Wright State University
CORE Scholar
Physics Faculty Publications
1990
A Signature of Auroral Precipitation in the
Nightside Ionosphere of Venus
Jane L. Fox
H. A. Taylor Jr.
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Repository Citation
Fox, J. L., & Taylor, H. A. (1990). A Signature of Auroral Precipitation in the Nightside Ionosphere of Venus. Geophysical Research
Letters, 17 (10), 1625-1628.
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Physics
GEOPHYSICAL
RESEARCH
LETTERS,
VOL. 17,NO. 10,PAGES1625-1628,
SEPTEMBER
1990
A SIGNATUREOF AUROtLiLPRECIPITATIONIN THE NIGHTSIDEIONOSPHEREOF VENUS
J. L. Fox
InstituteforTerrestrial
andPlanetary
Atmospheres
StateUniversity
ofNewYorkat StonyBrook
H. A. Taylor, Jr.
TaylorEnterprises,
BeaverRiver,NovaScotia
Abstract. We show here that the densities of mass-
plainedsinceN2
+ reactsrapidlywithO to produce
NO+
28 ionsmeasuredby the Pioneer Venus Orbiter ion mass
Nt + O -+ NO+ + N.
spectrometer
(OIMS) onthenightside
ofYenusarehighly
,ariab!e and show little correlation with the values of the
Becauseof the high ionizationpotentialsof both N2 and
CO, the chemicalsourcesare limited. Yet a survey of
early Pioneer Venus orbits with periapsisbelow 165 km
reveals a large variability in the maximum densitiesof
0 + densities.We have determinedthe total production
ratesof mass-28ionsin the chemicalequih'briumregion
and •nd that this production rate cannot be explained
by known chemicalproduction reactions. We propose
that the "excess"production is due to precipitation of
electronsinto the nightsidethermosphere.
mass-28
ionswithvalues
rangingfromabout10 cm-3 to
about104cm
-3. Tayloret al. [1982]hadearlierremarked
thattheoccasional
largedensities
of NO+ andO• mea-
The sourceof the Venus nightsideionospherehas been
disputedsincethe first radio occultationmeasurementsof
sured by the OIMS appeared to require a sourcein addition to ion transpo•; they suggestedthat the densities
were enhancedby particle precipitation, but the mechanism of the sourcewas not resolved. We propose that
thenightside
electrondensityprofileby Mariner5 [K!iore
the excess
NO+ is produced
fromN2
+ created
by parti-
etal.,1967].Sincethen,theviablemechanisms,have
been
cle precipitation,and that high densitiesof mass-28ions
in the nightsideionosphereare an in situ signature of
precipitation of electronsinto the nightsideatmosphere.
Introduction
reducedto two: transport of atomic ions, mostly O+,
from the daysideand precipitation of energetic electrons
that havebeen observedin the Venusumbra. Models appropriate to high solar activity have shown that plasma
transport is the dominant maintenance mechan•m, since
In the discussionthat follows, we identify the sources
and sinksof mass-28ions.in the chemicaleq 'ui!ibriumregion, that below approximately165-170kin. There the
ion productionand lossrates are equal, so the magnitude
it alonecanproducethe observeddensitiesof O+, with
electronprecipitation playing a possiblesecondaryrole
of the source can be estimated because the total loss rate
in enhancing
the lower02+ peak[Spenner
et al., 1981;
can be computed fairly accurately. From the loss rate
and the known chemical sources, the %xcess" production rate, that we ascribehere to particle precipitation,
Cravenset al., 1983]. Gringauzet al. [1977']maintain
that precipitation of electronsalone is sufficient to pro-
is determined.
ducethe main ion peak, as measuredby the Venera9 and
!0 radio occultationexperimentsat solar minimum.
We havebeeninvestigatingthe chemistryof the nightsideionosphere,with a view towazd elucidatingthe relative roles of electron precipitation and plasma trans-
Sources
ofN•+ andCO+
Reactions,referencesand rate coefficientsused in this
work are summarized in Table 1, and the reactions num-
port in its maintenance. Model calculations based on
bers cited below refer to the numbers in that table. Be-
ion transport as the sourceof the nightside ionosphere
causeof the highionizationpotentia!of N2 (15.58eV),
only a few chargetransferreactionsof atmosphericspe-
sho•ed that the reaction
ot + N -+
ciesareenergetically
capable
ofproducing
N•+, including
charge
transfer
fromHe+, 0 ++ andO+(2D)(reactions
3-
++ o
5). Reaction(3) • potentiallyimportant,sincenighttime
He+ densitiesare largerthan daytimevalues,peakingin
the the pre-dawnsector[Tayloret al., 1980]. Densities
of O++ weremeasuredby the PV OIMS, but are often
wasthe major sourceof NO+in the chemicalequilibrium
region.Ana!y-sis
of data from eight P•neer Venus(PV)
orbitschosen
lot lowperiapshand the availabilityof data
showedthat the NO + densities could be modeled with re-
above the noise level of the instrtunent only above the
action(1) asthe solesourceforsomeorbits,but forothers
it wascompletely
inadequate.A closelookat thoseorbits
showed
that they werean character• by high densities
chemicalequilibriumregion.In addition,the product•of
reaction(4) are unknown;the large exothermidtysuggeststhat the dissociativechanne!probablydominates.
ofmass-28
ions(N•+ + CO+)..Thisdeviation
is easilyex-
Charge
transfer
from0+(2/)) to N2,reaction
(5), isa major source
of N2
+ in thedayside
ionosphere
[Fax,1•2],
and the importanceof t]:dsreaction dependson whet.her
Copyright 1990 by the American Geophysical Union.
the roetastable
O+(2D) ioncouldsurvivetransportfrom
the dayside. A detailed analysiswo•d be required to
Paper number 90GL01458
0094-8276 / 90 / 9 0GL-0! 4 5:8503.00
modelthe roleof reaction(5) in producing
the observed
!625
! 626
Fox and Taylor: Auroral Signaturein VenusIonosphere
Table1. Ratecoefficients
forreactions
involved
in thechemistry
ofN•+ andCO+ in theVenus
nightside
ionosphere.
No.
Reaction
Rate Coefficient
(cm3s
-•) References
(2)
N•+ + O --,NO+ + N
1.4(-10)a(300/Ti)
0'44
McFarland
et al. (197'4)
_• 1.6(-9)b
Anicichet al. (1977)
Howorkaet al. (1979)
Johnsenand Biondi(1980)
(3)
(4)
He+ + Ns--*N2
+ + He
O++ + Ns-• N•+ + 0 +
O+(2D)+N• • N•+ + O
(6)
He+ + COs • He + CO+ + 0
O++ + COs -• CO+ + O+ + O
O ++ + CO -• CO + + O +
(9)
C+ + CO•. -+ CO+ + CO
N + + COs -* CO + + NO
(11)
N + + CO -+ CO + + N
CO + + O -, CO + 0 +
5.9(--10)
(13)
(14)
++
+ CO*
CO+ + CO• -• CO•
+ + CO
N•++e•N+N
?.?(--10)
1(-9)
(16)
CO +.+ e-* C+ 0
a. Read 1.4 x 10- xø
AdamsandSmith(1976);Rakshitet al. (1978)
_<2(-9)
_• 1.6(-9)b
(8)
-
Johnsen and Biondi c
(assumed
the sameasfor reaction5)
z)(3oo/i.
2(- ?)(300/T,)ø'4s
Fahey et al. (1981)
Smith et al. (197'8)
Miller et al. (1984)
Fehsenfeld
and Ferguson(197'2)
Smith et al. (1978)
Adamset al. (1978)
(98o)
MitchellandHus (1985)
b. The productsin the reactionsof 0 ++ are unknown,sothe rate coefficients
are upperlimits only.
c. Private communication
to Fox and Victor (1981).
N•+, butwewillshowbelowthatit canberuledouton
imate the total lossrate of mass-28ionswe usedaveraged
morphologicalgrounds.
CO also has a fairly high ionization potential of 14.01
values for the rate coefficients. Errors of less than 15%
eV. CO+ can be producedby the dissociativecharge
transferreactionof He+ with CO•. (reaction6) with a
yield of about 79%. The reactionof He+ with CO producesverylittle CO+ , resultingalmostcompletelyin dissociativeionization[Rakshitet al., 1978]. Productionof
CO+ in thereactionof O++ with CO2 and with CO (reactions7 and 8) is energeticallyfeasible,but the products
in these reactions are unknown. Other possible production mechanismsare reactionsof C+ and N + with COs
and CO (reactions9-11). Thesereactionsare potentially
important,sinceC+ andN+ shouldbe transportedfrom
the daysidealongwith O+, althoughwith muchsmaller
or so, substantially lessthan the uncertainty in the rate
coefficientsthemselves,result from this assumption.
220
200
180
J•
The source due to chemical reactions
can be computedif the densitiesof the speciesinvolved
are measured. The OIMS measureddensitiesof He+,
O++, C+, and N +, but valuesare not availablefor all
of the nightsidepassesof the PVO, nor at all altitudes.
Nonetheless,it is possibleto computethe sourceof mass28 ionsdueto reactions(3-11) for a numberof orbitsover
somepart of the chemicalequilibrium region.
Total Production
Rate of Mass-28 Ions
;,"'..,,-'?-'
: _+
,'
..•
•__-'.,.'-'.•--.
...... :: ....... •,•..... •-.-•,.
140
1
160
2
3
4
5
LOGIONDENSITIES
(CM-•)
b. --
'"'ol'':' I'' "'I''' '.....
'....
cially electronimpact, on Ns, CO and COs remains a
viable mechanism.
';
_.•.
'•,
2•'
.-,.NO..-'
F
160
fluxes.
If chemical sourcesare insufficient to produce the observed mass-28ion densities, only particle impact, espe-
L
150
o
140 ,,,
0
l,.... , ,, )',, , , ) , ,,,
1
2
3
4
LOGPRODUCTION
ORLOSSRATE(CM-aS
-•)
Fig. la. Densities
of majorionsfor orbit505 (inbound)
from PV orbiter ion massspectrometerdata. lb. Total
productionrate of mass-28ions(filledcircles),production
rate due to chemicalreactions(opencircles)and excess
production
(opensquares),
for Orbit 505(inbound).
SincebothN•+ andCO+ contribute
to the measured
Figuresla and 2a showthe major ion densitiesfor or-
density,it is not possibleto computethe total lossrate
exactly. Fortunately,however,the rate coefcients for
bits 505 and 529. Periapsesfor orbits 505 and 529 were
the majorreactions
of CO+ andN2
+ arenearlyequal,
at 22.1 and0.7 hourslocaltime andsolarzenith anglesof
so an approximatevaluefor the total lossrate can be
!49.5 ø and 163.2ø, respectively,so both orbits were near
obtained. The most important lossmechanismfor both
the antisolarpoint. The maximum mass-28ion density
is a factorof morethan 25 largerfor orbit 529 than orbit
ionsis reactionwith 0 (reactions2 and 12). The rate
coefficients
for both reactions(2) and (12) are closeto
1.4x 10-•øcm3s
-z. N•+ andCO+ alsoreactat nearlygas
kineticrateswith C02 (reactions
13 and 14). Dissociariverecombination
(reactions
15and16)isthemajorloss
mechanism
for both ionsat high altitudes. Rate coefcientsof 2.2 x 10-z and 2.0 x 10-7 cm3s-x at 300 K have
beenreported
forN•+ andCO+, respectively.
Toapprox-
505. Figureslb and 2b comparethe total lossratesof
mass-28ions(reactions2 and 12-16) to the production
ratesdueto reactions(3-11) in the chemicalequilibrium
regionfor the two orbits. The maximummass-28ion
lossrate for orbit 529 is about 104cm-3s-•, whereasthe
productionrate dueto chemical
reactionsis only 10- 20
cm-as
-•. Theexcess
production
is onthe orderof 104
FoxandTaylor:AuroralSignature
in Venus
Ionosphere
220
titude below 1(35kin, and data at the mass-28 ion peak
,
for thedensities
of O+, O•+, He+, CO•, O, andN•. In
-
-
this calcui•tion,only the reactionsof He+ (reactions3
200
-
and •) wereincludedaschemicalsourcesof mass-28ions,
-
-
since experiencewith individual orbits showed the other
sourcesto be unimportant. The electron temperature
was assumedto be about I000 K for all the data points.
180
-
.
-
1•0
_
--
140
-,
1
• , • I • • • , I ,,,,• ,,,,, I • • f ,-
2
3
4
LOGION DENSITIES
(CM-•)
160
ß
Errors resultingfrom this assumptionshould be small,
since the electron temperature dependenceof the dissociative recombination coefficientis weak, and dissociative
recombinationis not the major lossmechanism. In order
to reduce the effectsof variability due to changesin the
neutral atmosphereand in the altitude of periapsis, the
excessproduction"frequency"was obtainedby dividing
150
the rate by the factor$[N•.]4-0.1[CO•]. The first term
ø%
140
o
1627
1
2
in the factor representsionization of N• and CO, and
was chosenbecausethe CO density is about twice the N•
•
Fig. 2. Sameas for Fig. 1, but for Orbit 529 (inbound).
In Fig. 2b, the total productionrate and excessproduction rate axe essentially superimposed.
cm-as-x. For orbit 505, the maximumauroralproduction rate is about a factor of 40 less than for orbit 529,
althoughthe production rates due to chemicalreactions
are about the same for the two orbitsß
Other evidence for the precipitation of electrons into
the nightsidethermosphereexists. Fluxes of su,pratherreal electronshave been measuredabovethe atmosphere
in the Venusumbraby the PV retardingpotentialanalyzer(ORPA) [e.g. Knudsen"andMiller, •985] as well
as by the plasma analyzers on the Soviet Venera 9 and
10 spacecraft[e.g. Gringauzet al., 1977]. Continuous
but highly variable emissionsof atomic oxygen at 1304
and1356]k.havebeenobserved
in images
of the nightsideof Venusby the P V orbiter ultraviolet spectrometer
densityon the nightsideabovethe homopause
tHedinet
al., 1983]. The secondterm representsthe contribution
of dissociativeionization of CO•; the factor 0.10 is the
high energy ratio of the electron impact crosssections
for productionof CO+ from CO• and CO. On the dayside, the excessproduction is due to direct ionization and
chemicalreactionotherthan reactions(3) and (6). The
production frequencyappearslarge on the dayside also
becauseCO is moreabundantrelativeto N•. there tHedin
et al., 1983].The variabilityon the daysideis small,because all the data were taken near solar maximum, but
somescatter remainsfrom the variation of periapsisalti-
tude. The largevariabilityin the sourceon the nightside,
ranging over more than two orders of magnitude, itself
suggestsparticle precipitation as the sourcemechanism.
Some excessproductionappears to be present in nearly
all the orbits. This is consistentwith the morphologyof
the1304• aurora[Phillips
et al., !986],whichis nearly
alwaysvisibleon the nightside,but exhibitslarge spatial
and temporal variations.
(OUVS) [phillipset al., 1986]. Fox and Stewart[1990]
haveproposedthat theseemissionsaxecausedby the precipitationof soft electronsinto the nightsideatmosphere.
ø
-4
' ß '
d,'•
"..,..• I ;- J-' ;,"
ß
' ,,,ira,
.h•/"
..
.
'•-' '•
; --
:":... !
, _
.
o
'1'
0
14
8
!2
it)
20
24
LOCAL TIME
o
o
Fig. 4. Ratioof [28+][0]/[N•]at the mass-28
ionpeakto
the densityof O+ •t its peak,as a functionof local•ime
for the first 600 orbits. The variability on the nightsideis
substantiallygreaterthan •he •riability on the dayside.
Fig. 3. Productionfrequency
of mass-28ionsin excess
of that that can be accountedfor by chemicalreactions
at the mass-28ion peak as a functionof local time for
-all orbits of the first 600 for which sut•ci.ent data are
available.
Figure 3 showsthe exce• .t)roduction" --u•'ncy
-•' of
mass-28ionsat the mass-28ion peak for all orbits among
•hefirst600that meetthefollowing
criteria:periapsis
Since models have shown that the measured 0 + densi-
ties can only be accountedfor by transport from the dayside, and if mass-28ionsare mostly producedby auroral
processes,the relative values of the maximum m•28
ion and 0 + densities would be indicative of the relative
importance of ion transport and particle precipitation in
the nightsideionosphere.
Figure4 is a plot of the ratio of
thepeakmass-28
iondensity(dividedby [N•]/[O]) to the
peak0 + densityasa functionof localtime for all orbits
1628
Fox and Taylor: Auroral Signaturein VenusIonosphere
with periapsi$below 165 km and with availablemass-28
Fox, J. L. and G. A. Victor, O++ in the Venusianiono-
ion, 0 +, N2 and Oder.sities.Here the factor [N2]/[O] is
sphere,J. Geophys.Res., 86, 2438, 1981.
Gringauz, K. I., M. I. Verigin, T. K. Breus and T. Gom-
chosento limit the scatter due to the variability of the
neutral atmosphereand the changein the altitude of peflapsis. The remaininglarge orbit to orbit variability is
itself ir.dicative of particle precipitation as the source of
the mass-28ion. The occurrenceof very high values is
of particular interest, as it showsthat sufficientelectron
precipitation is present, at least occasionally,to affect
the ion densitiessubstantially. The large variation in the
ratio also showsthat high mass-28 ion densities are not
correlatedwith high O+ densitiesand that transport of
0 + (2D) is unlikelyasa sourceof mass-28
ions,sincethe
transport of the metastable ion would be expected to be
correlatedwith the transportof O+.
Conclusions
We have shown that the total production rate of mass28 ions in the Venus nightside ionosphere cannot be accountedfor by chemicalproduction resulting from transport of atomic ions from the dayside for a substantial
number of orbits of the Pioneer Venus spacecraft. We
ascribethe missingproduction to precipitation of particles, possibly the same electrons that are implicated in
the production of the ultraviolet "auroral" emissions. A
study of the production rate at the mass-28 ion density
maximum for all orbits of the first 600 for which sufficient
data are available shows that the auroral production rate
is highlyvariable,with maximumvaluesof about 10't
cm-Ss-•. Thisgreatvariabilitypointsto particleprecipitation
as the source.
Acknowledgments.JLF thanks NASA Goddard Space
Flight Center for their hospitality and aid in accessing
the UADS data base. This work has been supported in
part by NASA grants NAGW-665 and NAG2-523 to the
State University of New York.
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