Implications of Ionic Charge State Measurements on Particle Acceleration in the 20 January 2005 Solar Particle Event

The 20 January 2005 solar event was a large solar energetic particle event with particle spectra comparable to the hardest events of solar cycle 23, with proton spectra extending to over 1 GeV. The event was also characterized in part by a large x-ray flare followed within minutes by a rapid rise in particle intensity. The rapid arrival of particles from the event made observations of the CME difficult, with only one clean SOHO/LASCO image of the CME available. Protons arrived at Earth at 6:49 UT, and the single available LASCO image places the CME shock at ~3-3.5 solar radii above the solar surface at 6:54 UT. Estimates of the shock front velocity vary from ~2500 km/s to ~3500 km/s at this height, implying that the shock was at ~1.5-1.8 solar radii at the time the particles were emitted. Measurements of low mean ionic charge states imply coronal source material for particles in this event. If these particles were accelerated that close to the sun, they would have to have been accelerated and released within a very few minutes in order to avoid further stripping and higher charge states. We will discuss limits imposed on the acceleration times by the ionic charge state measurements and by stripping during acceleration and transport.