The Class of Type III-L Solar Radio Bursts and Their Associations with Solar Energetic Proton Events

Abstract

The source protons of Solar Energetic particle Proton events (defined as “SEP” events for this research) not associated with the Coronal Mass Ejection (CME) shock front are thought to come from either the flare site or the reconnection region beneath the CME. The Type III-L, a new class of solar radio burst has been defined by Cane et al. (2002) and MacDowall et al. (2003) as a sub–set of the Type III burst, beginning after the onset of the soft X–ray (SXR) flare, is long lasting and extends down to at least 1 MHz. The emission source region of Type III-Ls is believed to be at the reconnection region beneath the CME or on the flanks of the CME. Past association studies between SEP events and Type III-Ls began with a biased SEP–selected sample set to see if there can be found support for the emission source region of Type III-Ls and SEPs to come from the same accelerator site at the reconnection region beneath the CME. Unlike previous studies using an SEP–selected sample, I find that when using a radio–selected sample for well–connected SEP events with a solar source in the western hemisphere, the majority of the Type III-L events are associated with SEP events, but not all, and that Type III-L events associated with M– and X– class SXR flares, do not appear to be better predictors of SEP events than do Type II bursts which are associated with the CME shock. Also, I find that the occurrence of Type II events in the radio spectra of SEPs is just as common as the occurrence of Type III-Ls. This indicates that Type III-Ls should not be used as a predictor for SEP events, that the emission source region of Type III-Ls might not be at the reconnection region beneath the CME and reduces the strength of the support found by previous SEP – Type III-L association studies, that the source protons for SEP events necessarily come from the reconnection region beneath the CME. I found that Type III-L events have no strong longitude preference, but SEP events do have a 60% preference between W30 and W90 solar longitude. New data from new long wavelength arrays will help with position mapping the emission source regions of Type III-L bursts. An investigation was done on the internal structure of Type III-Ls. An implication of the result that the separation between components of the Type III-L burst was found to be longer than the separation between the components of Impulsive–Phase Type IIIs (defined as “Imp-Type IIIs”), is that the duration of the Type III-L components appears to be longer than those of Imp-Type IIIs. The result that the components of the Imp-Type IIIs have a faster frequency–time drift–rate than those of the Type III-Ls, shows that the source electrons for the Imp-Type IIIs appear to have a faster source emission velocity than do Type III-Ls. This is understandable as the source electrons for the Imp-Type IIIs are thought to come directly from the active region flare site, whereas the source electrons for Type III-Ls have a longer path along the neutral current sheet to either the reconnection region beneath the CME or up to the flanks of the CME. This gives us a reason as to why the Type III-L emission is delayed in respect to the Imp-Type IIIs. With their energy decreased, these Type III-L source electrons would form emission with a source velocity and frequency–time drift–rate slower than that of the Imp-Type IIIs. Data with better time and frequency resolution should help determine if there are additional weaker Type III-L components. Timing studies between SXR flares, the expansion of CMEs and the evolution of Type III-L components, should determine if electron accelerator sites for the Type III-L components are at a reconnection region beneath the CME or on the flanks of the CME.