Relativistic particle injection and interplanetary transport during the January 20, 2005 ground level enhancement

Abstract

Besides producing the largest ground level enhancement (GLE) in half a century, the relativistic solar particles detected during the event of January 20, 2005 showed some interesting temporal and directional features, including extreme anisotropy. In this paper we analyze the time evolution of cosmic ray density and anisotropy as characterized by data from the "Spaceship Earth" network of neutron monitors by using numerical solutions of the Fokker-Planck equation for particle transport. We find that a sudden change in the transport conditions during the event is needed to explain the data, and we propose that this change was caused by the solar particles themselves.