Unusual solar energetic proton fluxes at 1 AU within an interplanetary CME

In mid December 2006 several flares on the Sun occurred in rapid succession, spawning several CMEs and bathing the Earth in multiple solar energetic particle (SEP) events. O ne such SEP occurring on December 15th was observed at the Earth just as an interplanetary CME (ICME) from a previous flare on December 13th was transiting the Earth. Although solar wind observations during this time show typical energetic proton fluxes from the prior SEP and IP shock driven ahead of the ICME, as the ICME passes the Earth unusual energetic particle signatures are observed. Measurem ents from ACE, Wind, and STEREO show unusual proton flux variations at energies ranging from ∼3 MeV up to greater than 70 MeV. Within the Earth’s magnetosphere Polar HIST also sees unusual proton flux v ariations at energies greater than 10 MeV while crossing open field lines in the southern polar cap. However, no such variation in the energetic proton flux is observed at the GOES 10 or GOES 11 spacecraft in geosy nchronous orbit. Differential fluxes observed at GOES 12 in the 15-40 MeV energy range show some variation. However, the overall energetic particle signature within the ICME at GEO orbits remains unclear. This event illustrates the need for caution when using GEO data in detailed studies of SEP events and in interplanetary models of energetic particle transport to 1 AU