CONFIGURATION OF THE NIGHTSIDE VENUS IONOPAUSE

Nightside measurements of electron and ion concentrations by instruments on the Pioneer Venus Orbiter have revealed an ionopause that is highly dynamic and whose global configuration is unexpectedly complex. Inbound crossings which occur at high northern latitudes encounter the ionopause at an average altitude of 870 km in the late afternoon sector. The average ionopause altitude increases slightly near 110° solar zenith angle, SZA, followed by a nightside compression to well below 500 km. The outbound crossings of the ionopause occur about 20° south of the equator. The ionopause at this latitude is found at an average altitude of 800 km in the afternoon‐evening sector between 65° and 110° SZA, followed by a pronounced rise to altitudes as high as 3000 km in a pronounced bulge near 130° SZA. A gradual decrease in average ionopause height occurs between 130° and 150° SZA with the lowest ionopause heights observed beyond 150°. Superposed on this average behavior is an orbit to orbit variability of height by a factor of two or more. The difference in ionopause height at high and low latitudes near the terminator appears to confirm the asymmetry of the obstacle and shock surface observed by the Venera spacecraft and earlier predicted on the basis of theoretical arguments. The post‐terminator bulge of the ionopause at the equator and its extreme inward movement on the nightside are qualitatively consistent with the same type of theoretical arguments but require in addition the existence of strong electric fields that penetrate to very low altitudes in the cavity behind Venus. This paper is not subject to U.S. copyright.