Geomagnetic cutoffs during SEP events

SAMPEX’s 82 degree inclination low-Earth orbit is ideal for direct determination of the geomagnetic cutoffs for ions with energies ~1-100 MeV/nucleon. By observing the cutoff of ion intensities each time the spacecraft passes through the polar cap boundary, a time history of cutoff values can be compiled for each solar particle event. During the November 1992 and November 1997 solar particle events, for example, we found that the cutoffs varied by up to several degrees during the event, and that the cutoff changes were often correlated with changes in Dst. These changes in cutoff are relevant to issues of space weather and astronaut dose rate as the International Space Station undergoes construction.

We have continued this study to include several powerful solar particle events in 1998, and have found that the cutoff is indeed a function of the Dst and more interestingly that in at least one event the decrease in cutoff preceded the Dst decrease. The cutoff decrease began several hours before the Dst decrease and has important space weather consequences especially in the days of the international space station. The accompanying figure shows the cutoff for various SEP events recorded by SAMPEX since launch up to 1998. The cutoffs for each event are displaced by 5 degrees.

Contributed by Shri Kanekal, Goddard Space Flight Center