Topside ionosphere at sunrise

Abstract

The sunrise behavior of the midlatitude topside ionosphere at low sunspot numbers was examined by using Alouette 1 electron density and plasma scale height profiles obtained at Stanford University during May and June 1963. Under the assumption that observed variations result from solar control, the data were averaged to form height profiles of plasma scale height and electron density from 450 to 750 km altitude, which vary smoothly with solar zenith angle χ. The first manifestation of sunrise is a sudden and rapid decrease in plasma scale height, beginning between χ=105° and 102° and ending between χ=98° and 96°. Electron densities increase rapidly between χ=98° and 95°. Our interpretation of the observations is that the sudden decrease in plasma scale heights is due to an increase in percentage concentration of O+ ions at all altitudes, due primarily to a thermal expansion of the ionosphere that causes an upward movement of the topside ionization. The velocity of the thermal motion is estimated, and we attempt to resolve conflicting evidence from other observations that the topside ionosphere is controlled by either geomagnetic latitude or solar zenith angle.