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Laboratory for Atmospheric and Space Physics

Deep Cutoff Suppression During the “Bastille Day” Event

September 24, 2012

In Earth orbit, low energy solar particles are excluded from low latitudes by the geomagnetic field, while over the poles they have free access. In an ongoing study, we use SAMPEX rate measurements in large solar energetic particle (SEP) events to determine the location of the transition between these two regions, known as the geomagnetic cosmic ray cutoff, and observe how this location varies with geomagnetic activity. Examples of the varying cutoff latitude during 3 SEP time periods, including the very large 14 July 2000 “Bastille Day” event, are shown in the accompanying figure . The cutoff latitudes shown here were measured using ~8-15 MeV/nucleon He nuclei (which have the same rigidity as ~30-60 MeV protons), and represent the geomagnetic latitude at which the rate drops to half its value over the poles. For comparison, also shown in blue (on the right-hand scale) is the geomagnetic activity index Dst during these time periods.

Our previous observations of large SEP events have shown that the cutoff location is highly variable, changing by more than 7 degrees in less than one day during the August 1998 event, for example, and that the variability is generally well-correlated with changes in Dst. The Bastille Day event, however, was remarkable and unexpected in several respects. Not only was the cutoff suppression extremely large, with a swing of ~12 degrees during the corresponding -300 nT Dst geomagnetic storm, but the cutoff also recovered far more rapidly than Dst. The cause for this difference is not entirely clear. Perhaps the cutoff really tracks only one aspect of the changing geomagnetic field represented by Dst, as suggested by recent detailed Dst modeling by Temerin and others. However, it is not obvious why this appears not to be true for the other events in the figure.

One consequence of these cutoff suppressions is an increase in radiation exposure at the Space Station. Calculations indicate that a mere 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, while if a 15 degree suppression were to occur, the radiation exposure increases by more than a factor of 8 over what it would have been in the same event with the cutoff at its nominal location. Although the ability to predict Dst is improving, examples such as the Bastille Day event suggest that Dst may be a poor proxy for actual cutoff measurements. While in this case the quicker recovery of the cutoff meant that the radiation environment was less hazardous than expected from Dst, there are times when the reverse is true. On several occasions, such as in November 1992, we have seen the cutoff suppression occur up to several hours before the sudden onset of a geomagnetic storm. Further observations of the cutoff variability during large SEP events in the declining phase of this solar cycle should help to better establish the relationship between the cutoff and geomagnetic indices such as Dst.

Contributed by Rick Leske, Caltech

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