Our knowledge of the upper atmosphere and ionosphere of Venus and its interaction with the solar wind has advanced dramatically over the last decade, largely due to the data obtained during the Pioneer Venus mission and to the theoretical work that was motivated by this data. Most of this information was obtained during the period 1978 through 1981, when the periapsis of the Pioneer Venus Orbiter (PVO) was still in the measurable atmosphere. However, solar gravitational perturbations will again lower the PVO periapsis into the upper atmosphere in September 1992, prior to the destruction of the spacecraft toward the end of this year. The physics and chemistry of the thermosphere and ionosphere of Venus are reviewed

Cravens, T. E.

English

NASA Technical Reports Server

_; "International Colloquium on Venus
IMAGE PROCESSING AND PRODUCTS FOR THE
MAGELLAN MISSION TO VENUS. Jerry Clark, Doug
Alexander, Paul Andres, Scott Lewicki, and Myche McAuley,
Magellan Image Data Processing Team, Jet Propulsion Laboratory,
Mail Stop 168-514, 4800 Oak Grove Drive, Pasadena CA 91109,
USA.
The Magellan mission to Venus is providing planetary scientists
with massive amounts of new data about the surface geology of
Venus. Digital image processing is an integral pan of the ground
data system that provides data products to the investigators. The
mosaicking of synthetic aperture radar (SAR) image data from the
spacecraft is being performed at JPL's Multimission Image Pro-
cessing Laboratory (MIPL). MIPL hosts and supports the Image
Data Processing Subsystem (IDPS), which was developed in a
VAXcluster environment of hardware and software that includes
optical disk jukeboxes and the TAE-VICAR (Transportable Appli-
cations Executive-Video Image Communiation and Retrieval) sys-
tem. The IDPS is being used by processing analysts of the Image
Data Processing Team to produce Magellan image data products.
Data arrive at the IDPS via the fiber optic Imaging Local Area
Network from the SAR Data Processing Subsystem that correlates
raw SAR data into image data. The input SAR image swaths, called
F-BIDRs (Full-resolution Basic Image Data Record). are long. thin
swaths of imagery covering an area on the surface about 20 km in
width by about 17,000 km in length,extending from thenorth pole
tonear the southpole ofVenus. Systematic procedures wcrc wrlttcn
for the automatic mosaicking of multiple orbitsof data into image
frames covering predetermined regions of the planet.Algorithms
wcrc devclopod to perform such functions as the correction of
radiomctric differences at the edges of adjacent orbits and the
aurora atic tiepointing ofoverl apping data to correct for nay ig ational
errors between orbits. After mosaicking, the images arc contrast
enhanced, annotated, and masked to create photo products for
scientific analysis. Other versions of mosaics arc created from
reduced resolution image swaths. Dcpending on the product, the
data are output from the MIPL as archive tapes; working WORM
(Write-Once-Read- Many) optical disks; premastcred tapes for stamp-
ing, by a vendor, ofCD -ROMs (Compact- Disc- Read-Only-Memory)
that are used to distribute mosaics and ancillary data to the scientific
community; and exposed film that is used for the production of
prints and copy negatives. Numerous special products have been
made at the re.quest of thc investigators, including SAR mosaics
merged with non-SAR data (such as altimetry and radiomctric
emissivity data) that are displayed as color composites and stereo
anaglyphs that use SAR data collected at different look-angles to
view surface topography through stereo parallax. In addition,
mosaicked data are used to make other products, including terrain
rendering that shows SAR mosaics combined with digital elevation
data in perspective views and videos that uscthousands of incremented
rendered scenes to cream simulated flights over the surface of
Venus.
(This work is being performed at JPL, California Institute of
Technology, under contract with NASA.)
N93-14307
THE THERMOSPHERE AND IONOSPHERE OF VENUS.
T. E. Cravens, Department of Physics and Astronomy, University of
Kansas. Lawrence KS 66045. USA.
Our knowledge of the upper atmosphere and ionosphere of
Venus and its interaction with the solar wind has advanced dramati-
cally over the last decade, largely due to the data obtained during the
Pioneer Venus mission and to the theoretical work that was moti-
vated by thisdata.Most of thisinformation was obtained during the
period 1978 through 1981, when theperiapsisofthe Pioneer Venus
Orbiter (PVO) was stillin the measurable ata'nosphcre.However,
solargravitationalperturbationswillagain lower the PVO periapsis
intothe upper atmosphere inSeptember 1992, priorto the destruc-
tionof the spacecrafttoward the end of thisyear.The physics and
chemistry of the thcrrnosphcre and ionosphere of Venus will be
reviewed in thispaper. The book entitled Venus Aeronomy [I]
contains severalchapters thattogether provide a good overview of
thissubject.
The neutralatmosphere isprimarily composed of carbon diox-
ide,but for altitudesabove about 160 krn atomic oxygen, which is
produced via photodissociation of CO 2by solarphotons, becomes
the dominant neutralspecies.The PVO neutralmass spoetrometcr
has provided most of our information on the composition of the
Venus thermosphere. The thermosphere of Venus isquite cold in
comparison with Earth'sthermosphere with an cxospherlc tempera-
tureon the daysidc ofTes = 300 K, whereas atEarth Tex - 1500 K.
The nightside thermosphere of Venus isextremely cold with Tex --
100 K; ithas bccn suggested, in fact,that this region of the
atmosphere bc called the "cryosphcre" rather than the thermo-
sphere.Itisnot fullyunderstood why the upper atmosphere ofVenus
is so cold, although part of the answer is contained in the CO 2
15-l.tmcooling mechanism and also in the nature of the thcrmo-
spbere dynamics. Thermospheric wind speeds of several hundred
mctcrs per second have bccn theoreticallycalculatedfor altitudes
above about 150 kin,and indirectobservational cvidcncc, such as
compositional gradients,supports the validityof thcsccalculations.
The cxobasc of Venus is located at about 180 kin; above this
altitude,in the cxosphcrc, neutral atoms and molecules largely
follow ballistictrajectories.Both atomic oxygen and atomic hydro-
gcn arc abundant in the Venus cxosphcrc. The H density isabout
10s cm -3on the daysidc and about 107 cm -3on thc nightsidc.Two
populations of cxosphcric H existat Venus: (I) a cold thermal
component and (2)a hot (i.e.,an cffcctivctemperature of---1200K)
nonthcrmal component. The hot hydrogen isthought tobe mainly
produced by thc charge cxchangc ofhot H* ions with neutralH and
O atoms. Both cold and hot populations ofatomic oxygen alsoexist
in the Vcnus cxosphcrc, and the nonthcrmal hot population has a
larger dcnsity than filecold population for altitudesabove about
300 kin.The hot oxygen corona, as itiscalled,was observed by the
ultraviolctspectrometeronboard PVO via rcsonantlyscatteredsolar
130.4-nm photons. The major sourcc of hot oxygen isthe dissocia-
tiverecombination of ionospheric 02+ ions. Hot oxygen plays an
important role in the solar wind interactionwith Venus, becansc
photoionizationof oxygen atoms thatarc prcscntout in the Venus
magnctoshcath createshcavy ions that"mass load,'"and thus slow
down, the solarwind flow.
The ionosphcre of Vcnus forms duc to the ionizationofneutrals.
Photoionization by solarextreme ultraviolet(EUV)photons isthe
main ionizationprocess, although some contributionis alsomade
by electronimpact ionizationby photoelectrons on thedaysidc and
by "'auroral"clecta'onson the nightsidc. Itshould bc noted that
supcrthcrmal electronsappear tobc precipitatingintothe nightside
atmosphere of Venus, generating emissions obscrvcd by the PVO
ultravioletspectrometer and alsocreating ionization.However, the
Venus aurora isvery weak incomparison with theterrestrialaurora,
and isthought tobe caused by relativelylow energy electrons(i.e.,
energies of = 100 cV orIcss,whereas auroralcleclyonsatEarth have
energies of thousands of cV).
https://ntrs.nasa.gov/search.jsp?R=19930005119 2020-03-17T08:47:47+00:00Z
LP) Contribution No. 789 23
The ionization process for the major neutral species, CO 2 and O,
in the ionosphere of Venus can be represented by the reactions
hv + CO 2 --_ CO 2, + •
-_CO+O*+e
hv+O_O,+e
where hv represents an ultraviolet photon, h is Planck's constant and
v is the photon frequency, hv must exceed the ionization potential
of the molecule for these photochemical reactions to proceed.
A peak electron density in the Venus ionosphere of_ 6 x l0 s cm -3
was observed on the dayside by the Pioneer Venus radio occultation
experiment, as well as by the radio occultation experiments on other
missions. The altitude of the peakis located at z =140 kin.The major
ion species in the lower ionosphere of Venus was observed by the
PVO ion mass spectrometer and by the retarding potential analyzer
experiments to be 02+ and not COl+, even though the abundance of
neutral O 2 in the atmosphere of Venus is negligible, because of the
following rapid ion-neutral reaction
COl+ + O -_ 02+ + CO
The O2* ions are removed from the ionosphere by means of the
following dissociative recombination reaction with ionospheric
electa'ons
Ol+ +e--_O + O
The neutral oxygen atoms produced by this reaction have energies
of a few eV and arc the source of the hot oxygen corona. The major
ion species observed by instruments on PVO for altitudes above
= 160 krn is O +. Many minor ion species, including H ÷, CO ÷. N2+ .
COz+, NO ÷. He+, C*, and N ÷. were observed by the PVO ion mass
spectrometer.
Electron and ion temperatures of several thousand degrees were
observed in the ionosphere of Venus by the PVO electron tempera-
ture probe and by the retarding potential analyzer. Theoretical
models indicate that most of the energy required to heat the
ionospheric plasma to these temperatures, which greatly exceed the
neutral temperature, is derived from the solar wind interaction with
the ionosphere.
The nightside ionosphere of Venus is quite variable, both spa-
tially and temporally, with peak densities typically observed to be
about 104 cm -3. The main source of nigh tside ionization is thought
to be transport of ions from the dayside to the night.side. O* ions dri ft
upward on the dayside, then flow horizontally with speed s of several
kilometers per second above 200 kin, and then subside to lower
altitudes on thenightside. Large ion drift speeds near the terminator
of Venus were measured by the PVO retarding potential analyzer.
Auroral ionization also contributes to the nightside ionosphere,
especially during time periods of large solar wind dynamic pressure,
when it is known that the nightside ionosphere at higher altitudes
virtually disappears. A variety of other nightside ionospheric phe-
nomena have also been observed, such as tall rays. ionospheric
clouds, and ionospheric holes.
The solar wind interacts very strongly with the ionosphere of
Venus. In fact, two types of ionosphere exist: (1) unmagnetizEd and
(2) magnetized. The former ionospheric state is observed to be
present whenever the solar wind dynamic pressure is low, and in this
case large-scale magnetic fields are excluded from the ionosphere,
although small-scale magnetic structures called flux ropes were
observed to be present in the ionosphere by the PVO magnetometer.
The boundary between the solar wind and ionosphere, called the
ionopause, is rather narrow and is located at higher altitudes for
unrnagnetized ionosphere cases. However, the dayside ionosphere
is observed to be permeated by large-scale magnetic fields during
conditions of high solar wind dynamic pressure. In this case,
electrical currents flow throughout the ionosphere and both the
density structure and the dynamics of the ionosphere are strongly
affected by the solar wind interaction. The ionopause in this case is
located below about 300 km and is rather broad.
References: [1] Russell C. T., cal. (1991) Venus Aeronomy,
Kluwer, Dordrecht.
I• !N93 14308.^,
NEAR-INFRARED OXYGEN AIRGLOW FROM THE VE-
NUS NIGHTSIDE. D. Crisp t, V. S. Meadows 2. D. A. Allen 3.
B. Bezard 't, C. DeBergh 't, and L-P. Maillard 5, tJet Propulsion
Laboratory, USA, 2University of Sydney, Australia, 3Anglo-Aus-
tralian Observatory, q3bserv. Paris-Meudon, France, SlAP Paris,
France.
Cn-oundbased imaging and spectroscopic observations of Venus
reveal intense near-infrared oxygen airglow emission from the
upper atmosphere [ 1.2] and provide new constraints on the oxygen
photochemistry and dynamics near the mesopause (-100 krn).
Atomic oxygen is produced by the photolysis of CO2on the dayside
of Venus. These atoms are transported by the general circulation,
and eventually recombine to form molecular oxygen, Because this
recombination reaction is exothermie, many of these molecules are
created in an excited state known as O2(IA). The airglow is produced
as these molecules emit a photon and return to their ground state.
Connes et al. [1] found that the airglow intensity is comparable on
the dayside (1.5 x 10 n photons/cmVs) and night.side (1.2 x I012
photons/cm2/s) of the planet. They concluded that the O2(IA )
emission is spatially uniform, and that chemical reactions involving
O2(IA ) provide a major pathway for the recombination of oxygen
atoms in the venusian atmosphere. The intensity and apparent
uniformity of this emission has puzzled aunospheric chemists for
more than a decade because these properties cannot be explained by
existing models [3].
New imaging and spectroscopic observations acquired during
the summer and fall of 1991 show unexpected spatial and temporal
variations in the O_(IA) airglow [4..5]. High-resolution (0.4 cm -1)
spectra of selected regions of the dayside and nightside of Venus
were obtained with the Fourier Transform Spectrometer on the
Canada-France-Hawaii Telescope (Mauna Kea Hawaii) on 27 June
and 1 July 1991 (Fig. I). Individual oxygen emission lines of the
O2(1A ) band near 7880 cm -I (1.269 p.m) were resolved, allowing us
to distinguish the airglow emission from the deep-atmosphere
thermal emission peak near 7830 cm -l (1.277 p.m). The intensity of
the nigbtside O20A ) emission increased by a factor of 4 between
10N to 5S latitude on 27 June. The emission measured near 15S
latitude on 1 July was almost six times brighter than that seen four
days earlier, and about three times brighter than that reported by
Connes et at. [ 1 ]. Comparisons of intensities o f individual rotation al
transitions in the P and R branches of the emission spectrum indicate
a rotational temperature of 186 + 6 K (Fig. 2).
This temperature is comparable to that derived from the Oz(tA )
observations made in the mid 1970s [1]. When Pioneer Venus
arrived in 1978, the Venus mesosphere was characterized by an
anomalous thermal structure, with warm poles and a relatively cool
equator. Dynamical models showed that this temperature structure
was consistent with a rapid decrease in the amplitude of the cloud-


The thermosphere and ionosphere of Venus

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