VENUS AERONOMY

Edited by: C. T. Russell

Institute of Geophysics and Planetary Physics, University of California at Los Angeles.

Reprinted from Space Science Reviews, Volume 55, Nos.1-4, 1991

489pp, KLUWER ACADEMIC PUBLISHERS
DORDRECHT/BOSTON/LONDON

TABLE OF CONTENTS

Introduction vii

J. L. PHILLIPS and D. J. McCOMAS/The Magnetosheath and Magnetotail of Venus (p1)

L. H. BRACE and A. J. KLIORE/The Structure of the Venus lonosphere (p81)

K. L. MILLER and R. C. WHITTEN/Ion Dynamics in the Venus Ionosphere (p165)

J. G. LUHMANN and T. E. CRAVENS/Magnetic Fields in the Ionosphere of Venus (p201)

R. J. STRANGEWAY / Plasma Waves at Venus (p275)

C. T. RUSSELL/Venus Lightning (p317)

J. L. FOX and S. W. BOUGHER/The Structure, Luminosity and Dynamics of the Venus Atmosphere (p357)

INTRODUCTION

      For almost three decades since Mariner 2 flew by the planet in December 1962, Venus has been the subject of intense investigation by both the Soviet and American space programs. Since the intrinsic magnetic field of Venus is exceedingly weak, if it exists at all, we expect many phenomena of the upper atmosphere and ionosphere of Venus to differ from their terrestrial counterparts. While flybys and landings of the many Venus missions provided useful data on these phenomena, orbital missions were needed for their detailed investigation. Such orbital missions were provided by the Soviet program with Veneras 9 and 10 in October 1975 and by the United States with the Pioneer Venus Orbiter in December 1978. Originally designed for a prime mission of only 243 days, the Pioneer Venus Orbiter is still functioning over a decade later, providing data nearly 24 hours a day through one of the most active solar cycles to date. We expect these transmissions to continue until September 1992 when gravitation perturbations will lower the periapsis of the PVO orbit so that the spacecraft will be lost to the atmosphere.

      Venera 9 and 10 and the Pioneer Venus observations have led to an explosion of knowledge about the upper atmosphere and ionosphere of Venus and their interaction with the solar wind. The availability of data over a full solar cycle has led to greater understanding than had been planned prior to launch and has resolved many of the initial differences between the Venera and the early PVO observations which were obtained at different phases of the solar cycle. The seven articles which follow in this special issue attempt to capture this explosion in our understanding of Venus. We have divided the task into seven topical areas. We begin with the solar wind interaction. In the paper "The Magnetosheath and Magnetotail of Venus", J. L. Phillips and D. J. McComas explain how the planet slows and diverts the solar wind flow with the formation of a bow shock and show how the magnetotail is formed. In the paper "The Structure of the Venus Ionosphere", L. H. Brace and A. J. Kliore combine the results of "in-situ" and radio occultation investigations to reveal the density, temperature and composition of the ionosphere and how it varies over the solar cycle. This is followed by K. L. Miller's and R. C. Whitten's manuscript, "Ion Dynamics in the Venus Ionosphere" which describes and explains the flows in the Venus ionosphere.

      One of the puzzles of the early PVO observations was the variation in the state of magnetization of the ionosphere. Some days the ionosphere was field free and at other times it was strongly magnetized. Thus behavior was difficult to understand if the current systems in the ionosphere were directly driven by the solar wind. The interaction was more subtle than this simple picture predicted. In their paper "Magnetic Fields in the Ionosphere of Venus", J. G. Luhmann and T. E. Cravens review the observations and the theoretical explanation of this behavior. One of the important diagnostics of the physical processes occurring in a plasma is the waves it produces. Pioneer Venus included a simple plasma wave instrument measuring wave power in four narrow bands. R. J. Strangeway in his paper "Plasma Waves at Venus" reviews the results of this investigation from the bow shock to lowest altitudes in the night ionosphere. One of the earliest interpretations of these latter signals was that they were caused by electrical discharges, or lightning, in the upper atmosphere. F. L. Scarf, who was invited to be a coauthor on this article before his untimely death in July 1988, was one of the strongest proponents of this view. Strangeway examines the arguments for and against this interpretation.

      In a paper solicited independently of this collection of papers, but included here because of its appropriateness, I review the totality of evidence for lightning on Venus from the Venera landers, the Venera 9 orbiter and the Pioneer Venus Orbiter. Included in this review is a discussion of the properties of terrestrial lightning so that we may know what we should expect at another planet. While alternate explanations are still being examined for these waves, the atmospheric electric source is still the strongest candidate explaining simply most of the observed properties. If this explanation is indeed correct, then lightning on Venus is a very prevalent phenomenon, probably much more so than on Earth.

      In the last paper of the series, "The Structure, Luminosity and Dynamics of the Venus Atmosphere", J. L. Fox and S. W. Bougher review observations and models related to the chemical and thermal structures, airglow and auroral emissions and dynamics of the Venus atmosphere. This discussion includes a treatment of the extended exospheres of hydrogen and oxygen that surround Venus as well as phenomena such as the unexpectedly cold night side thermosphere. Finally they review the major aspects of the circulation and dynamics of the thermosphere: subsolar to antisolar convection, super rotation and turbulent processes.

      These articles represent the present state of our understanding of the upper atmosphere, ionosphere and solar wind interaction with Venus. Much effort has been expended in preparing these reviews and we thank the authors for their exhaustive reviews. We are also grateful for the assistance of many reviewers, especially W. C. Knudsen, who also gave unselfishly of their time to assist in ensuring the quality and accuracy of these papers. We caution the reader that knowledge, like the Venus ionosphere, is dynamic, and that our understanding may continue to evolve and improve. If readers have any questions about these papers, I am confident that the authors would like to hear from them. Please do not hesitate to begin a dialogue.

C. T. RUSSELL March 1990

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