases. Relativistic electrons (>=~1 MeV) are nearly equally likely to produce enhancement, depletion, and no-change events in the outer belt. We also show that the L-shell of peak flux correlates to storm magnitude only for 100s of keV electrons. Turner, D.; O\textquoterightBrien, T.; Fennell, J.; Claudepierre, S.; Blake, J.; Kilpua, E.; Hietala, H.; Published by: Geophysical Research Letters Published on: 07/2015 YEAR: 2015 DOI: 10.1002/2015GL064747 electrons; Van Allen Probes; Geomagnetic storms; Radiation belts |
|
It is well known that the plasmapause is influenced by the solar wind and magnetospheric conditions. Empirical models of its location have been previously developed such as those by O\textquoterightBrien and Moldwin (2003) and Larsen et al. (2006). In this study, we identified the locations of the plasmapause using the plasma density data obtained from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites. We used the data for the period (2008\textendash2012) corresponding to the ascending phase of Solar Cycle 24. Our database includes data from over a year of unusually weak solar wind conditions, correspondingly covering the plasmapause locations in a wider L range than those in previous studies. It also contains many coronal hole stream intervals during which the plasmasphere is eroded and recovers over a timescale of several days. The plasmapause was rigorously determined by requiring a density gradient by a factor of 15 within a radial distance of 0.5 L. We first determined the statistical correlation of the plasmapause locations with several solar wind parameters as well as geomagnetic indices. We found that the plasmapause locations are well correlated with the solar wind speed and the interplanetary magnetic field Bz, therefore the y component of the convective electric field, and some energy coupling functions such as the well-known Akasofu\textquoterights epsilon parameter. The plasmapause locations are also highly correlated with the geomagnetic indices, Dst, AE, and Kp, as recognized previously. Finally, we suggest new model fit functions for the plasmapause locations in terms of the solar wind parameters and geomagnetic indices. When applied to a new data interval outside the model training interval, our model fit functions work better than existing ones. The new model fit functions developed here extend the range of conditions from those used in previous works. Cho, Junghee; Lee, Dae-Young; Kim, Jin-Hee; Shin, Dae-Kyu; Kim, Kyung-Chan; Turner, Drew; Published by: Journal of Geophysical Research: Space Physics Published on: 04/2015 YEAR: 2015 DOI: 10.1002/2015JA021030 |
|
We present a case study of eight successive plasma sheet (PS) activations (usually referred to as bursty bulk flows or dipolarization fronts ) associated with small individual inline image increases on 31 March 2009 (0200\textendash0900 UT), observed by the THEMIS mission. This series of events happens during very quiet solar wind conditions, over a period of 7 hours preceding a substorm onset at 1230 UT. The amplitude of the dipolarizations increases with time. The low-amplitude dipolarization fronts are associated with few (1 or 2) rapid flux transport events (RFT, Eh > 2mV/m), whereas the large-amplitude ones encompass many more RFT events. All PS activations are associated with small and localized substorm current wedge (SCW) like current system signatures, which seems to be the consequence of RFT arrival in the near tail. The associated ground magnetic perturbations affect a larger part of the contracted auroral oval when, in the magnetotail, more RFT are embedded in PS activations (> 5). Dipolarization fronts with very low amplitude, a type usually not included in statistical studies, are of particular interest because we found even those to be associated with clear small SCW-like current system and particle injections at geosynchronous orbit. This exceptional dataset highlights the role of flow bursts in the magnetotail and leads to the conclusion that we may be observing the smallest form of a substorm, or rather its smallest element. This study also highlights the gradual evolution of the ionospheric current disturbance as the plasma sheet is observed to heat-up. Palin, L.; Jacquey, C.; Opgenoorth, H.; Connors, M.; Sergeev, V.; Sauvaud, J.-A.; Nakamura, R.; Reeves, G.D.; Singer, H.J.; Angelopoulos, V.; Turc, L.; Published by: Journal of Geophysical Research: Space Physics Published on: 04/2015 YEAR: 2015 DOI: 10.1002/2015JA021040 bursty bulk flow; dipolarization front; field-aligned currents; substorm; substorm current wedge; wedgelet |
|
Unraveling the drivers of the storm time radiation belt response We present a new framework to study the time evolution and dynamics of the outer Van Allen belt electron fluxes. The framework is entirely based on the large-scale solar wind storm drivers and their substructures. The Van Allen Probe observations, revealing the electron flux behavior throughout the outer belt, are combined with continuous, long-term (over 1.5 solar cycles) geosynchronous orbit data set from GOES and solar wind measurements A superposed epoch analysis, where we normalize the timescales for each substructure (sheath, ejecta, and interface region) allows us to avoid smearing effects and to distinguish the electron flux evolution during various driver structures. We show that the radiation belt response is not random: The electron flux variations are determined by the combined effect of the structured solar wind driver and prestorm electron flux levels. In particular, we find that loss mechanisms dominate during stream interface regions, coronal mass ejection (CME) ejecta, and sheaths while enhancements occur during fast streams trailing the stream interface or the CME. Kilpua, E.; Hietala, H.; Turner, D.; Koskinen, H.; Pulkkinen, T.; Rodriguez, J.; Reeves, G.; Claudepierre, S.; Spence, H.; Published by: Geophysical Research Letters Published on: 04/2015 YEAR: 2015 DOI: 10.1002/2015GL063542 coronal mass ejections; Magnetic Storms; Radiation belts; solar wind storm drivers; stream interaction regions; Van Allen Probes |
|
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 |
|
Pulsating auroras show quasi-periodic intensity modulations caused by the precipitation of energetic electrons of the order of tens of keV. It is expected theoretically that not only these electrons but also sub-relativistic/relativistic electrons precipitate simultaneously into the ionosphere owing to whistler-mode wave\textendashparticle interactions. The height-resolved electron density profile was observed with the European Incoherent Scatter (EISCAT) Troms\o VHF radar on 17 November 2012. Electron density enhancements were clearly identified at altitudes >68 km in association with the pulsating aurora, suggesting precipitation of electrons with a broadband energy range from ~10 keV up to at least 200 keV. The riometer and network of subionospheric radio wave observations also showed the energetic electron precipitations during this period. During this period, the footprint of the Van Allen Probe-A satellite was very close to Troms\o and the satellite observed rising tone emissions of the lower-band chorus (LBC) waves near the equatorial plane. Considering the observed LBC waves and electrons, we conducted a computer simulation of the wave\textendashparticle interactions. This showed simultaneous precipitation of electrons at both tens of keV and a few hundred keV, which is consistent with the energy spectrum estimated by the inversion method using the EISCAT observations. This result revealed that electrons with a wide energy range simultaneously precipitate into the ionosphere in association with the pulsating aurora, providing the evidence that pulsating auroras are caused by whistler chorus waves. We suggest that scattering by propagating whistler simultaneously causes both the precipitations of sub-relativistic electrons and the pulsating aurora. Miyoshi, Y.; Oyama, S.; Saito, S.; Kurita, S.; Fujiwara, H.; Kataoka, R.; Ebihara, Y.; Kletzing, C.; Reeves, G.; Santolik, O.; Clilverd, M.; Rodger, C.; Turunen, E.; Tsuchiya, F.; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2015 YEAR: 2015 DOI: 10.1002/2014JA020690 EISCAT; pitch angle scattering; pulsating aurora; Van Allen Probes |
|
We used the fluxgate magnetometer data from Combined Release and Radiation Effects Satellite (CRRES) to estimate the power spectral density (PSD) of the compressional component of the geomagnetic field in the \~1 mHz to \~8 mHz range. We conclude that magnetic wave power is generally higher in the noon sector for quiet times with no significant difference between the dawn, dusk, and the midnight sectors. However, during high Kp activity, the noon sector is not necessarily dominant anymore. The magnetic PSDs have a very distinct dependence on Kp. In addition, the PSDs appear to have a weak dependence on McIlwain parameter L with power slightly increasing as L increases. The magnetic wave PSDs are used along with the Fei et al. (2006) formulation to compute inline image as a function of L and Kp. The L dependence of inline image is systematically studied and is shown to depend on Kp. More significantly, we conclude that inline imageis the dominant term driving radial diffusion, typically exceeding inline image by 1\textendash2 orders of magnitude. Ali, Ashar; Elkington, Scot; Tu, Weichao; Ozeke, Louis; Chan, Anthony; Friedel, Reiner; Published by: Journal of Geophysical Research: Space Physics Published on: 02/2015 YEAR: 2015 DOI: 10.1002/2014JA020419 |
|
Recent results by the Van Allen Probes mission showed that the occurrence of energetic ion injections inside geosynchronous orbit could be very frequent throughout the main phase of a geomagnetic storm. Understanding, therefore, the formation and evolution of energetic particle injections is critical in order to quantify their effect in the inner magnetosphere. We present a case study of a substorm event that occurred during a weak storm (Dst ~ - 40 nT) on 14 July 2013. Van Allen Probe B, inside geosynchronous orbit, observed two energetic proton injections within ten minutes, with different dipolarization signatures and duration. The first one is a dispersionless, short timescale injection pulse accompanied by a sharp dipolarization signature, while the second one is a dispersed, longer timescale injection pulse accompanied by a gradual dipolarization signature. We combined ground magnetometer data from various stations, and in-situ particle and magnetic field data from multiple satellites in the inner magnetosphere and near-Earth plasma sheet to determine the spatial extent of these injections, their temporal evolution, and their effects in the inner magnetosphere. Our results indicate that there are different spatial and temporal scales at which injections can occur in the inner magnetosphere and depict the necessity of multipoint observations of both particle and magnetic field data in order to determine these scales. Gkioulidou, Matina; Ohtani, S.; Mitchell, D.; Ukhorskiy, A.; Reeves, G.; Turner, D.; Gjerloev, J.; e, Nos\; Koga, K.; Rodriguez, J.; Lanzerotti, L.; Published by: Journal of Geophysical Research: Space Physics Published on: 02/2015 YEAR: 2015 DOI: 10.1002/2014JA020872 |
|
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 |
|
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 |
| 2014 |
|
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 |
|
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 |
|
We present simulations of the outer electron radiation belt using a new ULF wave-driven radial diffusion model, including empirical representations of loss due to chorus and plasmaspheric hiss. With an outer boundary condition constrained by in situ electron flux observations, we focus on the impacts of magnetopause shadowing and outward radial diffusion in the heart of the radiation belt. Third invariant conserving solutions are combined to simulate the L shell and time dependence of the differential flux at a fixed energy. Results for the geomagnetically quiet year of 2008 demonstrate not only remarkable cross L shell impacts from magnetopause shadowing but also excellent agreement with the in situ observations even though no internal acceleration source is included in the model. Our model demonstrates powerful utility for capturing the cross-L impacts of magnetopause shadowing with significant prospects for improved space weather forecasting. The potential role of the plasmasphere in creating a third belt is also discussed. Ozeke, Louis; Mann, Ian; Turner, Drew; Murphy, Kyle; Degeling, Alex; Rae, Jonathan; Milling, David; Published by: Geophysical Research Letters Published on: 10/2014 YEAR: 2014 DOI: 10.1002/2014GL060787 magnetopause shadowing; Radiation belt; ULF wave radial diffusion |
|
Acceleration and loss driven by VLF chorus: Van Allen Probes observations and DREAM model results For over a decade now we have understood the response of the Earth\textquoterights radiation belts to solar wind driving are a delicate balance of acceleration and loss processes. Theory has shown that the interaction of relativistic electrons with VLF whistler mode chorus can produce both energization through momentum diffusion and loss through pitch angle diffusion. Recent results from the Van Allen Probes mission has confirmed observationally that chorus can produce both acceleration and loss. The Van Allen Probes satellites are able to measure all the critical particle populations and wave fields with unprecedented precision and resolution but only at the two spacecraft locations. Those spatially-localized observations can be extended globally using three-dimensional diffusion codes such as the DREAM model. We will discuss some of the recent Van Allen Probes observations that firmly demonstrate local acceleration by chorus and losses due to chorus-produced pitch angle scattering (as well as outward radial diffusion). We will look at observational evidence for the complex chain of processes that inject a \textquotedblleftseed population\textquotedblright, generate chorus, and ultimately drive radiation belt acceleration and loss. We will also discuss how local satellite observations can be generalized to simulate global dynamics using data-driven input and boundary conditions. RW1/J/IEIE0175/0001 Reeves, G.; Spence, H.; Henderson, M.; Tu, W.; Cunningham, G.; Chen, Y.; Blake, J.; Fennell, J.; Baker, D.; Published by: Published on: 08/2014 YEAR: 2014 DOI: 10.1109/URSIGASS.2014.6929879 |
|
Quantifying the radiation belt seed population in the 17 March 2013 electron acceleration event We present phase space density (PSD) observations using data from the Magnetic Electron Ion Spectrometer instrument on the Van Allen Probes for the 17 March 2013 electron acceleration event. We confirm previous results and quantify how PSD gradients depend on the first adiabatic invariant. We find a systematic difference between the lower-energy electrons ( Boyd, A.; Spence, H.; Claudepierre, S.; Fennell, J.; Blake, J.; Baker, D.; Reeves, G.; Turner, D.; Published by: Geophysical Research Letters Published on: 04/2014 YEAR: 2014 DOI: 10.1002/2014GL059626 |
|
On the cause and extent of outer radiation belt losses during the 30 September 2012 dropout event On 30 September 2012, a flux \textquotedblleftdropout\textquotedblright occurred throughout Earth\textquoterights outer electron radiation belt during the main phase of a strong geomagnetic storm. Using eight spacecraft from NASA\textquoterights Time History of Events and Macroscale Interactions during Substorms (THEMIS) and Van Allen Probes missions and NOAA\textquoterights Geostationary Operational Environmental Satellites constellation, we examined the full extent and timescales of the dropout based on particle energy, equatorial pitch angle, radial distance, and species. We calculated phase space densities of relativistic electrons, in adiabatic invariant coordinates, which revealed that loss processes during the dropout were > 90\% effective throughout the majority of the outer belt and the plasmapause played a key role in limiting the spatial extent of the dropout. THEMIS and the Van Allen Probes observed telltale signatures of loss due to magnetopause shadowing and subsequent outward radial transport, including similar loss of energetic ring current ions. However, Van Allen Probes observations suggest that another loss process played a role for multi-MeV electrons at lower L shells (L* < ~4). Turner, D.; Angelopoulos, V.; Morley, S.; Henderson, M.; Reeves, G.; Li, W.; Baker, D.; Huang, C.-L.; Boyd, A.; Spence, H.; Claudepierre, S.; Blake, J.; Rodriguez, J.; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2014 YEAR: 2014 DOI: 10.1002/2013JA019446 dropouts; inner magnetosphere; loss; Radiation belts; relativistic electrons; Van Allen Probes |
|
Drastic variations of Earth\textquoterights outer radiation belt electrons ultimately result from various competing source, loss, and transport processes, to which wave-particle interactions are critically important. Using 15 spacecraft including NASA\textquoterights Van Allen Probes, THEMIS, and SAMPEX missions and NOAA\textquoterights GOES and POES constellations, we investigated the evolution of the outer belt during the strong geomagnetic storm of 30 September to 3 October 2012. This storm\textquoterights main phase dropout exhibited enhanced losses to the atmosphere at L* < 4, where the phase space density (PSD) of multi-MeV electrons dropped by over an order of magnitude in <4 h. Based on POES observations of precipitating >1 MeV electrons and energetic protons, SAMPEX >1 MeV electrons, and ground observations of band-limited Pc1-2 wave activity, we show that this sudden loss was consistent with pitch angle scattering by electromagnetic ion cyclotron waves in the dusk magnetic local time sector at 3 < L* < 4. At 4 < L* < 5, local acceleration was also active during the main and early recovery phases, when growing peaks in electron PSD were observed by both Van Allen Probes and THEMIS. This acceleration corresponded to the period when IMF Bz was southward, the AE index was >300 nT, and energetic electron injections and whistler-mode chorus waves were observed throughout the inner magnetosphere for >12 h. After this period, Bz turned northward, and injections, chorus activity, and enhancements in PSD ceased. Overall, the outer belt was depleted by this storm. From the unprecedented level of observations available, we show direct evidence of the competitive nature of different wave-particle interactions controlling relativistic electron fluxes in the outer radiation belt. Turner, D.; Angelopoulos, V.; Li, W.; Bortnik, J.; Ni, B.; Ma, Q.; Thorne, R.; Morley, S.; Henderson, M.; Reeves, G.; Usanova, M.; Mann, I.; Claudepierre, S.; Blake, J.; Baker, D.; Huang, C.-L.; Spence, H.; Kurth, W.; Kletzing, C.; Rodriguez, J.; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2014 YEAR: 2014 DOI: 10.1002/jgra.v119.310.1002/2014JA019770 |
|
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 |
|
Event-specific chorus wave and electron seed population models in DREAM3D using the Van Allen Probes The DREAM3D diffusion model is applied to Van Allen Probes observations of the fast dropout and strong enhancement of MeV electrons during the October 2012 \textquotedblleftdouble-dip\textquotedblright storm. We show that in order to explain the very different behavior in the two \textquotedblleftdips,\textquotedblright diffusion in all three dimensions (energy, pitch angle, and L*) coupled with data-driven, event-specific inputs, and boundary conditions is required. Specifically, we find that outward radial diffusion to the solar wind-driven magnetopause, an event-specific chorus wave model, and a dynamic lower-energy seed population are critical for modeling the dynamics. In contrast, models that include only a subset of processes, use statistical wave amplitudes, or rely on inward radial diffusion of a seed population, perform poorly. The results illustrate the utility of the high resolution, comprehensive set of Van Allen Probes\textquoteright measurements in studying the balance between source and loss in the radiation belt, a principal goal of the mission. Tu, Weichao; Cunningham, G.; Chen, Y.; Morley, S.; Reeves, G.; Blake, J.; Baker, D.; Spence, H.; Published by: Geophysical Research Letters Published on: 03/2014 YEAR: 2014 DOI: 10.1002/2013GL058819 |
|
Space science: Near-Earth space shows its stripes Using some of the first scientific satellites put into orbit during the late 1950s, teams led by physicists James Van Allen in the United States and Sergei Vernov in the Soviet Union independently reported1, 2 on defined regions of radiation in near-Earth space. These regions came to be known as Earth\textquoterights radiation belts, and they represent the first major scientific discovery of the space age. However, despite decades of study, many questions in radiation-belt physics remain unanswered, mostly concerning the nature of the inner and outer belts, which are populated by electrons moving at near the speed of light. As society becomes ever more dependent on satellite-based technology, it is increasingly important to understand the variability in the radiation belts, because the highest-energy \textquotedblleftkiller electrons\textquotedblright3 can result in potentially fatal damage to sensitive spacecraft electronics4. On page 338 of this issue, Ukhorskiy et al.5 present observations and a model of a previously unexplained structured feature of the inner radiation belt, which they call zebra stripes. Published by: Nature Published on: 03/2014 YEAR: 2014 DOI: 10.1038/507308a |
|
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 |
|
Generation of electromagnetic waves in the very low frequency band by velocity gradient It is shown that a magnetized plasma layer with a velocity gradient in the flow perpendicular to the ambient magnetic field is unstable to waves in the Very Low Frequency band that spans the ion and electron gyrofrequencies. The waves are formally electromagnetic. However, depending on wave vector k⎯⎯=kc/ωpe (normalized by the electron skin depth) and the obliqueness, k⊥/k|| , where k⊥,|||| are wave vectors perpendicular and parallel to the magnetic field, the waves are closer to electrostatic in nature when k⎯⎯>>1 and k⊥>>k|| and electromagnetic otherwise. Inhomogeneous transverse flows are generated in plasma that contains a static electric field perpendicular to the magnetic field, a configuration that may naturally arise in the boundary layer between plasmas of different characteristics. Ganguli, G.; Tejero, E.; Crabtree, C.; Amatucci, W.; Rudakov, L.; Published by: Physics of Plasmas Published on: 01/2014 YEAR: 2014 DOI: 10.1063/1.4862032 |
|
A nonstorm time enhancement of relativistic electrons in the outer radiation belt Despite the lack of a geomagnetic storm (based on the Dst index), relativistic electron fluxes were enhanced over 2.5 orders of magnitude in the outer radiation belt in 13 h on 13\textendash14 January 2013. The unusual enhancement was observed by Magnetic Electron Ion Spectrometer (MagEIS), onboard the Van Allen Probes; Relativistic Electron and Proton Telescope Integrated Little Experiment, onboard the Colorado Student Space Weather Experiment; and Solid State Telescope, onboard Time History of Events and Macroscale Interactions during Substorms (THEMIS). Analyses of MagEIS phase space density (PSD) profiles show a positive outward radial gradient from 4 < L < 5.5. However, THEMIS observations show a peak in PSD outside of the Van Allen Probes\textquoteright apogee, which suggest a very interesting scenario: wave-particle interactions causing a PSD peak at ~ L* = 5.5 from where the electrons are then rapidly transported radially inward. This letter demonstrates, for the first time in detail, that geomagnetic storms are not necessary for causing dramatic enhancements in the outer radiation belt. Schiller, Quintin; Li, Xinlin; Blum, Lauren; Tu, Weichao; Turner, Drew; Blake, J.; Published by: Geophysical Research Letters Published on: 01/2014 YEAR: 2014 DOI: 10.1002/2013GL058485 |
|
A nonstorm time enhancement of relativistic electrons in the outer radiation belt Despite the lack of a geomagnetic storm (based on the Dst index), relativistic electron fluxes were enhanced over 2.5 orders of magnitude in the outer radiation belt in 13 h on 13\textendash14 January 2013. The unusual enhancement was observed by Magnetic Electron Ion Spectrometer (MagEIS), onboard the Van Allen Probes; Relativistic Electron and Proton Telescope Integrated Little Experiment, onboard the Colorado Student Space Weather Experiment; and Solid State Telescope, onboard Time History of Events and Macroscale Interactions during Substorms (THEMIS). Analyses of MagEIS phase space density (PSD) profiles show a positive outward radial gradient from 4 < L < 5.5. However, THEMIS observations show a peak in PSD outside of the Van Allen Probes\textquoteright apogee, which suggest a very interesting scenario: wave-particle interactions causing a PSD peak at ~ L* = 5.5 from where the electrons are then rapidly transported radially inward. This letter demonstrates, for the first time in detail, that geomagnetic storms are not necessary for causing dramatic enhancements in the outer radiation belt. Schiller, Quintin; Li, Xinlin; Blum, Lauren; Tu, Weichao; Turner, Drew; Blake, J.; Published by: Geophysical Research Letters Published on: 01/2014 YEAR: 2014 DOI: 10.1002/2013GL058485 |
| 2013 |
|
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 |
|
Radiation Belt Storm Probes (RBSP) Education and Public Outreach Program The Radiation Belt Storm Probes (RBSP) Education and Public Outreach (E/PO) program serves as a pipeline of activities to inspire and educate a broad audience about Heliophysics and the Sun-Earth system, specifically the Van Allen Radiation Belts. The program is comprised of a variety of formal, informal and public outreach activities that all align with the NASA Education Portfolio Strategic Framework outcomes. These include lesson plans and curriculum for use in the classroom, teacher workshops, internship opportunities, activities that target underserved populations, collaboration with science centers and NASA visitors\textquoteright centers and partnerships with experts in the Heliophysics and education disciplines. This paper will detail the activities that make up the RBSP E/PO program, their intended audiences, and an explanation as to how they align with the NASA education outcomes. Additionally, discussions on why these activities are necessary as part of a NASA mission are included. Finally, examples of how the RBSP E/PO team has carried out some of these activities will be discussed throughout. Turney, D.; Novak, Matiella; Beisser, K.; Fox, N.; Published by: Space Science Reviews Published on: 11/2013 YEAR: 2013 DOI: 10.1007/s11214-012-9945-6 |
|
Unusual stable trapping of the ultrarelativistic electrons in the Van Allen radiation belts Radiation in space was the first discovery of the space age. Earth\textquoterights radiation belts consist of energetic particles that are trapped by the geomagnetic field and encircle the planet1. The electron radiation belts usually form a two-zone structure with a stable inner zone and a highly variable outer zone, which forms and disappears owing to wave\textendashparticle interactions on the timescale of a day, and is strongly influenced by the very-low-frequency plasma waves. Recent observations revealed a third radiation zone at ultrarelativistic energies2, with the additional medium narrow belt (long-lived ring) persisting for approximately 4 weeks. This new ring resulted from a combination of electron losses to the interplanetary medium and scattering by electromagnetic ion cyclotron waves to the Earth\textquoterights atmosphere. Here we show that ultrarelativistic electrons can stay trapped in the outer zone and remain unaffected by the very-low-frequency plasma waves for a very long time owing to a lack of scattering into the atmosphere. The absence of scattering is explained as a result of ultrarelativistic particles being too energetic to resonantly interact with waves at low latitudes. This study shows that a different set of physical processes determines the evolution of ultrarelativistic electrons. Shprits, Yuri; Subbotin, Dmitriy; Drozdov, Alexander; Usanova, Maria; Kellerman, Adam; Orlova, Ksenia; Baker, Daniel; Turner, Drew; Kim, Kyung-Chan; Published by: Nature Physics Published on: 11/2013 YEAR: 2013 DOI: 10.1038/nphys2760 |
|
Measurements from the Relativistic Electron and Proton Telescope integrated little experiment (REPTile) on board the Colorado Student Space Weather Experiment (CSSWE) CubeSat mission, which was launched into a highly inclined (65\textdegree) low Earth orbit, are analyzed along with measurements from the Relativistic Electron and Proton Telescope (REPT) and the Magnetic Electron Ion Spectrometer (MagEIS) instruments aboard the Van Allen Probes, which are in a low inclination (10\textdegree) geo-transfer-like orbit. Both REPT and MagEIS measure the full distribution of energetic electrons as they traverse the heart of the outer radiation belt. However, due to the small equatorial loss cone (only a few degrees), it is difficult for REPT and MagEIS to directly determine which electrons will precipitate into the atmosphere, a major radiation belt loss process. REPTile, a miniaturized version of REPT, measures the fraction of the total electron population that has small enough equatorial pitch angles to reach the altitude of CSSWE, 480 km \texttimes 780 km, thus measuring the precipitating population as well as the trapped and quasi-trapped populations. These newly available measurements provide an unprecedented opportunity to investigate the source, loss, and energization processes that are responsible for the dynamic behavior of outer radiation belt electrons. The focus of this paper will be on the characteristics of relativistic electrons measured by REPTile during the October 2012 storms; also included are long-term measurements from the Solar Anomalous and Magnetospheric Particle Explorer to put this study into context. Li, X.; Schiller, Q.; Blum, L.; Califf, S.; Zhao, H.; Tu, W.; Turner, D.; Gerhardt, D.; Palo, S.; Kanekal, S.; Baker, D.; Fennell, J.; Blake, J.; Looper, M.; Reeves, G.; Spence, H.; Published by: Journal of Geophysical Research: Space Physics Published on: 10/2013 YEAR: 2013 DOI: 10.1002/2013JA019342 |
|
Measurements from the Relativistic Electron and Proton Telescope integrated little experiment (REPTile) on board the Colorado Student Space Weather Experiment (CSSWE) CubeSat mission, which was launched into a highly inclined (65\textdegree) low Earth orbit, are analyzed along with measurements from the Relativistic Electron and Proton Telescope (REPT) and the Magnetic Electron Ion Spectrometer (MagEIS) instruments aboard the Van Allen Probes, which are in a low inclination (10\textdegree) geo-transfer-like orbit. Both REPT and MagEIS measure the full distribution of energetic electrons as they traverse the heart of the outer radiation belt. However, due to the small equatorial loss cone (only a few degrees), it is difficult for REPT and MagEIS to directly determine which electrons will precipitate into the atmosphere, a major radiation belt loss process. REPTile, a miniaturized version of REPT, measures the fraction of the total electron population that has small enough equatorial pitch angles to reach the altitude of CSSWE, 480 km \texttimes 780 km, thus measuring the precipitating population as well as the trapped and quasi-trapped populations. These newly available measurements provide an unprecedented opportunity to investigate the source, loss, and energization processes that are responsible for the dynamic behavior of outer radiation belt electrons. The focus of this paper will be on the characteristics of relativistic electrons measured by REPTile during the October 2012 storms; also included are long-term measurements from the Solar Anomalous and Magnetospheric Particle Explorer to put this study into context. Li, X.; Schiller, Q.; Blum, L.; Califf, S.; Zhao, H.; Tu, W.; Turner, D.; Gerhardt, D.; Palo, S.; Kanekal, S.; Baker, D.; Fennell, J.; Blake, J.; Looper, M.; Reeves, G.; Spence, H.; Published by: Journal of Geophysical Research: Space Physics Published on: 10/2013 YEAR: 2013 DOI: 10.1002/2013JA019342 |
|
Electron Acceleration in the Heart of the Van Allen Radiation Belts The Van Allen radiation belts contain ultrarelativistic electrons trapped in Earth\textquoterights magnetic field. Since their discovery in 1958, a fundamental unanswered question has been how electrons can be accelerated to such high energies. Two classes of processes have been proposed: transport and acceleration of electrons from a source population located outside the radiation belts (radial acceleration) or acceleration of lower-energy electrons to relativistic energies in situ in the heart of the radiation belts (local acceleration). We report measurements from NASA\textquoterights Van Allen Radiation Belt Storm Probes that clearly distinguish between the two types of acceleration. The observed radial profiles of phase space density are characteristic of local acceleration in the heart of the radiation belts and are inconsistent with a predominantly radial acceleration process. Reeves, G.; Spence, H.; Henderson, M.; Morley, S.; Friedel, R.; Funsten, H.; Baker, D.; Kanekal, S.; Blake, J.; Fennell, J.; Claudepierre, S.; Thorne, R.; Turner, D.; Kletzing, C.; Kurth, W.; Larsen, B.; Niehof, J.; Published by: Science Published on: 07/2013 YEAR: 2013 DOI: 10.1126/science.1237743 |
|
A quantitative analysis is performed on the decay of an unusual ring of relativistic electrons between 3 and 3.5 RE, which was observed by the Relativistic Electron Proton Telescope instrument on the Van Allen probes. The ring formed on 3 September 2012 during the main phase of a magnetic storm due to the partial depletion of the outer radiation belt for L > 3.5, and this remnant belt of relativistic electrons persisted at energies above 2 MeV, exhibiting only slow decay, until it was finally destroyed during another magnetic storm on 1 October. This long-term stability of the relativistic electron ring was associated with the rapid outward migration and maintenance of the plasmapause to distances greater than L = 4. The remnant ring was thus immune from the dynamic process, which caused rapid rebuilding of the outer radiation belt at L > 4, and was only subject to slow decay due to pitch angle scattering by plasmaspheric hiss on timescales exceeding 10\textendash20 days for electron energies above 3 MeV. At lower energies, the decay is much more rapid, consistent with the absence of a long-duration electron ring at energies below 2 MeV. Thorne, R.; Li, W.; Ni, B.; Ma, Q.; Bortnik, J.; Baker, D.; Spence, H.; Reeves, G.; Henderson, M.; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Turner, D.; Angelopoulos, V.; Published by: Geophysical Research Letters Published on: 06/2013 YEAR: 2013 DOI: 10.1002/grl.50627 |
| 2012 |
|
Explaining sudden losses of outer radiation belt electrons during geomagnetic storms The Van Allen radiation belts were first discovered in 1958 by the Explorer series of spacecraft1. The dynamic outer belt consists primarily of relativistic electrons trapped by the Earth\textquoterights magnetic field. Magnetospheric processes driven by the solar wind2 cause the electron flux in this belt to fluctuate substantially over timescales ranging from minutes to years3. The most dramatic of these events are known as flux \textquoterightdropouts\textquoteright and often occur during geomagnetic storms. During such an event the electron flux can drop by several orders of magnitude in just a few hours4, 5 and remain low even after a storm has abated. Various solar wind phenomena, including coronal mass ejections and co-rotating interaction regions6, can drive storm activity, but several outstanding questions remain concerning dropouts and the precise channels to which outer belt electrons are lost during these events. By analysing data collected at multiple altitudes by the THEMIS, GOES, and NOAA\textendashPOES spacecraft, we show that the sudden electron depletion observed during a recent storm\textquoterights main phase is primarily a result of outward transport rather than loss to the atmosphere. Turner, Drew; Shprits, Yuri; Hartinger, Michael; Angelopoulos, Vassilis; Published by: Nature Physics Published on: 01/2012 YEAR: 2012 DOI: 10.1038/nphys2185 |
1 2