The entry of solar protons over the polar caps

Abstract

Observations of solar protons at energies from 1 MeV to 360 MeV are examined in relation to the information that these particles give about the magnetosphere, magnetotail and magnetopause. Trajectory integrations in a realistic model of the geomagnetic field out to 25 R E and a tail field model fitted to observations from 15 R E to 80 R E are used to obtain a better understanding of the particle motion. The mean free path of protons in the tail is found to be 700 R E and 200 R E for 100 MeV and 1 MeV protons respectively, which indicates that trajectory calculations in a static field model are valid. Structure over the polar caps at low altitudes due to interplanetary anisotropies was observed for protons > 30 MeV during the solar flares of November 18th 1968 and February 25th 1969. Apart from an easily explained direct impact zone structure, fluxes corresponding to peak anisotropic interplanetary intensity were observed to come from deep in the magnetotail. A magnetopause model of the generalised rotational discontinuity type, with a diverging tail field, is proposed and it is shown that this allows particle access into the tail with conservation of the distribution function. Particles from the direction of maximum flux (usually the Parker spiral direction) reverse their direction of motion during multiple magnetopause crossings. They eventually stay in the tail with small pitch angles and are thus observable at low altitudes. Observations show that 1 MeV protons become isotropic at the magnetopause. It is suggested that this is due to irregularities in the magnetopause, possibly caused by Kelvin Helmholtz instabilities, which are of comparable dimensions to the Larmor radius of these particles. Intensity differences over the polar cap of ▪40 per cent are expected due to the overall radial intensity gradient, and rapid local gradients can produce even greater differences. It is proposed here that sharp local intensity changes bounded by tangentional discontinuities, which convect past the Earth with the solar wind account for most of the observed polar cap intensity variations for 1 MeV protons. Anisotropies in flux produce gradients across the tail which can also be observed over the polar caps. Particle trajectories crossing the neutral sheet are analysed, and it is shown that dispersion of trajectories produces broadening of sharp interplanetary intensity changes when observed in the tail, not by a diffusion process, but through different trajectory lengths. Results of trajectory integrations at three energies are presented. These can be used to analyse polar cap structure in future work. Results of nonadiabatic particle motion in the pseudo-trapping region are shown. This motion produces effects similar to strong pitch angle scattering although it takes place in a static field.