Comparing observations and expectations of SEP composition in the two December 2006 events

Abstract

In early December 2006, a large active region (number 10930) rotated over the eastern limb of the Sun. As it crossed the disk, it produced 4 X-class flares and at least 3 halo coronal mass ejections. Two large SEP events were generated when the region was at ~E70 and ~W25 and were observed by several spacecraft, including ACE and STEREO. We have combined observations from the Solar Isotope Spectrometer (SIS) and the Ultra-Low Energy Isotope Spectrometer (ULEIS) on ACE and the Low Energy Telescope (LET) on STEREO for both SEP events. By integrating the SEP data over the duration of each event, the energy sp ectra for many heavy ion species (including Mg, Si, and Ca) can be analyzed in detail and the elemental SEP composition can be compared for the two events. We find significant differences in the composition of the two events, with the second event having a high Fe/O ratio, similar to that of impulsive SEP events. Both events are examined in terms of expectations from two prevailing models, but neither model completely predicts the observations.