Abstract
Abstract. Energetic radiation belt electron fluxes can undergo sudden dropouts in response to different solar wind drivers. Many physical processes contribute to the electron flux dropout, but their respective roles in the net electron depletion remain a fundamental puzzle. Some previous studies have qualitatively examined the importance of magnetopause shadowing in the sudden dropouts either from observations or from simulations. While it is difficult to directly measure the electron flux loss into the solar wind, radial diffusion codes with a fixed boundary location (commonly utilized in the literature) are not able to explicitly account for magnetopause shadowing. The exact percentage of its contribution has therefore not yet been resolved. To overcome these limitations and to determine the exact contribution in percentage, we carry out radial diffusion simulations with the magnetopause shadowing effect explicitly accounted for during a superposed solar wind stream interface passage, and quantify the relative contribution of the magnetopause shadowing coupled with outward radial diffusion by comparing with GPS-observed total flux dropout. Results indicate that during high-speed solar wind stream events, which are typically preceded by enhanced dynamic pressure and hence a compressed magnetosphere, magnetopause shadowing coupled with the outward radial diffusion can explain about 60–99% of the main-phase radiation belt electron depletion near the geosynchronous orbit. While the outer region (L* > 5) can nearly be explained by the above coupled mechanism, additional loss mechanisms are needed to fully explain the energetic electron loss for the inner region (L* ≤ 5). While this conclusion confirms earlier studies, our quantification study demonstrates its relative importance with respect to other mechanisms at different locations.
Highlights
Reductions of energetic electron flux in the outer radiation belt can generally be attributed to (a) adiabatic motion (i.e., Dst effect) (Kim and Chan, 1997) that radially transports particles adiabatically following a configuration change in the magnetosphere to conserve the three adiabatic invariants (μ, K, φ) and (b) nonadiabatic processes, such as the loss caused by pitch-angle scattering via various cyclotron wave–particle interaction, which leads to electron precipitation to the lowaltitude atmosphere (e.g., Lyons et al, 1972; Thorne et al, 2005; Summers et al, 2007a, b; Millan et al, 2007) as well as the loss across the magnetopause into the interplanetary space (e.g., Desorgher et al, 2000; Ohtani et al, 2009; Ukhorskiy et al, 2006, 2011)
This paper primarily focuses on quantifying the relative contribution of magnetopause shadowing in the electron flux dropouts during 67 high-speed solarwind-stream- (HSS) events by simulating the magnetopause shadowing effect in the 1D radial diffusion model
The one-dimensional radial diffusion model simplified from the Fokker–Plank equation with a loss term is used to simulate the evolution of phase space density (PSD) distribution f (L∗, t) of the trapped radiation belt energetic electron during the above superposed HSS event:
Summary
Reductions of energetic electron flux in the outer radiation belt can generally be attributed to (a) adiabatic motion (i.e., Dst effect) (Kim and Chan, 1997) that radially transports particles adiabatically following a configuration change in the magnetosphere to conserve the three adiabatic invariants (μ, K, φ) and (b) nonadiabatic processes, such as the loss caused by pitch-angle scattering via various cyclotron wave–particle interaction, which leads to electron precipitation to the lowaltitude atmosphere (e.g., Lyons et al, 1972; Thorne et al, 2005; Summers et al, 2007a, b; Millan et al, 2007) as well as the loss across the magnetopause (i.e., magnetopause shadowing) into the interplanetary space (e.g., Desorgher et al, 2000; Ohtani et al, 2009; Ukhorskiy et al, 2006, 2011).
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