Bibliography

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

Energy limits of electron acceleration in the plasma sheet during substorms: A case study with the Magnetospheric Multiscale (MMS) mission

We present multipoint observations of earthward moving dipolarization fronts and energetic particle injections from NASA\textquoterights Magnetospheric Multiscale mission with a focus on electron acceleration. From a case study during a substorm on 02 August 2015, we find that electrons are only accelerated over a finite energy range, from a lower energy threshold at ~7\textendash9 keV up to an upper energy cutoff in the hundreds of keV range. At energies lower than the threshold energy, electron fluxes decrease, potentially due to precipitation by strong parallel electrostatic wavefields or initial sources in the lobes. Electrons at energies higher than the threshold are accelerated cumulatively by a series of impulsive magnetic dipolarization events. This case demonstrates how the upper energy cutoff increases, in this case from ~130 keV to >500 keV, with each dipolarization/injection during sustained activity. We also present a simple model accounting for these energy limits that reveals that electron energization is dominated by betatron acceleration.

Turner, D.; Fennell, J.; Blake, J.; Clemmons, J.; Mauk, B.; Cohen, I.; Jaynes, A.; Craft, J.; Wilder, F.; Baker, D.; Reeves, G.; Gershman, D.; Avanov, L.; Dorelli, J.; Giles, B.; Pollock, C.; Schmid, D.; Nakamura, R.; Strangeway, R.; Russell, C.; Artemyev, A.; Runov, A.; Angelopoulos, V.; Spence, H.; Torbert, R.; Burch, J.;

Published by: Geophysical Research Letters      Published on: 08/2016

YEAR: 2016     DOI: 10.1002/2016GL069691

energetic particle injections; magnetotail; Particle acceleration; plasma sheet; reconnection; substorm; Van Allen Probes

Dipolarizing flux bundles in the cis-geosynchronous magnetosphere: relationship between electric fields and energetic particle injections

Dipolarizing flux bundles (DFBs) are small flux tubes (typically < 3 RE in XGSM and YGSM) in the nightside magnetosphere that have magnetic field more dipolar than the background. Although DFBs are known to accelerate particles, creating energetic particle injections outside geosynchronous orbit (trans-GEO), the nature of the acceleration mechanism and the importance of DFBs in generating injections inside geosynchronous orbit (cis-GEO) are unclear. Our statistical study of cis-GEO DFBs using data from the Van Allen Probes reveals that just like trans-GEO DFBs, cis-GEO DFBs occur most often in the pre-midnight sector, but their occurrence rate is ~1/3 that of trans-GEO DFBs. Half the cis-GEO DFBs are accompanied by an energetic particle injection and have an electric field three times stronger than that of the injectionless half. All DFB injections are dispersionless within the temporal resolution considered (11 seconds). Our findings suggest that these injections are ushered or produced locally by the DFB, and the DFB\textquoterights strong electric field is an important aspect of the injection generation mechanism.

Liu, Jiang; Angelopoulos, V.; Zhang, Xiao-Jia; Turner, D.; Gabrielse, C.; Runov, A.; Li, Jinxing; Funsten, H.; Spence, H.;

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

YEAR: 2016     DOI: 10.1002/2015JA021691

dipolarization front; dipolarizing flux bundle; energetic particle injection; geosynchronous orbit; magnetic storm; Particle acceleration

Acceleration of ions by electric field pulses in the inner magnetosphere

Intense (~5-15 mV/m), short-lived (<=1 min) electric field pulses have been observed to accompany earthward-propagating, dipolarizing flux bundles (DFB; flux tubes with a strong magnetic field) before they are stopped by the strong dipole field. Using Time History of Events and Macroscale Interactions During Substorms (THEMIS) observations and test particle modeling, we investigate particle acceleration around L-shell ~7-9 in the nightside magnetosphere and demonstrate that such pulses can effectively accelerate ions with tens of keV initial energy to hundreds of keV. This acceleration occurs because the ion gyroradius is comparable to the spatial scale of the localized electric field pulse at the leading edge of the flux bundle before it stops. The proposed acceleration mechanism can reproduce observed spectra of high-energy ions. We conclude thatthe electric field associated with dipolarizing flux bundles prior to their stoppage in the inner magnetosphere provides a natural site for intense local ion acceleration.

Artemyev, A.V.; Liu, J.; Angelopoulos, V.; Runov, A.;

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

YEAR: 2015     DOI: 10.1002/2015JA021160

injections; inner magnetosphere; ion acceleration

Energetic electron injections deep into the inner magnetosphere associated with substorm activity

From a survey of the first nightside season of NASA\textquoterights Van Allen Probes mission (Dec/2012 \textendash Sep/2013), 47 energetic (10s to 100s of keV) electron injection events were found at L-shells <= 4, all of which are deeper than any previously reported substorm-related injections. Preliminary details from these events are presented, including how: all occurred shortly after dipolarization signatures and injections were observed at higher L-shells; the deepest observed injection was at L~2.5; and, surprisingly, L<=4 injections are limited in energy to <=250 keV. We present a detailed case study of one example event revealing that the injection of electrons down to L~3.5 was different from injections observed at higher L and likely resulted from drift resonance with a fast magnetosonic wave in the Pi 2 frequency range inside the plasmasphere. These observations demonstrate that injections occur at very low L-shells and may play an important role for inner zone electrons.

Turner, D.; Claudepierre, S.; Fennell, J.; O\textquoterightBrien, T.; Blake, J.; Lemon, C.; Gkioulidou, M.; Takahashi, K.; Reeves, G.; Thaller, S.; Breneman, A.; Wygant, J.; Li, W.; Runov, A.; Angelopoulos, V.;

Published by: Geophysical Research Letters      Published on: 02/2015

YEAR: 2015     DOI: 10.1002/2015GL063225

energetic particle injections; inner magnetosphere; Radiation belts; substorms; THEMIS; Van Allen Probes

Testing a two-loop pattern of the substorm current wedge (SCW2L)

Recent quantitative testing of the classical (region 1 sense) substorm current wedge (SCI) model revealed systematic discrepancies between the observed and predicted amplitudes, which suggested us to include additional region 2 sense currents (R2 loop) earthward of the dipolarized region (SCW2L model). Here we discuss alternative circuit geometries of the 3-D substorm current system and interpret observations of the magnetic field dipolarizations made between 6.6RE and 11RE, to quantitatively investigate the SCW2L model parameters. During two cases of a dipole-like magnetotail configuration, the dipolarization/injection front fortuitously stopped at r ~ 9RE for the entire duration of ~ 30 min long SCW-related dipolarization within a unique, radially distributed multispacecraft constellation, which allowed us to determine the locations and total currents of both SCW2L loops. In addition, we analyzed the dipolarization amplitudes in events, simultaneously observed at 6.6RE, 11RE and at colatitudes under a wide range of magnetograph conditions. We infer that the ratio I2/I1 varies in the range 0.2 to 0.6 (median value 0.4) and that the equatorial part of the R2 current loop stays at r>6.6RE in the case of a dipole-like field geometry (BZ0>75 nT at 6.6RE prior to the onset), but it is located at r<6.6RE in the case of a stretched magnetic field configuration (with BZ0<60 nT). Since the ground midlatitude perturbations are sensitive to the combined effect of the R1 and R2 sense current loops with the net current roughly equal to I1-I2, the ratio I2/I1 becomes an important issue when attempting to monitor the current disruption intensity from ground observations.

Sergeev, V.; Nikolaev, A.; Tsyganenko, N.; Angelopoulos, V.; Runov, A.; Singer, H.; Yang, J.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 02/2014

YEAR: 2014     DOI: 10.1002/2013JA019629

injections; magnetotail; substorm current wedge; substorms