Multi-spacecraft Observations of 3He-rich Solar Energetic Particle Events

Solar energetic particle events with large abundance enhancements of 3He relative to 4He have been extensively studied since the 1970s using in situ observations from near-Earth spacecraft. These events, in which the compositional fractionation is believed to be the result of resonant wave-particle interactions, are thought to occur at the site of a solar flare when magnetic reconnection leads to the dissipation of a large amount of energy in plasma waves and the release of some previously-trapped particles onto open field lines. The very localized acceleration implied in such a model should result in energetic particle events that extend over relatively narrow range of heliolongitudes when observed near 1 AU. Several studies supporting this view have reported longitude distributions covering several tens of degrees for solar flares associated with 3He-rich events observed with a single, near-Earth spacecraft. The twin STEREO spacecraft, launched in October 2006 and moving away from Earth at 22.5 degrees per year, allow observations of individual 3He-rich events from two separated vantage points. We report observations of He isotopes in the MeV energy range from the first year of operation of the Low Energy Telescope (LET) instruments on STEREO-A and -B and compare with data from the ACE/SIS instrument, situated midway between the two STEREO spacecraft. This work was supported by NASA under grants NAS5-12929 and NAS5-03131.