Observation of GeV Solar Energetic Particles from the 1997 November 6 Event Using Milagrito

Abstract

Milagrito was an extensive air-shower observatory that served as a prototype for the larger Milagro instrument. Milagrito operated from 1997 February to 1998 May. Although it was designed as a very high energy (few hundred GeV threshold) water-Cerenkov gamma-ray observatory, it could also be used to study solar energetic particles (SEPs). By recording sealer data, which correspond to photomultiplier tube singles rates, it was sensitive to muons and small showers from hadronic primary particles with rigidities above ∼4 GV. Milagrito simultaneously recorded air-shower trigger data of primary particles with energies greater than ∼100 GeV that provide the data to help reconstruct event directions. The Milagrito sealers registered a ground-level enhancement associated with the 1997 November 6 SEP event and X9 solar flare. At its peak, the enhancement was 22 times the background rms fluctuations. Based on comparisons to neutron monitor and satellite data, we find evidence that the rigidity power-law spectrum for the differential flux of energetic protons steepened above ∼4 GV and that the acceleration site was high in the corona (at ∼3 R⊙above the photosphere), assuming that a CME-driven shock was responsible for the ground-level enhancement.