Electrons and Protons in Solar Energetic Particle Events

A plot of 0.5 MeV peak electron intensity versus >10 MeV peak proton intensity for well-connected solar energetic particle (SEP) events from 1997 to 2003 reveals two distinct populations: (1) a group of events with peak proton intensities <3 protons cm -2 s -1 sr -1 that have electron-to-proton ( e / p ) ratios ranging from ~10 2 to 2 × 10 4 and (2) a well-defined branch spanning peak proton intensities from ~3 to 10 4 protons cm -2 s -1 sr -1 with e / p ratios ranging from ~10 1 to 2 × 10 2 . Events with strong abundance enhancements of trans-Fe elements form a prominent subset of "population 1" and are absent from "population 2." For a sample of poorly connected SEP events, population 1 largely disappears, and population 2 is observed to extend down to low (<10 -1 protons cm -2 s -1 sr -1 ) proton intensities. Plots of 0.5 MeV peak electron intensity versus >30 MeV peak proton intensity yield comparable results. The SEP events in population 2 are highly (~90%) associated with dekametric/hectometric (DH) type II bursts versus only a ~20% association rate for population 1 events. Population 2 events have flatter electron (0.5-4.4 MeV) and proton spectra (10-30 MeV) than those in population 1. Based on their high e / p ratios, trans-Fe enhancements, poor association with DH type IIs, and inferred small "emission cones," population 1 events are attributed to acceleration in solar flares. For population 2 events, evidence for a dominant shock process includes their flatter spectra, apparent widespread sources, and high association with DH type II bursts.