Thermal and suprathermal plasma densities in the outer magnetosphere

By using the low‐frequency cutoff of electromagnetic noise trapped in the magnetosphere at frequencies above the local plasma frequency it is now possible to make very accurate, ±1%, electron density measurements in the low density region between the magnetopause and plasmapause. This technique for measuring the total plasma density has been used, together with measurements of the suprathermal proton intensities with the Lepedea instrumentation on the Imp 6 spacecraft, to determine the thermal proton densities in the region between the plasmapause and magnetopause. Although the thermal protons usually account for a significant fraction, ∼50%, of the total proton density in this region, in some cases, particularly at the larger radial distances, the density of the thermal protons sometimes drops to a very small fraction, <5%, of the total density, and nearly all of the plasma consists of suprathermal particles. Usually, the high‐energy tail of the thermal proton distribution is sufficiently intense to be detected at low energies within the Lepedea energy range. When the high‐energy tail of the thermal distribution can be detected, it is possible to determine the temperature of the thermal protons. The temperature in a typical case is found to be 80,000°K. This high temperature for the thermal protons, which are presumably of ionospheric origin, agrees with earlier Ogo 5 measurements reported by Serbu and Maier.