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Found 882 entries in the Bibliography.

Showing entries from 401 through 450


Large-Amplitude Extremely Low Frequency Hiss Waves in Plasmaspheric Plumes

Su, Zhenpeng; Liu, Nigang; Zheng, Huinan; Wang, Yuming; Wang, Shui;

Published by: Geophysical Research Letters      Published on: 01/2018

YEAR: 2018     DOI: 10.1002/2017GL076754

electron instability; ELF hiss; generation mechanism; pitch angle scattering; precipitation loss; Radiation belt; Van Allen Probes

Modeling the Proton Radiation Belt With Van Allen Probes Relativistic Electron-Proton Telescope Data

An empirical model of the proton radiation belt is constructed from data taken during 2013\textendash2017 by the Relativistic Electron-Proton Telescopes on the Van Allen Probes satellites. The model intensity is a function of time, kinetic energy in the range 18\textendash600 MeV, equatorial pitch angle, and L shell of proton guiding centers. Data are selected, on the basis of energy deposits in each of the nine silicon detectors, to reduce background caused by hard proton energy spectra at low L. Instrument response functions are computed by Monte Carlo integration, using simulated proton paths through a simplified structural model, to account for energy loss in shielding material for protons outside the nominal field of view. Overlap of energy channels, their wide angular response, and changing satellite orientation require the model dependencies on all three independent variables be determined simultaneously. This is done by least squares minimization with a customized steepest descent algorithm. Model uncertainty accounts for statistical data error and systematic error in the simulated instrument response. A proton energy spectrum is also computed from data taken during the 8 January 2014 solar event, to illustrate methods for the simpler case of an isotropic and homogeneous model distribution. Radiation belt and solar proton results are compared to intensities computed with a simplified, on-axis response that can provide a good approximation under limited circumstances.

Selesnick, R.; Baker, D.; Kanekal, S.; Hoxie, V.; Li, X.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 01/2018

YEAR: 2018     DOI: 10.1002/2017JA024661

data; protons; Radiation belt; Van Allen Probes

One-Dimensional Full Wave Simulation of Equatorial Magnetosonic Wave Propagation in an Inhomogeneous Magnetosphere

The effect of the plasmapause on equatorially radially propagating fast magnetosonic (MS) waves in the Earth\textquoterights dipole magnetic field is studied by using finite difference time domain method. We run 1-D simulation for three different density profiles: (1) no plasmapause, (2) with a plasmapause, and (3) with a plasmapause accompanied with fine-scale density irregularity. We find that (1) without plasmapause the radially inward propagating MS wave can reach ionosphere and continuously propagate to lower altitude if no damping mechanism is considered. The wave properties follow the cold plasma dispersion relation locally along its trajectory. (2) For simulation with a plasmapause with a scale length of 0.006 RE compared to wavelength, only a small fraction of the MS wave power is reflected by the plasmapause. WKB approximation is generally valid for such plasmapause. (3) The multiple fine-scale density irregularities near the outer edge of plasmapause can effectively block the MS wave propagation, resulting in a terminating boundary for MS waves near the plasmapause.

Liu, Xu; Chen, Lunjin; Yang, Lixia; Xia, Zhiyang; Malaspina, David;

Published by: Journal of Geophysical Research: Space Physics      Published on: 01/2018

YEAR: 2018     DOI: 10.1002/2017JA024336

fine-scale density structure; finite difference time domain; magnetosonic wave; Plasmapause; Van Allen Probes

Prompt Disappearance and Emergence of Radiation Belt Magnetosonic Waves Induced by Solar Wind Dynamic Pressure Variations

Magnetosonic waves are highly oblique whistler mode emissions transferring energy from the ring current protons to the radiation belt electrons in the inner magnetosphere. Here we present the first report of prompt disappearance and emergence of magnetosonic waves induced by the solar wind dynamic pressure variations. The solar wind dynamic pressure reduction caused the magnetosphere expansion, adiabatically decelerated the ring current protons for the Bernstein mode instability, and produced the prompt disappearance of magnetosonic waves. On the contrary, because of the adiabatic acceleration of the ring current protons by the solar wind dynamic pressure enhancement, magnetosonic waves emerged suddenly. In the absence of impulsive injections of hot protons, magnetosonic waves were observable even only during the time period with the enhanced solar wind dynamic pressure. Our results demonstrate that the solar wind dynamic pressure is an essential parameter for modeling of magnetosonic waves and their effect on the radiation belt electrons.

Liu, Nigang; Su, Zhenpeng; Zheng, Huinan; Wang, Yuming; Wang, Shui;

Published by: Geophysical Research Letters      Published on: 01/2018

YEAR: 2018     DOI: 10.1002/2017GL076382

magnetosonic waves; Radiation belt; ring current; solar wind dynamic pressure; Van Allen Probes; Wave-particle interaction

Space Weather Operation at KASI with Van Allen Probes Beacon Signals

The Van Allen Probes (VAPs) are the only modern NASA spacecraft broadcasting real-time data on the Earth\textquoterights radiation belts for space weather operations. Since 2012, the Korea Astronomy and Space Science Institute (KASI) has contributed to the receipt of this data via a 7-m satellite tracking antenna and used these data for space weather operations. An approximately 15-min period is required from measurement to acquisition of Level-1 data. In this paper, we demonstrate the use of VAP data for monitoring space weather conditions at geostationary orbit (GEO) by highlighting the Saint Patrick\textquoterights Day storm of 2015. During that storm, Probe-A observed a significant increase in the relativistic electron flux at 3 RE. Those electrons diffused outward resulting in a large increase of the electron flux > 2 MeV at GEO, which potentially threatened satellite operations. Based on this study, we conclude that the combination of VAP data and National Oceanic and Atmospheric Administration-Geostationary Operational Environmental Satellite (NOAA-GOES) data can provide improved space environment information to geostationary satellite operators. In addition, the findings obtained indicate that more data-receiving sites would be necessary and data connections improved if this or a similar system were to be used as an operational data service.

Lee, Jongkil; Kim, Kyung-Chan; Romeo, Giuseppe; Ukhorskiy, Sasha; Sibeck, David; Kessel, Ramona; Mauk, Barry; Giles, Barbara; Gu, Bon-Jun; Lee, Hyesook; Park, Young-Deuk; Lee, Jaejin;

Published by: Space Weather      Published on: 01/2018

YEAR: 2018     DOI: 10.1002/2017SW001726

Electron acceleration; Radiation belt; Relativistic electron; Space weather; Van Allen Probes

Survey of the Favorable Conditions for Magnetosonic Wave Excitation

The ratio of the proton ring velocity (VR) to the local Alfven speed (VA), in addition to proton ring distributions, plays a key factor in the excitation of magnetosonic waves at frequencies between the proton cyclotron frequency fcp and the lower hybrid resonance frequency fLHR in the Earth\textquoterights magnetosphere. Here we investigate whether there is a statistically significant relationship between occurrences of proton rings and magnetosonic waves both outside and inside the plasmapause using particle and wave data from Van Allen Probe-A during the time period of October 2012 to December 2015. We also perform a statistical survey of the ratio of the ring energy (ER, corresponding to VR) to the Alfven energy (EA, corresponding to VA) to determine the favorable conditions under which magnetosonic waves in each of two frequency bands (fcp < f <= 0.5 fLHR and 0.5 fLHR < f < fLHR) can be excited. The results show that the magnetosonic waves in both frequency bands occur around the postnoon (12\textendash18 magnetic local time, MLT) sector outside the plasmapause when ER is comparable to or lower than EA, and those in lower-frequency bands (fcp < f <= 0.5 fLHR) occur around the postnoon sector inside the plasmapause when ER/EA > ~9. However, there is one discrepancy between occurrences of proton rings and magnetosonic waves in low-frequency bands around the prenoon sector (6\textendash12 MLT) outside the plasmapause, which suggests either that the waves may have propagated during active time from the postnoon sector after being excited during quiet time, or they may have locally excited in the prenoon sector during active time.

Kim, Kyung-Chan; Shprits, Yuri;

Published by: Journal of Geophysical Research: Space Physics      Published on: 01/2018

YEAR: 2018     DOI: 10.1002/2017JA024865

magnetosonic equatorial noise; proton ring distribution; Van Allen Probes

Three-Step Buildup of the 17 March 2015 Storm Ring Current: Implication for the Cause of the Unexpected Storm Intensification

We examine the spatiotemporal variations of the energy density and the energy spectral evolution of energetic ions in the inner magnetosphere during the main phase of the 17 March 2015 storm, using data from the RBSPICE and EMFISIS instruments onboard Van Allen Probes. The storm developed in response to two southward IMF intervals separated by about 3 h. In contrast to two steps seen in the Dst/SYM-H index, the ring current ion population evolved in three steps: the first subphase was apparently caused by the earlier southward IMF, and the subsequent subphases occurred during the later southward IMF period. Ion energy ranges that contribute to the ring current differed between the three subphases. We suggest that the spectral evolution resulted from the penetration of different plasma sheet populations. The ring current buildup during the first subphase was caused by the penetration of a relatively low-energy population that had existed in the plasma sheet during a prolonged prestorm northward IMF interval. The deeper penetration of the lower-energy population was responsible for the second subphase. The third subphase, where the storm was unexpectedly intensified to a Dst/SYM-H level of <-200 nT, was caused by the penetration of a hot, dense plasma sheet population. We attribute the hot, dense population to the entry of hot, dense solar wind into the plasma sheet and/or ion heating/acceleration in the near-Earth plasma sheet associated with magnetotail activity such as reconnection and dipolarization.

Keika, Kunihiro; Seki, Kanako; e, Masahito; Miyoshi, Yoshizumi; Lanzerotti, Louis; Mitchell, Donald; Gkioulidou, Matina; Manweiler, Jerry;

Published by: Journal of Geophysical Research: Space Physics      Published on: 01/2018

YEAR: 2018     DOI: 10.1002/2017JA024462

enhancements of oxygen ions of ionospheric origin; plasma transport from the plasma sheet into the inner magnetosphere; RBSPICE; unexpected intensification of the magnetic storm; Van Allen Probes

Van Allen Probes Observations of Second Harmonic Poloidal Standing Alfv\ en Waves

Long-lasting second-harmonic poloidal standing Alfv\ en waves (P2 waves) were observed by the twin Van Allen Probes (Radiation Belt Storm Probes, or RBSP) spacecraft in the noon sector of the plasmasphere, when the spacecraft were close to the magnetic equator and had a small azimuthal separation. Oscillations of proton fluxes at the wave frequency (\~10 mHz) were also observed in the energy (W) range 50\textendash300 keV. Using the unique RBSP orbital configuration, we determined the phase delay of magnetic field perturbations between the spacecraft with a 2nπ ambiguity. We then used finite gyroradius effects seen in the proton flux oscillations to remove the ambiguity and found that the waves were propagating westward with an azimuthal wave number (m) of \~-200. The phase of the proton flux oscillations relative to the radial component of the wave magnetic field progresses with W, crossing 0 (northward moving protons) or 180\textdegree (southward moving protons) at W \~ 120 keV. This feature is explained by drift-bounce resonance (mωd \~ ωb) of \~120 keV protons with the waves, where ωd and ωb are the proton drift and bounce frequencies. At lower energies, the proton phase space density ( math formula) exhibits a bump-on-tail structure with math formula occurring in the 1\textendash10 keV energy range. This math formula is unstable and can excite P2 waves through bounce resonance (ω \~ ωb), where ω is the wave frequency.

Takahashi, Kazue; Oimatsu, Satoshi; e, Masahito; Min, Kyungguk; Claudepierre, Seth; Chan, Anthony; Wygant, John; Kim, Hyomin;

Published by: Journal of Geophysical Research: Space Physics      Published on: 01/2018

YEAR: 2018     DOI: 10.1002/2017JA024869

bounce and drift-bounce resonances; energetic protons; plasmasphere; poloidal ULF waves; second harmonic; Van Allen Probes


Empirical modeling of the quiet and storm-time geosynchronous magnetic field

A dynamical empirical model of the near-geosynchronous magnetic field has been constructed, based on a recently developed RBF approach and a multi-year set of spacecraft data taken by THEMIS, Polar, Cluster, and Van Allen Probes missions including 133 geomagnetic storms in the time interval between 1996 and 2016. The model describes the field as a function of Cartesian solar-magnetic coordinates, dipole tilt angle, solar wind ram pressure, and of a set of dynamic variables representing the response of the magnetosphere to the external driving/loading during the active phase of a space weather event, followed by the internal relaxation/dissipation during the storm recovery. In terms of the disturbance level, the model\textquoterights validity range extends to intense storms with peak Sym-H values down to -150 nT. The spatial validity domain is a toroidal volume bounded by the inner (L\~4) and outer (L\~9) dipolar L-shells, which allows the model to be used for tracing field lines to magnetically map geosynchronous spacecraft locations down to low altitudes. The model has been validated on independent out-of-sample magnetic field data and compared with an earlier empirical model and GOES-15 data taken in 2012 and 2015.

Andreeva, V.; Tsyganenko, N.;

Published by: Space Weather      Published on: 12/2017

YEAR: 2017     DOI: 10.1002/2017SW001684

geomagnetic field; geostationary orbit; Modeling; spacecraft data; Van Allen Probes

The Radiation Belt Electron Scattering by Magnetosonic Wave: Dependence on Key Parameters

Magnetosonic (MS) waves have been found capable of creating radiation belt electron butterfly distributions in the inner magnetosphere. To investigate the physical nature of the interactions between radiation belt electrons and MS waves, and to explore a preferential condition for MS waves to scatter electrons efficiently, we performed a comprehensive parametric study of MS wave-electron interactions using test particle simulations. The diffusion coefficients simulated by varying the MS wave frequency show that the scattering effect of MS waves is frequency insensitive at low harmonics (f < 20 fcp), which has great implications on modeling the electron scattering caused by MS waves with harmonic structures. The electron scattering caused by MS waves is very sensitive to wave normal angles, and MS waves with off 90\textdegree wave normal angles scatter electrons more efficiently. By simulating the diffusion coefficients and the electron phase space density evolution at different L shells under different plasma environment circumstances, we find that MS waves can readily produce electron butterfly distributions in the inner part of the plasmasphere where the ratio of electron plasma-to-gyrofrequency (fpe/fce) is large, while they may essentially form a two-peak distribution outside the plasmapause and in the inner radiation belt where fpe/fce is small.

Lei, Mingda; Xie, Lun; Li, Jinxing; Pu, Zuyin; Fu, Suiyan; Ni, Binbin; Hua, Man; Chen, Lunjin; Li, Wen;

Published by: Journal of Geophysical Research: Space Physics      Published on: 12/2017

YEAR: 2017     DOI: 10.1002/2016JA023801

magnetosonic wave; parametric study; Radiation belt; Van Allen Probes; Wave-particle interaction


On December 11, 2016 at 00:12:30 UT, Van Allen Probe-B, at the equator and near midnight, and AC6-B, in the ionosphere, were on magnetic field lines whose 100 km altitude foot points were separated by 600 km. Van Allen Probe-B observed a 30 second burst of lower band chorus waves (with maximum amplitudes >1 nT) at the same time that AC6-B observed intense microburst electrons in the loss cone. One-second averaged variations of the chorus intensity and the microburst electron flux were well-correlated. The low altitude electron flux expected from quasi-linear diffusion of the equatorial electrons by the equatorial chorus is in excellent agreement with the observed, one second averaged, low altitude electron flux. However the large amplitude, <0.5 second duration, low altitude electron pulses require non-linear processes for their explanation.

Mozer, F.; Agapitov, O.; Blake, J.; Vasko, I;

Published by: Geophysical Research Letters      Published on: 12/2017

YEAR: 2017     DOI: 10.1002/2017GL076120

chorus makes microbursts; Van Allen Probes

Synthetic empirical chorus wave model from combined Van Allen Probes and Cluster statistics

Chorus waves are among the most important natural electromagnetic emissions in the magnetosphere as regards their potential effects on electron dynamics. They can efficiently accelerate or precipitate electrons trapped in the outer radiation belt, producing either fast increases of relativistic particle fluxes, or auroras at high latitudes. Accurately modeling their effects, however, requires detailed models of their wave power and obliquity distribution as a function of geomagnetic activity in a particularly wide spatial domain, rarely available based solely on the statistics obtained from only one satellite mission. Here, we seize the opportunity of synthesizing data from the Van Allen Probes and Cluster spacecraft to provide a new comprehensive chorus wave model in the outer radiation belt. The respective spatial coverages of these two missions are shown to be especially complementary and further allow a good cross-calibration in the overlap domain. We used 4 years (2012-2016) of Van Allen Probes VLF data in the chorus frequency range up to 12 kHz at latitudes lower than 20 degrees, combined with 10 years of Cluster VLF measurements up to 4 kHz in order to provide a full coverage of geomagnetic latitudes up to 45 degrees in the chorus frequency range 0.1fce-0.8fce. The resulting synthetic statistical model of chorus wave amplitude, obliquity, and frequency is presented in the form of analytical functions of latitude and Kp in three different MLT sectors and for two ranges of L-shells outside the plasmasphere. Such a synthetic and reliable chorus model is crucially important for accurately modeling global acceleration and loss of electrons over the long run in the outer radiation belt, allowing a comprehensive description of electron flux variations over a very wide energy range.

Agapitov, O.; Mourenas, D.; Artemyev, A.; Mozer, F.; Hospodarsky, G.; Bonnell, J.; Krasnoselskikh, V.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 12/2017

YEAR: 2017     DOI: 10.1002/2017JA024843

chorus waves model; Van Allen Probes

Temporal evolution of ion spectral structures during a geomagnetic storm: Observations and modeling

Using the Van Allen Probes/Helium, Oxygen, Proton, and Electron (HOPE) mass spectrometer, we perform a case study of the temporal evolution of ion spectral structures observed in the energy range of 1-~50 keV throughout the geomagnetic storm of 2 October 2013. The ion spectral features are observed near the inner edge of the plasma sheet and are signatures of fresh transport from the plasma sheet into the inner magnetosphere. We find that the characteristics of the ion structures are determined by the intensity of the convection electric field. Prior to the beginning of the storm, the plasma sheet inner edge exhibits narrow nose spectral structures that vary little in energy across L values. Ion access to the inner magnetosphere during these times is limited to the nose energy bands. As convection is enhanced and large amounts of plasma are injected from the plasma sheet during the main phase of the storm, ion access occurs at a wide energy range, as no nose structures are observed. As the magnetosphere recovers from the storm, single noses and then multiple noses are observed once again. We use a model of ion drift and losses due to charge exchange to simulate the ion spectra and gain insight into the main observed features.

Ferradas, C.; Zhang, J.-C.; Spence, H.; Kistler, L.; Larsen, B.; Reeves, G.; Skoug, R.; Funsten, H.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 12/2017

YEAR: 2017     DOI: 10.1002/2017JA024702

Geomagnetic storm; ion injection; ion nose structure; numerical modeling; Van Allen Probes; Weimer electric field model

Very Oblique Whistler Mode Propagation in the Radiation Belts: Effects of Hot Plasma and Landau Damping

Satellite observations of a significant population of very oblique chorus waves in the outer radiation belt have fueled considerable interest in the effects of these waves on energetic electron scattering and acceleration. However, corresponding diffusion rates are extremely sensitive to the refractive index N, controlled by hot plasma effects including Landau damping and wave dispersion modifications by suprathermal (15\textendash100 eV) electrons. A combined investigation of wave and electron distribution characteristics obtained from the Van Allen Probes shows that peculiarities of the measured electron distribution significantly reduce Landau damping, allowing wave propagation with high N \~ 100\textendash200. Further comparing measured refractive indexes with theoretical estimates incorporating hot plasma corrections to the wave dispersion, we provide the first experimental demonstration that suprathermal electrons indeed control the upper limit of the refractive index of highly oblique whistler mode waves. Such results further support the importance of incorporating very oblique waves into radiation belt models.

Ma, Q.; Artemyev, A.; Mourenas, D.; Li, W.; Thorne, R.; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Reeves, G.; Spence, H.; Wygant, J.;

Published by: Geophysical Research Letters      Published on: 12/2017

YEAR: 2017     DOI: 10.1002/2017GL075892

Landau damping; maximum refractive index; oblique chorus waves; thermal electron effects; Van Allen Probes; Van Allen Probes observation

Analysis of the Duration of Rising Tone Chorus Elements

The duration of chorus elements is an important parameter to understand chorus excitation and to quantify the effects of nonlinear wave-particle interactions on energetic electron dynamics. In this work, we analyze the duration of rising tone chorus elements statistically using Van Allen Probes data. We present the distribution of chorus element duration (τ) as a function of magnetic local time (MLT) and the geomagnetic activity level characterized by auroral electrojet (AE) index. We show that the typical value of τ for nightside and dawnside is about 0.12 s, smaller than that for dayside and duskside by about a factor of 2 to 4. Using a previously developed hybrid code, DAWN, we suggest that the background magnetic field inhomogeneity might be an important factor in controlling the chorus element duration. We also report that τ is larger during quiet times and shorter during moderate and active periods; this result is consistent with the MLT dependence of τ and the occurrence pattern of chorus waves at different levels of geomagnetic activity. We then investigate the correlation between τ and the frequency chirping rate (Γ). We show that, from observation, τ scales with Γ as math formula, suggesting that statistically the frequency range of chorus elements (τΓ) should be roughly the same for different elements. These findings should be useful to the further development of a theoretical model of chorus excitation and to the quantification of nonlinear wave-particle interactions on energetic electron dynamics.

Teng, S.; Tao, X.; Xie, Y.; Zonca, F.; Chen, L.; Fang, W.; Wang, S.;

Published by: Geophysical Research Letters      Published on: 12/2017

YEAR: 2017     DOI: 10.1002/2017GL075824

chorus element duration; DAWN; frequency chirping rate; Van Allen Probes

Automated Identification and Shape Analysis of Chorus Elements in the Van Allen Radiation Belts

An important goal of the Van Allen Probes mission is to understand wave-particle interaction by chorus emissions in terrestrial Van Allen radiation belts. To test models, statistical characterization of chorus properties, such as amplitude variation and sweep rates, is an important scientific goal. The Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) instrumentation suite provides measurements of wave electric and magnetic fields as well as DC magnetic fields for the Van Allen Probes mission. However, manual inspection across terabytes of EMFISIS data is not feasible and as such introduces human confirmation bias. We present signal processing techniques for automated identification, shape analysis, and sweep rate characterization of high-amplitude whistler-mode chorus elements in the Van Allen radiation belts. Specifically, we develop signal processing techniques based on the radon transform that disambiguate chorus elements with a dominant sweep rate against hiss-like chorus. We present representative results validating our techniques and also provide statistical characterization of detected chorus elements across a case study of a 6 s epoch.

Gupta, Ananya; Kletzing, Craig; Howk, Robin; Kurth, William; Matheny, Morgan;

Published by: Journal of Geophysical Research: Space Physics      Published on: 12/2017

YEAR: 2017     DOI: 10.1002/2017JA023949

Chorus; Van Allen Probes; Van Allen radiation belt

Chorus Wave Modulation of Langmuir Waves in the Radiation Belts

Using high-resolution waveforms measured by the Van Allen Probes, we report a novel observation in the radiation belts. Namely, we show that multiband, discrete, rising-tone whistler mode chorus emissions exhibit a one-to-one correlation with Langmuir wave bursts. Moreover, the periodic Langmuir wave bursts are generally observed at the phase location where the chorus wave E|| component is oriented opposite to its propagation direction. The electron measurements show a beam in phase space density at the particle velocity that matches the parallel phase velocity of the chorus waves. Based on this evidence, we conclude that the chorus waves accelerate the suprathermal electrons via Landau resonance and generate a localized electron beam in phase space density. Consequently, the Langmuir waves are excited locally and are modulated by the chorus wave phase. This microscale interaction between chorus waves and high-frequency electrostatic waves provides a new insight into the nonlinear wave-particle interaction process.

Li, Jinxing; Bortnik, Jacob; An, Xin; Li, Wen; Thorne, Richard; Zhou, Meng; Kurth, William; Hospodarsky, George; Funsten, Herbert; Spence, Harlan;

Published by: Geophysical Research Letters      Published on: 12/2017

YEAR: 2017     DOI: 10.1002/2017GL075877

Chorus wave; Landau resonance; Langmuir wave; nonlinear interaction; Radiation belt; Van Allen Probes; wave modulation

Conjugate Ground-Spacecraft Observations of VLF Chorus Elements

We present results of simultaneous observations of VLF chorus elements at the ground-based station Kannuslehto in Northern Finland and on board Van Allen Probe A. Visual inspection and correlation analysis of the data reveal one-to-one correspondence of several (at least 12) chorus elements following each other in a sequence. Poynting flux calculated from electromagnetic fields measured by the Electric and Magnetic Field Instrument Suite and Integrated Science instrument on board Van Allen Probe A shows that the waves propagate at small angles to the geomagnetic field and oppositely to its direction, that is, from northern to southern geographic hemisphere. The spacecraft was located at L≃4.1 at a geomagnetic latitude of -12.4o close to the plasmapause and inside a localized density inhomogeneity with about 30\% density increase and a transverse size of about 600 km. The time delay between the waves detected on the ground and on the spacecraft is about 1.3 s, with ground-based detection leading spacecraft detection. The measured time delay is consistent with the wave travel time of quasi-parallel whistler-mode waves for a realistic profile of the plasma density distribution along the field line. The results suggest that chorus discrete elements can preserve their spectral shape during a hop from the generation region to the ground followed by reflection from the ionosphere and return to the near-equatorial region.

Demekhov, A.; Manninen, J.; ik, O.; Titova, E.;

Published by: Geophysical Research Letters      Published on: 12/2017

YEAR: 2017     DOI: 10.1002/2017GL076139

ground-spacecraft observations; Magnetosphere; Van Allen Probes; VLF chorus

Observations Directly Linking Relativistic Electron Microbursts to Whistler Mode Chorus: Van Allen Probes and FIREBIRD II

We present observations that provide the strongest evidence yet that discrete whistler mode chorus packets cause relativistic electron microbursts. On 20 January 2016 near 1944 UT the low Earth orbiting CubeSat Focused Investigations of Relativistic Electron Bursts: Intensity, Range, and Dynamics (FIREBIRD II) observed energetic microbursts (near L = 5.6 and MLT = 10.5) from its lower limit of 220 keV, to 1 MeV. In the outer radiation belt and magnetically conjugate, Van Allen Probe A observed rising-tone, lower band chorus waves with durations and cadences similar to the microbursts. No other waves were observed. This is the first time that chorus and microbursts have been simultaneously observed with a separation smaller than a chorus packet. A majority of the microbursts do not have the energy dispersion expected for trapped electrons bouncing between mirror points. This confirms that the electrons are rapidly (nonlinearly) scattered into the loss cone by a coherent interaction with the large amplitude (up to \~900 pT) chorus. Comparison of observed time-averaged microburst flux and estimated total electron drift shell content at L = 5.6 indicate that microbursts may represent a significant source of energetic electron loss in the outer radiation belt.

Breneman, A.; Crew, A.; Sample, J.; Klumpar, D.; Johnson, A.; Agapitov, O.; Shumko, M.; Turner, D.; Santolik, O.; Wygant, J.; Cattell, C.; Thaller, S.; Blake, B.; Spence, H.; Kletzing, C.;

Published by: Geophysical Research Letters      Published on: 11/2017

YEAR: 2017     DOI: 10.1002/2017GL075001

Chorus; conjunction; FIREBIRD; microburst; Van Allen Probes

Empirical modeling of the plasmasphere dynamics using neural networks

We propose a new empirical model for reconstructing the global dynamics of the cold plasma density distribution based only on solar wind data and geomagnetic indices. Utilizing the density database obtained using the NURD (Neural-network-based Upper hybrid Resonance Determination) algorithm for the period of October 1, 2012 - July 1, 2016, in conjunction with solar wind data and geomagnetic indices, we develop a neural network model that is capable of globally reconstructing the dynamics of the cold plasma density distribution for 2<=L<=6 and all local times. We validate and test the model by measuring its performance on independent datasets withheld from the training set and by comparing the model predicted global evolution with global images of He+ distribution in the Earth\textquoterights plasmasphere from the IMAGE Extreme UltraViolet (EUV) instrument. We identify the parameters that best quantify the plasmasphere dynamics by training and comparing multiple neural networks with different combinations of input parameters (geomagnetic indices, solar wind data, and different durations of their time history). The optimal model is based on the 96-hour time history of Kp, AE, SYM-H, and F10.7 indices. The model successfully reproduces erosion of the plasmasphere on the night side and plume formation and evolution. We demonstrate results of both local and global plasma density reconstruction. This study illustrates how global dynamics can be reconstructed from local in-situ observations by using machine learning techniques.

Zhelavskaya, Irina; Shprits, Yuri; c, Maria;

Published by: Journal of Geophysical Research: Space Physics      Published on: 10/2017

YEAR: 2017     DOI: 10.1002/2017JA024406

inner magnetosphere; Machine learning; Models; neural networks; plasmasphere; Van Allen Probes

The Evolution of the Plasma Sheet Ion Composition: Storms and Recoveries

The ion plasma sheet (~few hundred eV to ~few 10s keV) is usually dominated by H+ ions. Here, changes in ion composition within the plasma sheet are explored both during individual events, and statistically during 54 calm-to-storm events and during 21 active-to-calm events. Ion composition data from the HOPE (Helium, Oxygen, Proton, Electron) instruments onboard Van Allen Probes satellites provide exceptional spatial and temporal resolution of the H+, O+, and He+ ion fluxes in the plasma sheet. H+ shown to be the dominant ion in the plasma sheet in the calm-to-storm transition. However, the energy-flux of each ion changes in a quasi-linear manner during extended calm intervals. Heavy ions (O+ and He+) become increasingly important during such periods as charge-exchange reactions result in faster loss for H+ than for O+ or He+. Results confirm previous investigations showing that the ion composition of the plasma sheet can be largely understood (and predicted) during calm intervals from knowledge of: (a) the composition of previously injected plasma at the onset of calm conditions, and (b) use of simple drift-physics models combined with calculations of charge-exchange losses.

Denton, M.; Thomsen, M.; Reeves, G.; Larsen, B.; Henderson, M.; Jordanova, V.; Fernandes, P.; Friedel, R.; Skoug, R.; Funsten, H.; MacDonald, E.; Spence, H.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 10/2017

YEAR: 2017     DOI: 10.1002/2017JA024475

plasma sheet; Van Allen Probes

Examining coherency scales, substructure, and propagation of whistler-mode chorus elements with Magnetospheric Multiscale (MMS)

Whistler-mode chorus waves are a naturally occurring electromagnetic emission observed in Earth\textquoterights magnetosphere. Here, for the first time, data from NASA\textquoterights Magnetospheric Multiscale (MMS) mission were used to analyze chorus waves in detail, including the calculation of chorus wave normal vectors, k. A case study was examined from a period of substorm activity around the time of a conjunction between the MMS constellation and NASA\textquoterights Van Allen Probes mission on 07 April 2016. Chorus wave activity was simultaneously observed by all six spacecraft over a broad range of L-shells (5.5 < L < 8.5), magnetic local time (06:00 < MLT < 09:00), and magnetic latitude (-32\textdegree < MLat < -15\textdegree), implying a large chorus active region. Eight chorus elements and their substructure were analyzed in detail with MMS. These chorus elements were all lower band and rising tone emissions, right-handed and nearly circularly polarized, and propagating away from the magnetic equator when they were observed at MMS (MLat ~ -31\textdegree). Most of the elements had \textquotedbllefthook\textquotedblright like signatures on their wave power spectra, characterized by enhanced wave power at flat or falling frequency following the peak, and all the elements exhibited complex and well organized substructure observed consistently at all four MMS spacecraft at separations up to 70 km (60 km perpendicular and 38 km parallel to the background magnetic field). The waveforms in field-aligned coordinates also demonstrated that these waves were all phase coherent allowing for the direct calculation of k. Error estimates on calculated k revealed that the plane wave approximation was valid for six of the eight elements and most of the subelements. The wave normal vectors were within 20-30\textdegree from the direction anti-parallel to the background field for all elements and changed from subelement to subelement through at least two of the eight elements. The azimuthal angle of k in the perpendicular plane was oriented earthward and was oblique to that of the Poynting vector, which has implications for the validity of cold plasma theory.

Turner, D.; Lee, J.; Claudepierre, S.; Fennell, J.; Blake, J.; Jaynes, A.; Leonard, T.; Wilder, F.; Ergun, R.; Baker, D.; Cohen, I.; Mauk, B.; Strangeway, R.; Hartley, D.; Kletzing, C.; Breuillard, H.; Le Contel, O.; Khotyaintsev, Yu; Torbert, R.; Allen, R.; Burch, J.; Santolik, O.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 10/2017

YEAR: 2017     DOI: 10.1002/2017JA024474

chorus waves; inner magnetosphere; Magnetospheric multiscale; MMS; Radiation belts; Van Allen Probes

Improving empirical magnetic field models by fitting to in situ data using an optimized parameter approach

A method for comparing and optimizing the accuracy of empirical magnetic field models using in situ magnetic field measurements is presented. The optimization method minimizes a cost function - τ - that explicitly includes both a magnitude and an angular term. A time span of 21 days, including periods of mild and intense geomagnetic activity, was used for this analysis. A comparison between five magnetic field models (T96, T01S, T02, TS04, TS07) widely used by the community demonstrated that the T02 model was, on average, the most accurate when driven by the standard model input parameters. The optimization procedure, performed in all models except TS07, generally improved the results when compared to unoptimized versions of the models. Additionally, using more satellites in the optimization procedure produces more accurate results. This procedure reduces the number of large errors in the model, i.e. it reduces the number of outliers in the error distribution. The TS04 model shows the most accurate results after the optimization in terms of both the magnitude and direction, when using at least 6 satellites in the fitting. It gave a smaller error than its unoptimized counterpart 57.3\% of the time and outperformed the best unoptimized model (T02) 56.2\% of the time. Its median percentage error in |B| was reduced from 4.54\% to 3.84\%. The difference among the models analyzed, when compared in terms of the median of the error distributions, is not very large. However, the unoptimized models can have very large errors, which are much reduced after the optimization.

Brito, Thiago; Morley, Steven;

Published by: Space Weather      Published on: 10/2017

YEAR: 2017     DOI: 10.1002/2017SW001702

comparison; Empirical Model; magnetic field model; optimization; Van Allen Probes

Intelligent Sampling of Hazardous Particle Populations in Resource-Constrained Environments

Sampling of anomaly-causing space environment drivers is necessary for both real-time operations and satellite design efforts, and optimizing measurement sampling helps minimize resource demands. Relating these measurements to spacecraft anomalies requires the ability to resolve spatial and temporal variability in the energetic charged particle hazard of interest. Here we describe a method for sampling particle fluxes informed by magnetospheric phenomenology so that, along a given trajectory, the variations from both temporal dynamics and spatial structure are adequately captured while minimizing oversampling. We describe the coordinates, sampling method, and specific regions and parameters employed. We compare resulting sampling cadences with data from spacecraft spanning the regions of interest during a geomagnetically active period, showing that the algorithm retains the gross features necessary to characterize environmental impacts on space systems in diverse orbital regimes while greatly reducing the amount of sampling required. This enables sufficient environmental specification within a resource-constrained context, such as limited telemetry bandwidth, processing requirements, and timeliness.

McCollough, J.; Quinn, J.; Starks, M.; Johnston, W.;

Published by: Space Weather      Published on: 10/2017

YEAR: 2017     DOI: 10.1002/2017SW001629

data sampling; magnetospheric plasma; measurement; Solar Energetic Protons; trapped electrons; trapped protons; Van Allen Probes

Relativistic electron increase during chorus wave activities on the 6-8 March 2016 geomagnetic storm

There was a geomagnetic storm on 6\textendash8 March 2016, in which Van Allen Probes A and B separated by \~2.5 h measured increase of relativistic electrons with energies \~ several hundred keV to 1 MeV. Simultaneously, chorus waves were measured by both Van Allen Probes and Magnetospheric Multiscale (MMS) mission. Some of the chorus elements were rising-tones, possibly due to nonlinear effects. These measurements are compared with a nonlinear theory of chorus waves incorporating the inhomogeneity ratio and the field equation. From this theory, a chorus wave profile in time and one-dimensional space is simulated. Test particle calculations are then performed in order to examine the energization rate of electrons. Some electrons are accelerated, although more electrons are decelerated. The measured time scale of the electron increase is inferred to be consistent with this nonlinear theory.

Matsui, H.; Torbert, R.; Spence, H.; Argall, M.; Alm, L.; Farrugia, C.; Kurth, W.; Baker, D.; Blake, J.; Funsten, H.; Reeves, G.; Ergun, R.; Khotyaintsev, Yu.; Lindqvist, P.-A.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 10/2017

YEAR: 2017     DOI: 10.1002/2017JA024540

chorus waves; Geomagnetic storm; relativistic electrons; Van Allen Probes

SC-associated electric field variations in the magnetosphere and ionospheric convective flows

We examine magnetic and electric field perturbations associated with a sudden commencement (SC), caused by an interplanetary (IP) shock passing over the Earth\textquoterights magnetosphere on 16 February 2013. The SC was identified in the magnetic and electric field data measured at THEMIS-E (THE-E: MLT = 12.4, L = 6.3), Van Allen Probe-A (VAP-A: MLT = 3.2, L = 5.1), and Van Allen Probe-B (VAP-B: MLT = 0.2. L= 4.9) in the magnetosphere. During the SC interval, THE-E observed a dawnward-then-duskward electric (E) field perturbation around noon, while VAP-B observed a duskward E-field perturbation around midnight. VAP-A observed a dawnward-then-duskward E-field perturbation in the postmidnight sector, but the duration and magnitude of the dawnward E-perturbation are much shorter and weaker than that at THE-E. That is, the E-field signature changes with local time during the SC interval. The SuperDARN radar data indicate that the ionospheric plasma motions during the SC are mainly due to the E-field variations observed in space. This indicates that the SC-associated E-field in space plays a significant role in determining the dynamic variations of the ionospheric convection flow. By comparing previous SC MHD simulations and our observations, we suggest that the E-field variations observed at the spacecraft are produced by magnetospheric convection flows due to deformation of the magnetosphere as the IP shock sweeps the magnetopause.

Kim, S.-I.; Kim, K.-H.; Kwon, H.-J.; Jin, H.; Lee, E.; Jee, G.; Nishitani, N.; Hori, T.; Lester, M.; Wygant, J.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 10/2017

YEAR: 2017     DOI: 10.1002/2017JA024611

electric field; Sudden commencement; Van Allen Probes

Shock-induced disappearance and subsequent recovery of plasmaspheric hiss: Coordinated observations of RBSP, THEMIS and POES satellites

Plasmaspheric hiss is an extremely low frequency whistler-mode emission contributing significantly to the loss of radiation belt electrons. There are two main competing mechanisms for the generation of plasmaspheric hiss: excitation by local instability in the outer plasmasphere and origination from chorus outside the plasmasphere. Here, on the basis of the analysis of an event of shock-induced disappearance and subsequent recovery of plasmaspheric hiss observed by RBSP, THEMIS and POES missions, we attempt to identify its dominant generation mechanism. In the pre-shock plasmasphere, the local electron instability was relatively weak and the hiss waves with bidirectional Poynting fluxes mainly originated from the dayside chorus waves. On arrival of the shock, the removal of pre-existing dayside chorus and the insignificant variation of low-frequency wave instability caused the prompt disappearance of hiss waves. In the next few hours, the local instability in the plasmasphere was greatly enhanced due to the substorm injection of hot electrons. The enhancement of local instability likely played a dominant role in the temporary recovery of hiss with unidirectional Poynting fluxes. These temporarily recovered hiss waves were generated near the equator and then propagated toward higher latitudes. In contrast, both the enhancement of local instability and the recurrence of pre-noon chorus contributed to the substantial recovery of hiss with bidirectional Poynting fluxes.

Liu, Nigang; Su, Zhenpeng; Gao, Zhonglei; Reeves, G.; Zheng, Huinan; Wang, Yuming; Wang, Shui;

Published by: Journal of Geophysical Research: Space Physics      Published on: 10/2017

YEAR: 2017     DOI: 10.1002/2017JA024470

Chorus; interplanetary shock; Plasmaspheric Hiss; Radiation belt; substorm injection; Van Allen Probes; Wave-particle interaction

A Statistical Study of the Spatial Extent of Relativistic Electron Precipitation with Polar Orbiting Environmental Satellites.

Relativistic Electron Precipitation (REP) in the atmosphere can contribute significantly to electron loss from the outer radiation belts. In order to estimate the contribution to this loss, it is important to estimate the spatial extent of the precipitation region. We observed REP with the zenith pointing (0o) Medium Energy Proton Electron Detector (MEPED) on board Polar Orbiting Environmental Satellites (POES), for 15 years (2000-2014) and used both single and multi satellite measurements to estimate an average extent of the region of precipitation in L shell and Magnetic Local Time (MLT). In the duration of 15 years (2000-2014), 31035 REP events were found in this study. Events were found to split into two classes; one class of events coincided with proton precipitation in the P1 channel (30-80 keV), were located in the dusk and early morning sector, and were more localized in L shell (dL<0.5), whereas the other class of events did not coincide with proton precipitation, were located mostly in the midnight sector and were wider in L shell (dL \~ 1-2.5). Both classes were highly localized in MLT (dMLT <= 3 hrs), occuring mostly during the declining phase of the solar cycle and geomagnetically active times. The events located in the midnight sector for both classes were found to be associated with tail magnetic field stretching which could be due to the fact that they tend to occur mostly during geomagnetically active times, or could imply that precipitation is caused by current sheet scattering.

Shekhar, Sapna; Millan, Robyn; Smith, David;

Published by: Journal of Geophysical Research: Space Physics      Published on: 10/2017

YEAR: 2017     DOI: 10.1002/2017JA024716

Magnetosphere; precipitation; Radiation belts; relativistic electrons; spatial scale of REP; Van Allen Probes; wave particle scattering

On the effect of geomagnetic storms on relativistic electrons in the outer radiation belt: Van Allen Probes observations

Using Van Allen Probes ECT-REPT observations we performed a statistical study on the effect of geomagnetic storms on relativistic electrons fluxes in the outer radiation belt for 78 storms between September 2012 and June 2016. We found that the probability of enhancement, depletion and no change in flux values depends strongly on L and energy. Enhancement events are more common for \~ 2 MeV electrons at L \~ 5, and the number of enhancement events decreases with increasing energy at any given L shell. However, considering the percentage of occurrence of each kind of event, enhancements are more probable at higher energies, and the probability of enhancement tends to increases with increasing L shell. Depletion are more probable for 4-5 MeV electrons at the heart of the outer radiation belt, and no change events are more frequent at L < 3.5 for E\~ 3 MeV particles. Moreover, for L > 4.5 the probability of enhancement, depletion or no-change response presents little variation for all energies. Because these probabilities remain relatively constant as a function of radial distance in the outer radiation belt, measurements obtained at Geosynchronous orbit may be used as a proxy to monitor E>=1.8 MeV electrons in the outer belt.

Moya, Pablo.; Pinto, \; Sibeck, David; Kanekal, Shrikanth; Baker, Daniel;

Published by: Journal of Geophysical Research: Space Physics      Published on: 10/2017

YEAR: 2017     DOI: 10.1002/2017JA024735

Geomagnetic storms; Radiation belts; relativistic electrons; Van Allen Probes

Energetic proton spectra measured by the Van Allen Probes

We test the hypothesis that pitch-angle scattering by electromagnetic ion cyclotron (EMIC) waves can limit ring current proton fluxes. For two chosen magnetic storms, during March 17-20, 2013 and March 17-20, 2015, we measure proton energy spectra in the region 3 <= L <= 6 using the RBSPICE B instrument on the Van Allen Probes. The most intense proton spectra are observed to occur during the recovery periods of the respective storms. Using proton precipitation data from the POES (NOAA and MetOp) spacecraft, we deduce that EMIC wave action was prevalent at the times and L-shell locations of the most intense proton spectra. We calculate limiting ring current proton energy spectra from recently developed theory. Comparisons between the observed proton energy spectra and the theoretical limiting spectra show reasonable agreement. We conclude that the measurements of the most intense proton spectra are consistent with self-limiting by EMIC wave scattering.

Summers, Danny; Shi, Run; Engebretson, Mark; Oksavik, Kjellmar; Manweiler, Jerry; Mitchell, Donald;

Published by: Journal of Geophysical Research: Space Physics      Published on: 09/2017

YEAR: 2017     DOI: 10.1002/2017JA024484

EMIC-wave -proton scattering; proton ring current; Van Allen Probes

Generation of Highly Oblique Lower-band Chorus via Nonlinear Three-wave Resonance

Chorus in the inner magnetosphere has been observed frequently at geomagnetically active times, typically exhibiting a two-band structure with a quasi-parallel lower-band and an upper-band with a broad range of wave normal angles. But recent observations by Van Allen Probes confirm another type of lower-band chorus, which has a large wave normal angle close to the resonance cone angle. It has been proposed that these waves could be generated by a low-energy beam-like electron component or by temperature anisotropy of keV electrons in the presence of a low-energy plateau-like electron component. This paper, however, presents an alternative mechanism for generation of this highly oblique lower-band chorus. Through a nonlinear three-wave resonance, a quasi-parallel lower-band chorus wave can interact with a mildly oblique upper-band chorus wave, producing a highly oblique quasi-electrostatic lower-band chorus wave. This theoretical analysis is confirmed by 2D electromagnetic particle-in-cell simulations. Furthermore, as the newly generated waves propagate away from the equator, their wave normal angle can further increase and they are able to scatter low-energy electrons to form a plateau-like structure in the parallel velocity distribution. The three-wave resonance mechanism may also explain the generation of quasi-parallel upper-band chorus which has also been observed in the magnetosphere.

Fu, Xiangrong; Gary, Peter; Reeves, Geoffrey; Winske, Dan; Woodroffe, Jesse;

Published by: Geophysical Research Letters      Published on: 09/2017

YEAR: 2017     DOI: 10.1002/2017GL074411

oblique whistler; PIC simulation; Ray Tracing; three-wave resonance; Van Allen Probes

Multiple-satellite observation of magnetic dip event during the substorm on 10 October, 2013

We present a multiple-satellite observation of the magnetic dip event during the substorm on October 10, 2013. The observation illustrates the temporal and spatial evolution of the magnetic dip and gives a compelling evidence that ring current ions induce the magnetic dip by enhanced plasma beta. The dip moves with the energetic ions in a comparable drift velocity and affects the dynamics of relativistic electrons in the radiation belt. In addition, the magnetic dip provides a favorable condition for the EMIC wave generation based on the linear theory analysis. The calculated proton diffusion coefficients show that the observed EMIC wave can lead to the pitch angle scattering losses of the ring current ions, which in turn partially relax the magnetic dip in the observations. This study enriches our understanding of magnetic dip evolution and demonstrates the important role of the magnetic dip for the coupling of radiation belt and ring current.

He, Zhaoguo; Chen, Lunjin; Zhu, Hui; Xia, Zhiyang; Reeves, G.; Xiong, Ying; Xie, Lun; Cao, Yong;

Published by: Geophysical Research Letters      Published on: 09/2017

YEAR: 2017     DOI: 10.1002/2017GL074869

EMIC wave; magnetic dip; radiation belt electrons; Ring current ions; Van Allen Probes

Multipoint observations of energetic particle injections and substorm activity during a conjunction between Magnetospheric Multiscale (MMS) and Van Allen Probes

This study examines multipoint observations during a conjunction between MMS and Van Allen Probes on 07 April 2016 in which a series of energetic particle injections occurred. With complementary data from THEMIS, Geotail, and LANL-GEO (16 spacecraft in total), we develop new insights on the nature of energetic particle injections associated with substorm activity. Despite this case involving only weak substorm activity (max. AE < 300 nT) during quiet geomagnetic conditions in steady, below-average solar wind, a complex series of at least six different electron injections was observed throughout the system. Intriguingly, only one corresponding ion injection was clearly observed. All ion and electron injections were observed at < 600 keV only. MMS reveals detailed substructure within the largest electron injection. A relationship between injected electrons with energy < 60 keV and enhanced whistler-mode chorus wave activity is also established from Van Allen Probes and MMS. Drift mapping using a simplified magnetic field model provides estimates of the dispersionless injection boundary locations as a function of universal time, magnetic local time, and L-shell. The analysis reveals that at least five electron injections, which were localized in magnetic local time, preceded a larger injection of both electrons and ions across nearly the entire nightside of the magnetosphere near geosynchronous orbit. The larger, ion and electron injection did not penetrate to L < 6.6, but several of the smaller, electron injections penetrated to L < 6.6. Due to the discrepancy between the number, penetration depth, and complexity of electron vs. ion injections, this event presents challenges to the current conceptual models of energetic particle injections.

Turner, D.; Fennell, J.; Blake, J.; Claudepierre, S.; Clemmons, J.; Jaynes, A.; Leonard, T.; Baker, D.; Cohen, I.; Gkioulidou, M.; Ukhorskiy, A; Mauk, B.; Gabrielse, C.; Angelopoulos, V.; Strangeway, R.; Kletzing, C.; Le Contel, O.; Spence, H.; Torbert, R.; Burch, J.; Reeves, G.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 09/2017

YEAR: 2017     DOI: 10.1002/2017JA024554

energetic particles; injections; inner magnetosphere; plasma sheet; substorms; Van Allen Probes; wave-particle interactions

Radiation-Induced Single-Event Effects on the Van Allen Probes Spacecraft

Electronic devices on the Van Allen Probes mission have experienced more than a thousand single-event effects (SEE) during the 4.5 years of transit through the inner and outer earth trapped radiation belts. The majority of these SEE have been due to trapped protons determined by the orbit timing and the dose rate response of the engineering radiation monitor. Fault tolerant systems engineering and spacecraft operation have enabled a successful mission to date without a safe mode or spacecraft emergency.

Maurer, Richard; Fretz, Kristin; Angert, Matthew; Bort, David; Goldsten, John; Ottman, Geffrey; Dolan, Jeff; Needell, Gerald; Bodet, David;

Published by: IEEE Transactions on Nuclear Science      Published on: 09/2017

YEAR: 2017     DOI: 10.1109/TNS.2017.2754878

Space vehicles; Probes; Belts; Orbits; Monitoring; protons; Observatories; Van Allen Probes

Ring Current He-Ion Control by Bounce Resonant ULF Waves

Ring current energy He-ion (\~65 keV to \~520 keV) differential flux data from the Radiation Belt Storm Probe Ion Composition Experiment (RBSPICE) instrument aboard the Van Allan Probes spacecraft show considerable variability during quiet solar wind and geomagnetic time periods. Such variability is apparent from orbit to orbit (\~9 hours) of the spacecraft and is observed to be \~50\textendash100\% of the nominal flux. Using data from the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) instrument, also aboard the Van Allen Probes spacecraft, we identify that a dominant source of this variability is from ULF waveforms with periods of 10\textquoterights of sec. These periods correspond to the bounce resonant timescales of the ring current He-ions being measured by RBSPICE. A statistical survey using the particle and field data for one full spacecraft precession period (approximately two years) shows that the wave and He-ion flux variations are generally anti-correlated, suggesting the bounce resonant pitch-angle scattering process as a major component in the scattering of He-ions.

Kim, Hyomin; Gerrard, Andrew; Lanzerotti, Louis; Soto-Chavez, Rualdo; Cohen, Ross; Manweiler, Jerry;

Published by: Journal of Geophysical Research: Space Physics      Published on: 09/2017

YEAR: 2017     DOI: 10.1002/2017JA023958

bounce resonance; Helium ion; ring current; ULF waves; Van Allen Probes

Signatures of Ultrarelativistic Electron Loss in the Heart of the Outer Radiation Belt Measured by Van Allen Probes

Up until recently, signatures of the ultrarelativistic electron loss driven by electromagnetic ion cyclotron (EMIC) waves in the Earth\textquoterights outer radiation belt have been limited to direct or indirect measurements of electron precipitation or the narrowing of normalized pitch angle distributions in the heart of the belt. In this study, we demonstrate additional observational evidence of ultrarelativistic electron loss that can be driven by resonant interaction with EMIC waves. We analyzed the profiles derived from Van Allen Probe particle data as a function of time and three adiabatic invariants between 9 October and 29 November 2012. New local minimums in the profiles are accompanied by the narrowing of normalized pitch angle distributions and ground-based detection of EMIC waves. Such a correlation may be indicative of ultrarelativistic electron precipitation into the Earth\textquoterights atmosphere caused by resonance with EMIC waves.

Aseev, N.; Shprits, Y; Drozdov, A; Kellerman, A.; Usanova, M.; Wang, D.; Zhelavskaya, I.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 09/2017

YEAR: 2017     DOI: 10.1002/2017JA024485

electron loss; EMIC waves; Radiation belts; ultrarelativistic electrons; Van Allen Probes; wave-particle interactions

Simulated prompt acceleration of multi-MeV electrons by the 17 March 2015 interplanetary shock

Prompt enhancement of relativistic electron flux at L = 3-5 has been reported from Van Allen Probes Relativistic Electron Proton Telescope (REPT) measurements associated with the 17 March 2015 interplanetary shock compression of the dayside magnetosphere. Acceleration by \~ 1 MeV is inferred on less than a drift time scale as seen in prior shock compression events, which launch a magetosonic azimuthal electric field impulse tailward. This impulse propagates from the dayside around the flanks accelerating electrons in drift resonance at the dusk flank. Such longitudinally localized acceleration events produce a drift echo signature which was seen at >1 MeV energy on both Van Allen Probe spacecraft, with sustained observations by Probe B outbound at L = 5 at 2100 MLT at the time of impulse arrival, measured by the Electric Fields and Waves instrument. MHD-test particle simulations are presented which reproduce drift echo features observed in the REPT measurements at Probe B, including the energy and pitch angle dependence of drift echoes observed. While the flux enhancement was short-lived for this event due to subsequent inward motion of the magnetopause, stronger events with larger electric field impulses, as observed in March 1991 and the Halloween 2003 storm, produce enhancements which can be quantified by the inward radial transport and energization determined by the induction electric field resulting from dayside compression.

Hudson, Mary; Jaynes, Allison; Kress, Brian; Li, Zhao; Patel, Maulik; Shen, Xiaochen; Thaller, Scott; Wiltberger, Michael; Wygant, John;

Published by: Journal of Geophysical Research: Space Physics      Published on: 09/2017

YEAR: 2017     DOI: 10.1002/2017JA024445

17 March 2015; MeV electron acceleration; Radiation belt; test-particle simulation; Van Allen Probes

The Warm Plasma Composition in the Inner Magnetosphere during 2012-2015

Ionospheric heavy ions play an important role in the dynamics of Earth\textquoterights magnetosphere. The greater mass and gyro radius of ionospheric oxygen differentiates its behavior from protons at the same energies. Oxygen may have an impact on tail reconnection processes, and it can at least temporarily dominate the energy content of the ring current during geomagnetic storms. At sub-keV energies, multi-species ion populations in the inner magnetosphere form the warm plasma cloak, occupying the energy range between the plasmasphere and the ring current. Lastly, cold lighter ions from the mid-latitude ionosphere create the co-rotating plasmasphere whose outer regions can interact with the plasma cloak, plasma sheet, ring current, and outer electron belt. In this paper we present a statistical view of warm, cloak-like ion populations in the inner magnetosphere, contrasting in particular the warm plasma composition during quiet and active times. We study the relative abundances and absolute densities of warm plasma measured by the Van Allen Probes, whose two spacecraft cover the inner magnetosphere from plasmaspheric altitudes close to Earth to just inside geostationary orbit. We observe that warm (>30 eV) oxygen is most abundant closer to the plasmasphere boundary whereas warm hydrogen dominates closer to geostationary orbit. Warm helium is usually a minor constituent, but shows a noticeable enhancement in the near-Earth dusk sector.

Jahn, J.-M.; Goldstein, J.; Reeves, G.; Fernandes, P.; Skoug, R.; Larsen, B.; Spence, H.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 09/2017

YEAR: 2017     DOI: 10.1002/2017JA024183

geomagnetic activity; inner magnetosphere; plasma composition; plasma density; statistics; Van Allen Probes

Bounce resonance scattering of radiation belt electrons by low-frequency hiss: Comparison with cyclotron and Landau resonances

Bounce-resonant interactions with magnetospheric waves have been proposed as important contributing mechanisms for scattering near-equatorially mirroring electrons by violating the second adiabatic invariant associated with the electron bounce motion along a geomagnetic field line. This study demonstrates that low-frequency plasmaspheric hiss with significant wave power below 100 Hz can bounce-resonate efficiently with radiation belt electrons. By performing quantitative calculations of pitch-angle scattering rates, we show that low-frequency hiss induced bounce-resonant scattering of electrons has a strong dependence on equatorial pitch-angle αeq. For electrons with αeq close to 90\textdegree, the timescale associated with bounce resonance scattering can be comparable to or even less than 1 hour. Cyclotron- and Landau-resonant interactions between low-frequency hiss and electrons are also investigated for comparisons. It is found that while the bounce and Landau resonances are responsible for the diffusive transport of near-equatorially mirroring electrons to lower αeq, pitch-angle scattering by cyclotron resonance could take over to further diffuse electrons into the atmosphere. Bounce resonance provides a more efficient pitch-angle scattering mechanism of relativistic (>= 1 MeV) electrons than Landau resonance due to the stronger scattering rates and broader resonance coverage of αeq, thereby demonstrating that bounce resonance scattering by low-frequency hiss can contribute importantly to the evolution of the electron pitch-angle distribution and the loss of radiation belt electrons.

Cao, Xing; Ni, Binbin; Summers, Danny; Zou, Zhengyang; Fu, Song; Zhang, Wenxun;

Published by: Geophysical Research Letters      Published on: 09/2017

YEAR: 2017     DOI: 10.1002/2017GL075104

bounce resonance; Low-frequency hiss; Radiation Belt Dynamics; Van Allen Probes; wave-particle interactions

Butterfly distribution of Earth\textquoterights radiation belt relativistic electrons induced by dayside chorus

Previous theoretical studies have shown that dayside chorus can produce butterfly distribution of energetic electrons in the Earth\textquoterights radiation belts by preferentially accelerating medium pitch angle electrons, but this requires the further confirmation from high-resolution satellite observation. Here, we report correlated Van Allen Probes data on wave and particle during the 11\textendash13 April, 2014 geomagnetic storm. We find that a butterfly pitch angle distribution of relativistic electrons is formed around the location L = 4.52, corresponding to the presence of enhanced dayside chorus. Using a Gaussian distribution fit to the observed chorus spectra, we calculate the bounce-averaged diffusion rates and solve two-dimensional Fokker-Planck equation. Numerical results demonstrate that acceleration by dayside chorus can yield the electron flux evolution both in the energy and butterfly pitch angle distribution comparable to the observation, providing a further evidence for the formation of butterfly distribution of relativistic electrons driven by very low frequency (VLF) plasma waves.

Jin, YuYue; Yang, Chang; He, Yihua; Liu, Si; Zhou, Qinghua; Xiao, Fuliang;

Published by: Science China Technological Sciences      Published on: 09/2017

YEAR: 2017     DOI: 10.1007/s11431-017-9067-y

butterfly distribution relativistic electrons radiation belts wave-particle interaction dayside chorus; Van Allen Probes

The characteristic response of whistler mode waves to interplanetary shocks

Magnetospheric whistler mode waves play a key role in regulating the dynamics of the electron radiation belts. Recent satellite observations indicate a significant influence of interplanetary (IP) shocks on whistler mode wave power in the inner magnetosphere. In this study, we statistically investigate the response of whistler mode chorus and plasmaspheric hiss to IP shocks based on Van Allen Probes and THEMIS satellite observations. Immediately after the IP shock arrival, chorus wave power is usually intensified, often at post-midnight to pre-noon sector, while plasmaspheric hiss wave power predominantly decreases near the dayside but intensifies near the nightside. We conclude that chorus wave intensification outside the plasmasphere is probably associated with the suprathermal electron flux enhancement caused by the IP shock. Through a simple ray tracing modeling assuming the scenario that plasmaspheric hiss is originated from chorus, we find that the solar wind dynamic pressure increase changes the magnetic field configuration to favor ray penetration in the nightside and promote ray refraction away from the dayside, potentially explaining the magnetic local time (MLT) dependent responses of plasmaspheric hiss waves following IP shock arrivals.

Yue, Chao; Chen, Lunjin; Bortnik, Jacob; Ma, Qianli; Thorne, Richard; Angelopoulos, Vassilis; Li, Jinxing; An, Xin; Zhou, Chen; Kletzing, Craig; Reeves, Geoffrey; Spence, Harlan;

Published by: Journal of Geophysical Research: Space Physics      Published on: 09/2017

YEAR: 2017     DOI: 10.1002/2017JA024574

IP shocks; MLT dependent; Plasmaspheric Hiss; Ray Tracing; Van Allen Probes; whistler mode chorus

Diffusive transport of several hundred keV electrons in the Earth\textquoterights slot region

We investigate the gradual diffusion of energetic electrons from the inner edge of the outer radiation belt into the slot region. The Van Allen Probes observed slow inward diffusion and decay of ~200-600 keV electrons following the intense geomagnetic storm that occurred on 17 March 2013. During the 10-day non-disturbed period following the storm, the peak of electron fluxes gradually moved from L~2.7 to L~2.4, and the flux levels decreased by a factor of ~2-4 depending on the electron energy. We simulated the radial intrusion and decay of electrons using a 3-dimentional diffusion code, which reproduced the energy-dependent transport of electrons from ~100 keV to 1 MeV in the slot region. At energies of 100-200 keV, the electrons experience fast transport across the slot region due to the dominance of radial diffusion; at energies of 200-600 keV, the electrons gradually diffuse and decay in the slot region due to the comparable rate of radial diffusion and pitch angle scattering by plasmaspheric hiss; at energies of E > 700 keV, the electrons stopped diffusing near the inner edge of outer radiation belt due to the dominant pitch angle scattering loss. In addition to plasmaspheric hiss, magnetosonic waves and VLF transmitters can cause the loss of high pitch angle electrons, relaxing the sharp \textquotelefttop-hat\textquoteright shaped pitch angle distributions created by plasmaspheric hiss. Our simulation indicates the importance of balance between radial diffusion and loss through pitch angle scattering in forming the diffusive intrusion of energetic electrons across the slot region.

Ma, Q.; Li, W.; Thorne, R.; Bortnik, J.; Reeves, G.; Spence, H.; Turner, D.; Blake, J.; Fennell, J.; Claudepierre, S.; Kletzing, C.; Kurth, W.; Hospodarsky, G.; Baker, D.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 09/2017

YEAR: 2017     DOI: 10.1002/2017JA024452

Electron transport; Energetic electron diffusion; pitch angle scattering; Slot region dynamics; Van Allen Probes; Van Allen Probes observation; Waves in plasmasphere

Empirical radiation belt models: Comparison with in-situ data and implications for environment definition

The empirical AP8/AE8 model has been the de-facto Earth\textquoterights radiation belts engineering reference for decades. The need from the community for a better model incubated the development of AP9/AE9/SPM, which addresses several shortcomings of the old model. We provide additional validation of AP9/AE9 by comparing in-situ electron and proton data from Jason-2, POES, and the Van Allen Probes spacecraft with the 5th, 50th, and 95th percentiles from AE9/AP9 and with the model outputs from AE8/AP8. The relatively short duration of Van Allen Probes and Jason-2 missions means that their measurements are most certainly the result of specific climatological conditions. In LEO, the Jason-2 proton flux is better reproduced by AP8 compared to AP9, while the POES electron data are well enveloped by AE9 5th and 95th percentiles. The shape of the SAA from Jason-2 data is better captured by AP9 compared to AP8, while the peak SAA flux is better reproduced by AP8. The <1.5 MeV inner belt electrons from MagEIS are well enveloped by AE9 5th and 95th percentiles while AE8 over-predicts the measurements. In the outer radiation belt, MagEIS and REPT electrons closely follow the median estimate from AE9, while AP9 5th and 95th percentiles generally envelope REPT proton measurements in the inner belt and slot regions. While AE9/AP9 offer the flexibility to specify the environment with different confidence levels, the dose and trapped proton peak flux for POES and Jason-2 trajectories from the AE9/AP9 50th percentile and above are larger than the estimates from the AE8/AP8 models.

Pich, Maria; Jun, Insoo; Evans, Robin;

Published by: Space Weather      Published on: 08/2017

YEAR: 2017     DOI: 10.1002/2017SW001612

Empirical Models; Radiation belts; Radiation effects; Van Allen Probes

Low-energy (< 200 eV) electron acceleration by ULF waves in the plasmaspheric boundary layer: Van Allen Probes observation

We report observational evidence of cold plamsmaspheric electron (< 200 eV) acceleration by ultra-low-frequency (ULF) waves in the plasmaspheric boundary layer on 10 September 2015. Strongly enhanced cold electron fluxes in the energy spectrogram were observed along with second harmonic mode waves with a period of about 1 minute which lasted several hours during two consecutive Van Allen Probe B orbits. Cold electron (<200 eV) and energetic proton (10-20 keV) bi-directional pitch angle signatures observed during the event are suggestive of the drift-bounce resonance mechanism. The correlation between enhanced energy fluxes and ULF waves leads to the conclusions that plasmaspheric dynamics is strongly affected by ULF waves. Van Allen Probe A and B, GOES 13, GOES 15 and MMS 1 observations suggest ULF waves in the event were strongest on the dusk-side magnetosphere. Measurements from MMS 1 contain no evidence of an external wave source during the period when ULF waves and injected energetic protons with a bump-on-tail distribution were detected by Van Allen Probe B. This suggests that the observed ULF waves were probably excited by a localized drift-bounce resonant instability, with the free energy supplied by substorm-injected energetic protons. The observations by Van Allen Probe B suggest that energy transfer between particle species in different energy ranges can take place through the action of ULF waves, demonstrating the important role of these waves in the dynamical processes of the inner magnetosphere.

Ren, Jie; Zong, Q.; Miyoshi, Y.; Zhou, X.; Wang, Y.; Rankin, R.; Yue, C.; Spence, H.; Funsten, H.; Wygant, J.; Kletzing, C.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 08/2017

YEAR: 2017     DOI: 10.1002/2017JA024316

Cold plasmaspheric electrons; drift-bounce resonance; Plasma instability; Plasmaspheric boundary layer; Substorm-injected protons; ULF waves; Van Allen Probes

The plasma environment inside geostationary orbit: A Van Allen Probes HOPE survey

The two full precessions in local time completed by the Van Allen Probes enable global specification of the near-equatorial inner magnetosphere plasma environment. Observations by the Helium-Oxygen-Proton-Electron (HOPE) mass spectrometers provide detailed insight into the global spatial distribution of electrons, H+, He+, and O+. Near-equatorial omnidirectional fluxes and abundance ratios at energies 0.1\textendash30 keV are presented for 2 <= L <= 6 as a function of L shell, magnetic local time (MLT), and geomagnetic activity. We present a new tool built on the UBK modeling technique for classifying plasma sheet particle access to the inner magnetosphere. This new tool generates access maps for particles of constant energy for more direct comparison with in situ measurements, rather than the traditional constant μ presentation typically associated with UBK. We present for the first time inner magnetosphere abundances of O+ flux relative to H+ flux as a function of Kp, L, MLT, and energy. At L = 6, the O+/H+ ratio increases with increasing Kp, consistent with previous results. However, at L < 5 the O+/H+ ratio generally decreases with increasing Kp. We identify a new \textquotedblleftafternoon bulge\textquotedblright plasma population enriched in 10 keV O+ and superenriched in 10 keV He+ that is present during quiet/moderate geomagnetic activity (Kp < 5) at ~1100\textendash2000 MLT and L shell 2\textendash4. Drift path modeling results are consistent with the narrow energy and approximate MLT location of this enhancement, but the underlying physics describing its formation, structure, and depletion during higher geomagnetic activity are currently not understood.

Fernandes, Philip; Larsen, Brian; Thomsen, Michelle; Skoug, Ruth; Reeves, Geoffrey; Denton, Michael; Friedel, Reinhard; Funsten, Herbert; Goldstein, Jerry; Henderson, Michael; Jahn, örg-Micha; MacDonald, Elizabeth; Olson, David;

Published by: Journal of Geophysical Research: Space Physics      Published on: 08/2017

YEAR: 2017     DOI: 10.1002/2017JA024160

inner magnetosphere; magnetospheric composition; plasma access; plasma convection; UBK modeling; Van Allen Probes

Pulsating auroras produced by interactions of electrons and time domain structures

Previous evidence has suggested that either lower band chorus waves or kinetic Alfven waves scatter equatorial kilovolt electrons that propagate to lower altitudes where they precipitate or undergo further low-altitude scattering to make pulsating auroras. Recently, time domain structures (TDSs) were shown, both theoretically and experimentally, to efficiently scatter equatorial electrons. To assess the relative importance of these three mechanisms for production of pulsating auroras, 11 intervals of equatorial THEMIS data and a 4 h interval of Van Allen Probe measurements have been analyzed. During these events, lower band chorus waves produced only negligible modifications of the equatorial electron distributions. During the several TDS events, the equatorial 0.1\textendash3 keV electrons became magnetic field-aligned. Kinetic Alfven waves may also have had a small electron scattering effect. The conclusion of these studies is that time domain structures caused the most important equatorial scattering of ~1 keV electrons toward the loss cone to provide the main electron contribution to pulsating auroras. Chorus wave scattering may have provided part of the highest energy (>10 keV) electrons in such auroras.

Mozer, F.; Agapitov, O.; Hull, A.; Lejosne, S.; Vasko, I;

Published by: Journal of Geophysical Research: Space Physics      Published on: 08/2017

YEAR: 2017     DOI: 10.1002/2017JA024223

pulsating auroras; Van Allen Probes; wave scattering

Rapid loss of radiation belt relativistic electrons by EMIC waves

How relativistic electrons are lost is an important question surrounding the complex dynamics of the Earth\textquoterights outer radiation belt. Radial loss to the magnetopause and local loss to the atmosphere are two main competing paradigms. Here, on the basis of the analysis of a radiation belt storm event on 27 February 2014, we present new evidence for the EMIC wave-driven local precipitation loss of relativistic electrons in the heart of the outer radiation belt. During the main phase of this storm, the radial profile of relativistic electron phase space density was quasi-monotonic, qualitatively inconsistent with the prediction of radial loss theory. The local loss at low L-shells was required to prevent the development of phase space density peak resulting from the radial loss process at high L-shells. The rapid loss of relativistic electrons in the heart of outer radiation belt was observed as a dip structure of the electron flux temporal profile closely related to intense EMIC waves. Our simulations further confirm that the observed EMIC waves within a quite limited longitudinal region was able to reduce the off-equatorially mirroring relativistic electron fluxes by up to 2 orders of magnitude within about 1.5 h.

Su, Zhenpeng; Gao, Zhonglei; Zheng, Huinan; Wang, Yuming; Wang, Shui; Spence, H.; Reeves, G.; Baker, D.; Wygant, J.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 08/2017

YEAR: 2017     DOI: 10.1002/2017JA024169

electron loss; EMIC waves; pitch angle scattering; radial diffusion; Radiation belts; Van Allen Probes; Wave-particle interaction

Storm time empirical model of O + and O 6+ distributions in the magnetosphere

Recent studies have utilized different charge states of oxygen ions as a tracer for the origins of plasma populations in the magnetosphere of Earth, using O+ as an indicator of ionospheric-originating plasma and O6+ as an indicator of solar wind-originating plasma. These studies have correlated enhancements in O6+ to various solar wind and geomagnetic conditions to characterize the dominant solar wind injection mechanisms into the magnetosphere but did not include analysis of the temporal evolution of these ions. A sixth-order Fourier expansion model based empirically on a superposed epoch analysis of geomagnetic storms observed by Polar is presented in this study to provide insight into the evolution of both ionospheric-originating and solar wind-originating plasma throughout geomagnetic storms. At high energies (~200 keV) the flux of O+ and O6+ are seen to become comparable in the outer magnetosphere. Moreover, while the density of O+ is far higher than O6+, the two charge states have comparable pressures in the outer magnetosphere. The temperature of O6+ is generally higher than that of O+, because the O6+ is injected from preheated magnetosheath populations before undergoing further heating once in the magnetosphere. A comparison between the model results with O+ observations from the Magnetospheric Multiscale mission and the Van Allen Probes provides a validation of the model. In general, this empirical model agrees qualitatively well with the trends seen in both data sets. Quantitatively, the modeled density, pressure, and temperature almost always agree within a factor of at most 10, 5, and 2, respectively.

Allen, R.; Livi, S.; Vines, S.; Goldstein, J.; Cohen, I.; Fuselier, S.; Mauk, B.; Spence, H.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 08/2017

YEAR: 2017     DOI: 10.1002/2017JA024245

MMS mission; Polar mission; solar wind injection; storm time dynamics; Van Allen Probes; Van Allen Probes mission

Sub-Auroral Polarization Stream (SAPS) duration as determined from Van Allen Probe successive electric drift measurements

We examine a characteristic feature of the magnetosphere-ionosphere coupling, namely, the persistent and latitudinally narrow bands of rapid westward ion drifts called the Sub-Auroral Polarization Streams (SAPS). Despite countless works on SAPS, information relative to their durations is lacking. Here, we report on the first statistical analysis of more than 200 near-equatorial SAPS observations based on more than two years of Van Allen Probe electric drift measurements. First, we present results relative to SAPS radial locations and amplitudes. Then, we introduce two different ways to estimate SAPS durations. In both cases, SAPS activity is estimated to last for about nine hours on average. However, our estimates for SAPS duration are limited either by the relatively long orbital periods of the spacecraft or by the relatively small number of observations involved. 50 \% of the events fit within the time interval [0;18] hours.

Lejosne, ène; Mozer, F.;

Published by: Geophysical Research Letters      Published on: 08/2017

YEAR: 2017     DOI: 10.1002/2017GL074985

duration; electric drift measurements; magnetosphere-ionosphere coupling; SAPS; Van Allen Probes

Understanding the Mechanisms of Radiation Belt Dropouts Observed by Van Allen Probes

To achieve a better understanding of the dominant loss mechanisms for the rapid dropouts of radiation belt electrons, three distinct radiation belt dropout events observed by Van Allen Probes are comprehensively investigated. For each event, observations of the pitch angle distribution of electron fluxes and electromagnetic ion cyclotron (EMIC) waves are analyzed to determine the effects of atmospheric precipitation loss due to pitch angle scattering induced by EMIC waves. Last closed drift shells (LCDS) and magnetopause standoff position are obtained to evaluate the effects of magnetopause shadowing loss. Evolution of electron phase space density (PSD) versus L* profiles and the μ and K (first and second adiabatic invariants) dependence of the electron PSD drops are calculated to further analyze the dominant loss mechanisms at different L*. Our findings suggest that these radiation belt dropouts can be classified into distinct classes in terms of dominant loss mechanisms: magnetopause shadowing dominant, EMIC wave scattering dominant, and combination of both mechanisms. Different from previous understanding, our results show that magnetopause shadowing can deplete electrons at L* < 4, while EMIC waves can efficiently scatter electrons at L* > 4. Compared to the magnetopause standoff position, it is more reliable to use LCDS to evaluate the impact of magnetopause shadowing. The evolution of electron PSD versus L* profile and the μ, K dependence of electron PSD drops can provide critical and credible clues regarding the mechanisms responsible for electron losses at different L* over the outer radiation belt.

Xiang, Zheng; Tu, Weichao; Li, Xinlin; Ni, Binbin; Morley, S.; Baker, D.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 08/2017

YEAR: 2017     DOI: 10.1002/2017JA024487

EMIC wave; last closed drift shell; magnetopause shadowing; Phase space density; radiation belt dropout; Van Allen Probes

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