Long‐term loss and re‐formation of the outer radiation belt

Abstract

[1] The Earth's outer radiation belt is known to vary often and significantly on various time scales. In this study, we have used the data of various instruments onboard the THEMIS spacecraft to study long-term changes of the outer radiation belt electrons around the year 2009. We find that the entire outer belt became extremely weak for nearly a year and was practically lost a few times, each time lasting~20 days up to~2 months, before eventually re-forming. This was revealed at a wide energy range from several tens of keV to up to 719 keV, which was covered by the THEMIS spacecraft measurements. The loss of the outer belt was associated with extremely weak solar wind conditions, i.e., low interplanetary magnetic field magnitude and slow solar wind speed. In particular, this set greatly reduced magnetospheric convection and/or injections for a prolonged time interval, which led to a large expansion of the plasmasphere, even beyond geosynchronous altitude and thus invading the majority of the typical outer belt territory for the same prolonged time interval. Consequently, preexisting electrons inside the plasmasphere had enough time to be lost into the atmosphere gradually over a time scale of several days without being supplied with fresh electrons from the plasma sheet under the same reduced convection and/or injections. Plasmaspheric hiss waves with an amplitude of up to a few tens of pT persisted to exist during the gradual decay periods, implying that they are likely responsible for the continual loss of the electrons inside the plasmasphere. A complete re-formation of the outer belt to full intensity was then realized over an interval of a few months. During the re-formation process, the magnetospheric convection and/or injections increased, which led to a gradual increase of whistler chorus wave activity, contraction of the plasmasphere, and supply of the plasma sheet electrons at high L shells. This set first an outward increasing profile of the phase space density, which eventually developed into a profile with a peak at low L of~5 over a time scale of 1-2 days. In this latter stage, a local acceleration at low L shells is found to be clearly needed although the radial diffusion process can contribute to some extent, in particular, for particles with a low first adiabatic invariant value.

An Energetic Electron Flux Dropout Due to Magnetopause Shadowing on 1 June 2013

“…Even though EMIC waves generally do not cause very strong pitch angle scattering for low pitch angle electrons with energies between 226 keV and 1 MeV, hiss waves can interact with these electrons and result in precipitation loss to the atmosphere (Li et al, ; Ni et al, ; Thorne, ). However, hiss waves are not responsible for rapid dropouts in general, since timescales of pitch angle scattering to the loss cone due to hiss are approximately 10 h to several days (Lee et al, ; Ma et al, ; Summers et al, ).…”

A prediction model for the global distribution of whistler chorus wave amplitude developed separately for two latitudinal zones

“…For this study we use the chorus wave data observed on the THEMIS satellites over a prolonged period [ Angelopoulos , ]. Specifically, we first construct the prediction model of chorus amplitudes using the data for about a 5 year period, from July 2007 to July 2012—excluding about 4 months from 1 June 2009 when the solar wind conditions and the magnetospheric activities were unusually weak [ Lee et al ., ] and the determination of the plasmapause location could not be made appropriately (see below). Then, the developed prediction model is tested for a separate 1.5 years period from August 2012 to January 2014.…”

“…The bulk of current radiation belt research is largely based on the interaction of electrons with plasma waves of a wide frequency range within the inner magnetosphere [e.g., Bortnik and Thorne, 2007;Shprits et al, 2008Shprits et al, , 2013Kim et al, 2012;Lee et al, 2013;Ni et al, 2013;Glauert et al, 2014]. Various types of waves have been observed in different regions and wide frequency domains and are believed to play a role in various aspects and to different degrees.…”

Variability of the pitch angle distribution of radiation belt ultrarelativistic electrons during and following intense geomagnetic storms: Van Allen Probes observations

“…The Earth's radiation belt electron dynamics is highly complex, resulting from a delicate, competitive balance between their transport, energization, and loss processes, and also shows strong dependence on a number of factors including solar wind driving condition, geomagnetic activity, electron kinetic energy, spatial location, and time [e.g., Li et al, 1997;Reeves et al, 1998Reeves et al, , 2003Meredith et al, 2003;Lee et al, 2013;Ni et al, 2013;Thorne et al, 2013aThorne et al, , 2013bBaker et al, 2013aBaker et al, , 2014aBaker et al, , 2014b. It has been known that geomagnetic storms can either increase or decrease the fluxes of radiation belt relativistic electrons, with about half of all storms increasing the relativistic electron fluxes, one quarter decreasing the fluxes, and the remaining quarter producing little or no change in the fluxes [Reeves et al, 2003].…”