Drift Orbit Bifurcations and Cross-field Transport in the Outer Radiation Belt: Global MHD and Integrated Test-Particle Simulations

TL;DR

This study uses advanced simulations to analyze drift orbit bifurcations in the outer radiation belt, revealing their dependence on magnetic field orientation and electric fields, and their role in particle flux variations.

Contribution

It introduces a high-precision test-particle simulation approach to map cross-field transport and incorporates electric field effects into DOB analysis, advancing understanding of radiation belt dynamics.

Findings

DOBs occur within an Earth radius of the magnetopause

Magnetic field orientation affects DOB occurrence and characteristics

Electric fields significantly influence long-term particle transport

Abstract

Energetic particle fluxes in the outer magnetosphere present a significant challenge to modelling efforts as they can vary by orders of magnitude in response to solar wind driving conditions. In this article, we demonstrate the ability to propagate test particles through global MHD simulations to a high level of precision and use this to map the cross-field radial transport associated with relativistic electrons undergoing drift orbit bifurcations (DOBs). The simulations predict DOBs primarily occur within an Earth radius of the magnetopause loss cone and appears significantly different for southward and northward interplanetary magnetic field orientations. The changes to the second invariant are shown to manifest as a dropout in particle fluxes with pitch angles close to 90$^\circ$ and indicate DOBs are a cause of butterfly pitch angle distributions within the night-time sector. The convective electric field, not included in previous DOB studies, is found to have a significant effect on the resultant long term transport, and losses to the magnetopause and atmosphere are identified as a potential method for incorporating DOBs within Fokker-Planck transport models.