Relativistic‐electron dropouts and recovery: A superposed epoch study of the magnetosphere and the solar wind

Abstract

[1] During 124 high-speed-stream-driven storms from two solar cycles, a multispacecraft average of the 1.1-1.5 MeV electron flux measured at geosynchronous orbit is examined to study global dropouts of the flux. Solar wind and magnetospheric measurements are analyzed with a superposed epoch technique, with the superpositions triggered by storm-convection onset, by onset of the relativistic-electron dropouts, and by recovery of the dropouts. It is found that the onset of dropout occurs after the passage of the IMF sector reversal prior to the passage of the corotating interaction region (CIR) stream interface. The recovery from dropout commences during the passage of the compressed fast wind. Relativistic-electron-dropout onset is temporally associated with the onset of the superdense ion and electron plasma sheet, with the onset of the extra-hot ion and electron plasma sheet and with the formation of the plasmaspheric drainage plume. Dropout recovery is associated with the termination of the superdense plasma sheet and with a decay of the plasmaspheric drainage plume. When there is appreciable spatial overlap of the superdense ion plasma sheet with the drainage plume, dropouts occur, and when that overlap ends, dropouts recover. This points to pitch-angle scattering by electromagnetic ion-cyclotron (EMIC) waves as the primary cause of the relativistic-electron dropouts, with the waves residing in the lumpy drainage plumes driven by the superdense ion plasma sheet. The drainage plume is caused by enhanced magnetospheric convection associated with southward (GSM) magnetic field after the IMF sector reversal. The superdense plasma sheet has its origin in the compressed slow wind of the CIR.

On the cause and extent of outer radiation belt losses during the 30 September 2012 dropout event

“…Here we use the definition of dropouts described in Turner et al [2012b], which includes both adiabatic effects and nonadiabatic losses. , possibly within plasmaspheric plumes that form during active periods and span the radial extent of the outer radiation belt [Borovsky and Denton, 2009]. Typically, only electrons with energy >~2 MeV can resonate with EMIC waves [Meredith et al, 2003], though during very active conditions this resonant energy limit might decrease to~400 keV [Ukhorskiy et al, 2010].…”

Explaining sudden losses of outer radiation belt electrons during geomagnetic storms

“…Mechanisms resulting in atmospheric loss include pitch-angle scattering through wave particle interactions with magnetospheric plasma waves 13 and violation of the first adiabatic invariant due to highly stretched magnetotail fields (that is, those in which the field line curvature becomes comparable to the particle's gyro-radius 14 ). Several recent statistical studies have concluded that losses to the atmosphere are probably responsible for main phase 5,11 and recovery phase 15,16 electron loss during certain types of geomagnetic storms. However, the mainphase dropout results were inferred in those studies, with no clear observations showing that the primary loss was to the atmosphere.…”

Radial diffusion simulations of the 20 September 2007 radiation belt dropout

“…Electromagnetic ion cyclotron (EMIC) waves can lead to very rapid pitch angle diffusion of relativistic electrons, and are expected to be proximate to the plasmapause (Summers et al, 1998;Li et al, 2007). Borovsky and Denton (2009) found from a study of geosynchronous data that dropouts, attributed to EMIC waves, coincide with the formation of a plasmaspheric drainage plume, in conjunction with a dense and hot plasma sheet. Usanova et al (2012Usanova et al ( , 2013 found from THEMIS and Cluster surveys that EMIC within plumes correlates with solar wind dynamic pressure, though the occurrence rate is still in the 5-10 % range inside of geosynchronous orbit.…”