GEOPHYSICALRESEARCH
LETTERS, VOL. 15, NO. 5, PAGES433-435,
HOT
OXYGEN
ATOMS
IN THE
UPPER
ATMOSPHERES
MAY 1988
OF VENUS
AND
MARS
Andrew F. Nagy and ThomasE. Cravens
SpacePhysicsResearch
Laboratory,Departmentof Atmospheric,
Oceanic,andSpaceSciences
The Universityof Michigan
Abstract.The altitudedistribution
of thehotoxygendensity
wascalculatedfor the upperatmospheres
of VenusandMars
for solar cycle maximum and minimum conditions,
respectively.The calculatedvaluesfor Venus are in good
agreementwith the observedvaluesandare abouta factorof
twenty larger than the calculatedvalues for Mars at the
exobase.
the dissociativerecombinationof 02 + ions. The branching
ratiosfor the dissociativerecombinationof 02 + in the original
modelwere adoptedfrom the work of RohrbaughandNisbet
(1973); somewhatdifferent branchingratioswere proposed
morerecenfiyby Paxton(1983). Both setsof branchingratios
are presentedin Table I and were used in the calculations
presentedin thispaper.We alsoincreasedthe valueusedfor
the cross section of elastic collision
between
the hot and
thermaloxygenatomsto 1.2x10
-15cm-2as suggested
by
Introduction
The purposeof thisbrief noteis to presentthe resultsof
calculationsof the hot oxygenatompopulationsin the upper
atmospheresof both Venus and Mars. Hot atoms were
predictedto bepresenton bothplanetsovera decadeago(e.g.
McElroy, 1972, Sze andMcElroy, 1975;Ferrin 1976;Wallis,
1978). A hot hydrogenpopulationhas been observedby
Lymanetobservations
froma numberof differentspacecraft
at
Venus( e.g. Anderson,1976) but not at Mars .The presence
of hot oxygen at Venus was confirmed by the 1304 A
measurements
of the ultravioletspectrometer
on the Pioneer
Venus Orbiter (Nagy et a1,1981;Paxton,1983). The initial
calculationsof the hot oxygen atom populationof Venus,
publishedby Nagy et al (1981), usedsomepreliminaryion
densitydata, which were higher than the sinceestablished
mean values and thus led to relatively high predictedhot
oxygendensities.The calculationscarded out by McElroy
(1982) and Paxton (1983), using more recent input
parameters,gavelower densities,in generalagreementwith
the published observedvalues. We repeated our earlier
calculations
for Venusin orderto showthatby simplyusing
the acceptedvaluesfor the ionosphericdensitiesour model
valuesagreewith theobservedones.In thispaper,we usethe
samemodelto calculatethehotoxygenpopulationexpectedto
McElroy et al.(1982).
The hot oxygenmodelvaluespublishedby us earlier for
Venus used somepreliminary ionosphericdata and led to
relativelyhighvalues.The modelresultspresented
herearefor
typical solar cycle maximum conditions; the neutral
atmosphere
valueswere adoptedfrom the modelof Hedin et
a1.(1983)and the ionosphericparameterswere taken from
Theis et al. (1980). In orderto allow futurecomparisons/tests
of theseresultsthe most importantinput parametervalues
chosenare shownin Figure 1. The calculatedhot oxygen
densitiesfor typicalsolarcyclemaximumdaytimeconditions
arepresented
in Figure2. The resultsobtainedfor bothsetsof
branching
ratios,aswell asthemeasured
valuesof Paxtonand
Stewart(1988) are shownfor comparison.
We also calculated,usingour model, the expectedsolar
cycle minimum hot oxygen population for Mars. The
atmospheric
andionospheric
inputparameters
neededfor these
calculationswere takenfrom the Viking database(Nier and
McElroy, 1977; Hanson et al., 1977, Chen et al., 1978),
except for hydrogen, which came from the Lyman ot
observations (Anderson, 1974) The results of these
calculations
areshownin Figure3 alongwith thecoldoxygen
and hydrogendensitiesfor comparison.
The calculatedhot
oxygendensityneartheexobase
is abouta factorof 20 smaller
be found at Mars, near solar minimum conditions, when the
at Mars than at Venus. In the case of Mars, the effect of the
two Phobosspacecraft
will reachit in 1989.The terrestrial
hot
oxygencorona was reviewedanddiscussed
by Yee (1980)
and Yee et a1.(1980)and will not be addressedin this brief
differentbranching
ratiosis muchsmallerthanthatfor Venus
(not visible in Figure 3), becauseof the significantlysmaller
escape energy.
note.
Discussion
Model Calculations and Results
The calculations
presented
hereuseda two streammodel
(cf. Chandrasekhar,1960, Nagy and Banks, 1970) to
calculatetheequilibriumfluxesandenergydistribution
of the
nonthermalcomponent
of atomicoxygen.The resultsfrom
thistwo streammodelwerecompared,
usingthe sameinput
parameters,
to the resultsof a numericaltransportmodel,
basedon momentsof the Boltzmannequation,developedby
Yee (1980),andwerefoundto bein goodagreement
(Nagyet
al, 1981),thusgivingusconfidence
in our model.Oncethe
energydependent
hotoxygenfluxesarecalculated
belowthe
exosphere,
thedensities
abovetheexobase
areobtained
using
Liouville'sequation(Chamberlain,1978).
The various assumptionsgoing into the model were
discussed
in our originalpaper(Nagy et al., 1981) andwill
not be repeatedhere exceptfor brief commentson the
followingtwoitems.Themainsourceof hotoxygenatomsis
We calculatedthe meandaytimedensitydistributionof the
hot oxygen corona expectedto be foundat bothVenusand
Input Parameters for Venus Calculations
1012
10TM
1010
109
108
-e- [O]
-•- [02+]
105
-e-[O+]
104
103
102
100
Copyright
1988 by the American Geophysical
Union.
-a-[CO2]
107
106
200
300
Altitude (krn)
Fig. 1. Major input parametersusedin calculatingthe hot
oxygencoronafor solarcyclemaximumconditionsat Venus.
Paper number 8L6759.
0094-827 6/88 / 008L-67 59503. O0
433
434
NagyandCravens:
HotOxygen
Table 1. BranchingRatiosfor theDissociative
Recombination
of 02 +
Products
Excess
Energy Rohrbaugh
andNisbet
1
(eV)
Paxton
2 Values
Values
O(3p)+ O(3P)
6.96
0.325
0.22
O(3P)+ O(1D)
5.00
0.30
0.55
O(3p)+ O(1S)
2.78
0.05
0.00
O(1D)+ O(1D)
3.02
0.275
0.13
O(1D)+ O(1S)
0.8
0.05
0.10
,
1Rohrbaugh,
R.P.andJ.S.Nisbet,
Effectof energetic
oxygen
atoms
onneutral
density
models,J. Geophys.Res., 78 6788, 1973.
2paxton,
L.J.,AtomicCarbon
in theVenusThermosphere:
Observations
andTheory_,
Ph.D.
thesis,Universityof Colorado,1983.
Mars. The Venuscalculations
werecardedout for solarcycle
energy at Mars is approximatelya quarterof what it is at
maximum conditions, becausemost of the Pioneer Venus data
Venus and this difference accounts for the other factor of about
is from that epoch.On the other handthe Mars calculations
correspond
to solarcycleminimumconditionsbecause1) the
only appropriateinput data baseis from the Viking landers,
flown in 1976, and 2) the model'smostlikely nearterm use
will be in association with the Phobos spacecraft
measurementswhich will reach Mars in early 1989. The
densitiescalculatedfor Venus are abouta factor of twenty
largerthanthoseobtainedfor Mars,whichis to be expectedas
the ionosphericdensitiesare also significantlygreateron
Venus.On Venus,hot oxygenbecomesthemajorneutralgas
constituentaboveabout500 km on the dayside;only above
about 2500 km does hydrogenbecomethe major neutral
species.
On thedaysideof Marshotoxygenplaysa muchless
two in the calculateddensities.
Ip (1989),in a paperrecently
submittedfor publication,presents
an excellentdiscussion
of
the potentialimportanceand implicationsof a hot oxygen
coronaat Mars; he calculated
thedistribution
functionusinga
Monte Carlotechniqueandobtaineda hotoxygenpopulation
abouta factorof five greaterthantheonepresented
here.
An extendedneutralcoronaabouta planetarybodyhasa
major impact on the interactionof the solar wind with that
body (e.g. Russellet al., 1983, Breus, 1986). This effect is
most significant if the neutral gas constituentis "heavy"
(heavierthan hydrogen)and extendsto "great"(in termsof
planetaryradii) distances.
The mostextendedneutralcoronain
the solarsystemare the onesassociated
with activecomets.
importantrole,at leastduringsolarcycleminimumconditions The solar wind interaction region extends millions of
(seeFigure3). Even duringsolarcyclemaximum,the hot kilometersupstreamof activecometssuchas P/Halley (e.g.
oxygenpopulationis not expectedto dominateat Mars as it
Somogyiet al., 1986).The knowledgethata hot hydrogen
will onlyincreaseby a factorwhichis roughlythesameasthat andoxygencoronaexistsaroundVenushasleadto numberof
of theionizingsolarflux. The ionizingEUV solarflux at Mars studiesof their effect on the solarwind interactionwith Venus
duringsolarminimumis approximatelya factorof ten less (e.g. Gombosi et al.; 1980, Wallis, 1982; Breus, 1986;
than the solarflux at Venusduringsolarcycle maximum.
However
there is a further factor which reduces the hot
oxygendensitiesat Mars; namelythe fact that the escape
Calculated
and Measured
Densities
for Mars
lO7
Hot Oxygen Densities at Venus
105
lO6
E 104
E 105
-a- Hot Oxygen
-e.. Cold Oxygen
-a- Hydrogen
[]
ß• 104
:*2_-103
•3 102
ß.a..
Paxton B.R.
-e..
R.& N. B.R.
---
PV Obs.
103
102
O 101
o
.
0
0
1000
2000
3000
4000
5000
Altitude (km)
Fig. 2. Measuredand calculatedhot oxygen densitiesat
Venus.
1000
2000
3000
Altitude (km)
100
Fig. 3. Hot oxygendensities
calculated
for solarcycle
minimum
conditions
at Mars;coldoxygenandhydrogen
values deduced from observations are also shown for
comparison.
NagyandCravens:Hot Oxygen
t•35
Belotserkovskii et al., 1987), which have established that
McElroy, M. B., M. J. PratherandJ. M. Rodriguez,Lossof
photoionizationand/or chargeexchangeof the hot oxygen
atomsinside the magnetosheath
is significant.There is also
experimental
evidencethattheseprocesses
areoperatingin the
magnetosheathand that they play an importantrole in the
interactionprocesses(e.g. Mihalov and Barnes,1981). Mars
and Venus are very similar in many ways; therefore, the
questionof the importanceof the hot oxygencoronaat Mars
hasalsobeenraised.Howeverthe expecteddensities,at least
during solarcycle minimum, are too low to affect the solar
wind interactionprocesses
to any significant
degree(Russellet
oxygenfrom Venus, Geo•hvs. Res. Lett.. 9_,649, 1982
Mihalov,J. D. andA. Bames•E{,idence
for theacceleration
of
ionosphericO+ in themagnetosheath
of Venus,Geophys.
al., 1983; Breus, 1986).
Acknowledgments.The work presentedin thispaperwas
carded out with the supportof NASA GrantsNGR 23-005015, NAG 2-491 and NAGW-15.
Res. Lett.. 8, 1277, 1981.
Nagy, A.F. and P.M. Banks, Photoelectronfluxes in the
ionosphere,J. Geophys.Res.,75, 6260, 1970.
Nagy, A.F., T.E. Cravens,J.H. Yee and A.I.P. Stewart, Hot
oxygen atoms in the upper atmosphere of Venus,
Geor•hvs.Res. Lett., 8, 629, 1981.
Nier,A• 0., andM. B. McElroy,Composition
andstructure
of Mars'upperatmosphere:
Resultsfromtheneutralmass
spectrometers
on Viking 1 and 2, J. Geophys.Res.. 82,
4341, 1977.
Paxton,L.J., Atomic Carbonin the Venus Thermosphere:
Observations
andTheory,Ph.D. Thesis,Universityof
Colorado, 1983.
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Michigan,SpacePhysicsResearch
Laboratory,Departmentof
Atmospheric, Oceanic, and Space Sciences,'Ann Arbor,
Michigan 48109-2143,
(ReceivedFebruary5, 1988;
revised March 15, 1988;
acceptedApril 12, 1988.)