Table Of ContentVENUS AERONOMY
VENUS AERONOMY
Edited by
C. T. RUSSELL
Institute o/Geophysics and Planetary Physics,
University o/California at Los Angeles.
Reprinted from Space Science Reviews, Volume 55, Nos. 1-4, 1991
SPRINGER SCIENCE+BUSINESS MEDIA, B.V.
Llbrary of Congress Cataloglng-ln-Publlcatlon Data
Venus aeronomy I [edited] by C.T. Russell.
p. cm.
ISBN 978-94-010-5457-7 ISBN 978-94-011-3300-5 (eBook)
DOI 10.1007/978-94-011-3300-5
1. Venus (P 1a net ) --Atmosphere . 1. Russell, C. T.
QB621.V47 1991
551.5'0999'22--dc2G 90-26481
Printed on acid-free paper
AII Rights Reserved
© 1991 Springer Science+Business Media Dordrecht
Originally published by Kluwer Academic Publishers in 1991
Softcover reprint of the hardcover 1 st edition 1991
No part of the material protected by this copyright notice may be reproduced or
utilized in any form or by any means, electronic Of mechanical,
including photocopying, recording or by any information storage and
retrieval system, without written permission from the copyright owner.
TABLE OF CONTENTS
Introduction vii
J. L. PHILLIPS and D. J. McCOMAS / The Magnetosheath and Magnetotai1 of
Venus
L. H. BRACE and A. J. KLIORE / The Structure of the Venus Ionosphere 81
K. L. MILLER and R. C. WHITTEN / Ion Dynamics in the Venus Ionosphere 165
J. G. LUHMANN and T. E. CRAV ENS / Magnetic Fields in the Ionosphere of
Venus 201
R. J. STRANGEWAY /P1asma Waves at Venus 275
C. T. RUSSELL / Venus Lightning 317
J. L. FOX and S. W. BOUGHER / The Structure, Luminosity and Dynamics of
the Venus Atmosphere 357
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 transmis
sions 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.
The 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
Space Science Reviews 55: vii-viii, 1991.
© 1991 Kluwer Academic Publishers.
viii INTRODUcnON
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 co-author 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 strdngest 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 unexpect
edly cold nightside thermosphere. Finally they review the major aspects of the circulation
and dynamics of the thermosphere: subsolar to anti-solar convection, superrotation 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
THE MAGNETOSHEATH AND MAGNETOTAIL OF VENUS
JOHN L.-PHILLIPS and DAVID J. McCOMAS
Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A.
Abstract. We describe the observational history and assess the current understanding of the magnetosheath
and magnetotail of Venus , stressing recent developments. We make recommendations for research that can
be done using existing observations, as well as desirable trajectory and instrumentation characteristics for
future spacecraft missions.
Table of Contents
I. Introduction
2. Observational History
3. Boundaries and Building Blocks
3.1. The Solar Wind at Venus
3.2. The Bow Shock
3.3. The Ionopause
3.4. The Neutral Exosphere
4. The Dayside and Near-Terminator Magnetosheath
4.1. Macroscopic Properties
4.2. Turbulence and Waves
4.3. The Magnetic Barrier
4.4. Ion Pickup Theory and Observations
4.5. The Magnetosheath Boundary Layer
5. The Near-Planet Wake and Nightside Magnetosheath
5.1. Flow and Magnetic Field Configuration
5.2. Plasma Observations
6. The Magnetotail
6.1. Formation and Configuration
6.2. Magnetic Field Observations
6.3. Plasma and Plasma Wave Observations
7. Conclusions
7.1. Current Understanding
7.2. Recommendations for Near-Term Research
7.3. Recommendations for Future Spacecraft
1. Introduction
The subjects to be discussed in this review, the Venus magnetosheath and magnetotail,
make up a major portion, but by no means the entirety, of the interaction of the solar
wind with Venus. For discussions of other aspects of the Venus atmosphere and the
solar wind interaction the reader is referred to the companion review papers in this
volume. Previous reviews of these subjects have been presented by Breus (1979), Russell
and Vaisberg (1983), and Luhmann (1 986a). Collections of original papers can be found
in a special Pioneer Venus issue of Journal of Geophysical Research (December 1980),
Space Science Reviews 55: 1-80, 1991.
© 1991 Kluwer Academic Publishers.
2 JOHN L. PHILLIPS AND DAVID J. McCOMAS
two special Venus issues of Icarus (August and December 1982), and a Venus
atmosphere issue of Advances in Space Research (Volume 5, No.9, 1985). Additionally,
the 1983 University of Arizona volume entitled Venus features a comprehensive series
of articles on the solid planet, the atmosphere, and the solar wind interaction.
Of the solar-planetary interactions thus far explored, all are unique in some respect.
Each planet has a combination of atmospheric characteristics, magnitude and
orientation of the planetary magnetic field, and the presence or absence of satellites
which defines its surrounding plasma environment. At the magnetized planets, the
primary obstacle to the solar plasma is the planetary magnetic field itself, and the region
dominated by the planetary field is called the magnetosphere. A standing bow shock
wave is formed upstream of the obstacle, and the region of shocked solar wind plasma
surrounding the magnetosphere is called the magnetosheath. The planetary field of
Venus, if one exists at all, has been shown to be inconsequential in diverting the solar
wind (e.g., Phillips and Russell, 1987). Our current understanding is that the obstacle
to the solar wind consists primarily of shielding currents carried by the ionosphere,
modified by a comet-like pickup process involving exospheric and ionospheric ions.
Thus, there is no distinct boundary, or magnetopause, separating regions dominated by
planetary and solar magnetic fields. Some authors have used the term 'magnetosphere'
to describe certain portions of the interaction region. We will avoid this term, as it
implies the existence of a planetary magnetic field, and thus is a misnomer in the case
of Venus. The 'ionopause' has been defined in various ways by various authors, but
generally can be considered to be an altitude above which the ion density decreases
sharply. While some authors have referred to the region of shocked solar plasma and
lesser amounts of planetary plasma, lying between the bow shock and the ionopause,
as the 'ionosheath', we will use the more common term of ' magnetos heath'.
Just as the obstacle to the solar wind at Venus is fundamentally different from those
of the magnetized planets thus far explored by spacecraft, so too the magnetotail is
unique. It resembles a draped cometary tail more than it does those at the magnetized
planets: Mercury, Earth, Jupiter, Saturn, Uranus, and Neptune. While the polarities of
the tail lobes at the magnetized planets are controlled primarily by the planetary fields,
the lobes at Venus respond directly to changes in the orientation of the interplanetary
magnetic field (lMF). The presence of a neutral exosphere which extends far into the
regions dominated by the solar wind plasma adds an additional comet-like aspect to
the interaction. The ionization and subsequent assimilation by the shocked solar wind
of the exospheric particles produces observable effects in both the magnetosheath and
the magnetotail. Figure 1 contains a schematic of the Venus-solar wind interaction, with
the major plasma regimes and typical locations indicated.
Before proceeding to the detailed discussions of phenomenology, we will digress
briefly to describe the basic numerical simulation types applicable to modeling the
Venus-solar wind interaction. Modeling results will be mentioned often in subsequent
sections, so it is important that the reader be aware of the physics included in, and
excluded from, various model types. At present, three types of models have been used
to study the Venus magnetosheath. The first is the gas dynamic model developed by
THE MAGNETOSHEATH AND MAGNETOTAIL OF VENUS
4
" ,
" B\MF
"
"
"
"
2 "
"
.(-I) "
"
«X
E
o Magnetotail
o
~ --
I.&..
CD
-o
,
oc: -2 " " , , , , , " ...
.(-I) " " , , " ... ... ...
o " ...
" " ....
" ............
"
" "
-4 L-__~ __L -__~ ~"L -__~ __~ ~~ __L -" ~~ __L -~~~
4 2 0 -2 -4 -6 - 8
Distance Along Venus-Sun Axis (RV)
Fig. I. Schematic of the solar wind interaction with Venus for a typical IMF 'toward sector' configuration,
in a plane defined by the solar wind V and B vectors. Aberration due to planetary motion is not incorporated.
The momentum flux of the solar wind, including inertial, thermal and magnetic contributions, is withstood
by the thermal pressure of the ionosphere. Unlabeled solid and dashed lines represent streamlines and
magnetic field lines, respectively. The connection of the magnetotail to the near-planet interaction region
is not yet well understood, and so is not shown here.
Spreiter and Stahara (1980). In this model the fluid equations for hypersonic flow
around an impenetrable obstacle are solved. The magnetic field is not explicitly included
in calculation of the model flow field, but instead is determined subsequently based on
the distortion of the fluid elements and thus has no effect on the flow parameters. A
variation of this model which has been used to assess the behavior of planetary pickup
ions and their effects consists of the gas dynamic model with test particle planetary ions.
The trajectories of newborn ions are determined by the background electric and
magnetic fields calculated for the magnetosheath. Such a model is not self consistent
in that it does not include the reaction of the flow parameters to the mass addition, but
it gives a first-order picture of pickup ion behavior (cf. Luhmann et al., 1985; Phillips
et ai., 1987). A third type is the 'mass-loaded' gas dynamic model where a source term
added to the continuity equation allows assessment of the effects of the addition of a
heavy, cold photoion 'fluid' at low altitudes (cf. Berlotserkovskii et ai., 1987; Stahara
et al., 1987). Such a simulation, while treating the planetary ion source self-consistently,
does not take finite gyroradius effects into account and, therefore, can underestimate
planetary ion effects at higher altitudes. We note that none of the afore-mentioned
4 JOHN L. PHILLIPS AND DAVID J. McCOMAS
simulations incorporate realistic tail properties, and that, therefore, these models are
applicable only to the upstream and near-terminator regions.
Two model types which can answer certain questions left by those mentioned above
are an MHD model and a hybrid model. We are currently aware of progress in
simulating the interaction with both of these techniques, though published results are
not yet available. The former method adds the magnetic terms to the fluid equations and
thus describes self-consistently the effects of the magnetic field on the plasma flow.
Observed and predicted asymmetries due to MHD effects in the interaction region
should be obtainable with such a model. Test particle pickup ions or a fluid mass loading
source term can be added to an MHD simulation in the same manner, and with the same
limitations, as for a gasdynamic simulation. A hybrid model, which would treat ions as
individual particles and electrons as a fluid, would incorporate not only magnetic effects
but also the finite Larmor radius effects not available with a fluid mass loading
treatment. Such a simulation could presumably model asymmetries related to ion pickup
in a self-consistent manner.
2. Observational History
Since the beginning of the spacecraft era, Venus has been visited by a total of 20
spacecraft missions, 15 from the Soviet Union and 5 from the United States. Another
U.S. spacecraft, the Magellan radar mapper, is enroute to Venus at the time of this
writing. The trajectories and/or instrument packages for many of the Soviet missions,
as well as for the Pioneer Venus multiprobe, were optimized for atmospheric or surface
studies, and therefore have not contributed significantly to our knowledge of the Venus
magnetosheath, wake and magnetotail. The remainder of this review will be concerned
primarily with observations by the following spacecraft: Mariner 5 and 10; Venera 4,
6, 9, and 10; Pioneer Venus Orbiter. Other spacecraft whose measurements have
provided indirect evidence concerning the Venus-solar wind interaction include
Mariner 2 and Venera 11, 12, 15, and 16. Contributions of these latter spacecraft will
be mentioned as part of the historical development, but will not be described in detail.
The first observational constraint on the Venus-solar wind interaction was
established by the Mariner 2 spacecraft, which flew by Venus on December 14, 1962.
Closest approach was at 41000 km from the center of the planet, just sunward of the
terminator plane. The lack of any planetary perturbation of the interplanetary magnetic
field (1M F) led the observers (Smith et aI., 1963, 1965) to conclude that the spacecraft
had never passed through a planetary bow shock. Using theoretical predictions for solar
wind flow around a magnetized obstacle (Spreiter and Jones, 1963), they then concluded
that the planetary magnetic moment must be less than one-tenth ofthat of Earth. Similar
results, that is no perturbation of the interplanetary medium, were observed by the
Mariner 2 ion spectrometer (Neugebauer and Snyder, 1965). Both magnetometer and
plasma experimenters estimated that the subsolar bow shock, if it existed, was 4 R v or
less from the center of the planet. Additionally, the absence of energetic protons and
electrons, which would have suggested magnetospheric trapping, led the energetic
