The solar proton event of April 16, 1970, 2. Transformation of proton pitch angle distributions from the solar wind into the magnetosheath

During the event of April 16, 1970, energetic (∼0.7 MeV) protons were observed by Vela 6B in the solar wind and by Vela 5B in the magnetosheath. During one 4-hour period the omnidirectional flux measured by 5B was a factor of 3 larger than that measured by 6B. The pitch angle distribution observed at this time by 6B was characterized by a strong peak at 0° pitch angle. Transformation of this distribution according to the Liouville equation accounts for the enhanced omnidirectional flux and the shape of the pitch angle distributions observed at 5B. At a later time in the event, distorted pitch angle distributions, which were observed in the magnetosheath, are accounted for by a Liouville transformation of an assumed unidirectional distribution in interplanetary space. We conclude that particle entry into the magnetosheath was approximately adiabatic.