Classification of solar energetic particles

Abstract

While it is now common to refer to a solar event as “impulsive” or “gradual,” the classification of the event can be difficult in practice and does not completely clarify the characteristics of solar energetic particles. In particular, a single event can produce multiple particle populations with distinct physical properties. We therefore propose to classify solar energetic particle populations of a given species, energy range, and time period in terms of their origin and final acceleration sites, i.e., in terms of their acceleration history. The former can be inferred from low-energy ionic charge states, indicating a solar energetic particle origin in the solar wind ( w), the open field line corona ( c), or a hot flare loop ( f), respectively. Presumably the site of origin is also the site of initial stochastic ( f) or coronal mass ejection-driven shock ( c or w) acceleration. The other dimension of our classification scheme is the final site of (substantial) acceleration, also indicated by f, c, or w. This can frequently be determined by a qualitative examination of the time-intensity or time-energy profile. We demonstrate this classification scheme using publicly available heavy ion data from the Advanced Composition Explorer mission. Distinct types of SEP populations can be identified: ff (impulsive), and cc, cw, and ww (gradual). Thus, when the usual classification refers to a gradual event, this fails to distinguish between particle populations of three distinct acceleration histories. By focusing on observational indicators of the acceleration history, this solar energetic particle classification scheme can help to identify outstanding physics questions and facilitate the interpretation of solar energetic particle abundances and spectra.