Penetration of low-energy solar protons to low geomagnetic latitudes

Changes in the geomagnetic cutoff latitude for low-energy (>2 Mev) solar protons have been compared with changes in the high-latitude boundary of trapped energetic electrons during a magnetically disturbed period. These changes have been measured with good time resolution. The observations suggest that the depression of the cutoff latitude is partly due to an enlargement of the region of direct access, as indicated by the depression of the trapped electron boundary. In addition, solar protons become trapped and are transported to lower latitudes during magnetospheric substorms.