Measurements of the 2005 January 20 GLE with the Milagro Water Čerenkov Detector

The Sun is capable of accelerating particles to high energies, the highest of which are detectable at the Earth’s surface. The physical process responsible for the acceleration continues to be debated. Essential information about the location and nature of the acceleration mechanism can be obtained from measurements of the highest-energy particles. The Milagro instrument was designed as a TeV γ-ray telescope, but it was also sensitive to cosmic rays and high-energy solar particles. Milagro operated from January 2000 to March 2008. It registered a strong ground-level enhancement (GLE) for the 2005 January 20 solar event. The multiple data channels in Milagro, combined with complementary neutron monitor data, enables a detailed analysis of the energetic proton spectra even during the brief anisotropic phase of the GLE. The results for the time-resolved proton spectrum are consistent with a coronal shock origin. A timing analysis reveals that the prompt arrival of the relativistic protons and the rapid evolution of the observed profiles measured at ground-level stations can be attributed to a coronal shock origin and does not require an additional or different process, i.e., direct solar-flare acceleration.