Interaction of ring current ions with the magnetopause

We have analyzed phase space density observations of ring current protons (30–130 keV) during a magnetopause crossing of ISEE 1 on July 5, 1978. Plasma and magnetic field data obtained during this crossing have been reported to be consistent with quasi-steady reconnection and a reconnection line south of the satellite crossing (Sonnerup et al., 1981). We find that within the magnetosphere the phase space densities of ∼30- to ∼65-keV protons are higher for particles streaming parallel to the field (i.e., inflowing particles) than antiparallel to the field (i.e., outward flowing particles). In the magnetosheath the phase space densities parallel to the field are higher than the densities of both outward and inward flowing particles parallel to the field within the magnetosphere. The observations are discussed in terms of acceleration of the reflected and transmitted ring current protons in the magnetopause current layer.