Understanding Electron Dropout Echoes Induced by Interplanetary Shocks: Test Particle Simulations

摘要

Recently, interplanetary shocks have been reported to cause "electron dropout echoes" in the outer radiation belt, which is manifested as repeated dropout and recovery signals in electron fluxes. Both previous case and statistical studies have shown that electron dropout echoes are mostly found for high-energy (>300 keV) electrons, and the initial dropout region is mainly located at the dusk magnetosphere, regardless of shock parameters such as shock normal. To understand these properties, we model the electron dropout echoes at geosynchronous orbit by tracing electrons in the analytic field model of the shock-induced propagating pulse. It is shown that the characteristics of shock-induced electron dropout echo events including energy dependence and localization are well reproduced by our model. By analyzing the trajectories of typical electrons, we find that electrons are inward transported and accelerated through "drift-resonance-like" interactions with the magnetosonic pulse. Two causes of the dawn-dusk asymmetric response are presented: (1) the difference between the interaction time of electrons with the magnetosonic pulse and (2) the opposite radial ?B drift of the electrons at dawnside and duskside. Further, we calculate the contributions to electron dynamics and phase space density variations from three terms: E × B drift, radial ?B drift, and gyrobetatron acceleration. The details of electron flux variations could vary with the form of the shock-induced pulse and the initial electron distribution, thus be different from our results; however, the basic ingredients of the electron interaction with the pulse could provide a general frame for understanding and evaluating electron flux responses.