Mean diurnal variation of the topside ionosphere at mid-latitudes

About 500 ionograms obtained with the Alouette satellite have been used to construct the mean diurnal variation of the mid-latitude topside ionosphere. Since this diurnal variation is based on observations from October to December 1962, quasi-seasonal effects had to be removed. The data are presented in the form of electron-density contours at fixed altitudes for two latitude ranges, 35°N−40°N and 40°N−45°N. These ranges, which are approximately separated at the 75°W meridian by the 70° magnetic dip line, show remarkable differences in the detailed behavior of electron density as a function of time and altitude. From the scale heights of these mean distributions it is apparent that there is absence of thermal equilibrium, at least during the day, and that light ionic constituents become important in the altitude region from 500 to 1000 km, especially during the night. There is also the suggestion of latitude gradients in the electron temperature and mean ionic mass.