Radial diffusion of ionized helium and protons: A probe for magnetospheric dynamics

The comparison of radial diffusion of magnetospheric helium ions and protons is a sensitive test of the diffusion process assumed (magnetic or electrostatic fluctuations) and thus valuable as a probe of magnetospheric dynamics for protons, electrons, and heavy ions, as well as for the helium ions themselves. In this paper radial profiles of ionized helium flux and protons are calculated, and certain physical effects ignored in previous treatments are taken into account. These effects include (a) charge-exchange processes (He++ → He+ → He or He++), which provide a loss mechanism as well as a way of converting solar-wind alpha particles to He+, and (b) contribution of fluctuating electrostatic fields to the radial-diffusion coefficient. Like nearly all other treatments of radial diffusion, other processes such as pitch-angle scattering have been omitted for lack of quantitative knowledge. The combination of a solar-wind source, of losses dominated by charge exchange, and of electrostatic radial diffusion (with a plausible power spectrum for the fluctuations) leads to (a) fast conversion of alphas to He+, and (b) a radial-diffusion coefficient proportional, at fixed energy per nucleon, to (Z/A)². Thus helium and other heavier ions (C, N, O) diffuse much more slowly than protons in the magnetosphere, and this slow diffusion can explain the very small α/p ratios observed near L = 3.