Enhancement or Drop-out Drift-Echoes? An Investigation of Magnetospheric Pre-Conditions that Influence the Prompt Radiation Belt Response to IP Shocks, using a 3D MHD Wave Model and Test Particle Simulations.

This presentation investigates the process of rapid enhancements and depletions in relativistic electron flux associated with IP shocks. The action of an IP shock on the magnetosphere is to launch a pulse of MHD fast mode waves from the magnetopause. These waves are understood to drive rapid enhancements in electron flux associated with IP shocks at lower L, due to the drift-resonant radial transport of a seed population at higher L (e.g. Li et al, GRL [1993]). The injection of localized depletions can be described by essentially the same physical process if the seed population at higher L is absent [Schiller et al, JGR 2017]. Recent observational studies [Liu et. al., JGR 2017] indicate that IP shock driven depletions are localized in MLT, and are initiated preferentially in the afternoon/dusk sector. These then drift in MLT as drop-out drift-echoes and gradually disperse in energy. It has also been argued [Degeling et al, JGR 2013] that electron "PSD holes" can be injected from the afternoon sector magnetopause by (Pc5) MHD fast mode waves. Once launched, the propagation and accessibility of Pc5 waves within the magnetosphere is determined by the local Alfvén speed, and hence the distribution of cold plasma density (for example, the presence or absence of a plasmaspheric drainage plume [Degeling et. al., JGR 2018]).

Therefore, it could be argued that two potentially important factors prior to the arrival of an IP shock that affect its geo-effectiveness are: i) the presence or absence of a seed population of electrons at high L, and ii) the distribution of cold plasma density within the magnetosphere. This is investigated using a 3D model for MHD waves in the magnetosphere to inform a test-particle simulation for relativistic electrons, in which the response to an idealized shock front impacting the magnetosphere is modelled, and the initial energetic electron distribution and the cold plasma density structure are varied parametrically. We find that the presence or absence of a seed population at high L in general determines the generation of either drift-echo enhancement or drop-out. The cold plasma density structure (e.g. the presence or absence of a plume) affects the MLT location (across the dayside /afternoon sector) and also the extent in L over-which the drift-resonant radial transport that produces the drift-echo occurs.