Observations of geomagnetic cutoff variations during solar energetic particle events and implications for the radiation environment at the Space Station

Data from the polar‐orbiting Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) satellite have been used to measure the location of the geomagnetic cutoff for low‐energy protons and alpha particles during several large solar energetic particle events from mid‐1992 to late 1998. When fluxes are sufficiently high, the cutoff latitude can be measured up to four times per orbit, allowing the variability of the cutoff to be observed on relatively short timescales. We find significant changes in the cutoff location, often by more than 5° in less than 1 day, and these changes are well correlated with geomagnetic activity as measured by either Dst or Kp . Spacecraft in intermediate‐inclination orbits such as the International Space Station (ISS) graze the geomagnetic polar cap at certain longitudes each day. Calculations show that a 5° suppression in the average geomagnetic cutoff increases by more than a factor of 2.5 the time that the ISS spends in the polar cap exposed to energetic particles. Since the Station is only vulnerable at certain longitudes, however, real‐time monitoring of the cutoff location from a polar‐orbiting spacecraft could be used to provide advance notice of the polar cap location and conditions, sometimes hours before the Space Station itself reaches high magnetic latitudes.