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Published August 25, 2008 | Published
Book Section - Chapter Open

Examination of the last large solar energetic particle events of solar cycle 23


The last two large solar energetic particle (SEP) events of solar cycle 23 were observed in December 2006 by several spacecraft including ACE and STEREO. Active region number 10930 rotated over the eastern limb of the Sun already generating intense x-ray flares. As it crossed the disk, it produced 4 X-class flares and at least 3 halo coronal mass ejections. The two dominant SEP events occurred when the region was at ~E70 and ~W25. We have combined particle 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 each event. Energy spectra for many heavy ion species integrated over the duration of each SEP event show distinct differences between the two events. We find the second event (on December 13) has a much harder spectrum above 10 MeV/nucleon and a 12-60 MeV/nucleon composition substantially enriched in elements with Z>14 as compared to the first event (on December 6). While the December 6 event is similar in Fe/O to other events with comparable fluence in solar cycle 23, the December 13 event has the highest Fe/O ratio of all events with Si fluence > 100 (cm^2 sr MeV/n)^-1. In composition, this second event is most similar to the event of November 6, 1997.

Additional Information

© 2008 American Institute of Physics. This work was supported by NASA under grants NAG5-12929 and NNX06AC21G, and by NSF under grant ATM-0454428.

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