Drift shell splitting by internal geomagnetic multipoles

Abstract

Computations on an 80–coefficient model of the earth's field illustrate the ‘topography’ of the magnetic equatorial surface (on which B · ∇B = 0) and the geometry of the drift shells of geomagnetically trapped particles. Individual terms in the spherical harmonic expansion (offset dipole coordinates) of the geomagnetic scalar potential V(γ, θ, ϕ) are either even or odd in cos θ, where θ = π/2 denotes the dipole equator. Terms that (unlike the dipole) are even in cos θ tend to ‘warp’ the equatorial surface, but do not (in first order) distort particle drift shells radially nor split the drift shells of particles having different equatorial pitch angles. Azimuthally asymmetric terms that are odd in cos θ do cause shell splitting in first order. Shell splitting at large L values (neglecting deformation of the earth's field by the solar wind) is found to be dominated by the geomagnetic octupole. At L ∼ 1, shell splitting is strongly enhanced by the South American and South African anomalies. When combined with pitch angle diffusion caused by atmospheric scattering, these results may be able to account for anomalous radial diffusion of inner zone electrons (as observed at L ≲ 1.2).

Neoclassical Diffusion of Radiation‐Belt Electrons Across Very Low L‐Shells

“…The radial diffusion mechanism proposed by Roederer et al () for deeply inner‐belt electrons is conceptually analogous to the collisional transport process known as “neoclassical diffusion” in the theory of laboratory plasma tokamaks (Balescu, ; Galeev et al, ; Galeev & Sagdeev, , ; Rosenbluth et al, ). This is why we call the corresponding magnetospheric radial‐diffusion process neoclassical.…”

“…The result in requires numerical computation of the same integral as for the particle's bounce period 2π/Ω 2 , which varies with φ 3 in a way that would be of interest in any case (cf. Roederer et al, ).…”

On the location of trapped particle populations in quadrupole magnetospheres

“…Furthermore, it has been demonstrated that the geometrical criterion defining the magnetic equatorial surface in the dipoledominated case [Roedeter et al, 1973] can be extended to provide a physical description-of trapping center surfaces, that is, the location of trapped particle populations in arbitrary magnetic field geometries. Thus this concept may well be applied to more realistic paleomagnetic field configurations in future investigations.…”

Advances in magnetospheric physics: Energetic particles

“…There also have been several significant theoretical papers on the drift motions of energetic particles in a distorted magnetosphere to produce driftshell splitting [Shabansky, 1971[Shabansky, ,1972ifosik, 1971;Antonova, 1972;Roederer, 1972;Schulz, 1972;Alekseyev, '973;Roederer et al, 1973). All the above papers discuss particles in guiding-center raolion along the field lines.…”