.4dv. Space Res. Vol. 10. No. 5. pp. (5)41—(5)44, 1990 0273—1177/90 $0.00 + .50 Printed in Great Britain. All rights reserved. Copyright © 1959 COSPAR DYNAMICAL FEATURES OF VENUS IONOSPHERE FROM COMPARATIVE STUDY OF THEORETICAL AND IN-SITU MEASURED ELECTRON DENSITY PROFILES Han Om Upadhyay and R. N. Singh Department of Applied Physics, institute of Technology, Banaras Hindu University, Varanasi — 221 005, india ABSTRACT A modeling of neutral density of Venusian gases for different solar activity and solar zenith angles has been made. Taking appropriate EUV solar flux data and selecting appropriate ionization and absorption cross—sections. The electron—ion pair production rates have been computed. Considering various reactions and their appropriate reaction rates the steady state ion density and electron density profiles have been computed in the altitude range of 100—800 Km. It is shown that the steady state ion densities and electron density below 250 Km. compare well with the theoretical results reported by various other workers and the in situ measurements by electron temperature probe (OETP) aboard Pioneer Venus Orbiter (PVO). However, the computed results above 250 Km. are shown to be considerably large. Using appropriate transport processes and matching the theoretical and in situ measured electron density profiles above 250 Km. the dynamical features of Venus ionosphere has been studied. The results thus obtained have been compared with other reported dynamical results. INTRODUCTION The solar XEUV radiations and solar wind protons are known to play an important role in controlling the ionization of planetary atmospheres. Further, the solar wind inter- action with the planetary atmospheres give rise to modifications in subsolar ionospheric region. In this paper, we have made an effort to model the Venus neutral atmosphere as a function of changing solar activity. Using the neutral atmospere and choosing appropriate solar flux data /1/. relevant cross—sections /2.3,4/ and corresponding react- ions rates, /5.6/ we have theoretically computed the electron density profiles in the subsolar region of the Venus ionosphere. In general, the theoretically computed steady state electron density profiles do not compare well with the in situ measured electron density profiles. Because of lack of Intrinsic magnetic field of Venus, the plasma trans- port phenomena is likely to play an important role in controlling the dynamical features of electron density profiles. We have made theoretical computation of electron density profiles for three types of Venus neutral atmospheric models. The in situ electron density is found to compare well with the theoretically computed electron density pro- files at lower altitudes, whereas significant differences are oftentimes observed at higher altitudes. Taking illustrative profiles of in situ measured electron density profiles from OETP /7/ and considering that the vertical transport is mainly responsible for modifying the photoionized ionospheric plasma and making it compatible with the steady state electron density profile, the contribution of vertical transport in the Venus ion- osphere has been computed using the continuity equation. The vertical velocity gradient profiles are found to change in accordance with the chosen in situ measured electron density profiles. The vertical transport velocity gradient pr5TIl~ire found to change qu.i.te significantly below 800 Km. We have also analysed orbit 422 for which the ionopause was formed at 2800 Km. We find that the vertical transport velocity gradient becomes slower whenever the ionopause is formed at higher altitudes, whereas at low altitudes the transport velocity gradients do not change much with changing ionopause altitudes. IONIZING SOLAR FLUX MODELS The XEUV ernmisslons and nicz’ov;ave radiatio’.~s during the solar flare, when monitored from the ground-based stations, are found to vary due to the presence of cloud top and other simultaneous atmospheric effects. The microwave radiation at 2800 MHz (10.7 cm) reaches the planetary atmospheres almost unattenuated and are extensively used to classify the effect of solar activity. These solar activity indices are used to study the variations induced in the neutral density composition of the planetary (5)41