LOW-FREQUENCY TYPE III BURSTS AND SOLAR ENERGETIC PARTICLE EVENTS

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We compare the ∼1 MHz type III bursts of flares associated with samples of "impulsive" and "gradual" solar energetic particle (SEP) events from cycle 23. While large gradual SEP events had much higher > 30 MeV proton intensities, the median-integrated intensities, peak intensities, and durations of the two groups of radio bursts were comparable. Thus, the median "proton yield" (peak > 30 MeV proton intensity of an SEP event divided by its associated integrated ∼1 MHz intensity) of type III bursts associated with gradual SEP events was ∼280 times larger than that for impulsive SEP events. A similar yield difference of ∼250 was observed for 4.4 MeV electron events. Only for extrapolated electron energies ∼5 keV, corresponding to the energy of the electrons that excite type III emission, does the median yield converge to the same value for both groups of events. The time profiles of ∼1 MHz bursts associated with impulsive SEP events are characteristically shorter and simpler than those associated with the gradual SEP events, reflecting the development of the second stage of radio emission in large eruptive flares. The gradual SEP events were highly associated (96%) with decametric-hectometric (DH) type II bursts versus only a 5% association rate for the impulsive events. Large favorably located ∼1 MHz type III bursts with associated DH type IIs had an ∼60% association rate with large (≥ 1 pfu) > 30 MeV SEP events versus ∼5% for ∼1 MHz bursts without accompanying DH II emission. These results are interpreted in terms of two distinct types of particle acceleration at the Sun, a flare-resident process that produces relatively few > 30 MeV protons and ∼4 MeV electrons in space and a shock process that dominates the large gradual proton events.