What happens when the geomagnetic field reverses?

TL;DR

This paper explores how geomagnetic field reversals affect radiation belt particle populations and cosmic ray entry, using an analytical approach to extend Stormer's theory and analyze the implications of magnetic field changes.

Contribution

It introduces a simple analytical extension of Stormer's theory to study particle entry during geomagnetic reversals, contrasting with previous numerical simulation approaches.

Findings

Radiation belt high-energy protons are depleted during reversals.

Energy spectra of trapped particles soften and recover over millennia.

Entry of galactic cosmic rays and solar energetic particles is facilitated during reduced magnetic dipole moments.

Abstract

During geomagnetic field reversals the radiation belt high-energy proton populations become depleted. Their energy spectra become softer, with the trapped particles of highest energies being lost first, and eventually recovering after a field reversal. The radiation belts rebuild in a dynamical way with the energy spectra flattening on the average during the course of many millennia, but without ever reaching complete steady state equilibrium between successive geomagnetic storm events determined by southward turnings of the IMF orientation. Considering that the entry of galactic cosmic rays and the solar energetic particles with energies above a given threshold are strongly controlled by the intensity of the northward component of the interplanetary magnetic field, we speculate that at earlier epochs when the geomagnetic dipole was reversed, the entry of these energetic particles into the geomagnetic field was facilitated when the interplanetary magnetic field was directed northward. Unlike in other complementary work where intensive numerical simulations have been used, our demonstration is based on a simple analytical extension of Stormer's theory. The access of GCR and SEP beyond geomagnetic cut-off latitudes is enhanced during epochs when the Earth's magnetic dipole is reduced, as already demonstrated earlier.