Solar proton entry to the magnetosphere on 18 November 1968 and 25 February 1969—I. Interpretation of satellite data using trajectory computations in a model magnetosphere

Abstract

Latitudinal distributions of high-energy (∼100 MeV) solar protons measured over the Earth's polar caps by low-altitude polar-orbiting satellites are described. The data, which refer to the initial phases of the 18 November 1968 and 25 February 1969 solar events, are related to interplanetary particle and magnetic-field measurements by means of trajectory computations using a model magnetosphere. A feature of this model magnetosphere, which is based on the Hones-Taylor model, is its realistic representation of the geomagnetic tail under relatively quiet magnetic conditions. It is shown that the proton distributions over the polar caps are consistent with the field-aligned particle anisotropies measured outside the magnetosphere in the initial phases of the two events. Lower-latitude maxima in the proton count rates are associated with particle entry through the dawn side of the magnetopause, and higher-latitude minima are associated with particle entry through the dusk side. Particle entry also occurs through the magnetotail and this mode of access produces enhancements of the proton count rates in the central polar caps. These results imply that solar protons had direct access to the magnetosphere during the two events.