Observations of ring current protons at low altitudes

Variable intensities of geomagnetically trapped protons with energies between 12.4 and 500 kev were observed during times encompassing the magnetic storms on March 20 and 24, 1969. These proton fluxes were measured for 1.0 < L < 1.1 near the magnetic equator at local midnight with solid state detectors and an electrostatic analyzer on the low-altitude satellite OV1-17. Marked increases of proton intensities at energies below 100 kev occurred during this magnetically disturbed period. In contrast, proton intensities with E > 200 kev were observed to be a relatively stable and permanent feature of this region of the near-earth magnetosphere. The altitude range for these measurements was 400-470 km, a region where the atmospheric density is such that charge exchange lifetimes are comparable with the bounce period and short in relation to the longitudinal drift period. The OV1-17 observations of proton intensities with E > 200 kev are in substantial agreement with similar measurements taken with the Azur satellite and reported recently by Moritz (1972). Further, the lower-energy proton intensities, measured with OV1-17 instrumentation, support Moritz's suggestion that the source of these low-altitude protons is the ring current.