A COMPARATIVE-STUDY OF ELECTRON COOLING RATES IN THE VENUS IONOSPHEREDURING MAGNETIZED AND UNMAGNETIZED CONDITIONS

When the solar wind dynamic pressure (P-sw) is high, the ''top'' of th e Venus ionosphere (defined by the altitude where the steep density gr adient begins) moves down to altitudes between 200 and 300 km. Thick i onopauses are formed in the region between 200 and 400 km and here the electron density decreases with a scale height of about 20 km. Furthe r these regions get permeated by strong horizontal magnetic fields. Ho wever during conditions of low P-sw, the ''top'' of the ionosphere mov es to higher attitudes and the region between 200 and 400 km is nearly free of magnetic fields. This region then forms a part of the ''main' ' ionosphere where electron density decreases with a scare height of a bout 200 km. In this paper, we study the electron cooling processes an d their rates in the region between 200 and 400 km for these two condi tions, namely, (1) when this region is fully magnetized and (2) when t his region is unmagnetized. We use Langmuir probe measurements of elec tron density and electron temperature from the Pioneer Venus Orbiter f or these studies. We find that the dominant electron cooling process i n the ionopause region is due to electronic excitation of the ground s tate of O to the O(D-1) level. In the main ionosphere, the major proce sses are cooling due to the fine structure of O, and electron ion Coul omb collisions. Further, the cooling rates are lower in the ionopause region than in the ionospheric region. Only if thermal conduction is a ssumed to be inhibited in the presence of a horizontal magnetic field, the magnetized orbits can then be used to estimate heating rates by e quating these to cooling rates (i.e., local equilibrium). Under this a ssumption, we find that heating rates so estimated are far smaller tha n those used by several workers in the heat balance models. However, t hese rates are closer to model values which include inhibition of phot oelectron transport in the presence of a horizontal magnetic field.