Observations of Solar Energetic Particles from ^3He-rich Events over a Wide Range of Heliographic Longitude

Abstract

A prevailing model for the origin of ^3He-rich solar energetic particle (SEP) events attributes particle acceleration to processes associated with the reconnection between closed magnetic field lines in an active region and neighboring open field lines. The open field from the small reconnection volume then provides a path along which accelerated particles escape into a relatively narrow range of angles in the heliosphere. The narrow width (standard deviation <20°) of the distribution of X-ray flare longitudes found to be associated with ^3He-rich SEP events detected at a single spacecraft at 1 AU supports this model. We report multispacecraft observations of individual ^3He-rich SEP events that occurred during the solar minimum time period from 2007 January through 2011 January using instrumentation carried by the two Solar Terrestrial Relations Observatory spacecraft and the Advanced Composition Explorer. We find that detections of ^3He-rich events at pairs of spacecraft are not uncommon, even when their longitudinal separation is >60°. We present the observations of the ^3He-rich event of 2010 February 7, which was detected at all three spacecraft when they spanned 136° in heliographic longitude. Measured fluences of ^3He in this event were found to have a strong dependence on longitude which is well fit by a Gaussian with standard deviation ~48° centered at the longitude that is connected to the source region by a nominal Parker spiral magnetic field. We discuss several mechanisms for distributing flare-accelerated particles over a wide range of heliographic longitudes including interplanetary diffusion perpendicular to the magnetic field, spreading of a compact cluster of open field lines between the active region and the source surface where the field becomes radial and opens out into the heliosphere, and distortion of the interplanetary field by a preceding coronal mass ejection. Statistical studies of additional ^3He-rich events detected at multiple spacecraft will be needed to establish the relative importance of the various mechanisms.

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