Two classes of solar energetic particle events associated with impulsive and long-duration soft X-ray flares

Abstract

For the period 1978 September to 1983 December we have identified 67 solar particle events for which our instruments detected electrons above 3 MeV and for which there are soft X-ray observations. On the basis of their signature in soft X-rays, we have divided the events into two classes, impulsive and long duration, and find that they have different properties. The events originating with impulsive flares are associated with intense meter-wavelength type III bursts with associated type V continuum. The impulsive events are nearly all well connected, have high electron-to-proton ratios, and are almost never associated with interplanetary shocks. In contrast, the events associated with long-duration flares can originate anywhere on the solar disk, extend to much higher proton energies, and are well associated with coronal and interplanetary shocks. For about half of the long-duration events, the associated meter-wavelength events do not include type III bursts. We deduce that the two classes of particle events arise because different acceleration processes operate preferentially for the two classes of flares. In impulsive flares the acceleration is rapid. In some energetic events relativistic electrons are accelerated along with the lower energy electrons that are responsible for type III emission and other impulsive phenomena. The reported correlation of so-called electron-rich events with y-ray events is consistent with this suggestion. In long-duration flares the acceleration is more gradual, and we assume that acceleration takes place at extended shocks. These differences are attributed to the fact that impulsive flares occur low in the corona, are compact, and have high-energy densities, whereas long-duration flares occur high in the corona in extended regions. We find that occasionally a single flare event exhibits strong impulsive acceleration and also generates strong coronal and interplanetary shocks with their associated large fluxes of protons.