The “SEP clock”: A discussion of first proton arrival times in wide-spread solar energetic particle events

Abstract

This work analyzes the appearance of wide-spread deka-MeV solar energetic proton (SEP) events, in particular the arrival of the first protons within ~ 4.5–45 MeV measured at Earth-Sun L1, and their relationship with relative solar source longitude. The definition of “wide-spread SEP event” for this study refers to events that are observed as a 25 MeV proton intensity increase at near-1 AU locations that are separated by at least 130ᵒ in solar longitude. Many of these events are seen at all three of the spacecraft, STEREO A, STEREO B, and SOHO, and may therefore extend far beyond 130ᵒ in longitude around the Sun. A large subset of these events have already been part of a study by Richardson et al. (2014). The event source region identifications draw from this study; more recent events have also been added. Our focus is on answering two specific questions: (1) What is the maximum longitude over which SEP protons show energy dispersion, i.e., a clear sign of arrival of higher-energy protons before those of lower energy? (2) What implications can be drawn from the ensemble of events observed regarding either direct magnetic connectivity to shocks and/or cross-field transport from the site of the eruption in the onset phase of the event?