Solar energetic particle event onsets: Far backside solar sources and the east-west hemispheric asymmetry

Abstract

Prompt onsets and short rise times to peak intensities Ip have been noted in a few solar energetic (E > 10 MeV) particle (SEP) events from far behind the west limb. We discuss 15 archival and recent examples of these prompt events, giving their source longitudes, onset and rise times, and associated CME speeds. Their timescales and CME properties are not exceptional in comparison to a larger set of SEP events from behind the west limb. A further statistical comparison of observed timescales of SEP events from behind the west limb with events similarly poorly magnetically connected to the eastern hemisphere shows the longer timescales of the latter group. We interpret this result in terms of a difference between SEP production at parallel shocks on the east flanks of west backside events and at perpendicular shocks on the west flanks of eastern hemisphere events.