A Comparison of the Characteristics of 3He-Rich Solar Energetic Particle Events Observed at Single and at Multiple Spacecraft
Tuesday, 16 December 2014
An important objective of NASA's STEREO mission has been to understand the mechanisms responsible for the transport of solar energetic particles (SEPs) between the solar corona and 1 AU. For 3He-rich SEP events, in which particle acceleration is believed to be associated with a relatively small magnetic-reconnection region, it had been thought that the longitudinal distribution of particles arriving at 1 AU should be relatively narrow and single-spacecraft studies had been used to infer a standard deviation in longitude of about 20 degrees. However, measurements on ACE and STEREO showed that some 3He-rich SEP events are observable over a much broader range of longitudes. Whether the longitudinal spreading of the particles in these events occurs in the solar corona or in interplanetary space has not yet been conclusively established. It was originally anticipated that the twin STEREO spacecraft would be able to investigate the longitudinal transport problem as they began separating from one another shortly after their 2006 launch. However, during the long, quiet minimum between solar cycles 23 and 24 there were hardly any 3He-rich events on which to base such studies. The longitudinal separation of the two spacecraft on the back side of the Sun is now less than 35 degrees and decreasing, making it possible to carry out the studies that could not be done in 2007-2009. Although frequent detections of individual 3He-rich events at both spacecraft have become common, there are still instances where an event is clearly observed at one spacecraft but not detected at the other. We will report results from a study in which we investigate the particle transport by determining the characteristics of events observed at both spacecraft and comparing with the corresponding characteristics of events that are observable at only one of the two. These results are expected to aid in the development of SEP transport models that can be used for predicting the characteristics of events that will be observed by the Solar Probe Plus and Solar Orbiter missions.