Geomagnetic field distortions and their effects on radiation belt particles

Abstract

The study of trapped‐particle motion in the earth's magnetic field is usually geared toward building the foundation of radiation‐belt dynamics (particle injection, acceleration, diffusion, and loss). In this article we turn the situation around and use trapped‐particle motion to explore the properties of the geomagnetic field itself. One of the quadrupole terms in the main geomagnetic field is found to contribute to a north‐south ‘shear distortion’ of the particle‐drift shells, whereas one of the octupole components causes a longitude‐dependent radial deformation and associated ‘drift‐shell splitting.’ The collective action of all higher multipoles on trapped‐particle motion is then used to analyze the ‘true’ anomalies or distortions of the internal geomagnetic field that are independent of the quadrupole‐related eccentricity of the main dipole. These ‘true’ anomalies must originate in upper‐mantle or crustal perturbations that lie relatively near the earth's surface on both sides of the mid‐Atlantic ridge; they influence trapped‐particle drift shells only where the latter have their closest approach to the earth (South Atlantic and South African areas). The quadrupole and octupole perturbations, on the other hand, obviously originate deep in the earth's core. The most adequate space and particle coordinate systems are introduced, and their validity is briefly analyzed. In the final part of this review, we discuss the effects of external magnetospheric currents. A time‐dependent symmetric ring current causes drift shells to be displaced radially, with associated particle acceleration; magnetopause currents introduce a day‐night asymmetry, causing shell splitting. Time dependence of these effects introduces further complications; an invariant particle coordinate system is discussed that can be used for particle‐flux representations that are independent of the particular instantaneous field configuration (provided that the latter is known and that all changes occur slowly, i.e., adiabatically).