Quantifying the Precipitation Loss of Radiation Belt Electrons During a Rapid Dropout Event

Abstract

Relativistic electron flux in the radiation belt can drop by orders of magnitude within the timespan of hours. In this study, we used the drift‐diffusion model that includes azimuthal drift and pitch angle diffusion of electrons to simulate low‐altitude electron distribution observed by POES/MetOp satellites for rapid radiation belt electron dropout event occurring on 1 May 2013. The event shows fast dropout of MeV energy electrons at L > 4 over a few hours, observed by the Van Allen Probes mission. By simulating the electron distributions observed by multiple POES satellites, we resolve the precipitation loss with both high spatial and temporal resolutions and a range of energies. We estimate the pitch angle diffusion coefficients as a function of energy, pitch angle, and L‐shell and calculate corresponding electron lifetimes during the event. The simulation results show fast electron precipitation loss at L > 4 during the electron dropout, with estimated electron lifetimes on the order of half an hour for MeV energies. The electron loss rate shows strong energy dependence with faster loss at higher energies, which suggest that this dropout event is dominated by quick and localized scattering process that prefers higher energy electrons. The improved temporal and spatial resolutions of electron precipitation rates provided by multiple low‐altitude observations can resolve fast‐varying electron loss during rapid electron dropouts (over a few hours), which occur too fast for a single low‐altitude satellite. The capability of estimating the fast‐varying electron lifetimes during rapid dropout events is an important step in improving radiation belt model accuracy.

POES/MEPED Angular Response Functions and the Precipitating Radiation Belt Electron Flux

“…Measured flux ratios from the horizontal (90°) and vertical (0°) telescopes have been used to infer efficiency of loss‐cone filling by trapped electron scattering in several studies of radiation belt and atmospheric dynamics (Li et al, 2013; Ni et al, 2014; Nesse Tyssøy et al, 2016; Soria‐Santacruz et al, 2015); others have used data from either telescope orientation alone (Peck et al, 2015; Pham et al, 2017; Pettit et al, 2019; Rodger et al, 2010). Typically, they have relied on nominal detector response characteristics as specified by the electronic and geometrical configurations of the MEPED instrumentation, which provide an energy threshold and angular field of view (FOV) for each of several available data channels, or have combined more accurate energy response functions (Yando et al, 2011) with a nominal FOV angular response.…”

Cosmic Ray Albedo Neutron Decay (CRAND) as a Source of Inner Belt Electrons: Energy Spectrum Study

“…The conventional understanding of the source of quasitrapped electrons has been the pitch angle scattering of stably trapped electrons, where the intensity could be explained by a balance between azimuthal drift and pitch angle scattering. Both theoretical calculation and simulation works have been conducted to evaluate the pitch angle diffusion coefficient and electron lifetime quantitatively (e.g., Abel & Thorne, ; Lyons et al, ; Pham et al, ; Schiller et al, ; Selesnick et al, , ; Tu et al, ). Note that these studies only focused on the regions where stably trapped electrons are abundant.…”

On Energetic Electron Dynamics During Geomagnetic Quiet Times in Earth's Inner Radiation Belt due to Atmospheric Collisional Loss and CRAND as a Source

“…The initial electron flux as a function of geomagnetic longitude and equatorial PA is shown in Figure a. Based on the real satellite position and detector look direction (e.g., Pham et al, ; Xiang et al, ), we calculated the PA of electrons measured by DEMETER when it passed the SAA region and selected the maximum one for comparison, which is indicated by the green star point in Figure a (PA = 69° and longitude = 300°). The electron flux corresponding to the green star location is used to compare with DEMETER measurements as shown later in Figure .…”

“…Quasi‐trapped electrons have equatorial PAs between the local BLC and the maximum BLC over all longitudes on their drift shell, while trapped electrons have PA greater than the largest BLC across all longitudes. Based on electron measurements in these three categories, drift‐diffusion models have been developed to quantify the precipitation loss of radiation belt electrons under different geomagnetic conditions (Pham et al, ; Selesnick, ; Selesnick et al, ; Tu et al, ) and a drift‐source model has been built to reproduce the quasi‐trapped electron flux from CRAND in the inner radiation belt (Xiang et al, ). Here we report that a drift‐collision‐source model is developed for the first time to quantitatively determine the roles of atmospheric collisions and CRAND in the dynamics of the inner belt electrons.…”