Electrons and Protons in Solar Energetic Particle Events

Abstract

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 ~102 to 2×104 and (2) a well-defined branch spanning peak proton intensities from ~3 to 104 protons cm-2 s-1 sr-1 with e/p ratios ranging from ~101 to 2×102. 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.