concentration at 105 km is enhanced by factors of 1.2 and 2.6 when the energy distributions of the N atoms from electron impact dissociation of N 2 are chosen ...
JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 103, NO. A6, PAGES 11,579-11,594, JUNE 1, 1998
Nitric oxide abundance in the mesosphere/lowerthermosphere
region:Rolesof solarsoftX rays,suprathermal N(4S)atoms, and vertical transport P. K. Swaminathan, • D. F. Strobel,2,3D. G. Kupperman,• C. Krishna Kumar,•,4, 5 L. Acton,6 R. DeMajistre,• J.-H. Yee, • L. Paxton,• D. E. Anderson,• D. J. Strickland,7 and J. W. Duff 8 Abstract. This paper carefullyexaminesthe inabilityof photochemicalmodelsto account
for thelargenitricoxidedensities of --•108 cm-3 at --•105kmobtained fromIR, UV, and microwavemeasurements. A detailedand up-to-datephotochemicalmodel is constructed that incorporatesmeasuredYOHKOH soft X ray fluxes,hot N atom chemistrywith an energydependentthermalizationcrosssectionand sevenreactionsources,and laboratory-
constrained N(2D) yields. Theresulting modelwhichhaswell-constrained chemistry comparedto pastmodelsfails to generatehigh enoughNO densitiesin comparisonwith the most reliable
measurements
of absolute NO concentrations
in the lower
thermosphere.The sensitivityof the model resultsand the knownuncertaintiesin the inputsare usedto identifywherefuture effortsshouldbe focused.A deficitremains
despitean increase in theverticalmixingratesin thelowerthermosl•here fromthevery low Kzzprofileusedin our calculations and/oran increasein the N(•D) yield from electronimpactdissociation of N 2 from its nominalvalue of 0.54 to 0.62. The sensitivityof NO profilesto the nascentenergydistributionsof the atmosphericsourcesof suprathermalN atomsis illustratedby includingthe thermalizationof suprathermalN atomswith an updatedthermalizationcrosssection.The diurnallyaveragedNO concentrationat 105 km is enhancedby factorsof 1.2 and 2.6 when the energy distributions of the N atomsfrom electronimpactdissociation of N 2 are chosenwith peaksnear 0.6 eV or 3-4 eV, but deficitsof factorsof --•7 and --•3,respectively, remain. There is highersensitivityto verticaltransportthan to variationsof chemistrywithin known uncertainties.
1.
quantitativeyieldsfor theseprocesses on the basisof available laboratorystudiesand theoreticalestimates.However,the un-
Introduction
SinceBarth [1964] discoveredlarge amountsof NO in the Earth's upper atmosphere,it has been recognizedto be importantas a dominantionizableconstituentin the D region,a chargetransfer agent in the E region, a dynamictracer of mesosphere/lower thermospheremotion, and an ozone catalyst in the stratosphere.To accountfor the NO abundance, Nortonand Barth [1970] and Strobelet al. [1970] invokedthe
certainty in theN(2D) yieldwaslargeenough thatphotochemical models could match the abundances of NO inferred
from
observations [Barth,1992].Laboratorymeasurements overthe
pastdecadeanda half haveincreasingly refinedthe N(2D) yieldsin many chemicalreactions.The impact of the overall
constraint on N(2D) yieldhasbeento causephotochemical modelsto underestimatethe low-latitudenitric oxide (NO)
reaction of metastable N(2D) with02. Metastable N(2D) is abundancein the lower thermosphereand to require addigeneratedin a numberof highlyexothermicionosphericprocesses; Oran et al. [1975]made a first attemptto estimatethe
tional sourcesof NO throughenhancedsolarenergyinput in the softX raywavelengthregion[Siskindet al., 1990,1995]and addedproductionof nitric oxideby reactivesuprathermalni1AppliedPhysics Laboratory, JohnsHopkinsUniversity,Laurel, trogenatoms[Solomon,1983;Swaminathanet al., 1994;KupMaryland. permanet al., 1995;Gerardet al., 1995,1997;Shematovich et al., 2Department of EarthandPlanetary Sciences, JohnsHopkinsUni1991a, b; Sharma et al., 1996]. versity,Baltimore, Maryland. 3Department of Physics andAstronomy, JohnsHopkinsUniversity, This underestimate in calculated NO abundance has been Baltimore, Maryland. accentuatedby recentnitric oxidemeasurements obtainedby 4Alsoat Department of Physics andAstronomy, HowardUniversity, techniquesother than UV solarfluorescence. The analysesof Washington,D.C. SAlsoat Centerfor the Studyof TerrestrialandExtra-Terrestrial the 5.3 gm solaroccultationspectraldata obtainedby AtmosphericTraceMoleculeSpectroscopy/Atmospheric Laboratory Atmospheres,Howard University,Washington,D.C. 6Department of Physics, MontanaStateUniversity, Bozeman. for Applicationsand Science(ATMOS/ATLAS 1) during a 7Computational Physics Inc.,Fairfax,Virginia. shuttlemission(March 26 to April 4, 1992)yieldedpeak NO 8Spectral Sciences Inc.,Burlington, Massachusetts.
densities of-108 cm-3 at 105_+2.5km in thelatituderange
Copyright1998 by the American GeophysicalUnion. Paper number97JA03249. 0148-0227/98/97 JA-03249509.00
38øN-58øS[KrishnaKumar et al., 1995]. These densitylevels are severaltimeslarger than thosepreviouslyreported[Barth, 1992] from UV measurements.The previousanalysisof UV 11,579
11,580
SWAMINATHAN
ET AL.: LOWER THERMOSPHERIC
data underestimatedthe amount of NO due to the neglectof self absorptionin the calculationof an effectiveNO •/band emissionrate, even though Cravens[1977] pointed out that typicalinferredNO columndensitieswere sufficientto attenuate the solar flux that excitesthe NO •/bands. However, more recently,Eparvierand Barth [1992] have estimatedthis selfabsorptioncorrection.Also, Stevens[1995] has developeda comprehensivealgorithm to accuratelycompute these selfabsorptioneffects.It yieldsthe requiredeffective•/band emission rates and confirmsan upward adjustmentof a factor of 2-3. Correctionsfor self absorptionto earlier UV retrievals have sincebeen made, resultingin an increaseof the Barth [1992] reportedpeak NO densitiesby a factor of 2-3 (reprocessedSME data was presentedby Barth [1996]). As further
supportfor highpeakNO densities, Swaminathan et al. [1995] noted that contemporaneousNO measurementsfrom the HalogenOccultationExperiment(HALOE) instrumenton the UARS satellitewere in very goodagreementwith the ATLAS 1 NO results.The key impact of the new higher peak abundancesis to widenthe discrepancy with photochemical models, especiallyat low latitudeswheremodelsalreadyunderpredict
NITRIC OXIDE ABUNDANCE
spherequenchingcrosssection,have varied from 0.01% to 15%. However, it is shownhere that there were errors in those
estimates andthata corrected "hotN(4S) reactive fraction" usinga hard spherethermalizationcrosssectionis -6-8%. If one usesthe recentlycalculated[Gerardet al., 1997] energy dependentthermalization crosssections,it is found that the known sourcesof suprathermalN atoms can result in a hot
N(4S) reactivefractionof 3-15% at 110km and critically depends ontheadoptednascent N(4S) energydistributions. The primarypurposeof this paper is to investigatequantitativelythe importanceof three chemicalmechanisms of producingupper atmosphericNO which require quantification. Observations havehelpedconstrainthe modelinputs:(1) pho-
toelectron branching ratiosdetermining N(2D) yieldsfrom electronimpactdissociation of nitrogen,(2) the solarsoft X ray energy flux, and (3) the contributionof suprathermal
N(4S)atoms. Weconstrain solar softX rays(10-50•) onthe basis of YOHKOH
satellite
measurements
and use recent lab-
oratorymeasurements to fix the N(2D) branching ratioand theroleof translationally hotN(4S) atomsin theproduction
of nitric oxidein the lower thermosphere.It shouldbe noted that the mechanismsof X raysand hot atomsare related; that Siskindet al. [1990, 1995] have championedthe role of X is, the softX ray region of the solarflux leadsto a goodshare raysin the modelsto producemore NO. The importanceof of the suprathermalN atoms.This point maybe key to undersoftX raysarisesfrom ionizationof N2, 02, and O to produce standingsolarcyclevariationsin NO becausethe shorterwaveenergeticphotoelectronswhich further ionize and dissociate lengthregionsin the solarradiationdisplaylargeramplitudes N2 (formingN•-, N + + N, or two N atomswith possible of variabilityduringthe solarcycle.The emphasisof thispaper electronicexcitations)that can subsequently reactwith O and is on chemicalprocesses,but sensitivityto vertical diffusion, 02 toformNO.Thecontribution of solar(18-50•) softX which is known to play an importantrole in the altitude disrayswasexaminedby Siskindetal. [1990],whofoundthat large tribution of nitric oxide, is also examined. Section2 describesthe salient aspectsof the one-dimenfluxes(-50 times referencesolarminimumX ray flux or an 18-50• fluxof0.75ergscm-2 s-•) wereneeded toreproducesional(l-D) photochemical modeladoptedfor thisstudy.Sec1 nitric oxide measurethe high NO densitylevelsof previousrocket measurements. tion 3 reviews the ATMOS/ATLAS Recently,Siskindet al. [1995]havereducedthe requiredscale ments,and section4 describesthe soft X ray fluxesmeasured factor usingan improvedphotoelectronmodel and a broader with the YOHKOH/SXT instrumentduringthe time periodof NO.
solar X raywavelength range, 11-50•. However, these studies the did not have availablecontemporaneous solar soft X ray flux measurements.In the present researchthe solar soft X ray
ATLAS
1 mission. Section 5 discusses the various sources
of suprathermal N(4S) atomsandthemethodfor thecalcula-
tion of the reactiveprobabilitiesof these sources.Section6 presentsmodel resultsand comparisons with ATLAS 1 NO to assess the currentunderstanding of nitricoxide on the YOHKOH satelliteare usedto quantitativelyestimate observations chemistry.Section7 presentsthe concludingremarks. the role of X raysin the productionof NO. Another pathwayto enhanceNO productionis to produce
(11-503,) fluxes measured bythesoftX raytelescope (SXT)
suprathermal N(4S) atomsthatreactwith02. The latterre- 2. CT1D: One-Dimensional Model actionrate hasa high activationenergybarrier, sotranslationfor Photochemistry and Transport allyhotatomsreactfasterwith02 thanthermalN(4S) atoms Chemistryand transportin one dimension(CT1D) is the andproportionally morehotN(4S)thancoldN(4S)willform newlydevelopedphotochemistry and verticaltransportmodel of the Applied PhysicsLaboratory,JohnsHopkinsUniversity actionisalsokeyto modeling thecoldN(4S) population which (APL). Givena modelatmosphereof N2, 02, andtemperature, determines the NO loss rate via the "cannibalistic" odd nitroCT1D solvesthe standardminor speciescontinuityequations genlossmechanism N(4S) q-NO -->N2 q-O. Thepopulation involvingmolecularand eddydiffusioncoupledto photochemof translationally hot N(4S) at anygiventime is givenby a istryof all key trace speciesin the altituderegionof the stratoand lower thermosphere.It hasbeenvalcompositedistributioncomprisedof the tail of the Maxwell sphere,mesosphere, velocity distributionand the velocity distributionof nascent idatedusingresultsfrom the Jet PropulsionLaboratory(JPL) N(4S)produced byvarious exothermic reactions. A number of model [Allen et al., 1981, 1984; Strobelet al. 1987] for the exothermic sources of N(4S) havealreadybeeninvestigated.atmosphericaltitude regionof 40-150 km. CT1D includesall key speciesand their knownreactionsfor Solomon [1983]examined photodissociation of N2 by977• reactionsgiven solarphotonsand Shematovich et al. [1991a,b] expandedthe over50 species.In additionto the mesospheric studyto includeseveralother processes. The resultsare usually byAllenet al. [1984],the halogenspecies,oddnitrogenspecies and stratosphere, and sevenkey ions(N•-, expressedin terms of the fraction of suprathermalnitrogen in thermosphere atomsthat react with 02 to form NO rather than be collision- NO+, O•-, N+, O+(2P, 2D, 4S)) in the thermosphere are ally thermalized. Estimates from previous studies for this treated.It usesindividualspecieschemistry(no familyor phoquantity,if thermalizationoccursat a rate givenby the hard tochemicalequilibriumassumptions) which is a requirement NO rather than reactwith NO to recycleN 2 (reactionsP3 and L1 of Table A2). Reliably estimatingthe suprathermalN re-
SWAMINATHAN
ET AL.: LOWER THERMOSPHERIC
NITRIC
OXIDE
ABUNDANCE
11,581
dinalvariability(O+N ......-.." •+ • / -_••
/• +_ 0 '•: 0 I /
loo .
N(4S) Atom, 0.3
-- • •,... ,, • •.--•" .•- .."o
• • + -',"d • 10-2 10¸ 102 Production Rote(cm-3sec -1)
N2+e->N+N 0 00
110
see text
/
o,'• i"
•...•'
N2+hv->N+N•----
Nz+e->N+N+
Table 1. Sources of Hot N(4S) andInitialEnergies
eV
.......
....
90 10-4
120
Energy per
•
i •
',,
The N(4S) energydistribution fromelectronimpactdisso-
N2 + hv-->2N*(4S) + 0.55eV N2 + e -->N(4S) + N+ + 2e + energy N2 + e -->N*(4S) + N(2D, 2p) + e + energy NO+ + +e -->N*(4S) +, O(3p) + 2.75eV + 4
' i 0I / E
140 *2+O+->NO++N+[• // / o ,/ • 130 •+/ • o?: 1O0
H1 H2
+/
_
:N++O2->O2++N
Figure3. Fractionof hot N(4S) that reactswith 0 2 as a
Suprathermal N(4S) SourceReactions
(3)
+ +
FNO++e->N+O '
16
Velocity(10s cm/sec)
z)
.
..•'
../"
•
,/
i.I
.
•
(b)
ß
..
90
10-4
•
I•. "•
•.•
I
10-2 10¸ 102 Production Rote(cm-•sec-•)
104
Figure4. Hot N(4S) atomproduction ratesat (a) noonand (b) sunset(1800 LT) from processes in Table 1 as determined from the chemistryand transportin one dimension(CT1D) photochemicalmodel and AtmosphericUltraviolet Radiance IntegratedCode (AURIC) photoelectronmodel.
SWAMINATHAN
ET AL.: LOWER THERMOSPHERIC
NITRIC
OXIDE ABUNDANCE
11,585
2.0
2.5
(o) 90 km 110 km 130 km 150 km
2.0
(b)
........
-
90
1.5
.......
1.5 _
km
110
km
130
km
150
km
...........
.......
..(D
O
n
1 0
-,'
.
L:,,•
0.5F
0.5
k :i i•!l ' 0.0 0.0
0.5
1.0
2.0
1.5
0
2
4
'
2.5
2.0
'
'
•
90 km
]" '
110 k........
/
130k..... o
1.5
ß>- 1.o
6
8
Energy (eV)
Energy(eV)
:'/
2.5
\
",. \
(c)
-
2.0
": \
• 1..5
150 km ......'/-"./','.'"" /,:.....,.l •,,'.:,:, :.,..,..,.:...1 '.:.• 31.o ß/!
,,,
0..5
.'
0.5
0.0 0.0
0.5
1.0
1.5
2.0
0.0
Energy(eV)
O.S
1.0
1.5
2.0
Energy(eV)
Figure5. InitialhotN(4S) atomenergy distributions for 90,110,130,and150km.Thebroadening of delta functionenergies listedin Table1 is caused by transforming froma center-of-mass to a laboratory frame,(a) Case1 and (b) case2 at noonand (c) case1 and (d) case2 at sunset(1800LT).
and wasstudiedby Lie-Svendson et al. [1991]usingthe Boltzmann equationto obtain respective f* valuesof 1-4% and scenarios are dominated by N(2D) quenching andNO+ dis- 0.01%,respectively. As notedbyLie-Svendson et al. [1991],the sociativerecombination yieldingaf* --- 5 % (at 105km) since discrepancy with Solomon[1983]is mainlyexplainedby Sothe fragmentsare at ---1.4eV from theseprocesses. Electron lomon'suseof a reactivecrosssection100 timeslarger than the impactandphotodissociation of N2 arebothnegligibleat such acceptedvalue.Solomon[1983]useda hot atomreactionrate high solarzenith angles.However,it is evidentthat at noon ---2.0x 10-• cm3 molecule -• s-• basedon the hightemper(overheadsun)theseprocesses becomethe dominantmecha- ature Arhenius frequencyfactor from laboratory measurenisms for hotN(4S) production. TheN(4S) atomsproduced ments.The effectiverate determinedby Lie-Svendsonet al. from photodissocation are not energeticenoughto reactwith [1991]for0.3eV atomenergy is •'10-•3 cm3 molecule -• s-• 02 anddo not contributeto the fractionreacting.If the case1 whichmostlyexplainsthe changefrom 1% to 0.01%. electronimpactenergydistributionis adopted,then the fracThe Lie-Svendson et al. [1991]studydid not intendto focus tion reacted is 2.5-3% at 105 km. However, if the case 2 on process H1 but ratheron electronimpactprocesses H2 and distributionis adopted,the fractionreactedis 12-15% at 105 H3 (Table 1) for the aurora, but their assumption of 0.3 eV for km for noon conditions.
The resultingaltitude profile of f* is shownin Figure 6 for
sunrise,sunset,and noon conditions.The sunriseand sunset
The literatureon the role of suprathermal nitrogenatomsin producing NO issomewhat confusing withresultsrangingfrom one extremeto the other. The presentHSA resultsdisagree with the previousinvestigations of Solomon[1983]but agree with the resultsof Lie-Svendson et al. [1991]for the following
the translational energyof N(4S) atoms(following Solomon [1983])corresponds insteadto processHi. As discussed, processH3 resultsin energiescenterednear at least---0.65eV for
the N(4S) atoms.The present(algebraic andMonteCarlo) resultsfor the fate of 0.3 eV N(4S) atomsproduced from
agreeswith Lie-Svendson et al. [1991].The reasons. Process H1 ofTable1 (solar CIII 977• linephoto- photodissociation
dissociation of N2producing nascent N(4S) atoms withenergy conclusionfrom these0.3 eV analysesis that there is an insigof 0.3 eV) was initially studiedby Solomon[1983] usingan approximate algebraicapproachbasedon a simplerate ratio
nificant(f*
--- 0.01%) contributionto NO formationfrom
theN(4S)atoms produced bysolar CIII 977•linephotodis-
11,586
SWAMINATHAN
ET AL.: LOWER THERMOSPHERIC
150 :•
::
1401•! Iil, b! ! •i,
1;o il, 120
NITRIC
OXIDE
ABUNDANCE
rameter;in fact, it is usualpracticeto lump theseparameters together [e.g., Siskindet al., 1995] into a singleparameter
(fl
+ f*) whichis takento be the effectiveN(2D)-like
,200 LT (Cose 2) --
ooo.T
.........
•
1800 LT
---
__• NO densities to solarsoftX raysin the18-50• interval by :-
•'•
:
:
branchingratio. This studyinitially examinedthe model sensitivityof peak
varyingthe input X ray flux for the ATLAS 1 period about the
average YOHKOH valueof 0.055ergscm-2 s-1. The [NO] abundancewas found to be relativelyinsensitiveto the X ray
fluxin the18-50,• bandpass' a 50%change in thefluxlevel
110
leadsto only an ---10% changein the abundanceof NO. This
explainsthe large X ray scalingsrequiredby Siskindet al. [1990,1995].The sensitivityof the peak NO abundanceto the
1 O0
0.00
0.05
0.10 Fraction
0.15
0.20
0.2
Reacted
Figure6. Fractionof hot N(4S) reactedas a functionof altitude usingthe energydependentthermalizationcrosssection of Gerardet al. [1997].
18-50,• fluxwasfound toincrease slightly withthevalueoff* used (fixedf* runs were made for this comparison)thereby demonstratingthat the influenceof X raysentersmainly via
thesuprathermal N(4S) atommechanism. On thebasisof the
relative insensitivity to 18-50• fluxit isreasonable to concludethat YOHKOH data providedadequateinputvalue of
theimportant part,namely, the10-18• interval ofthesoftX ray fluxto whichthe peak [NO] densitieshavebeenpreviously reportedto be quite sensitive[Swaminathan et al., 1994]. sociationof N2, the processoriginallyconsideredby Solomon Figure 7 showsthe primary resultsof this paper: CT1D [1983]. profilesof [NO] for sunriseand sunsetconditionsusing the
6.
CT1D Model Results and Discussion
case1 (0.6eV peak)andcase2 (3-4 eV peak)N(4S) atom energy distributions fromelectron impact ofN2.Fordatacom-
parisonthe ATMOS measurementis used[the "egl" casefrom KrishnaKumar et al. [1995] is shown;the other caseswere In this subsectionthe CT1D modelresultsusingthe Strobel similar].To illustratethe sensitivitiesfor each case,Figure 7 et al. [1987] model C parameterizationfor the vertical eddy showsthe model profiles obtainedby varyingthe hot atom diffusioncoefficientKzz are presented.TableA2 givesa list of reactivefractionby an arbitraryscalefactors from 0 to 4 where nitric oxide source and sink reactions with their rates as ins = 1 correspondsto the nominal profile for each energy cluded in the CT1D model. Table A3 lists key rates and distribution case. All model runs are based on the Gerard et al. branching ratiosfortheproduction ofmetastable precursors of [1997] energydependentenergytransfercrosssections.For nitric oxide. A number of the additionalthermosphericpro- case1, there is onlya factor of 1.2 enhancement in peak [NO] cesses listed in Tables A4a-A4c also influence the nitric oxide betweens = 0 ands - 1. Case1 profilesunderpredictNO for abundance.Barth [1992] has reviewedthe photochemistryof all scalingsand indicatean overallminor role for suprathermal NO and highlightedthe sensitivityof its abundanceto various N(4S) atomsin the [NO] abundance problem.This strong rate parameters,soft X ray flux levels,and auroral energy conclusion appliesto themeasured N(4S) energydistribution inputs.However,until recently,one of the mostuncertainand of Cosby[1993]. The Cosby[1993] distributionis a likely sceimportantrate parametershasbeenthe branchingratiofl to nario if predissociation of N 2 representsall importantcontribproduceelectronicallyexcitedatomicnitrogenin one of the utingchannelsin the electronimpactdissociation of N 2 (howdoubletstatesuponelectronimpactdissociation of molecular ever, see Cosby'scommentsregardingother channelsin the nitrogen.The doubletyieldbranching ratio for N(2D, 2p) light of his descrepancywith Winters[1966] for the total cross productionby electronimpact dissociation of N 2 can be in- section).The case2 profiles,on the other hand,showa factor ferred from Table 2 of Zipf et al. [1980] (includingonly the of 2.5 betweens = 0 and s = 1 and the peak [NO] density neutraldissociation channels)to befl = 0.54. Note that this approachesthe observedNO densityfor a scalingof s = 4. branching ratio is also consistentwith recent experiments However, this is a large scalingfactor, and the known uncer[Walteret al., 1993;Cosby,1993]coveringelectronenergiesup taintiesin the presentenergydistributionsand crosssections to 148.5eV therebyprovidingadditionalconfidence. The yield do not promisesucha largescaling.Hencethe deficitremains uncertaintyhas been givenby Walteret al. [1993] as no more in spiteof a completetreatmentof the knownmechanisms of than2%. However,Ajelloand Ciocca[1996]haveveryrecently NO production. measuredadditionaldoubletproductionfrom the dissociative Detailsof the resultsare now examinedto gainsomeinsights eXCitation channel. Including thisadditional source andafter into the underlyingprocessescontrollingthe present model accountingfor the uncertaintyin the total crosssectionsmea- description.Plate 1 showscontoursof the constantproduction suredby Winters[1966] and Cosby[1993],we infer an upper rate of [NO] from the suprathermal N(4S) atomsourcefor limit of 0.62 for the doubletyield for this study.The sensitivity case 1. Note that the maximum rate is ---75 cm-3 s-• centered of model resultsto the value off 1 wasinvestigatedin the range slightlyafter localnoon and coincidentwith the lowerthermoof estimateduncertainty(0.54-0.62). The role of suprathermal spheric[NO] peakaltitude(---107km),showing thatthiseffect, N(4S) atomsisincluded byincorporating intotheCT1Dpho- thoughmodest(when diurnallyaveraged,---1.5%and 8% of tochemicalmodel the diurnallyvaryingheightprofile of f* themetastable N(2D) source of NO for cases 1 and2, respeccalculated as described in section 5. Previous studies have tively;see Table 2), is concentratedwhere it is most needed.
6.1. ReSUlts Usinga FixedK....Profile
considered thebranching ratiof l asanuncertain inputpa-
Similar contours are found for case 2 with a maximum
of 500
SWAMINATHAN
ET AL.: LOWER THERMOSPHERIC
NITRIC
ABUNDANCE
'"•'•! l/ ....... ',% .... i ........ i ........ is=1 ...... /,'•i ' '•',i\ s=0.5,2.0 L
,:1..,•
120
•
•.,.,.\
.: x,(..,,.
s=0.25,4.0
7 120kI'..'"7, '\\"•["..... ,,
x.X'.., (
-
•x
• ,'/
"-'
NO
(•
100
/
%, '. •
•
•
•
x
'x,x
,,,...:1 / I ' /
'•...... ., '..,
•' lOOk zl'"/;N(S)
•',,/
/.'
........ , ......:f?5 ,
8O
105
lO4
106
.I
/
09
108
104
10`5
Density(cm-•)
Sunset[N4S][NO] sensitivities
,
140
...... 0.2540 •.
120
120
........
,
-
N(4S) q- O(3p)) andhencean enhancement of thereactionof N(2D) with02 to augment the NO abundance.Note that the thermosphericpeak correlation with Kzz was not observedby Fesenet al. [1990] sincethey
140
preferred a higheddyrate(-1 x 107cm2 s-]) coupled witha slightlylowerquenching ratefor N(2D) + O. Bothof these renderatomicoxygenineffectivein quenching N(2D) to N(4S). Hence,in theirmodelmostN(2D) reactsto formNO.
120
In our model,with a lowerKzz and a slightlyhighervalue for
N(2D) quenching, atomicO iscompetitive with02 onthefate of N(2D). 100 Diurnal Averages
........ %[N(4S)] __ 8O
%[NO]
J
to) I
-100
-5O
o
50
oo
% Densitychangedue to N*(4S)
140 ' ' ' ' '/'• ' ' ' ' ,
.•.
'
'
12o
E
qD
,
'_•
