Global behavior of the ionosphere at 1000-kilometer altitude

Explorer 22 electrostatic probe measurements of electron temperature (Te) and concentration (Ne) at the 1000-km level during the period from November 1964 to May 1965 are presented and discussed. Precession of the orbital plane during this period has permitted the resolution of two full diurnal cycles of ionospheric behavior at this altitude, one centered on the winter solstice and one on the vernal equinox. In both seasons, the latitudinal distributions of Ne and Te reveal a diurnal modulation at middle and low latitudes that appears consistent with a mechanism of thermal expansion and contraction of the protonosphere along the geomagnetic field lines. Although there is a strong tendency toward symmetry about the magnetic equator, an important seasonal effect is evident as an enchancement of Ne throughout the summer hemisphere. Interpretation of the equatorial measurements in terms of the processes of photoelectron heating, local cooling via ions to neutrals, and electron thermal conduction permits the conclusions that: (1). diffusive equilibrium exists in the equatorial protonosphere, even at sunrise and sunset; (2) the observed latitudinal distributions of Te and Ne near the equator are mutually consistent with diffusive equilibrium, both day and night; (3) a local electron cooling exists in the equatorial region that is inconsistent with present neutral atmosphere models but suggests the existence of about 5 × 105 cm−3 atoms of neutral hydrogen at 1000 km. This article contains supplementary material.