A comparative study of electron cooling rates in the Venus ionosphere during magnetized and unmagnetized conditions

Abstract

When the solar wind dynamic pressure (P _sw) is high, the "top" of the Venus ionosphere (defined by the altitude where the steep density gradient begins) moves down to altitudes between 200 and 300 km. Thick ionopauses are formed in the region between 200 and 400 km and here the electron density decreases with a scale height of about 20 km. Further, these regions get permeated by strong horizontal magnetic fields. However, during conditions of low P _sw, the "top" of the ionosphere moves to higher altitudes 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 scale height of about 200 km. In this paper, we study the electron cooling processes and their rates in the region between 200 and 400 km for these two conditions, namely, (1) when this region is fully magnetized and (2) when this region is unmagnetized. We use Langmuir probe measurements of electron density and electron temperature from the Pioneer Venus Orbiter for these studies. We find that the dominant electron cooling process in the ionopause region is due to electronic excitation of the ground state of O to the O(¹ D) level. In the main ionosphere, the major processes are cooling due to the fine structure of O, and electron ion Coulomb collisions. Further, the cooling rates are lower in the ionopause region than in the ionospheric region. Only if thermal conduction is assumed to be inhibited in the presence of a horizontal magnetic field, the magnetized orbits can then be used to estimate heating rates by equating these to cooling rates (i.e., local equilibrium). Under this assumption, we find that heating rates so estimated are far smaller than those used by several workers in the heat balance models. However, these rates are closer to model values which include inhibition of photoelectron transport in the presence of a horizontal magnetic field.