The effect of a southward interplanetary magnetic field on St\"ormer's allowed regions

TL;DR

This paper extends Stormer's theory to include the effects of a uniform interplanetary magnetic field, analyzing how a southward IMF alters the allowed regions for charged particles in Earth's magnetic field.

Contribution

The authors derive a new Stormer potential expression incorporating a uniform IMF and analyze its impact on particle allowed zones, especially under southward IMF conditions.

Findings

Southward IMF expands particle allowed zones into inner geomagnetic regions.

A new Stormer potential expression accounts for IMF effects.

Southward IMF facilitates entry of energetic particles into Earth's inner magnetosphere.

Abstract

The motion of a charged particle in a magnetic dipole has first been studied by Stormer. The different applications of Stormer's theory to aurorae, cosmic rays and Van Allen radiation belt particles are recalled in an historical perspective. In this paper, we expand the Stormer theory in order to take into account the effects produced by an additional uniform and stationary interplanetary magnetic field (IMF) whose orientation is parallel or antiparallel to the magnetic moment of the dipole. A new expression is derived for the Stormer potential taking into account the additional IMF component. It is shown how Stormer's allowed and forbidden zones are influenced by the implementation of a northward or a southward IMF, and how a southward turning of the IMF orientation makes it easier for Solar Energetic Particle and Galactic Cosmic Rays to enter into the inner part of the geomagnetic field along interconnected magnetic field lines.