All of the OGO-5 light ion density measurements (covering the period from March 1968 to May 1969) obtained from the Lockheed Light Ion Mass Spectrometer were used to determine the average global topology of the equatorial plasmasphere density distribution. The variation of the light ion equatorial density at L≤3.2 with local time was deduced by determining the average density observed within one hour of a specific local time and within 0.1 of a given L coordinate. The average H+density showed a semidiurnal variation with peaks near noon and midnight. The He+observations also revealed multiple peaks throughout the day but with smaller amplitudes than those of H+. At L>3.2 plasma trough conditions increase the scatter of densities. The average variation of the H+density with L within the plasmasphere is found to be steepest near midnight and can be least squares fitted equally well to either an exponential variation exp (-bL) where b is between 0.85 and 1.5 or to a power law L-awhere a varies from 3.2 to 5. © 1976.
Chen, A. J., Grebowsky, J. M., & Marubashi, K. (1976). Diurnal variation of thermal plasma in the plasmasphere. Planetary and Space Science, 24(8), 765–769. https://doi.org/10.1016/0032-0633(76)90113-6