Low-energy (≥0.3 Mev) solar-particle observations at widely separated points (>0.1 AU) during 1967

Simultaneous observations of solar-particle events at low (<1 Mev) energies have been obtained by using intercalibrated solid-state detectors on Mariner 5, Explorer 35, and Mariner 4 during days 200 to 271, 1967. An examination of the intensity-time profiles, anisotropies, intensity ratios of protons to α particles (jp/jα), ratios of α particles to Z≥3 (M-nuclei) (jα/jM), and the proton energy spectrums from several events has led to the following results. (a) There are finite differences in particle intensity onsets following flares even when spacecraft are located close to the same interplanetary magnetic-field line. (b) When particle fluxes have reached a quasi-stationary state, identifiable features in the intensity profile can often be observed to be convected by the solar wind past widely separated (>0.1 AU) spacecraft. (c) Several flare-associated and nonflare intensity peaks are found to corotate with the sun. (d) Anisotropies are large during the onset phase of flare-associated events with the maximum intensity coming from the solar quandrant (±45° to the sun-spacecraft line) and still larger for corotating events. (e) The proton spectrum hardens during event onsets and becomes softer as the event progresses at both spacecraft. (f) The jp/jα time profile generally follows the proton profile and varies considerably during events; also jp/jα decreases as the proton spectrum hardens in accordance with Schatzman's (1963) model. (g) The jα/jM ratio is relatively less variable than the jp/jα ratio. (h) Few intensity increases can be reliably associated with specific flares on the sun. It is concluded that, in view of the observations, multi-spacecraft studies should allow the development of more sophisticated models for the understanding of low-energy solar-particle events.