Spatial Evolution of 20 MeV Solar Energetic Proton Events

Abstract

The longitudinal extents of solar energetic (E > 10 MeV) particle (SEP) events in the heliosphere are a characteristic important for understanding SEP acceleration and transport as well as their space weather effects. SEP detectors on the STEREO A and B spacecraft launched in 2008, combined with those on Earth-orbiting spacecraft, have enabled recent studies of this characteristic for many events. Each SEP event distribution has been characterized by a single central longitude, width, and amplitude derived from Gaussian fits to peak intensities or fluences at each spacecraft. To capture dynamic changes of those parameters through SEP events, we apply Gaussian fits in solar-based Carrington longitude coordinates with 1 hr resolution to four selected large 20 MeV proton events. The limitations of single-Gaussian fits for very extended events is discussed. In all four examples the widths are increasing throughout the event, as expected, while the projected Gaussian centers at SEP onset start from 30° to 100° east of the associated flare, move westward, then remain stationary well east of the flare for several days before moving west as the event amplitudes decrease. Late decay phases can be characterized by eastward movements away from the flare longitudes. We introduce schematic Buffett plots to show successive snapshots of event longitudes and amplitudes.