Solar energetic particle 3He-rich events from the nearly quiet sun in 2007-2008

Abstract

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. © 2009. The American Astronomical Society.