Modelling the Propagation of Solar Energetic Particles Injected by a Shock-like Source using Test Particle Simulations

Abstract

Solar Energetic Particles (SEPs) are ions and electrons accelerated during flare and Coronal Mass Ejection (CME) events. SEP events increase the particle radiation that permeates the heliosphere and are harmful to spacecraft equipment as well as human health. For this reason, the study of energetic particle propagation is an area of vital importance in understanding the local radiation environment.

In this thesis we developed the first shock-like particle injection for a full-orbit test particle code and studied two distinct scenarios of SEP propagation. Firstly, we considered instantaneous injections of energetic protons at CME-driven shock heights. We modelled the particle back-precipitation to the solar surface to study whether protons injected from a CME shock can explain the -ray emission associated with Long Duration Gamma-Ray Flares (LDGRFs). We calculated precipitation fractions for each injection to determine the proportion of the injected population that could contribute to the -ray emission for a range of interplanetary scattering conditions mean free path, � = 0:0025 1:0 au). We estimated upper limits for the total precipitation fractions for eight LDGRF events and found that they were considerably smaller than the minimum requirements for back-precipitation from a CME-driven shock to be the dominant mechanism for -ray production during LDGRFs.

Secondly, we simulated a shock-like particle injection for a variety of injection functions to derive SEP intensity and anisotropy proles at 1 au. It has been proposed by several studies that the long duration of SEP events at 1 au is due to temporally extended acceleration at an interplanetary shock. We compared SEP intensity proles modelled from instantaneous and shock-like injections and found that the link between injection duration and event duration is very weak, unlike what is commonly assumed. In addition the variation of injection efficiency along the shock front was found to play a minor role in shaping intensity proles for gradual SEP events. We modelled SEP propagation with and without including the effects of corotation, considering the shock-like injection and found that corotation plays a dominant role in the decay phase of SEP events. Corotation reduces the decay time constant, , significantly for both eastern and western events and it makes 's dependence on the mean free path negligible, in contrast to results from 1D focussed transport models. The work presented in this thesis provides useful steps to model radiation within the inner heliosphere more accurately.