Observations of precipitating protons with ring current energies

Precipitating protons with energies characteristic of the storm time ring current were measured during all phases of magnetic activity near the low-altitude trapping boundary at midnight. The objective of this study was to examine the energy dependence of the precipitating fluxes as a function of latitude (or L shell) and in relation to the plasmapause and the low-energy isotropic electron enhancements that are associated with the outer zone boundary. The low-energy measurements taken by the OV1-17 satellite covered the time period from March 19 to April 3, 1969. Data provided by the OV1-19 spacecraft during the same time period were used to extend the measurements to higher energies and to examine pitch angle distributions. Protons from 12.4 to 500 keV were observed to precipitate outside the plasmapause. The higher-energy fluxes peaked equatorward of the low-energy maximum intensities. Statistically, the 55-keV proton boundary tracks the midnight plasmapause with an average poleward displacement of 0.8L. From these and other related observations we conclude that ring current protons are in strong pitch angle diffusion on closed field lines between the midnight plasmasphere and the electron trapping boundary. These precipitating fluxes are probably responsible for H arcs, which are found at the low-latitude termination of the visual auroral forms during magnetic storms. In addition, we estimate that less than 1% of the total ring current energy is lost by direct particle precipitation.