Bibliography

Evidence of Alfvenic Poynting flux as the primary driver of auroral motion during a geomagnetic substorm

Abstract Geomagnetic substorms are major energy transfer events where energy stored in the Earths magnetotail is released into the ionosphere. Substorm phenomena, including auroral activities, earthward Poynting flux, magnetic field dipolarization, etc, have been extensively studied. However, the complex interplay among them is not fully understood. In a fortuitous event on June 07, 2013, the twin Van Allen Probes (separated by 0.4 hour in local time) observed bursts of earthward Alfvenic Poynting flux in the vicinity of the plasma sheet boundary layer (PSBL). The Poynting flux bursts correlate with enhancements of auroral brightness around the footpoints of both spacecraft. This indicates a temporal and spatial correlation between the auroral brightening and Poynting flux bursts, and that the auroral motion is directly linked to the perpendicular expansion of the Alfven wave. These observations suggest that the Alfvenic Poynting flux is a primary driver for the auroral electron acceleration. Around the time of auroral brightening, a dipolarization was seen to propagate more than 4 hours in local time during a 20 min period. The azimuthal phase speed of this dipolarization (2 deg/min) is too small to explain the azimuthal motion of the aurora (13.6 deg/min), but the dipolarization could be related to the generation of the Alfvenic Poynting flux through phase mixing at strong density gradients like those in the PSBL. This article is protected by copyright. All rights reserved.

Tian, S.; Colpitts, C.; Wygant, J.; Cattell, C.; Ferradas, C.; Igl, A.; Larsen, B.; Reeves, G.; Donovan, E.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 04/2021

YEAR: 2021     DOI: https://doi.org/10.1029/2020JA029019

Poynting flux; auroral physics; discrete arc; Dipolarization; Alfven waves; Van Allen Probes

Van Allen probe observations of disappearance, recovery and patchiness of plasmaspheric hiss following two consecutive interplanetary shocks: First results

Abstract We present, for the first time, a plasmaspheric hiss event observed by the Van Allen probes in response to two successive interplanetary shocks occurring within an interval of ∼2 hours on December 19, 2015. The first shock arrived at 16:16 UT and caused disappearance of hiss for ∼30 minutes. Combined effect of plasmapause crossing, significant Landau damping by suprathermal electrons and their gradual removal by magnetospheric compression led to the disappearance of hiss. Calculation of electron phase space density and linear wave growth rates showed that the shock did not change the growth rate of whistler waves within the core frequency range of plasmaspheric hiss (0.1 - 0.5 kHz) during this interval making conditions unfavorable for the generation of hiss. The recovery began at ∼16:45 UT which is attributed to an enhancement in local plasma instability initiated by the first shock-induced substorm and additional possible contribution from chorus waves. This time, the wave growth rate peaked within the core frequency range ( ∼350 Hz). The second shock arrived at 18:02 UT and generated patchy hiss persisting up to ∼19:00 UT. It is shown that an enhanced growth rate and additional contribution from shock-induced poloidal Pc5 mode (periodicity ∼240 sec) ULF waves resulted in the excitation of hiss waves during this period. The hiss wave amplitudes were found to be additionally modulated by background plasma density and fluctuating plasmapause location. The investigation highlights the important roles of interplanetary shocks, substorms, ULF waves and background plasma density in the variability of plasmaspheric hiss.

Chakraborty, S.; Chakrabarty, D.; Reeves, G.; Baker, D.; Claudepierre, S.; Breneman, A.; Hartley, D.; Larsen, B.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 03/2021

YEAR: 2021     DOI: https://doi.org/10.1029/2020JA028873

Plasmaspheric Hiss; Van Allen Probe; Interplanetary shocks; substorms; Whistlers; ULF waves; Van Allen Probes

RBSP-ECT Combined Pitch Angle Resolved Electron Flux Data Product

Abstract We describe a new data product combining pitch angle resolved electron flux measurements from the Radiation Belt Storm Probes (RBSP) Energetic Particle Composition and Thermal Plasma (ECT) suite on the National Aeronautics and Space Administration s Van Allen Probes. We describe the methodology used to combine each of the data sets and produce a consistent set of pitch-angle-resolved spectra for the entire Van Allen Probes mission. Three-minute-averaged flux spectra are provided spanning energies from 15 eV up to 20 MeV. This new data product offers researchers a consistent cross calibrated data set to explore the particle dynamics of the inner magnetosphere across a wide range of energies. This article is protected by copyright. All rights reserved.

Boyd, A.J.; Spence, H.E.; Reeves, G.D.; Funsten, H.O; Skoug, R.K.; Larsen, B.A.; Blake, J.B.; Fennell, J.F.; Claudepierre, S.G.; Baker, D.N.; Kanekal, S.K.; Jaynes, A.N.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 02/2021

YEAR: 2021     DOI: https://doi.org/10.1029/2020JA028637

Van Allen Probes; Radiation belts; ECT; MAGEis; REPT; HOPE

Observations of density cavities and associated warm ion flux enhancements in the inner magnetosphere

Abstract We present a statistical study of density cavities observed in the inner magnetosphere by the Van Allen Probes during four one-month periods: February 2013, July 2013, January 2014 and June 2014. These periods were chosen to allow the survey of all magnetic local times. We find that density cavities are a recurrent feature of the density profiles of in situ measurements in the inner magnetosphere. We further investigate the correlation between the density cavities and the enhancement of fluxes of warm ions with energies of 10-100 eV. The results show that warm ion flux enhancements associated with the density cavities were observed more frequently for H+, then for He+ and the least frequently for O+. The occurrences of the associated flux enhancements were increased when considering only the cavities inside the plasmasphere. Possible mechanisms responsible for the observed warm ion flux enhancements and the role of density cavities on these ion flux enhancements are discussed.

Ferradas, C.; Boardsen, S.; Fok, M.-C.; Buzulukova, N.; Reeves, G.; Larsen, B.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 02/2021

YEAR: 2021     DOI: https://doi.org/10.1029/2020JA028326

Magnetosphere: inner; plasmasphere; magnetospheric configuration and dynamics; plasma waves and instabilities; plasma sheet; density cavity; cold ion heating; cold ions; warm Plasma cloak; Van Allen Probes

Bayesian Model for HOPE Mass Spectrometers on Van Allen Probes

Abstract Space instruments rely heavily on modeling to predict and understand the instrument response, enabling a determination of the capabilities and resolution. The Bayesian approach provides a framework to incorporate prior knowledge and propagate uncertainty to predict the instrument response. We present an empirical Bayes model for the end-to-end performance of the Helium, Oxygen, Proton, and Electron (HOPE) mass spectrometers aboard the Van Allen Probes mission. In this model, we use a combination of external modeling, laboratory calibration, and expert opinion to construct the time-of-flight spectra and demonstrate good agreement with on-orbit data. The empirical Bayes model is applied to explore doubly charged ions and carbon, nitrogen, oxygen discrimination during the Van Allen Probes mission. This article is protected by copyright. All rights reserved.

Vira, A.; Larsen, B.; Skoug, R.; Fernandes, P.;

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

YEAR: 2021     DOI: https://doi.org/10.1029/2020JA028862

Van Allen Probes

Equatorial pitch angle distributions of 1 – 50 keV electrons in Earth s inner magnetosphere: an empirical model based on the Van Allen Probes observations

Using seven years of data from the HOPE instrument on the Van Allen Probes, equatorial pitch angle distributions (PADs) of 1 – 50 keV electrons in Earth s inner magnetosphere are investigated statistically. An empirical model of electron equatorial PADs as a function of radial distance, magnetic local time, geomagnetic activity, and electron energy is constructed using the method of Legendre polynomial fitting. Model results show that most equatorial PADs of 1 – 10s of keV electrons in Earth s inner magnetosphere are pancake PADs, and the lack of butterfly PADs is likely due to their relatively flat or positive flux radial gradients at higher altitudes. During geomagnetically quiet times, more anisotropic distributions of 1 – 10s of keV electrons at dayside than nightside are observed, which could be responsible for moderate chorus wave activities at dayside during quiet times as reported by previous studies. During active times, the anisotropy of 1 – 10s of keV electrons significantly enhances, consistent with the enhanced chorus wave activity during active times and suggesting the critical role of 1 – 10s of keV electrons in generating chorus waves in Earth s inner magnetosphere. Different enhanced anisotropy patterns of different energy electrons are also observed during active times: at R>∼4 RE, keV electrons are more anisotropic at dawn to noon, while 10s of keV electrons have larger anisotropy at midnight to dawn. These differences, combined with the statistical distribution of chorus waves shown in previous studies, suggest the differential roles of electrons with different energies in generating chorus waves with different properties. This article is protected by copyright. All rights reserved.

Zhao, H.; Friedel, R.; Chen, Y.; Baker, D.; Li, X.; Malaspina, D.; Larsen, B.; Skoug, R.; Funsten, H.; Reeves, G.; Boyd, A.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 12/2020

YEAR: 2020     DOI: https://doi.org/10.1029/2020JA028322

Pitch angle distribution; energetic electrons; Earth s inner magnetosphere; Anisotropy; Chorus wave; statistical analysis; Van Allen Probes

Defining Radiation Belt Enhancement Events Based on Probability Distributions

We present a methodology to define moderate, strong, and intense space weather events based on probability distributions. We have illustrated this methodology using a long-duration, uniform data set of 1.8–3.5 MeV electron fluxes from multiple LANL geosynchronous satellite instruments, but a strength of this methodology is that it can be applied uniformly to heterogeneous data sets. It allows quantitative comparison of data sets with different energies, units, orbits, and so forth. The methodology identifies a range of times, “events,” using variable flux thresholds to determine average event occurrence in arbitrary 11-year intervals (“cycles”). We define moderate, strong, and intense events as those that occur 100, 10, and 1 time per cycle and identify the flux thresholds that produce those occurrence frequencies. The methodology does not depend on any ancillary data set (e.g., solar wind or geomagnetic conditions). We show event probabilities using GOES > 2 MeV fluxes and compare them against event probabilities using LANL 1.8–3.5 MeV fluxes. We present some examples of how the methodology picks out moderate, strong, and intense events and how those events are distributed in time: 1989 through 2018, which includes the declining phases of solar cycles 22, 23, and 24. We also provide an illustrative comparison of moderate and strong events identified in the geosynchronous data with Van Allen Probes observations across all L-shells. We also provide a catalog of start and stop times of moderate, strong, and intense events that can be used for future studies.

Reeves, Geoffrey; Vandegriff, Elizabeth; Niehof, Jonathan; Morley, Steven; Cunningham, Gregory; Henderson, Michael; Larsen, Brian;

Published by: Space Weather      Published on: 06/2020

YEAR: 2020     DOI: https://doi.org/10.1029/2020SW002528

Radiation belts; methods; geosynchronous; energetic particles; hazards; Solar Cycle; Van Allen Probes

Global ENA Imaging and In Situ Observations of Substorm Dipolarization on 10 August 2016

Abstract This paper presents the first combined use of data from Magnetospheric Multiscale (MMS), Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS), and Van Allen Probes (RBSP) to study the 10 August 2016 magnetic dipolarization. We report the first correlation of MMS tail observations with TWINS energetic neutral atom (ENA) images of the ring current (RC). We analyze 15-min, 1° TWINS 2 images in 1–50 keV energy bins. To characterize the high-altitude RC we extract peak ENA flux from L= 2.5 to 5 in the postmidnight sector. We estimate peak low-altitude ion flux from ENAs near the Earth s limb. For a local perspective, we use spin-averaged proton fluxes from the RBSP A Helium Oxygen Proton Electron (HOPE) spectrometer. We find that the 1000 UT dipolarization triggered an abrupt and significant increase in low-altitude ions and a gradual but modest increase in the high-altitude RC. The relative strength and timing of the low versus high-altitude flux indicate that the dipolarization isotropized the injected ions and initially filled the loss cone. The substorm injection brought cooler ions in from the magnetotail, reducing the peak energy at both low and high altitudes. The post-dipolarization low-altitude flux exhibited a decay rate dispersion favoring longer decay times at lower energies, possibly caused by growth of the low energy RC providing enhanced flux into the loss cone. A variety of finer scale local injection structures were observed in the high-altitude RC both before and after the dipolarization, and the average system level RC intensity increased after 1000 UT.

Goldstein, J.; Valek, P.; McComas, D.; Redfern, J.; Spence, H.; Skoug, R.; Larsen, B.; Reeves, G.; Nakamura, R.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 04/2020

YEAR: 2020     DOI: 10.1029/2019JA027733

substorm dipolarization; cross-scale physics; imaging; multipoint in situ; ring current; Van Allen Probes

Global ENA Imaging and In Situ Observations of Substorm Dipolarization on 10 August 2016

This paper presents the first combined use of data from Magnetospheric Multiscale (MMS), Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS), and Van Allen Probes (RBSP) to study the 10 August 2016 magnetic dipolarization. We report the first correlation of MMS tail observations with TWINS energetic neutral atom (ENA) images of the ring current (RC). We analyze 15-min, 1° TWINS 2 images in 1–50 keV energy bins. To characterize the high-altitude RC we extract peak ENA flux from L= 2.5 to 5 in the postmidnight sector. We estimate peak low-altitude ion flux from ENAs near the Earth s limb. For a local perspective, we use spin-averaged proton fluxes from the RBSP A Helium Oxygen Proton Electron (HOPE) spectrometer. We find that the 1000 UT dipolarization triggered an abrupt and significant increase in low-altitude ions and a gradual but modest increase in the high-altitude RC. The relative strength and timing of the low versus high-altitude flux indicate that the dipolarization isotropized the injected ions and initially filled the loss cone. The substorm injection brought cooler ions in from the magnetotail, reducing the peak energy at both low and high altitudes. The post-dipolarization low-altitude flux exhibited a decay rate dispersion favoring longer decay times at lower energies, possibly caused by growth of the low energy RC providing enhanced flux into the loss cone. A variety of finer scale local injection structures were observed in the high-altitude RC both before and after the dipolarization, and the average system level RC intensity increased after 1000 UT.

Goldstein, J.; Valek, P.; McComas, D.; Redfern, J.; Spence, H.; Skoug, R.; Larsen, B.; Reeves, G.; Nakamura, R.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 04/2020

YEAR: 2020     DOI: https://doi.org/10.1029/2019JA027733

substorm dipolarization; cross-scale physics; imaging; multipoint in situ; ring current

Identifying STEVE\textquoterights Magnetospheric Driver Using Conjugate Observations in the Magnetosphere and on the Ground

The magnetospheric driver of strong thermal emission velocity enhancement (STEVE) is investigated using conjugate observations when Van Allen Probes\textquoteright footprint directly crossed both STEVE and stable red aurora (SAR) arc. In the ionosphere, STEVE is associated with subauroral ion drift features, including electron temperature peak, density gradient, and westward ion flow. The SAR arc at lower latitudes corresponds to regions inside the plasmapause with isotropic plasma heating, which causes redline-only SAR emission via heat conduction. STEVE corresponds to the sharp plasmapause boundary containing quasi-static subauroral ion drift electric field and parallel-accelerated electrons by kinetic Alfv\ en waves. These parallel electrons could precipitate and be accelerated via auroral acceleration processes powered by Alfv\ en waves propagating along the magnetic field with the plasmapause as a waveguide. The electron precipitation, superimposed on the heat conduction, could explain multiwavelength continuous STEVE emission. The green picket-fence emissions are likely optical manifestations of electron precipitation associated with wave structures traveling along the plasmapause.

Chu, Xiangning; Malaspina, David; Gallardo-Lacourt, Bea; Liang, Jun; Andersson, Laila; Ma, Qianli; Artemyev, Anton; Liu, Jiang; Ergun, Robert; Thaller, Scott; Akbari, Hassanali; Zhao, Hong; Larsen, Brian; Reeves, Geoffrey; Wygant, John; Breneman, Aaron; Tian, Sheng; Connors, Martin; Donovan, Eric; Archer, William; MacDonald, Elizabeth;

Published by: Geophysical Research Letters      Published on: 11/2019

YEAR: 2019     DOI: 10.1029/2019GL082789

aurora; kinetic Alfven wave; Plasmapause; STEVE; subauroral ion drift; table red auroral arc; Van Allen Probes

RBSP-ECT Combined Spin-Averaged Electron Flux Data Product

We describe a new data product combining the spin-averaged electron flux measurements from the Radiation Belt Storm Probes (RBSP) Energetic Particle Composition and Thermal Plasma (ECT) suite on the National Aeronautics and Space Administration\textquoterights Van Allen Probes. We describe the methodology used to combine each of the data sets and produce a consistent set of spectra for September 2013 to the present. Three-minute-averaged flux spectra are provided spanning energies from 15 eV up to 20 MeV. This new data product provides additional utility to the ECT data and offers a consistent cross calibrated data set for researchers interested in examining the dynamics of the inner magnetosphere across a wide range of energies.

Boyd, A.; Reeves, G.; Spence, H.; Funsten, H.; Larsen, B.; Skoug, R.; Blake, J.; Fennell, J.; Claudepierre, S.; Baker, D.; Kanekal, S.; Jaynes, A.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 10/2019

YEAR: 2019     DOI: 10.1029/2019JA026733

ECT; HOPE; MAGEis; Radiation belts; REPT; Van Allen Probes

Comparison of Electron Loss Models in the Inner Magnetosphere During the 2013~St. Patrick\textquoterights Day Geomagnetic Storm

Electrons with energies in the keV range play an important role in the dynamics of the inner magnetosphere. Therefore, accurately modeling electron fluxes in this region is of great interest. However, these calculations constitute a challenging task since the lifetimes of electrons that are available have limitations. In this study, we simulate electron fluxes in the energy range of 20 eV to 100 keV to assess how well different electron loss models can account for the observed electron fluxes during the Geospace Environment Modelling Challenge Event of the 2013 St. Patrick\textquoterights Day storm. Three models (Case 1, Case 2, and Case 3) of electron lifetimes due to wave-induced pitch angle scattering are used to compute the fluxes, which are compared with measurements from the Van Allen Probes. The three models consider electron losses due to interactions with whistler mode hiss waves inside the plasmasphere and with whistler mode chorus waves outside the plasmasphere. The Case 1 (historical) model produces excessive loss at low L shells before and after the storm, suggesting that it overestimates losses due to hiss during quiet times. During the storm main phase and early recovery all three models show good agreement with the observations, indicating that losses due to chorus during disturbed times are, in general, well accounted for by the models. Furthermore, the more recent Case 2 and Case 3 models show overall better agreement with the observed fluxes.

Ferradas, C.; Jordanova, V.; Reeves, G.; Larsen, B.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 09/2019

YEAR: 2019     DOI: 10.1029/2019JA026649

electron lifetime; electron loss; numerical modeling; pitch angle scattering; Van Allen Probes; Weimer electric field model

Temperature Dependence of Plasmaspheric Ion Composition

We analyze a database of Dynamics Explorer-1 (DE-1) Retarding Ion Mass Spectrometer densities and temperatures to yield the first explicit measure of how cold ion concentration depends on temperature. We find that cold H+ and He+ concentrations have very weak dependence on temperature, but cold O+ ion concentration increases steeply as these ions become warmer. We demonstrate how this result can aid in analyzing composition data from other satellites without spacecraft potential mitigation, by applying the result to an example using data from the Van Allen Probes mission. Measurement of light ion concentrations above 1 electron volt (eV) are a reasonable proxy for the concentrations of colder (eV) ions. Warmer O+ ion concentrations may be extrapolated to colder temperatures using our fit to the statistical distribution versus temperature.

Goldstein, J.; Gallagher, D.; Craven, P.; Comfort, R.; Genestreti, K.; Mouikis, C.; Spence, H.; Kurth, W.; Wygant, J.; Skoug, R.; Larsen, B.; Reeves, G.; De Pascuale, S.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 07/2019

YEAR: 2019     DOI: 10.1029/2019JA026822

composition; plasmasphere: ion; temperature; Van Allen Probes

Plasma anisotropies and currents in the near-Earth plasma sheet and inner magnetosphere

The region occupying radial distances of \~3 - 9 Earth radii (RE) in the night side, includes the near-Earth plasma sheet with stretched magnetic field lines and the inner magnetosphere with strong dipolar magnetic field. In this region, the plasma flow energy, which was injected into the inner magnetosphere from the magnetotail, is converted to particle heating and electromagnetic wave generation. These important processes are controlled by plasma anisotropies, which are the focus of this study. Using measurements of THEMIS and Van Allen Probes in this transition region we obtain radial profiles of ion and electron temperatures and anisotropies for various geomagnetic activity levels. Ion and electron anisotropies vary with the geomagnetic activity in opposite directions. Parallel anisotropic ions are observed together with transversely anisotropic electrons, whereas the change of ion anisotropy from parallel to transverse (with increasing Kp) is accompanied by the electron anisotropy changing from transverse to parallel. Based on plasma anisotropy observations, we estimate that the anisotropy-related currents (curvature currents) are about 10 - 20\% of the diamagnetic currents.

Artemyev, A.; Zhang, X.-J.; Angelopoulos, V.; Runov, A.; Spence, H.; Larsen, B.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 06/2018

YEAR: 2018     DOI: 10.1029/2018JA025232

injections; inner magnetosphere; plasma currents; plasma sheet; Van Allen Probes

Comparing simulated and observed EMIC wave amplitudes using in situ Van Allen Probes\textquoteright measurements

We perform a statistical study calculating electromagnetic ion cyclotron (EMIC) wave amplitudes based off in situ plasma measurements taken by the Van Allen Probes\textquoteright (1.1\textendash5.8 Re) Helium, Oxygen, Proton, Electron (HOPE) instrument. Calculated wave amplitudes are compared to EMIC waves observed by the Electric and Magnetic Field Instrument Suite and Integrated Science on board the Van Allen Probes during the same period. The survey covers a 22-month period (1 November 2012 to 31 August 2014), a full Van Allen Probe magnetic local time (MLT) precession. The linear theory proxy was used to identify EMIC wave events with plasma conditions favorable for EMIC wave excitation. Two hundred and thirty-two EMIC wave events (103 H+-band and 129 He+-band) were selected for this comparison. Nearly all events selected are observed beyond L = 4. Results show that calculated wave amplitudes exclusively using the in situ HOPE measurements produce amplitudes too low compared to the observed EMIC wave amplitudes. Hot proton anisotropy (Ahp) distributions are asymmetric in MLT within the inner (L < 7) magnetosphere with peak (minimum) Ahp, \~0.81 to 1.00 (\~0.62), observed in the dawn (dusk), 0000 < MLT <= 1200 (1200 < MLT <= 2400), sectors. Measurements of Ahp are found to decrease in the presence of EMIC wave activity. Ahp amplification factors are determined and vary with respect to EMIC wave-band and MLT. He+-band events generally require double (quadruple) the measured Ahp for the dawn (dusk) sector to reproduce the observed EMIC wave amplitudes.

Saikin, A.A.; Jordanova, V.K.; Zhang, J.C.; Smith, C.W.; Spence, H.E.; Larsen, B.A.; Reeves, G.D.; Torbert, R.B.; Kletzing, C.A.; Zhelavskaya, I.S.; Shprits, Y.Y.;

Published by: Journal of Atmospheric and Solar-Terrestrial Physics      Published on: 02/2018

YEAR: 2018     DOI: 10.1016/j.jastp.2018.01.024

EMIC waves Van Allen Probes Linear theory Wave generation; 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

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

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

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

Roles of hot electrons in generating upper-hybrid waves in the earth\textquoterights radiation belt

Electrostatic fluctuations near upper-hybrid frequency, which are sometimes accompanied by multiple-harmonic electron cyclotron frequency bands above and below the upper-hybrid frequency, are common occurrences in the Earth\textquoterights radiation belt, as revealed through the twin Van Allen Probe spacecrafts. It is customary to use the upper-hybrid emissions for estimating the background electron density, which in turn can be used to determine the plasmapause locations, but the role of hot electrons in generating such fluctuations has not been discussed in detail. The present paper carries out detailed analyses of data from the Waves instrument, which is part of the Electric and Magnetic Field Instrument Suite and Integrated Science suite onboard the Van Allen Probes. Combined with the theoretical calculation, it is shown that the peak intensity associated with the upper-hybrid fluctuations might be predominantly determined by tenuous but hot electrons and that denser cold background electrons do not seem to contribute much to the peak intensity. This finding shows that upper-hybrid fluctuations detected during quiet time are not only useful for the determination of the background cold electron density but also contain information on the ambient hot electrons population as well.

Hwang, J.; Shin, D.; Yoon, P.; Kurth, W.; Larsen, B.; Reeves, G.; Lee, D;

Published by: Physics of Plasmas      Published on: 06/2017

YEAR: 2017     DOI: 10.1063/1.4984249

Hot carriers; Magnetized plasmas; Radiation belts; Singing; Van Allen Probes; Whistler waves

Second harmonic poloidal waves observed by Van Allen Probes in the dusk-midnight sector

This paper presents observations of ultralow-frequency (ULF) waves from Van Allen Probes. The event that generated the ULF waves occurred 2 days after a minor geomagnetic storm during a geomagnetically quiet time. Narrowband pulsations with a frequency of about 7 mHz with moderate amplitudes were registered in the premidnight sector when Probe A was passing through an enhanced density region near geosynchronous orbit. Probe B, which passed through the region earlier, did not detect the narrowband pulsations but only broadband noise. Despite the single-spacecraft measurements, we were able to determine various wave properties. We find that (1) the observed waves are a second harmonic poloidal mode propagating westward with an azimuthal wave number estimated to be \~100; (2) the magnetic field fluctuations have a finite compressional component due to small but finite plasma beta (\~0.1); (3) the energetic proton fluxes in the energy ranging from above 10 keV to about 100 keV exhibit pulsations with the same frequency as the poloidal mode and energy-dependent phase delays relative to the azimuthal component of the electric field, providing evidence for drift-bounce resonance; and (4) the second harmonic poloidal mode may have been excited via the drift-bounce resonance mechanism with free energy fed by the inward radial gradient of \~80 keV protons. We show that the wave active region is where the plume overlaps the outer edge of ring current and suggest that this region can have a wide longitudinal extent near geosynchronous orbit.

Min, Kyungguk; Takahashi, Kazue; Ukhorskiy, Aleksandr; Manweiler, Jerry; Spence, Harlan; Singer, Howard; Claudepierre, Seth; Larsen, Brian; Soto-Chavez, Rualdo; Cohen, Ross;

Published by: Journal of Geophysical Research: Space Physics      Published on: 03/2017

YEAR: 2017     DOI: 10.1002/2016JA023770

drift-bounce resonance; high m ULF waves; Second harmonic poloidal mode; Van Allen Probes

On the origin of low-energy electrons in the inner magnetosphere: Fluxes and pitch-angle distributions

Accurate knowledge of the plasma fluxes in the inner magnetosphere is essential for both scientific and programmatic applications. Knowledge of the low-energy electrons (approximately tens to hundreds of eV) in the inner magnetosphere is particularly important since these electrons are acted upon by various physical processes, accelerating the electrons to higher energies, and also causing their loss. However, measurements of low-energy electrons are challenging, and as a result, this population has been somewhat neglected previously. This study concerns observations of low-energy electrons made by the Helium Oxygen Proton Electron instrument on board the Van Allen Probes satellites and also observations from geosynchronous orbit made by the Magnetospheric Plasma Analyzer on board Los Alamos National Laboratory satellites. The fluxes of electrons from ~30 eV to 1 keV are quantified as a function of pitch-angle, McIlwain L parameter, and local time for both quiet and active periods. Results indicate two sources for low-energy electrons in this energy range: the low-energy tail of the electron plasma sheet and the high-energy tail of the dayside ionosphere. These populations are identified primarily as a result of their different pitch-angle distributions. Field-aligned outflows from the dayside ionosphere are observed at all L shells during quiet and active periods. Our results also demonstrate that the dayside electron field-aligned fluxes at ~30 eV are particularly strong between L values of 6 and 7, indicating an enhanced source within the polar ionosphere.

Denton, M.; Reeves, G.; Larsen, B.; Friedel, R.; Thomsen, M.; Fernandes, P.; Skoug, R.; Funsten, H.; Sarno-Smith, L.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 02/2017

YEAR: 2017     DOI: 10.1002/2016JA023648

inner magnetosphere; Van Allen Probes

Second harmonic poloidal waves observed by Van Allen Probes in the dusk-midnight sector

This paper presents observations of ultra-low frequency (ULF) waves from Van Allen Probes. The event that generated the ULF waves occurred two days after a minor geomagnetic storm during a geomagnetically quiet time. Narrowband pulsations with a frequency of about 7 mHz with moderate amplitudes were registered in the pre-midnight sector when Probe A was passing through an enhanced density region near geosynchronous orbit. Probe B, which passed through the region earlier, did not detect the narrowband pulsations but only broadband noise. Despite the single-spacecraft measurements, we were able to determine various wave properties. We find that (1) the observed waves are a second harmonic poloidal mode propagating westward with an azimuthal wave number estimated to be \~100; (2) the magnetic field fluctuations have a finite compressional component due to small but finite plasma beta (\~0.1); (3) the energetic proton fluxes in the energy ranging from above 10 keV to about 100 keV exhibit pulsations with the same frequency as the poloidal mode and energy-dependent phase delays relative to the azimuthal component of the electric field, providing evidence for drift-bounce resonance; and (4) the second harmonic poloidal mode may have been excited via the drift-bounce resonance mechanism with free energy fed by the inward radial gradient of \~80 keV protons. We show that the wave active region is where the plume overlaps the outer edge of ring current and suggest that this region can have a wide longitudinal extent near geosynchronous orbit.

Min, Kyungguk; Takahashi, Kazue; Ukhorskiy, Aleksandr; Manweiler, Jerry; Spence, Harlan; Singer, Howard; Claudepierre, Seth; Larsen, Brian; Soto-Chavez, Rualdo; Cohen, Ross;

Published by: Journal of Geophysical Research: Space Physics      Published on: 02/2017

YEAR: 2017     DOI: 10.1002/2016JA023770

drift-bounce resonance; high m ULF waves; Second harmonic poloidal mode; Van Allen Probes

On the origin of low-energy electrons in the inner magnetosphere: Fluxes and pitch-angle distributions

Accurate knowledge of the plasma fluxes in the inner magnetosphere is essential for both scientific and programmatic applications. Knowledge of the low-energy electrons (approximately tens to hundreds of eV) in the inner magnetosphere is particularly important since these electrons are acted upon by various physical processes, accelerating the electrons to higher energies, and also causing their loss. However, measurements of low-energy electrons are challenging, and as a result, this population has been somewhat neglected previously. This study concerns observations of low-energy electrons made by the Helium Oxygen Proton Electron instrument on board the Van Allen Probes satellites and also observations from geosynchronous orbit made by the Magnetospheric Plasma Analyzer on board Los Alamos National Laboratory satellites. The fluxes of electrons from ~30 eV to 1 keV are quantified as a function of pitch-angle, McIlwain L parameter, and local time for both quiet and active periods. Results indicate two sources for low-energy electrons in this energy range: the low-energy tail of the electron plasma sheet and the high-energy tail of the dayside ionosphere. These populations are identified primarily as a result of their different pitch-angle distributions. Field-aligned outflows from the dayside ionosphere are observed at all L shells during quiet and active periods. Our results also demonstrate that the dayside electron field-aligned fluxes at ~30 eV are particularly strong between L values of 6 and 7, indicating an enhanced source within the polar ionosphere.

Denton, M.; Reeves, G.; Larsen, B.; Friedel, R.; Thomsen, M.; Fernandes, P.; Skoug, R.; Funsten, H.; Sarno-Smith, L.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 01/2017

YEAR: 2017     DOI: 10.1002/2016JA023648

inner magnetosphere; Van Allen Probes

Temperature of the plasmasphere from Van Allen Probes HOPE

Genestreti, K.; Goldstein, J.; Corley, G.; Farner, W.; Kistler, L.; Larsen, B.; Mouikis, C.; Ramnarace, C.; Skoug, R.; Turner, N.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 01/2017

YEAR: 2017     DOI: 10.1002/2016JA023047

plasmasphere; Van Allen Probes

Temperature of the plasmasphere from Van Allen Probes HOPE

We introduce two novel techniques for estimating temperatures of very low energy space plasmas using, primarily, in situ data from an electrostatic analyzer mounted on a charged and moving spacecraft. The techniques are used to estimate proton temperatures during intervals where the bulk of the ion plasma is well below the energy bandpass of the analyzer. Both techniques assume that the plasma may be described by a one-dimensional math formula drifting Maxwellian and that the potential field and motion of the spacecraft may be accounted for in the simplest possible manner, i.e., by a linear shift of coordinates. The first technique involves the application of a constrained theoretical fit to a measured distribution function. The second technique involves the comparison of total and partial-energy number densities. Both techniques are applied to Van Allen Probes Helium, Oxygen, Proton, and Electron (HOPE) observations of the proton component of the plasmasphere during two orbits on 15 January 2013. We find that the temperatures calculated from these two order-of-magnitude-type techniques are in good agreement with typical ranges of the plasmaspheric temperature calculated using retarding potential analyzer-based measurements\textemdashgenerally between 0.2 and 2 eV (2000\textendash20,000 K). We also find that the temperature is correlated with L shell and hot plasma density and is negatively correlated with the cold plasma density. We posit that the latter of these three relationships may be indicative of collisional or wave-driven heating of the plasmasphere in the ring current overlap region. We note that these techniques may be easily applied to similar data sets or used for a variety of purposes.

Genestreti, K.; Goldstein, J.; Corley, G.; Farner, W.; Kistler, L.; Larsen, B.; Mouikis, C.; Ramnarace, C.; Skoug, R.; Turner, N.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 01/2017

YEAR: 2017     DOI: 10.1002/jgra.v122.110.1002/2016JA023047

plasmasphere; Van Allen Probes

Drift paths of ions composing multiple-nose spectral structures near the inner edge of the plasma sheet

We present a case study of the H+, He+, and O+ multiple-nose structures observed by the Helium, Oxygen, Proton, and Electron instrument on board Van Allen Probe A over one complete orbit on 28 September 2013. Nose structures are observed near the inner edge of the plasma sheet and constitute the signatures of ion drift in the highly dynamic environment of the inner magnetosphere. We find that the multiple noses are intrinsically associated with variations in the solar wind. Backward ion drift path tracings show new details of the drift trajectories of these ions; i.e., multiple noses are formed by ions with a short drift time from the assumed source location to the inner region and whose trajectories (1) encircle the Earth different number of times or (2) encircle the Earth equal number of times but with different drift time, before reaching the observation site.

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

Published by: Geophysical Research Letters      Published on: 11/2016

YEAR: 2016     DOI: 10.1002/2016GL071359

drift path; ion injection; ion nose structure; numerical modeling; Van Allen Probes; Weimer electric field model

Ion nose spectral structures observed by the Van Allen Probes

We present a statistical study of nose-like structures observed in energetic hydrogen, helium, and oxygen ions near the inner edge of the plasma sheet. Nose structures are spectral features named after the characteristic shapes of energy bands or gaps in the energy-time spectrograms of in situ measured ion fluxes. Using 22 months of observations from the Helium Oxygen Proton Electron (HOPE) instrument onboard Van Allen Probe A, we determine the number of noses observed, and the minimum L-shell reached and energy of each nose on each pass through the inner magnetosphere. We find that multiple noses occur more frequently in heavy ions than in H+, and are most often observed during quiet times. The heavy-ion noses penetrate to lower L shells than H+ noses and there is an energy-magnetic local time (MLT) dependence in the nose locations and energies that is similar for all species. The observations are interpreted using a steady-state model of ion drift in the inner magnetosphere. The model is able to explain the energy and MLT dependence of the different types of nose structures. Different ion charge exchange lifetimes are the main cause for the deeper penetration of heavy-ion noses. The species dependence and preferred geomagnetic conditions of multiple-nose events indicate that they must be on long drift paths, leading to strong charge-exchange effects. The results provide important insight into the spatial distribution, species dependence, and geomagnetic conditions under which nose structures occur.

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: 11/2016

YEAR: 2016     DOI: 10.1002/2016JA022942

inner magnetosphere; ion injection; Ion structure; plasma sheet; ring current; Van Allen Probes

Ring Current Pressure Estimation with RAM-SCB using Data Assimilation and Van Allen Probe Flux Data

Capturing and subsequently modeling the influence of tail plasma injections on the inner magnetosphere is important for understanding the formation and evolution of the ring current. In this study, the ring current distribution is estimated with the Ring Current-Atmosphere Interactions Model with Self-Consistent Magnetic field (RAM-SCB) using, for the first time, data assimilation techniques and particle flux data from the Van Allen Probes. The state of the ring current within the RAM-SCB model is corrected via an ensemble based data assimilation technique by using proton flux from one of the Van Allen Probes, to capture the enhancement of the ring current following an isolated substorm event on July 18, 2013. The results show significant improvement in the estimation of the ring current particle distributions in the RAM-SCB model, leading to better agreement with observations. This newly implemented data assimilation technique in the global modeling of the ring current thus provides a promising tool to improve the characterization of particle distribution in the near-Earth regions.

Godinez, Humberto; Yu, Yiqun; Lawrence, Eric; Henderson, Michael; Larsen, Brian; Jordanova, Vania;

Published by: Geophysical Research Letters      Published on: 11/2016

YEAR: 2016     DOI: 10.1002/2016GL071646

data assimilation; ring current; Van Allen Probes

EMIC waves and associated relativistic electron precipitation on 25-26 January 2013

Using measurements from the Van Allen Probes and the Balloon Array for RBSP Relativistic Electron Losses (BARREL), we perform a case study of electromagnetic ion cyclotron (EMIC) waves and associated relativistic electron precipitation (REP) observed on 25\textendash26 January 2013. Among all the EMIC wave and REP events from the two missions, the pair of the events is the closest both in space and time. The Van Allen Probe-B detected significant EMIC waves at L = 2.1\textendash3.9 and magnetic local time (MLT) = 21.0\textendash23.4 for 53.5 min from 2353:00 UT, 25 January 2013. Meanwhile, BARREL-1T observed clear precipitation of relativistic electrons at L = 4.2\textendash4.3 and MLT = 20.7\textendash20.8 for 10.0 min from 2358 UT, 25 January 2013. Local plasma and field conditions for the excitation of the EMIC waves, wave properties, electron minimum resonant energy Emin, and electron pitch angle diffusion coefficient Dαα of a sample EMIC wave packet are examined along with solar wind plasma and interplanetary magnetic field parameters, geomagnetic activity, and results from the spectral analysis of the BARREL balloon observations to investigate the two types of events. The events occurred in the early main phase of a moderate storm (min. Dst* = -51.0 nT). The EMIC wave event consists of two parts. Unlike the first part, the second part of the EMIC wave event was locally generated and still in its source region. It is found that the REP event is likely associated with the EMIC wave event.

Zhang, Jichun; Halford, Alexa; Saikin, Anthony; Huang, Chia-Lin; Spence, Harlan; Larsen, Brian; Reeves, Geoffrey; Millan, Robyn; Smith, Charles; Torbert, Roy; Kurth, William; Kletzing, Craig; Blake, Bernard; Fennel, Joseph; Baker, Daniel;

Published by: Journal of Geophysical Research: Space Physics      Published on: 10/2016

YEAR: 2016     DOI: 10.1002/2016JA022918

BARREL; EMIC waves; FFT; Geomagnetic storm; relativistic electron precipitation (REP); Van Allen Probes

The complex nature of storm-time ion dynamics: Transport and local acceleration

Data from the Van Allen Probes Helium, Oxygen, Proton, Electron (HOPE) spectrometers reveal hitherto unresolved spatial structure and dynamics in ion populations. Complex regions of O+ dominance, at energies from a few eV to >10 keV, are observed throughout the magnetosphere. Isolated regions on the dayside that are rich in energetic O+ might easily be interpreted as strong energization of ionospheric plasma. We demonstrate, however, that both the energy spectrum and the limited MLT extent of these features can be explained by energy-dependent drift of particles injected on the night side 24 hours earlier. Particle tracing simulations show that the energetic O+ can originate in the magnetotail, not in the ionosphere. Enhanced wave activity is co-located with the heavy-ion rich plasma and we further conclude that the waves were not a source of free energy for accelerating ionospheric plasma but rather the consequence of the arrival of substorm-injected plasma.

Denton, M.; Reeves, G.; Thomsen, M.; Henderson, M.; Friedel, R.; Larsen, B.; Skoug, R.; Funsten, H.; Spence, H.; Kletzing, C.;

Published by: Geophysical Research Letters      Published on: 09/2016

YEAR: 2016     DOI: 10.1002/2016GL070878

plasmasheet; Van Allen Probes

Hiss or Equatorial Noise? Ambiguities in Analyzing Suprathermal Ion Plasma Wave Resonance

Previous studies have shown that low energy ion heating occurs in the magnetosphere due to strong equatorial noise emission. Observations from the Van Allen Probes Helium Oxygen Proton Electron (HOPE) instrument recently determined there was a depletion in the 1-10 eV ion population in the post-midnight sector of Earth during quiet times at L < 3. The diurnal variation of equatorially mirroring 1-10 eV H+ ions between 2 < L < 3 is connected with similar diurnal variation in the electric field component of plasma waves ranging between 150 and 600 Hz. Measurements from the Van Allen Probes Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) data set are used to analyze waves of this frequency in near-Earth space. However, when we examine the polarization of the waves in the 150 to 600 Hz range in the equatorial plane, the majority are right-hand polarized plasmaspheric hiss waves. The 1-10 eV H+ equatorially mirroring population does not interact with right hand waves, despite a strong statistical relationship suggesting the two is linked. We present evidence supporting the relationship, both in our own work and the literature, but we ultimately conclude that the 1-10 eV H+ heating is not related to the strong enhancement of 150 to 600 Hz waves.

Sarno-Smith, Lois; Liemohn, Michael; Skoug, Ruth; ik, Ondrej; Morley, Steven; Breneman, Aaron; Larsen, Brian; Reeves, Geoff; Wygant, John; Hospodarsky, George; Kletzing, Craig; Moldwin, Mark; Katus, Roxanne; Zou, Shasha;

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

YEAR: 2016     DOI: 10.1002/2016JA022975

equatorial noise; Low Energy Ions; plasma waves; plasmasphere; Plasmaspheric Hiss; Van Allen Probes

Storm time impulsive enhancements of energetic oxygen due to adiabatic acceleration of preexisting warm oxygen in the inner magnetosphere

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

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

Van Allen Probes observations of magnetic field dipolarization and its associated O ⁺ flux variations in the inner magnetosphere at L < 6.6

We investigate magnetic field dipolarization in the inner magnetosphere and its associated ion flux variations, using the magnetic field and energetic ion flux data acquired by the Van Allen Probes. From a study of 74 events that appeared at L = 4.5\textendash6.6 between 1 October 2012 and 31 October 2013, we reveal the following characteristics of the dipolarization in the inner magnetosphere: (1) its timescale is approximately 5 min, (2) it is accompanied by strong magnetic fluctuations that have a dominant frequency close to the O+ gyrofrequency, (3) ion fluxes at 20\textendash50 keV are simultaneously enhanced with larger magnitudes for O+ than for H+, (4) after a few minutes of the dipolarization, the flux enhancement at 0.1\textendash5 keV appears with a clear energy-dispersion signature only for O+, and (5) the energy-dispersed O+ flux enhancement appears in directions parallel or anti-parallel to the magnetic field. From these characteristics, we discuss possible mechanisms that can provide selective acceleration to O+ ions at >20 keV. We conclude that O+ ions at L = 5.4\textendash6.6 undergo nonadiabatic local acceleration caused by oscillating electric field associated with the magnetic fluctuations and/or adiabatic convective transport from the plasma sheet to the inner magnetosphere by the impulsive electric field. At L = 4.5\textendash5.4, however, only the former acceleration is plausible. We also conclude that the field-aligned energy-dispersed O+ ions at 0.1\textendash5 keV originate from the ionosphere and are extracted nearly simultaneously to the onset of the dipolarization.

e, M.; Keika, K.; Kletzing, C.; Spence, H.; Smith, C.; MacDowall, R.; Reeves, G.; Larsen, B.; Mitchell, D.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 07/2016

YEAR: 2016     DOI: 10.1002/2016JA022549

Dipolarization; inner magnetosphere; ionospheric outflow; Magnetic Fluctuation; O+ Acceleration; substorm; Van Allen Probes

The Source of O ⁺ in the Storm-time Ring Current

A stretched and compressed geomagnetic field occurred during the main phase of a geomagnetic storm on 1 June 2013. During the storm the Van Allen Probes spacecraft made measurements of the plasma sheet boundary layer, and observed large fluxes of O+ ions streaming up the field line from the nightside auroral region. Prior to the storm main phase there was an increase in the hot (>1 keV) and more isotropic O+ions in the plasma sheet. In the spacecraft inbound pass through the ring current region during the storm main phase, the H+ and O+ ions were significantly enhanced. We show that this enhanced inner magnetosphere ring current population is due to the inward adiabatic convection of the plasma sheet ion population. The energy range of the O+ ion plasma sheet that impacts the ring current most is found to be from ~5 to 60 keV. This is in the energy range of the hot population that increased prior to the start of the storm main phase, and the ion fluxes in this energy range only increase slightly during the extended outflow time interval. Thus, the auroral outflow does not have a significant impact on the ring current during the main phase. The auroral outflow is transported to the inner magnetosphere, but does not reach high enough energies to affect the energy density. We conclude that the more energetic O+ that entered the plasma sheet prior to the main phase and that dominates the ring current is likely from the cusp.

Kistler, L.M.; Mouikis, C.; Spence, H.E.; Menz, A.M.; Skoug, R.M.; Funsten, H.O.; Larsen, B.A.; Mitchell, D.G.; Gkioulidou, M.; Wygant, J.R.; Lanzerotti, L.J.;

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

YEAR: 2016     DOI: 10.1002/2015JA022204

Geomagnetic storm; Ionosphere; oxygen; plasma sheet; Plasma Sources; ring current; Van Allen Probes

Ring current electron dynamics during geomagnetic storms based on the Van Allen Probes measurements

Based on comprehensive measurements from Helium, Oxygen, Proton, and Electron Mass Spectrometer Ion Spectrometer, Relativistic Electron-Proton Telescope, and Radiation Belt Storm Probes Ion Composition Experiment instruments on the Van Allen Probes, comparative studies of ring current electrons and ions are performed and the role of energetic electrons in the ring current dynamics is investigated. The deep injections of tens to hundreds of keV electrons and tens of keV protons into the inner magnetosphere occur frequently; after the injections the electrons decay slowly in the inner belt but protons in the low L region decay very fast. Intriguing similarities between lower energy protons and higher-energy electrons are also found. The evolution of ring current electron and ion energy densities and energy content are examined in detail during two geomagnetic storms, one moderate and one intense. The results show that the contribution of ring current electrons to the ring current energy content is much smaller than that of ring current ions (up to ~12\% for the moderate storm and ~7\% for the intense storm), and <35 keV electrons dominate the ring current electron energy content at the storm main phases. Though the electron energy content is usually much smaller than that of ions, the enhancement of ring current electron energy content during the moderate storm can get to ~30\% of that of ring current ions, indicating a more dynamic feature of ring current electrons and important role of electrons in the ring current buildup. The ring current electron energy density is also shown to be higher at midnight and dawn while lower at noon and dusk.

Zhao, H.; Li, X.; Baker, D.; Claudepierre, S.; Fennell, J.; Blake, J.; Larsen, B.; Skoug, R.; Funsten, H.; Friedel, R.; Reeves, G.; Spence, H.; Mitchell, D.; Lanzerotti, L.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 04/2016

YEAR: 2016     DOI: 10.1002/2016JA022358

deep injections; Geomagnetic storms; ring current; ring current energy content; ring current electrons; Van Allen Probes

Spacecraft surface charging within geosynchronous orbit observed by the Van Allen Probes

Using the Helium Oxygen Proton Electron (HOPE) and Electric Field and Waves (EFW) instruments from the Van Allen Probes, we explored the relationship between electron energy fluxes in the eV and keV ranges and spacecraft surface charging. We present statistical results on spacecraft charging within geosynchronous orbit by L and MLT. An algorithm to extract the H+ charging line in the HOPE instrument data was developed to better explore intense charging events. Also, this study explored how spacecraft potential relates to electron number density, electron pressure, electron temperature, thermal electron current, and low-energy ion density between 1 and 210 eV. It is demonstrated that it is imperative to use both EFW potential measurements and the HOPE instrument ion charging line for examining times of extreme spacecraft charging of the Van Allen Probes. The results of this study show that elevated electron energy fluxes and high-electron pressures are present during times of spacecraft charging but these same conditions may also occur during noncharging times. We also show noneclipse significant negative charging events on the Van Allen Probes.

Sarno-Smith, Lois; Larsen, Brian; Skoug, Ruth; Liemohn, Michael; Breneman, Aaron; Wygant, John; Thomsen, Michelle;

Published by: Space Weather      Published on: 02/2016

YEAR: 2016     DOI: 10.1002/2015SW001345

EFW; HOPE; spacecraft charging; surface charging; Van Allen Probes

Nonlinearity in chorus waves during a geomagnetic storm on 1 November 2012

In this study, we investigate the possibility of nonlinearity in chorus waves during a geomagnetic storm on 1 November 2012. The data we use were measured by the Van Allen Probe B. Wave data and plasma sheet electron data are analyzed. Chorus waves were frequently measured in the morning side during the main phase of this storm. Large-amplitude chorus waves were seen of the order of \~0.6 nT and >7 mV/m, which are similar to or larger than the typical ULF waves. The waves quite often consist of rising tones during the burst sampling. Since the rising tone is known as a signature of nonlinearity, a large portion of the waves are regarded as nonlinear at least during the burst sampling periods. These results underline the importance of nonlinearity in the dynamics of chorus waves. We further compare the measurement and the nonlinear theories, based on the inhomogeneity ratio, our own calculation derived from the field equation and the backward wave oscillator model. The wave quantities examined are frequency, amplitude, frequency drift rate, and duration. This type of study is useful to more deeply understand wave-particle interactions and hence may lead to predicting the generation and loss of radiation belt electrons in the future.

Matsui, H.; Paulson, K.; Torbert, R.; Spence, H.; Kletzing, C.; Kurth, W.; Skoug, R.; Larsen, B.; Breneman, A.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 01/2016

YEAR: 2016     DOI: 10.1002/2015JA021772

chorus waves; Geomagnetic storm; nonlinearity; Van Allen Probes

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

\textquotedblleftTrunk-like\textquotedblright heavy ion structures observed by the Van Allen Probes

Dynamic ion spectral features in the inner magnetosphere are the observational signatures of ion acceleration, transport, and loss in the global magnetosphere. We report \textquotedbllefttrunk-like\textquotedblright ion structures observed by the Van Allen Probes on 2 November 2012. This new type of ion structure looks like an elephant\textquoterights trunk on an energy-time spectrogram, with the energy of the peak flux decreasing Earthward. The trunks are present in He+ and O+ ions but not in H+. During the event, ion energies in the He+ trunk, located at L = 3.6\textendash2.6, MLT = 9.1\textendash10.5, and MLAT = -2.4\textendash0.09\textdegree, vary monotonically from 3.5 to 0.04 keV. The values at the two end points of the O+ trunk are: energy = 4.5\textendash0.7 keV, L = 3.6\textendash2.5, MLT = 9.1\textendash10.7, and MLAT = -2.4\textendash0.4\textdegree. Results from backward ion drift path tracings indicate that the trunks are likely due to 1) a gap in the nightside ion source or 2) greatly enhanced impulsive electric fields associated with elevated geomagnetic activity. Different ion loss lifetimes cause the trunks to differ among ion species.

Zhang, J.-C.; Kistler, L.; Spence, H.; Wolf, R.; Reeves, G.; Skoug, R.; Funsten, H.; Larsen, B.; Niehof, J.; MacDonald, E.; Friedel, R.; Ferradas, C.; Luo, H.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 10/2015

YEAR: 2015     DOI: 10.1002/2015JA021822

inner magnetosphere; ion injection; Ion structure; magnetic cloud; magnetic storm; Van Allen Probes

The evolution of ring current ion energy density and energy content during geomagnetic storms based on Van Allen Probes measurements

Enabled by the comprehensive measurements from the MagEIS, HOPE, and RBSPICE instruments onboard Van Allen Probes in the heart of the radiation belt, the relative contributions of ions with different energies and species to the ring current energy density and their dependence on the phases of geomagnetic storms are quantified. The results show that lower energy (<50 keV) protons enhance much more often and also decay much faster than higher energy protons. During the storm main phase, ions with energies < 50 keV contribute more significantly to the ring current than those with higher energies; while the higher energy protons dominate during the recovery phase and quiet times. The enhancements of higher energy proton fluxes as well as energy content generally occur later than those of lower energy protons, which could be due to the inward radial diffusion. For the March 29, 2013 storm we investigated in detail, the contribution from O+ is ~25\% of the ring current energy content during the main phase, and the majority of that comes from < 50 keV O+. This indicates that even during moderate geomagnetic storms the ionosphere is still an important contributor to the ring current ions. Using the Dessler-Parker-Sckopke relation, the contributions of ring current particles to the magnetic field depression during this geomagnetic storm are also calculated. The results show that the measured ring current ions contribute about half of the Dst depression.

Zhao, H.; Li, X.; Baker, D.; Fennell, J.; Blake, J.; Larsen, B.; Skoug, R.; Funsten, H.; Friedel, R.; Reeves, G.; Spence, H.; Mitchell, D.; Lanzerotti, L.; Rodriguez, J.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 08/2015

YEAR: 2015     DOI: 10.1002/2015JA021533

Geomagnetic storms; Ring current energy content; Ring current ions; The DPS relation; The Dst index; Van Allen Probes

A background correction algorithm for Van Allen Probes MagEIS electron flux measurements

We describe an automated computer algorithm designed to remove background contamination from the Van Allen Probes MagEIS electron flux measurements. We provide a detailed description of the algorithm with illustrative examples from on-orbit data. We find two primary sources of background contamination in the MagEIS electron data: inner zone protons and bremsstrahlung X-rays generated by energetic electrons interacting with the spacecraft material. Bremsstrahlung X-rays primarily produce contamination in the lower energy MagEIS electron channels (~30-500 keV) and in regions of geospace where multi-MeV electrons are present. Inner zone protons produce contamination in all MagEIS energy channels at roughly L < 2.5. The background corrected MagEIS electron data produce a more accurate measurement of the electron radiation belts, as most earlier measurements suffer from unquantifiable and uncorrectable contamination in this harsh region of the near-Earth space environment. These background-corrected data will also be useful for spacecraft engineering purposes, providing ground truth for the near-Earth electron environment and informing the next generation of spacecraft design models (e.g., AE9).

Claudepierre, S.; O\textquoterightBrien, T.; Blake, J.; Fennell, J.; Roeder, J.; Clemmons, J.; Looper, M.; Mazur, J.; Mulligan, T.; Spence, H.; Reeves, G.; Friedel, R.; Henderson, M.; Larsen, B.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 06/2015

YEAR: 2015     DOI: 10.1002/2015JA021171

Background contamination; Inner radiation belt; outer radiation belt; Particle measurements; Radiation belt; Spacecraft engineering; Van Allen Probes

Multipoint observations of the open-closed field line boundary as observed by the Van Allen Probes and geostationary satellites during the November 14 ^th 2012 geomagnetic storm

The twin Van Allen Probes spacecraft witnessed a series of lobe encounters between 0200 and 0515 UT on November 14th 2012. Although lobe entry had been observed previously by the other spacecraft, the two Van Allen Probe spacecraft allow us to observe the motion of the boundary for the first time. Moreover, this event is unique in that it consists of a series of six quasi-periodic lobe entries. The events occurred on the dawn flank between 4 and 6.6 local time and at altitudes between 5.6 and 6.2 RE. During the events Dst dropped to less than -100nT with the IMF being strongly southward (Bz = -15nT) and eastward (By = 20 nT). Observations by LANL GEO spacecraft at geosynchronous orbit also show lobe encounters in the northern hemisphere and on the dusk flank. The two spacecraft configuration provides strong evidence that these periodic entries into the lobe are the result of local expansions of the OCB propagating from the tail and passing over the Van Allen Probes. Examination of pitch angle binned data from the HOPE instrument shows spatially large, accelerated ion structures occurring near simultaneously at both spacecraft, with the presence of oxygen indicating that they have an ionospheric source. The outflows are dispersed in energy and are detected when the spacecraft are on both open and closed field lines. These events provide a chance to examine the global magnetic field topology in detail, as well as smaller scale spatial and temporal characteristics of the OCB, allowing us to constrain the position of the open/closed field line boundary and compare it to a global MHD model using a novel method. This technique shows that the model can reproduce a periodic approach and retreat of the OCB from the spacecraft but can overestimate its distance by as much as 3 RE. The model appears to simulate the dynamic processes that cause the spacecraft to encounter the lobe but incorrectly maps the overall topology of the magnetosphere during these extreme conditions.

Dixon, P.; MacDonald, E.; Funsten, H.; Glocer, A.; Grande, M.; Kletzing, C.; Larsen, B.; Reeves, G.; Skoug, R.; Spence, H.; Thomsen, M.;

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

YEAR: 2015     DOI: 10.1002/2014JA020883

Lobes; Magnetosphere; Modelling; Open/closed field line boundary; Van Allen Probes

Electric field structures and waves at plasma boundaries in the inner magnetosphere

Recent observations by the Van Allen Probes spacecraft have demonstrated that a variety of electric field structures and nonlinear waves frequently occur in the inner terrestrial magnetosphere, including phase space holes, kinetic field line resonances, nonlinear whistler mode waves, and several types of double layer. However, it is unclear whether such structures and waves have a significant impact on the dynamics of the inner magnetosphere, including the radiation belts and ring current. To make progress toward quantifying their importance, this study statistically evaluates the correlation of such structures and waves with plasma boundaries. A strong correlation is found. These statistical results, combined with observations of electric field activity at propagating plasma boundaries, are consistent with the scenario that the sources of the free energy for the structures and waves of interest are localized near and comove with these boundaries. Therefore, the ability of these structures and waves to influence plasma in the inner magnetosphere is governed in part by the spatial extent and dynamics of macroscopic plasma boundaries in that region.

Malaspina, David; Wygant, John; Ergun, Robert; Reeves, Geoff; Skoug, Ruth; Larsen, Brian;

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

YEAR: 2015     DOI: 10.1002/2015JA021137

injection; inner magnetosphere; nonlinear electric field structures; plasma boundary; plasma sheet; Van Allen Probes

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

Study of EMIC wave excitation using direct ion measurements

With data from Van Allen Probes, we investigate EMIC wave excitation using simultaneously observed ion distributions. Strong He-band waves occurred while the spacecraft was moving through an enhanced density region. We extract from Helium, Oxygen, Proton, and Electron (HOPE) Mass Spectrometer measurement the velocity distributions of warm heavy ions as well as anisotropic energetic protons that drive wave growth through the ion cyclotron instability. Fitting the measured ion fluxes to multiple sinm-type distribution functions, we find that the observed ions make up about 15\% of the total ions, but about 85\% of them are still missing. By making legitimate estimates of the unseen cold (below ~2 eV) ion composition from cutoff frequencies suggested by the observed wave spectrum, a series of linear instability analyses and hybrid simulations are carried out. The simulated waves generally vary as predicted by linear theory. They are more sensitive to the cold O+ concentration than the cold He+ concentration. Increasing the cold O+ concentration weakens the He-band waves but enhances the O-band waves. Finally, the exact cold ion composition is suggested to be in a range when the simulated wave spectrum best matches the observed one.

Min, Kyungguk; Liu, Kaijun; Bonnell, John; Breneman, Aaron; Denton, Richard; Funsten, Herbert; Jahn, öerg-Micha; Kletzing, Craig; Kurth, William; Larsen, Brian; Reeves, Geoffrey; Spence, Harlan; Wygant, John;

Published by: Journal of Geophysical Research: Space Physics      Published on: 03/2015

YEAR: 2015     DOI: 10.1002/2014JA020717

EMIC wave excitation; observation; linear theory and hybrid simulation; Van Allen Probes

Formation of the oxygen torus in the inner magnetosphere: Van Allen Probes observations

We study the formation process of an oxygen torus during the 12\textendash15 November 2012 magnetic storm, using the magnetic field and plasma wave data obtained by Van Allen Probes. We estimate the local plasma mass density (ρL) and the local electron number density (neL) from the resonant frequencies of standing Alfv\ en waves and the upper hybrid resonance band. The average ion mass (M) can be calculated by M \~ ρL/neL under the assumption of quasi-neutrality of plasma. During the storm recovery phase, both Probe A and Probe B observe the oxygen torus at L = 3.0\textendash4.0 and L = 3.7\textendash4.5, respectively, on the morning side. The oxygen torus has M = 4.5\textendash8 amu and extends around the plasmapause that is identified at L\~3.2\textendash3.9. We find that during the initial phase, M is 4\textendash7 amu throughout the plasma trough and remains at \~1 amu in the plasmasphere, implying that ionospheric O+ ions are supplied into the inner magnetosphere already in the initial phase of the magnetic storm. Numerical calculation under a decrease of the convection electric field reveals that some of thermal O+ ions distributed throughout the plasma trough are trapped within the expanded plasmasphere, whereas some of them drift around the plasmapause on the dawnside. This creates the oxygen torus spreading near the plasmapause, which is consistent with the Van Allen Probes observations. We conclude that the oxygen torus identified in this study favors the formation scenario of supplying O+ in the inner magnetosphere during the initial phase and subsequent drift during the recovery phase.

e, Nos\; Oimatsu, S.; Keika, K.; Kletzing, C.; Kurth, W.; De Pascuale, S.; Smith, C.; MacDowall, R.; Nakano, S.; Reeves, G.; Spence, H.; Larsen, B.;

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

YEAR: 2015     DOI: 10.1002/2014JA020593

inner magnetosphere; magnetic storm; oxygen torus; plasmasphere; ring current; ULF waves; Van Allen Probes

Whistler Anisotropy Instabilities as the Source of Banded Chorus: Van Allen Probes Observations and Particle-in-Cell Simulations

Magnetospheric banded chorus is enhanced whistler waves with frequencies ωr < Ωe, where Ωe is the electron cyclotron frequency, and a characteristic spectral gap at ωr ≃ Ωe/2. This paper uses spacecraft observations and two-dimensional particle-in-cell (PIC) simulations in a magnetized, homogeneous, collisionless plasma to test the hypothesis that banded chorus is due to local linear growth of two branches of the whistler anisotropy instability excited by two distinct, anisotropic electron components of significantly different temperatures. The electron densities and temperatures are derived from HOPE instrument measurements on the Van Allen Probes A satellite during a banded chorus event on 1 November 2012. The observations are consistent with a three-component electron model consisting of a cold (a few tens of eV) population, a warm (a few hundred eV) anisotropic population, and a hot (a few keV) anisotropic population. The simulations use plasma and field parameters as measured from the satellite during this event except for two numbers: the anisotropies of the warm and the hot electron components are enhanced over the measured values in order to obtain relatively rapid instability growth. The simulations show that the warm component drives the quasi-electrostatic upper-band chorus, and that the hot component drives the electromagnetic lower-band chorus; the gap at \~ Ωe/2 is a natural consequence of the growth of two whistler modes with different properties.

Fu, Xiangrong; Cowee, Misa; Friedel, Reinhard; Funsten, Herbert; Gary, Peter; Hospodarsky, George; Kletzing, Craig; Kurth, William; Larsen, Brian; Liu, Kaijun; MacDonald, Elizabeth; Min, Kyungguk; Reeves, Geoffrey; Skoug, Ruth; Winske, Dan;

Published by: Journal of Geophysical Research: Space Physics      Published on: 10/2014

YEAR: 2014     DOI: 10.1002/2014JA020364

Chorus; HOPE; particle-in-cell simulation; Van Allen Probes

Simulation of Van Allen Probes Plasmapause Encounters

We use an E \texttimes B-driven plasmapause test particle (PTP) simulation to provide global contextual information for in situ measurements by the Van Allen Probes (RBSP) during 15\textendash20 January 2013. During 120 h of simulation time beginning on 15 January, geomagnetic activity produced three plumes. The third and largest simulated plume formed during enhanced convection on 17 January, and survived as a rotating, wrapped, residual plume for tens of hours. To validate the simulation, we compare its output with RBSP data. Virtual RBSP satellites recorded 28 virtual plasmapause encounters during 15\textendash19 January. For 26 of 28 (92\%) virtual crossings, there were corresponding actual RBSP encounters with plasmapause density gradients. The mean difference in encounter time between model and data is 36 min. The mean model-data difference in radial location is 0:40\textpm0:05 RE. The model-data agreement is better for strong convection than for quiet or weakly disturbed conditions. On 18 January, both RBSP spacecraft crossed a tenuous, detached plasma feature at approximately the same time and nightside location as a wrapped residual plume, predicted by the model to have formed 32 h earlier on 17 January. The agreement between simulation and data indicates that the model-provided global information is adequate to correctly interpret the RBSP density observations.

Goldstein, J.; De Pascuale, S.; Kletzing, C.; Kurth, W.; Genestreti, K.; Skoug, R.; Larsen, B.; Kistler, L.; Mouikis, C.; Spence, H.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 09/2014

YEAR: 2014     DOI: 10.1002/2014JA020252

observations; plasmasphere; residual plume; simulation; Van Allen Probes