Measurements of the Geomagnetic Cutoff Rigidity-Dependence for Solar Energetic Particle Events

During large solar energetic particle (SEP) events, charged particles can ionize the atmosphere and alter atmospheric chemistry. Geomagnetic cutoff latitudes bound the polar regions of the atmosphere exposed to solar particles. The size of the exposed polar regions may change significantly during SEP events, correlating well with geomagnetic activity. During SEP events, the geomagnetic cutoff latitude can vary by more than 10 degrees. Using data from the MAST instrument on SAMPEX, we have measured geomagnetic cutoffs for 17 large SEP events, including the 4/21/02 event. We will combine these measurements with data from other SAMPEX instruments to investigate how the size of the polar regions depends on particle rigidity, how this dependence may change with time during an event, and how the measurements compare with models.