Acceleration of ions by electric field pulses in the inner magnetosphere

Abstract

Intense (∼5–15 mV/m), short‐lived (≤1 min) electric field pulses have been observed to accompany earthward propagating, dipolarizing flux bundles (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 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 that the electric field associated with dipolarizing flux bundles prior to their stoppage in the inner magnetosphere provides a natural site for intense local ion acceleration.

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

“…The electric field magnitude is typically 3 times greater for the former than for the latter. This supports previous models suggesting that injections result from a DFB's electric field directly accelerating or ushering particles into the inner magnetosphere Gabrielse et al, 2012;Birn et al, 2013;Artemyev et al, 2015]. Consistent with their electric field difference, DFBs with injection move faster than DFBs without.…”

“…(Note that our electric fields are smaller than the values of Artemyev et al . [] because ours are averages over 1 min. The peaks of our electric fields are up to 42 mV/m.)…”

Drift‐Dispersed Flux Dropouts of Energetic Electrons Observed in Earth's Middle Magnetosphere by the Magnetospheric Multiscale (MMS) Mission

“…Multiple models have been proposed to explain observations of energetic particle injections (e.g., Artemyev et al, 2015;Birn et al, 1998;Gabrielse et al, 2012Gabrielse et al, , 2016Gabrielse et al, 2017;Li et al, 1998;Li et al, 2003;Mauk & McIlwain, 1974;Reeves et al, 1991;Sarris et al, 2002;Ukhorskiy et al, 2017). Previous studies (e.g., Angelopoulos et al, 2008;Baker et al, 1996) suggest that some energetic particle injections result from earthward moving plasma (e.g., plasma bubbles or dipolarizing flux bundles) associated with a sharp enhancement in the z component of the magnetic field (i.e., dipolarization or dipolarization front).…”

Ultralow Frequency Waves Deep Inside the Inner Magnetosphere Driven by Dipolarizing Flux Bundles

“…Such enhancements result most likely from particle acceleration by DFBs. This acceleration can be either adiabatic (Ashour‐Abdalla et al, ; Deng et al, ; Fu et al, ; Runov et al, ), most simply understood as caused by the electric field accompanying moving DFBs (e.g., Artemyev et al, ; Gabrielse et al, ; Zhou et al, ), or nonadiabatic due to large amplitude waves observed in the DFB vicinity (Zhou et al, ). For some DFBs that can penetrate into the inner magnetosphere, the electric field related to the DFB evolution can still be large and lead to significant local particle acceleration (Liu et al, ).…”