History of research on solar energetic particle (SEP) events: the evolving paradigm

Abstract

AbstractForbush initiated research on solar energetic particle (SEP) events in 1946 when he reported ionization chamber observations of the first three ground level events (GLEs). The next key development was the neutron monitor observation of the GLE of 23 February 1956. Meyer, Parker and Simpson attributed this high-energy SEP event to a short time-scale process associated with a solar flare and ascribed the much longer duration of the particle event to scattering in the interplanetary medium. Thus “flare particle” acceleration became the initial paradigm for SEP acceleration at the Sun. A more fully-developed picture was presented by the Australian radio astronomers Wild, Smerd, and Weiss in 1963. They identified two distinct SEP acceleration processes in flares: (1) the first phase accelerated primarily ~100 keV electrons that gave rise to fast-drift type III emission as they streamed outward through the solar atmosphere; (2) the second phase was produced by an outward moving (~1000 km s−1) magnetohydrodynamic shock, occurring in certain (generally larger) flares. The second phase, manifested by slow-drift metric type II emission, appeared to be required for substantial acceleration of protons and higher-energy electrons. This two-stage (or two-class) picture gained acceptance during the 1980s as composition and charge state measurements strengthened the evidence for two distinct types of particle events which were termed impulsive (attributed to flare-resident acceleration process(es)) and gradual (shock-associated). Reames championed the two-class picture and it is the commonly accepted paradigm today. A key error made in the establishment of this paradigm was revealed in the late 1990s by observations of SEP composition and charge states at higher energies (>10 MeV) than previously available. Specifically, some large and therefore presumably “gradual” SEP events looked “impulsive” at these energies. One group of researchers attributes these unusual events to acceleration of high-energy SEPs by flares and another school favors acceleration of flare seed particles by quasi-perpendicular shocks. A revised SEP classification scheme is proposed to accommodate the new observations and to include ideas on geometry and seed particle composition recently incorporated into models of shock acceleration of SEPs.