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The Relativistic Solar Proton Event of 11 June 1991
PHILLIPS LAB HANSCOM AFB MA
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The X1238 solar flare that occurred at heliographic coordinates N31, Wl7 in NOAA region 6659 on 11 June 1991 commencing at 01 56 UT was the source of a number of energetic phenomena including intense X-ray and gamma ray emission and the acceleration of ions to relativistic energies. The small associated ground-level enhancement GLE of about 6 hours duration was mildly anisotropic even though a geomagnetic storm was in progress. The relative increases at sea level atmospheric pressure ranged between approximately 3 and approximately 7 at high latitude neutron monitors. The largest increases were observed by neutron monitors with an asymptotic cone of acceptance viewing into the probable IMF direction. We find a differential rigidity spectrum with a slope of -5.5 provides a satisfactory fit to the observed neutron monitor increases at the GLE maximum.
APPROVED FOR PUBLIC RELEASE