Muon excess at sea level from solar flares in association with the Fermi GBM spacecraft detector

Abstract

This paper presents results of an ongoing survey on the association between muon excesses at ground level, registered by the Tupi telescopes, and transient solar events, whose gamma-ray and x-ray emissions were reported by the Fermi Gamma Burst Monitor and the Geostationary Operational Environmental Satellite 14, respectively. We show that solar flares of small scale, those with prompt x-ray emission classified by the Geostationary Operational Environmental Satellite as C-Class with power ${10}^{\ensuremath{-}6}$ to ${10}^{\ensuremath{-}5}\text{ }\text{ }\mathrm{Watts}\text{ }{\mathrm{m}}^{\ensuremath{-}2}$ at 1 AU, may give rise to muon excess probably associated with solar protons and ions emitted by the flare and arriving at the Earth as a coherent particle pulse. The Tupi telescopes are within the central region of the South Atlantic Anomaly, where the geomagnetic field intensity is the lowest on the Earth. Here we argue for the possibility of a ``scale-free'' power-law energy spectrum of particles accelerated by solar flares. For energies around and exceeding the pion production, large and small scale flares have the same power-law energy spectrum. The difference is only in the intensity. The Tupi events give support to this conjecture.