Variations of plasma scale height in the upper ionosphere during the sunrise period

Abstract

Alouette 1 topside soundings have been used to study predawn variations of plasma scale height for different seasons and altitudes. Plots of average plasma scale height against solar zenith angle show a difference in behavior for altitudes above and below 550 km, for solar zenith angles greater than 95°. Above 550 km there is an initial increase in plasma scale height that occurs at progressively higher solar zenith angles as the season changes from summer through equinox to winter. Subsequently, as the solar zenith angle changes from 105° to 95°, the plasma scale height decreases to its lowest value of the day. Below 550 km the plasma scale height is initially constant and then steadily increases during the period between solar zenith angles 105° and 95°. For solar zenith angles less than 95°, the behavior is of two types, either a sharp peak in the plasma scale height values, which is most pronounced at high altitudes, or fairly constant scale height values with a small altitude gradient in these values. It is shown that for the midnight and midday conditions the seasonal and altitude variations in plasma scale height can be explained in terms of the values of plasma temperatures, temperature gradients, and mean ionic masses. The way in which the values of the three quantities change during the sunrise period is studied, and it is seen that these changes can account for the plasma scale height variations during this time. Seasonal variations are explained in terms of photoelectron production in the conjugate ionosphere and possibly in the protonosphere. For solar zenith angles less than 95°, the plasma scale height variations are in agreement with the temperature changes predicted by da Rosa [1966].