Solar-Particle Energy Spectra during the Large Events of

We present observations of the energy spectra and timing of some of the largest solar particle events of solar cycle 23, including the events of October-November 2003 and the January 20, 2005 event, using data from ACE, SAMPEX, and GOES. These spectra are all well fit by a double power-law form. The January 20 event had the hardest spectrum observed during solar cycle 23 and also reached the highest>100 MeV intensity level in ~30 years, reaching its peak intensity at 1 AU within minutes, almost without warning. The timing and energy spectrum of this event are consistent with RHESSI solar γ-ray data due to particles accelerated in the flare. On the other hand, if this event was accelerated by a CME-driven shock, it must have formed low in the corona and accelerated particles to GeV energies within minutes. This, the January 20 event provides several new challenges to models of SEP events at the Sun and in the heliosphere. 1