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

Pitch Angle Scattering and Loss of Radiation Belt Electrons in Broadband Electromagnetic Waves

A magnetic conjunction between Van Allen Probes spacecraft and the Balloon Array for Radiation-belt Relativistic Electron Losses (BARREL) reveals the simultaneous occurrence of broadband Alfv\ enic fluctuations and multi-timescale modulation of enhanced atmospheric X-ray bremsstrahlung emission. The properties of the Alfv\ enic fluctuations are used to build a model for pitch angle scattering in the outer radiation belt on electron gyro-radii scale field structures. It is shown that this scattering may lead to the transport of electrons into the loss cone over an energy range from hundreds of keV to multi-MeV on diffusive timescales on the order of hours. This process may account for modulation of atmospheric X-ray fluxes observed from balloons and constitute a significant loss process for the radiation belts.

Chaston, C.; Bonnell, J.; Halford, A.; Reeves, G.; Baker, D.; Kletzing, C.; Wygant, J.;

Published by: Geophysical Research Letters      Published on: 09/2018

YEAR: 2018     DOI: 10.1029/2018GL079527

Alfven waves; drift-bounce resonance; energetic particles; Geomagnetic storms; pitch-angle scattering; Radiation belts; Van Allen Probes

Electron Distributions in Kinetic Scale Field Line Resonances: A Comparison of Simulations and Observations

Observations in kinetic scale field line resonances, or eigenmodes of the geomagnetic field, reveal highly field-aligned plateaued electron distributions. By combining observations from the Van Allen Probes and Cluster spacecraft with a hybrid kinetic gyrofluid simulation we show how these distributions arise from the nonlocal self-consistent interaction of electrons with the wavefield. This interaction is manifested as electron trapping in the standing wave potential. The process operates along most of the field line and qualitatively accounts for electron observations near the equatorial plane and at higher latitudes. In conjunction with the highly field-aligned plateaus, loss cone features are also evident, which result from the action of the upward-directed wave parallel electric field on the untrapped electron populations.

Damiano, P.A.; Chaston, C.C.; Hull, A.J.; Johnson, J.R.;

Published by: Geophysical Research Letters      Published on: 06/2018

YEAR: 2018     DOI: 10.1029/2018GL077748

Alfven waves; field line resonances; kinetic effects; numerical modeling; particle trapping; Radiation belts; Van Allen Probes

Radiation belt \textquotedblleftdropouts\textquotedblright and drift-bounce resonances in broadband electromagnetic waves

Observations during the main phase of geomagnetic storms reveal an anti-correlation between the occurrence of broadband low frequency electromagnetic waves and outer radiation belt electron flux. We show that the drift-bounce motion of electrons in the magnetic field of these waves leads to rapid electron transport. For observed spectral energy densities it is demonstrated that the wave magnetic field can drive radial diffusion via drift-bounce resonance on timescales less than a drift orbit. This process may provide outward transport sufficient to account for electron \textquotedblleftdropouts\textquotedblright during storm main phase and more generally modulate the outer radiation belt during geomagnetic storms.

Chaston, C.; Bonnell, J.; Wygant, J.; Reeves, G.; Baker, D.; Melrose, D.;

Published by: Geophysical Research Letters      Published on: 02/2018

YEAR: 2018     DOI: 10.1002/2017GL076362

Alfven waves; Geomagnetic storms; Radial Transport; Radiation belts; Van Allen Probes

Radial transport of radiation belt electrons in kinetic field-line resonances

A representative case study from the Van Allen Probes during a geomagnetic storm recovery phase reveals enhanced electron fluxes at intermediate pitch angles over energies from ~100 keV to 5 MeV coincident with broadband low frequency electromagnetic waves. The statistical properties of these waves are used to build a model for radial diffusion via drift-bounce resonances in kinetic Alfv\ en eigenmodes/kinetic field-line resonances. Estimated diffusion coefficients indicate timescales for radial transport of the order of hours in storm-time events at energies from <100 keV to MeVs over equatorial pitch angles from the edge of the loss cone to nearly perpendicular to the geomagnetic field. The correlation of kinetic resonances with electron depletions and enhancements during storm main phase and recovery, and the rapid diffusion these waves drive, suggest they may modulate the outer radiation belt.

Chaston, C.; Bonnell, J.; Wygant, J.; Reeves, G.; Baker, D.; Melrose, D.; Cairns, Iver.;

Published by: Geophysical Research Letters      Published on: 07/2017

YEAR: 2017     DOI: 10.1002/2017GL074587

Alfven waves; Diffusion; field line resonances; Radiation belts; Transport; Van Allen Probes

Driving ionospheric outflows and magnetospheric O ⁺ energy density with Alfv\ en waves

We show how dispersive Alfv\ en waves observed in the inner magnetosphere during geomagnetic storms can extract O+ ions from the topside ionosphere and accelerate these ions to energies exceeding 50 keV in the equatorial plane. This occurs through wave trapping, a variant of \textquotedblleftshock\textquotedblright surfing, and stochastic ion acceleration. These processes in combination with the mirror force drive field-aligned beams of outflowing ionospheric ions into the equatorial plane that evolve to provide energetic O+ distributions trapped near the equator. These waves also accelerate preexisting/injected ion populations on the same field lines. We show that the action of dispersive Alfv\ en waves over several minutes may drive order of magnitude increases in O+ ion pressure to make substantial contributions to magnetospheric ion energy density. These wave accelerated ions will enhance the ring current and play a role in the storm time evolution of the magnetosphere.

Chaston, C.; Bonnell, J.; Reeves, G.; Skoug, R.;

Published by: Geophysical Research Letters      Published on: 05/2016

YEAR: 2016     DOI: 10.1002/2016GL069008

Alfven waves; ion acceleration; Ionosphere; ionospheric outflow; ring current

Extreme ionospheric ion energization and electron heating in Alfv\ en waves in the storm-time inner magnetosphere

We report measurements of energized outflowing/bouncing ionospheric ions and heated electrons in the inner magnetosphere during a geomagnetic storm. The ions arrive in the equatorial plane with pitch angles that increase with energy over a range from tens of eV to > 50 keV while the electrons are field-aligned up to ~1 keV. These particle distributions are observed during intervals of broadband low frequency electromagnetic field fluctuations consistent with a Doppler-shifted spectrum of kinetic Alfv\ en waves and kinetic field-line resonances. The fluctuations extend from L≈3 out to the apogee of the Van Allen Probes spacecraft at L≈6.5. They thereby span most of the L-shell range occupied by the ring current. These measurements suggest a model for ionospheric ion outflow and energization driven by dispersive Alfv\ en waves that may account for the large storm-time contribution of ionospheric ions to magnetospheric energy density.

Chaston, C.; Bonnell, J.; Wygant, J.; Kletzing, C.; Reeves, G.; Gerrard, A.; Lanzerotti, L.; Smith, C.;

Published by: Geophysical Research Letters      Published on: 12/2015

YEAR: 2015     DOI: 10.1002/2015GL066674

Alfven waves; electron precipitation; Geomagnetic storms; ion acceleration; ion outflow; ion upflo

Broadband low frequency electromagnetic waves in the inner magnetosphere

A prominent yet largely unrecognized feature of the inner magnetosphere associated with particle injections, and more generally geomagnetic storms, is the occurrence of broadband electromagnetic field fluctuations over spacecraft frame frequencies (fsc) extending from effectively zero to fsc ≳ 100 Hz. Using observations from the Van Allen Probes we show that these waves most commonly occur pre-midnight but are observed over a range of local times extending into the dayside magnetosphere. We find that the variation of magnetic spectral energy density with fsc obeys inline image over several decades with a spectral break-point at fb ≈1 Hz. The values for α are log normally distributed with α = 1.9 \textpm 0.6 for fsc < fb andα = 2.9 \textpm 0.6 for fsc > fb. A is a function of geomagnetic activity with the largest values observed over intervals of decreasing Dst index during the main phase of geomagnetic storms. At these times these waves are nearly always present in the night-side inner magnetosphere and are commonly observed from L = 3 outward. The observed variation of the electric to magnetic field amplitude with fsc is well described by a dispersive Alfv\ en wave model under the assumption that fsc is primarily a consequence of the Doppler shift of plasma frame structures moving over the spacecraft. The robust anti-correlation between the time rate change of the Dst index and wave spectral energy density coupled with the ability of dispersive Alfv\ en waves to drive transverse ion acceleration suggests that these waves may boost ion energy density in the inner magnetosphere and intensify the ring current during storm times.

Chaston, C.; Bonnell, J.; Kletzing, C.; Hospodarsky, G.; Wygant, J.; Smith, C.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 09/2015

YEAR: 2015     DOI: 10.1002/2015JA021690

Alfven waves; Geomagnetic storms; ring current; turbulence; Van Allen Probes