What is Unusual About the 2005 January 20 SEP Event?

We report on the solar energetic particle (SEP) event of 2005 January 20 that had an associated ground level enhancement (GLE). The Solar and Heliospheric Observatory (SOHO) spacecraft observed a CME, which was among the fastest of cycle 23 CMEs. This event is consistent with the results that the GLE-associated CMEs represent the fastest known population of CMEs. A metric type II burst started before the proton injection time, suggesting that a coronal shock was formed before high-energy protons were released at the Sun. These 2005 January 20 event is consistent with the current paradigm that large SEP events originate in CME-driven shocks. We also determined the height of the CME at two time marks: the metric type II onset (2.1 Rs) and the proton injection time (4.5 Rs). At a height of 4.5 Rs the CME should have attained the maximum speed, thus driving the strongest shocks. We discuss the estimation of the CME speed given that the coronagraph observations were hampered by the SEPs arriving at the SOHO spacecraft. We also discuss the arrival of the CME-associated plasma and shock at 1 AU. Work supported by NASA/LWS and NSF/SHINE programs.