Two Classes of Solar Proton Events Derived from Onset Time Analysis

We analyze onset times for 26 solar energetic (from 30 keV to 6 MeV) proton events that exhibit clear velocity dispersion, as observed by the three-dimensional Plasma and Energetic Particles experiment on the Wind spacecraft. Assuming that the particles are injected simultaneously at all energies and travel the same path length, we find two classes of proton events: (1) for the 18 class 1 events, the derived path lengths are between 1.1 and 1.3 AU, indicating that the first arriving protons travel essentially scatter-free; and (2) the eight class 2 events show longer path lengths around 2 AU. For all proton events, the observed temporally related electron events all have electron path lengths around 1.1-1.3 AU. Relative to the electron injection time at the Sun, the protons of the first class are injected ~0.5-2 hr later. Assuming these particles are accelerated by the associated coronal mass ejection (CME) shock, the protons at all energies between 0.03 and 6 MeV appear to be accelerated (or released) simultaneously high in the corona, roughly ~1-10 R_☉ above the electrons. The pitch-angle distributions are observed to be similar for both classes of events, making it unlikely that propagation effects are responsible for the longer path length of class 2 events. The late proton onset times at 1 AU of class 2 are therefore more likely explained by a successively later solar release (or escape) of protons at successive lower energies. Assuming again acceleration at the CME shock, the release (or escape) of the protons of class 2 events appears to depend on energy and occur at a higher altitude for lower energies, with the most energetic protons possibly released simultaneously with the electrons.