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 KLUWER ACADEMIC PUBLISHERS DORDRECHT/BOSTON/LONDON X 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 VenusAtmosphere (p357) INTRODUCTION For almost three decades since Mariner 2 flew by the planet in December 1962, Venus hasbeen 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 expectmany phenomena of the upper atmosphere and ionosphere of Venus to differ from theirterrestrial counterparts. While flybys and landings of the many Venus missions provideduseful 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 ofthe most active solar cycles to date. We expect these transmissions to continue untilSeptember 1992 when gravitation perturbations will lower the periapsis of the PVO orbit sothat the spacecraft will be lost to the atmosphere. (0*0*0*Venera 9 and 10 and the Pioneer Venus observations have led to an explosion of knowledgeabout 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 thanhad been planned prior to launch and has resolved many of the initial differences between theVenera and the early PVO observations which were obtained at different phases of the solarcycle. The seven articles which follow in this special issue attempt to capture this explosionin our understanding of Venus. We have divided the task into seven topical areas. Webegin with the solar wind interaction. In the paper "The Magnetosheath and Magnetotail ofVenus", J. L. Phillips and D. J. McComas explain how the planet slows and diverts the solarwind flow with the formation of a bow shock and show how the magnetotail is formed. Inthe paper "The Structure of the Venus Ionosphere", L. H. Brace and A. J. Kliore combinethe results of "in-situ" and radio occultation investigations to reveal the density, temperatureand composition of the ionosphere and how it varies over the solar cycle. This is followedby K. L. Miller's and R. One of the puzzles of the early PVO observations was the variation in the state ofmagnetization of the ionosphere. Some days the ionosphere was field free and at other timesit was strongly magnetized. Thus behavior was difficult to understand if the current systemsin the ionosphere were directly driven by the solar wind. The interaction was more subtlethan this simple picture predicted. In their paper "Magnetic Fields in the Ionosphere ofVenus", 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 processesoccurring in a plasma is the waves it produces. Pioneer Venus included a simple plasmawave instrument measuring wave power in four narrow bands. R. J. Strangeway in hispaper "Plasma Waves at Venus" reviews the results of this investigation from the bow shockto lowest altitudes in the night ionosphere. One of the earliest interpretations of these lattersignals was that they were caused by electrical discharges, or lightning, in the upperatmosphere. 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. Strangewayexamines the arguments for and against this interpretation. In a paper solicited independently of this collection of papers, but included here because ofits appropriateness, I review the totality of evidence for lightning on Venus from the Veneralanders, the Venera 9 orbiter and the Pioneer Venus Orbiter. Included in this review is adiscussion of the properties of terrestrial lightning so that we may know what we shouldexpect at another planet. While alternate explanations are still being examined for thesewaves, the atmospheric electric source is still the strongest candidate explaining simply mostof the observed properties. If this explanation is indeed correct, then lightning on Venus is avery prevalent phenomenon, probably much more so than on Earth. In the last paper of the series, "The Structure, Luminosity and Dynamics of the VenusAtmosphere", 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 Venusatmosphere. This discussion includes a treatment of the extended exospheres of hydrogenand oxygen that surround Venus as well as phenomena such as the unexpectedly cold night side(0*0*0*thermosphere. Finally they review the major aspects of the circulation and dynamics of thethermosphere: 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 inpreparing these reviews and we thank the authors for their exhaustive reviews. We are alsograteful for the assistance of many reviewers, especially W. C. Knudsen, who also gaveunselfishly of their time to assist in ensuring the quality and accuracy of these papers. Wecaution the reader that knowledge, like the Venus ionosphere, is dynamic, and that ourunderstanding may continue to evolve and improve. If readers have any questions aboutthese papers, I am confident that the authors would like to hear from them. Please do nothesitate to begin a dialogue. C. T. RUSSELL March 1990