
Laboratory for Atmospheric and Space Physics

Time variability of Geomagnetic Cutoffs

September 24, 2012

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 four SEP events in 1998, as shown in the accompanying figure

The cutoff latitudes shown in the figure were 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. Blue crosses mark the average cutoffs for entering the south polar cap and exiting the north, while green diamonds show the average cutoff for exiting the south polar cap and entering the north, which occurs about 180 degrees in longitude away from the crosses. Also shown on the right-hand scale is the geomagnetic activity index Dst during these time periods (heavy red curves).

The cutoff location is found to be highly variable, changing by more than 7 degrees in less than one day during the August 1998 event, for example, and by 2 or 3 degrees from one side of the orbit to the other in the September 1998 event (presumably due to day-night asymmetries in the geomagnetic field). The temporal changes are generally well-correlated with changes in Dst, although large changes in the cutoff tend to occur several hours before the corresponding change in Dst during the April 1998 and August 1998 events.

Calculations indicate that a 5 degree suppression of the cutoff will, on average, increase the exposure time of the Space Station to the polar cap region by more than a factor of 2.5. If such a cutoff suppression, or an even greater one, were to occur during a major SEP event as shown here, occupants of the Station or their equipment would be exposed to a considerably increased radiation dose. Although the Station only enters into the polar cap at certain longitudes and spends most of its time completely shielded from particles of this rigidity, a polar orbiting satellite such as SAMPEX could monitor the location of the polar cap boundary four times each orbit. If the data were collected and analyzed in near real-time, this could potentially provide hours of warning to the Space Station of a cutoff suppression in progress.

Contributed by Rick Leske, Caltech

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