Preliminary investigation of the structure of the upper ionosphere as observed by the topside sounder satellite, Alouette

Abstract
Results are presented which illustrate the shape of the topside electron distributions at different local times and different latitudes. It is shown that the ionosphere appears to cool and contract during the night. The morphology and diurnal variation of the geomagnetic equatorial anomaly has been investigated; the structure of the equatorial topside ionosphere appears to be determined by the Earth’s magnetic field, and the geomagnetic anomaly is shown to exist in the topside only between 10.00 and 22.00 local time. Deductions about the composition or temperature of the atmosphere are shown to be complicated because diffusion occurs along field lines only and also because the temperature increases with height above ground. The scale height of the electron density distributions is found to increase with latitude, and it is shown that this effect is probably related to an increase of electron temperature with latitude. Five different localized phenomena have been observed in the ionosphere by the topside sounder, and three of these are shown to occur at the latitudes at which the magnetic field lines which pass through the hearts of the three radiation belts enter the Earth’s atmosphere. Other results are presented which also constitute important evidence that particles dumped from the radiation belts may be important sources of heat or ionization in the atmosphere. Observations made at sunrise and also during an eclipse indicate that the effects of electromagnetic movements and/or movements produced by temperature changes are very important in the upper ionosphere.

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