Observations from Ogo 5 of the thermal ion density and temperature within the magnetosphere

The Ogo 5 retarding potential analyzer experiment is operated in a voltage mode designed to yield the integral spectrum of particles in the energy interval from 0 to 500 ev. Detailed results obtained with the spacecraft inside 7 R E during the first 15 orbits (March–April 1968) are presented. We can identify and group the data into three types of density profile. First, a smooth decrease in ion density from approximately 10 4 ions/cm −3 at 1.5 R E down to the experiment threshold level of ∼20 ions/cm³ at approximately 5 to 6 R E . A second general type of observed density profile is typified by considerable structure within the plasmasphere, and the plasmapause is identified by a factor of 3 or more drop in ion density. The third type of profile shows a density drop of a factor of 20 or more inside the L = 7 R E radial distance to values below our minimal detectable level of 20 cm −3 . The ion temperature observations show that within the plasmasphere the day to night temperature ratio is about a factor of 2, with the temperature ranging from a low of 5×10³°K at 1.5 R E on the night side to as high as 3×10 4 °K at 3.6 R E on the day side. At the plasmapause a sudden increase in the ion temperature is observed. This increase is generally a factor of 5 or more within 0.8 R E , and it has been used to pinpoint the plasmapause location. Beyond the plasmapause we find a region of relatively high ion temperature, T i >10 5 °K.