SOLAR-WIND HE2-BEAM DISTRIBUTIONS DOWNSTREAM FROM THE EARTHS BOW SHOCK( RING)

The electrostatic potential at the shock causes incident solar wind He 2+ and H+ distributions to slow differentially across the boundary. Th is differential slowing produces a He2+ ring-beam distribution immedia tely downstream from the shock. The velocity (with respect to the down stream H+ distribution) and angle (with respect to the downstream magn etic field) of the ring-beam distribution are determined from a simple model that uses the change in velocity experienced by the protons acr oss the shock, the shock normal, and the downstream magnetic field dir ection. This simple model is tested with data from the ISEE 1 and 2 sp acecraft. Model and observations are found to agree reasonably well fo r quasi-perpendicular shocks over a wide range of plasma beta and Mach number. The predicted angle that the ring beam makes with the magneti c field can be significantly different from the observed angle for qua si-parallel shocks. This difference is probably due to the fluctuating magnetic field direction downstream from the shock.