Solar Energetic Particle ^3He-Rich Events from the Nearly Quiet Sun in 2007-2008
We have used Advanced Composition Explorer instruments to survey the period 2007 March through the end of 2008 for ^3He-rich solar energetic particle (SEP) events occurring during near solar minimum conditions. Four events were found, all associated with single solar active regions in the western hemisphere. They all show ^3He:^4He ratios of a few percent (i.e.,≳100 times solar system abundances), low intensities, and spectra extending up to at least 1 MeV nucleon^(–1). Two events, on 2008 February 4 and 2008 June 16, were devoid of signatures associated with ^3He-rich SEPs, namely they lacked associations with energetic electrons and type-III bursts. In addition, there were no clear coronal mass ejections and X-ray flare activity was very low or absent. We take this as evidence that the irreducible requirement for ^3He-rich SEPs is a western hemisphere solar active region where magnetic and plasma processes preferentially energize ^3He and heavy ions.
Additional Information© 2009. The American Astronomical Society. Received 2009 April 27; accepted 2009 June 18; published 2009 July 2. We gratefully acknowledge the STEREO Science Center, SOHO CDAW data center, ACE Science Center, and NOAA/ SWPC archives for data used in this study. We thank A. Vourlidas for comments on CME activity. This work was supported by NASA grants NNX07AP69G and NNG06GC21 at Johns Hopkins University/Applied Physics Laboratory, NASA Grant NNX8AI11G and NSF grant ATM-0454428 at the California Institute of Technology and the Jet Propulsion Laboratory, and NASA grants NNG05GD68G and NNX07AN13G at the Lockheed Martin Solar and Astrophysics Laboratory.
Published - Mason2009p5112Astrophys_J_Lett.pdf