Time Variability of the Geomagnetic Cutoff During Large Solar Particle Events

In its 82 degree inclination orbit, SAMPEX crosses a polar cap and is exposed to interplanetary energetic particles twice each orbit. During intense solar energetic particle (SEP) events, count rates of high energy particles change from near zero at the equator (deep inside the geomagnetic field) to hundreds or thousands per second at the poles, with a very clear transition between the two regions easily discernible. We have measured the location of this transition, or geomagnetic cutoff, at every polar cap boundary crossing during many large SEP events throughout the SAMPEX mission.

The cutoff latitudes shown in the figures were mostly obtained from the MAST "Z2" rate, which responds to ~8-15 MeV/nucleon He nuclei (or particles with a rigidity of ~250-340 MV, equivalent to about 30-60 MeV protons), and represent the geomagnetic latitude at which this rate drops to half its value over the poles. Also shown on the right-hand scales are the geomagnetic activity indices Dst and Kp during these time periods.

The cutoff location is found to be highly variable, changing by 10 degrees in less than one day during some events and sometimes by several degrees from one side of the orbit to the other (presumably due to day-night asymmetries in the geomagnetic field). The temporal changes are generally well-correlated with changes in Dst or Kp, although large changes in the cutoff sometimes occur several hours before the corresponding change in Dst.

The accompanying movies show the extent of the north polar region accessible to 8-15 MeV/n He nuclei (which have the same rigidity, and therefore cutoff latitude, as 30-60 MeV protons) in low Earth orbit during the large solar energetic particle (SEP) events of July 2000 and October 2003. The exposed oval was calculated by measuring the He cutoff latitudes as illustrated above, averaging the cutoff invariant latitudes measured upon entering and exiting the north polar cap, assuming the cutoff corresponds to a constant invariant latitude during a given polar cap passage, and converting that invariant latitude to geographic coordinates. The color of the oval indicates the particle intensity, as indicated on the color bar at the right. The blue ground track is the ground track of the International Space Station (ISS), which can suffer significant increases in radiation exposure when the cutoff moves to lower latitudes during large SEP events.

For more information on these cutoff measurements, see
Leske, R.A., Mewaldt, R.A., Stone, E.C., and von Rosenvinge, T.T., "Observations of Geomagnetic Cutoff Variations During Solar Energetic Particle Events and Implications for the Radiation Environment at the Space Station", J. Geophys. Res. 106, 30011-30022 (2001).


Plots of cutoff variation with time, for 19 different time periods

Plots of Cutoff latitude vs Dst and Kp, data from all events

above (except Oct/Nov 2003 events).