PARTICLE INJECTION FOLLOWING SOLAR-FLARES ON 1980 MAY 28 AND JUNE 8 - EVIDENCE FOR DIFFERENT INJECTION TIME HISTORIES IN IMPULSIVE AND GRADUAL EVENTS

We will discuss the injection time histories of one solar particle event on 1980 June 8 and a group of four homologous events on 1980 May 28 and compare them with the electromagnetic radiation emitted by the parent flares. Because of small coronal and interplanetary distances as well as relatively weak interplanetary scattering, in all five events the intensity-time profiles observed in interplanetary space are good approximations of the solar injection time history simply shifted by the rectilinear travel time along the smooth interplanetary magnetic field spiral. Comparison with electromagnetic radiation in all events shows electron injection to be nearly simultaneous with the particle acceleration as indicated by the increase in electromagnetic radiation. In contrast, the proton injection differs from one event to the other. In the June 8 event it is delayed by about 12 minutes with respect to the electron injection, occurring simultaneously with a second increase in electromagnetic radiation. This injection time history seems to be an example for particle acceleration occurring in two phases, as introduced by Wild et al. In the May 28 events not only the electrons, but also the nuclei, are injected simultaneously with the onset of the electromagnetic radiation. These observations are in agreement with a rapid acceleration of ions, as inferred from the observations of gamma-flares. Taking into account the classification of solar flares in impulsive and gradual or long-duration (LDE) events, we tentatively suggest an acceleration model in which both classes of events start with an initial energy release, called impulsive phase, and develop then in a different manner. In the long-duration event presented here a gradual phase accelerating relativistic particles by a coronal shock follows about 10 minutes after the initial energy release. In the impulsive events an intermediate phase or a second step in the impulsive phase occurs immediately after the impulsive phase accelerating electrons to relativistic energies and nuclei up to energies of some tens of MeV per nucleon on time scales of 1 s.