Date of Award

Fall 2010

Project Type


Program or Major


Degree Name

Doctor of Philosophy

First Advisor

James M Ryan


The Sun is capable of accelerating particles up to high energies, the highest of which (>1 GeV) are detectable at the Earth's surface. Although the accelerating source is tied to the solar magnetic field, the physical process responsible 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 originally designed as a very-high-energy gamma-ray detector, but was also sensitive to cosmic rays and high-energy solar particles. Milagro registered ground-level enhancements for the 2001 April 15 and 2005 January 20 solar events during its operation from January 2000 to March 2008, the latter of which was the most intense in 50 years. The multiple data channels in Milagro, combined with complementary neutron monitor data, enabled for a detailed analysis of the energetic proton spectra even during the brief anisotropic phase of the 2005 January 20 event.

For both events the results for the proton spectra are consistent with a coronal shock origin. A timing analysis for the controversial 2005 January 20 event reveals that the prompt arrival of the relativistic protons and rapid evolution of the observed profiles measured at ground-level stations can be attributed to a coronal shock origin and do not require an additional or different process, i.e., direct solar-flare acceleration.