Indications for diffusive coronal shock acceleration of protons in selected solar cosmic ray events

Four solar cosmic ray events which occurred on November 22, 1977, May 7, 1978, June 3, 1982, and February 16, 1984, have been analyzed using data from near‐Earth spacecraft and ground‐based neutron monitors to study the acceleration of solar protons. For May 7, 1978, and February 16, 1984, the scattering mean free path of the protons in the interplanetary medium between the Sun and the Earth was greater than or equal to 2 AU. Therefore, direct information was obtained about the acceleration of the flare particles at the Sun and their release into the interplanetary magnetic field. We found that the intensity‐time profiles observed at Earth for more than an hour after onsets for the May 7, 1978, and February 16, 1984, solar cosmic ray events represent the solar injection profiles even to energies as low as 20‐MeV. The energy dependence of the onset times for the May 1978, June 1982, and February 1984 events indicates that the first low energy protons (∼50‐MeV) were accelerated at least 5 min earlier than the first relativistic protons. The duration of the injection of the solar protons into the interplanetary medium (or acceleration) depended upon energy for the November 1977, May 1978, and February 1984 events, ranging from about 40 min at 2‐MeV to about 500 min at 10‐MeV and was proportional to . The deduced solar proton energy spectra at the Earth for these latter three events are similar. The energy spectra extrapolated back to the Sun for the anisotropic phases of the solar cosmic ray events on May 7, 1978, and February 16, 1984, exhibit time changes in both amplitude and shape. The energy spectra at the Sun also indicate that the first low energy particles were accelerated earlier than the first relativistic protons. The time from onset to maximum intensity was proportional to for the May 1978 and February 1984 events. The measured onset times of the solar protons at the Earth for these latter two events were compared with the observed onset times of the solar electromagnetic emissions and with predictions of the diffusive shock acceleration model proposed by Lee and Ryan [1986]. The various experimental and theoretical data are consistent with each other. The energy dependence of the duration of the acceleration and of the time from onset to maximum intensity was used to find that the diffusion coefficient of the solar protons in the solar corona was proportional to the size of the disturbed region of the shock and the radial distance of the shock from the Sun. The proton energy spectrum at 0945–1000 UT on February 16, 1984, at the Earth agrees well with the diffusive coronal shock acceleration model of Ellison and Ramaty [1985]. We, therefore, conclude that for the solar cosmic ray events on November 22, 1977, May 7, 1978, June 3, 1982, and February 16, 1984, there are strong indications for shock acceleration of the solar protons. The presence of large scale coronal shocks was directly observed in the May 1978 and February 1984 solar cosmic ray events. However, by the present analysis we cannot determine whether or not stochastic acceleration of the protons in the downstream region of the shock took place in addition to the diffusive coronal shock acceleration.