Space Weather Implications of the 20 January 2005 Solar Energetic Particle Event

The solar energetic particle event of January 20, 2005 has been called, by some measures, the most intense in 15 years, with a >100 MeV proton intensity comparable to that of the October 1989 event and the largest ground-level neutron monitor enhancement in many years. Using data from five instruments on the ACE, GOES, and SAMPEX spacecraft, we have measured the energy spectra of H, He, and heavier nuclei over the energy range from ~0.1 to several hundred MeV/nucleon. During the first 24 hours, when >90% of the ions >30 MeV/nucleon arrived, the energy spectra of species from H to Fe could be characterized by power laws, with a spectral index of -2.2 extending from 1 MeV/nucleon to at least 400 MeV/nucleon in the case of protons. Another surprising aspect of this event was the speed of its onset, reaching the intensity- maximum for >100 MeV protons within ~20 minutes of the first arriving particles - at essentially the same time as the maximum of the associated x-ray flare. On the other hand, the intensities of ions <1 MeV/nuc lagged many hours behind the expected arrival times, presumably as a result of trapping by the shock. The hard, power-law energy spectra and the rapid onset of the January 20 event make it a particular challenge for efforts to provide improved forecasts of solar particle radiation storms. We compare the time history, intensity, and energy spectra of the January 20 event to the largest events of solar cycle 23 and of the space era, and discuss the radiation hazard that events such as this pose to human spaceflight and space hardware.