The solar proton event of April 16, 1970, 1. Features in interplanetary space and in the magnetosheath

A description is given of the solar particle event of April 16, 1970, as recorded by two Vela satellites which were in the vicinity of the earth's bow shock at 18 R_E. Comparison with Explorer 35 in lunar orbit indicates that the early time profile of the event was due to a spatial structure convected past the earth, and the orientation of the convected front is deduced. Anisotropies and pitch angle distributions of energetic protons are obtained from a 32-point sampling of the particle flux over the celestial sphere with a time resolution of 1 min. The net flow patterns inferred in interplanetary space reveal two reversals in the north-south component, in general alignment with the magnetic field; neither was accompanied by a magnetic field reversal. Novel features of the pitch angle distributions recorded at proton energies of ≈ 1 MeV included (1) a narrow forward peak at 0° pitch angle observed in interplanetary space and accompanied by a smaller backscatter peak (we conclude that the forward peak is a real interplanetary effect uncontaminated by the presence of the bow shock), (2) accompanying this forward peak, a difference in the omnidirectional intensities at Vela 6B and at Vela 5B, of a factor of ≈3, when these satellites were situated in the solar wind and in the magnetosheath, respectively, (3) a pronounced depression over approximately one hemisphere which was recorded by both Vela 6B and 5B when each was in the magnetosheath and which represents a distortion of the interplanetary pitch angle distribution due to propagation of particles into the magnetosheath.