Ionospheres of the Terrestrial Planets -Schunk 1980

8/3/2019 Ionospheres of the Terrestrial Planets -Schunk 1980

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REVIEWS OF GEOPHYSICS AND SPACE PHYSICS, VOL. 18, NO. 4, PAGES 813-852, NOVEMBER 1980

Ionospheresof the Terrestrial PlanetsR. W. SCHUNK AND A. F. NAGY

Space hysics esearch aboratory,Universityf Michigan,AnnArbor,Michigan 8109The theoryand observationselating o the onospheresf the terrestrial lanetsVenus, he earth,andMars are reviewed.Emphasiss placedon comparing he basicdifferences nd similarities etween heplanetaryonospheres.he reviewcovershe plasma nd electric-magneticield environmentshat sur-round he planets,he heory eading o the creation nd ransport f ionizationn the onospheres,herelevantobservations,nd the most ecentmodelcalculations. he theorysectionncludes discussionof ambipolar iffusionn a partially onizedplasma, iffusionn a fully ionizedplasma, upersonicplasma low,photochemistry,nd heatingand coolingprocesses.he sections n observationsnd modelcalculationsoverheneutral tmosphereomposition,he oncomposition,heelectron ensity, nd heelectron, on, and neutral temperatures.

CONTENTSIntroduction .......................................................................................13Planetary environment .......................................... ............................813Venus ...............................................................................................814Earth ....................................................................... ......................... 16Mars ................................... ......................................... ..................... 19Theory .................................................................................................20Transportequations.......................................................................20Collision terms ...............................................................................22Collision requencies.....................................................................22Ambipolar diffusion ...................................................................... 23Diffusion n a fully ionizedplasma ..............................................25Supersonic lasma low .................................................................26Additional transporteffects...........................................................27Photochemistry.............................................................................. 29Heating and cooling ates..............................................................29Observations....................................................................................... 32Neutral atmospheric omposition...............................................32Ion composition ............. ............... ............... ............... ............... ....33Neutral gas emperatures..............................................................34Ion temperatures ............................................................................ 834Electron temperatures ................................................................... 35Electrondensity .............................................................................36Model calculations..............................................................................36Venus .............................................................................................. 37Earth ................................................................................................840Mars ................................... ......................................... ..................... 46Concluding emarks ..........................................................................47Appendix .................................... ......................................... ................ 47

1. INTRODUCTIONThe in situ exploration f the upper atmosphere/iono-sphere ystemsf planets ther han he earth,whichbeganwith the Viking I and 2 Mars anders Nier and McElroy,1977;Hanson t al., 1977], as akena greatstep orwardwiththecomprehensiveeasurementshicharebeingmadewiththe PioneerVenus-bornenstrument omplementcf. Colin,1979].This greatlyenhanced, lthough till limited,data basefromVenus ndMars,coupled ithourcontinuouslyncreas-ing understandingf the terrestrialonosphere, hichderivesfrom numerousocket,satellite, adar backscatter,nd theo-reticalstudies, rovides he impetus nd the material or thisreviewof the ionospheresf the terrestrial lanets. or.sim-plicity, hroughouthe review he term ionospheres'ill beused o mean he ionospheresf Venus, he earth,and Mars. Permanentddresss Centeror AtmosphericndSpace ciences,

Utah State University, Logan, Utah 84322.Copyright 1980by the AmericanGeophysical nion.

Ionospheric esearchduring the last two decades as shownthat the earth's onosphere xhibitssignificant ariation withsolar cycle, season,geomagnetic ctivity, altitude, latitude,and local time. Undoubtedly, he Venusianand Martian iono-spheres xhibitsimilarvariations, ut at present, he existingdata base s not sufficientor a comprehensiveomparison fthe ionospheresor sucha wide rangeof conditions.Thereforein this reviewwe have concentratedn comparinghe grossfeaturesof the ionospheres,mphasis eing placedon eluci-dating the chemicaland physicalprocesseshat control ono-sphericbehavior. n addition to pointing out the similaritiesand basic differences etween he ionospheres e have alsohighlighted omeunique eatures ssociated ith eachplanet.Our review is fairly comprehensive nd is directed towardthe noviceor nonexpertwho may wish o pursuestudiesn anexciting and rapidly changing area of planetary science.Thereforewe have attempted o presenta reasonably om-plete picture of the gross eaturesof ionospheric ehavior forVenus, he earth, and Mars. We have alsoattempted o avoidunnecessaryepetitionof materialcovered n previous eviewsand textson the ionospheresf Venusand Mars [WhittenandColin, 1974;Bauer, 1973;McConnell, 1976],and we have em-phasized esultsobtainedover the last few years.

2. PLANETARY ENVIRONMENTIn this sectionwe briefly discusshe physicalcharacteristicsof the terrestrialplanetsand the plasmaand electric-magneticenvironments hat surround he planets.The surroundingen-vironmentsand the physicalcharacteristics etermine he dy-

namical processes cting within and on the ionospheres, ndtherefore a descriptionof the planetary environments s cru-cial to an understanding of ionospheric behavior. A com-parison of the relevant physical characteristics f Venus, theearth, and Mars is shown n Table 1. Of the three planets, theearth and Venus are more similar to each other than to Marswith regard to most of the physicalcharacteristics, et as far asplanetary ionospheres re concerned, t is the Venusian andMartian ionospheres hat are most similar. This behavior canbe traced to the neutral atmosphereand the intrinsic magneticpropertiesof the planets,which play an important role in thechemistry and solar wind/ambient plasma interactions, re-spectively.Both Venus and Mars have at best a very smallmagnetic moment, while the earth possesses relatively largemagnetic moment.The solar wind is known to be highly variable in the vicin-

Paper number 80R0632.0034-6853/80/080R-0632506.00813


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814 SCHUNKAND NAGY: ONOSPHERESF TERRESTRIALLANETS

PlanetMass,1023

kgMean

Radius,103 m

TABLE 1. PhysicalCharacteristics f the Planets

Venus 48.7 6051Earth 59.8 6371Mars 6.43 3390

Gravita- Averagetional Distance LengthAccel- From Orbital oferation, Sun, Eccen- Year,m s 2 109m tricity days

MagneticPeriod of DipoleRotation, Moment,days G cm38.90 108 0.007 224.79.81 150 0.017 365.33.73 229 0.093 687

-243


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SCHUNKNDNAGY:ONOSPHERESFTERRESTRIALLANETS 815Pioneerenus TBow hock rossings

++. . + +

+

Ionopause//"I 0 -I

x'()Fig. 1. Locationof 172 PioneerVenusOrbiter bow shockcross-ingsand he oard ionopause,aken o be approximatelyircular ta mean altitude of 3 km [from Slavin et al., 1980].

planet the manner hown chematicay Figure2, andthe field strength creasesas the planet s approachedRus-sellet al., 1979].On early PioneerVenusorbits,peak fieldstrengthsf 460 y(1 y 10 gauss) eremeasured,he ex-actvalueberg highlyco,elatedwith the solarwd dynamicpressureWoe et al., 1979;Russell t al., 1979].The peakfieldstrengthsccur a regionwhere he flowstagnatesnd

IONOSPHERE

Fig. 2. Schematicllustration f the magneticield near Venus na plane containing he solarwind vectorand the magnetic ector,which s assumedo be perpendicularo the solarwind vector;B andE are magnetic nd electric ields, espectivelyfrom F. $. Johnsonand W. B. Hanson, 1979].

2OOO

1600 -

BOO-400-

0

PIONEER VENUS OIMS DECEMBER 1971] ' I ] ' I ' [ I ' ' I ' I ' '

0'"

ORBITi01

I J I I [ I t I 10 10 10 10 10

IONS crnFig. 3. Altitude profilesof O + measured y the orbiter ion massspectrometer n PioneerVenuson four differentdays, ndicating heextremevariability of the ionopause eight [from Taylor et al., 1979a]. 1'by the ' , ,k n ........... copyright ,,, American n oo ;,;. c,Science.

where he plasmadensity s low, the low density esultingfrom the escape f plasmaalongmagnetic ield lines Lees,1964;Zwanand Wolf, 1976;F. $. Johnsonnd W. B. Hanson,1979], sshownn Figure . Here hemagneticressureB2/8rr)approximatelyalanceshe solarwinddynamic ressure

Direct observations ave shown that for the suprathermalplasmaE > 10 eV) there s a finite ransitionegion orVenusmantle)between he ionosheathnd the ionosphere[Verigin t al., 1978; pennert al., 1980]. his ransitione-gionwas ound o surroundhe rontsideof the onosphereand extend to more than 8 Rg downstream.The ionopauseorrespondso the upperboundary f theionosphericlasma.On the daysidehere s a relativelyabrupt utoffn theconcentrationf ionosphericonsas hehigh-speedolarwindcompressesnd sweepsway' heam-bient ionization,producing zone of streaming nergeticplasmaTaylortal.,1979a]. n henightsidesharp ensitygradientionopause)s alsoobserved;owever,hemecha-nism or mechanisms)esponsibleor thisnightsideensitydiscontinuitys not well understoodt this ime.Both the thickness and the altitude of the ionopause arehighlyvariable, ependingrimarily n the solarwindrampressure,lthoughhe nterplanetaryagneticieldgeometrymayaffecthecoupling ith hesolarwind.The hicknessanvary roma fewkilometerso manyhundredsf kilometers[Bracetal.,1979a], hilehealtitudean ary rom bout 00to 2000km [Tayloret al., 1979a;Knudsent al., 1979a;Braceet al., 1979a]. he extremeariability f the onopauseltitudeis dramaticallyllustratedn Figure3, where t is shownhatlarge onopauseltitude hangesanoccurromday o day.The ionopauseccurs t the altitudewhere he ionosphericthermal ressure(Te + Ti) approximatelyalanceshemagneticressure2/8r,ustoutsidehe onopause,hichnturnapproximatelyqualshe solarwinddynamic ressure;thereforehe extreme ariabilityof the onopauseltitude s amanifestationof the extreme variability of the solar wind[Bracetal.,1979a;nudsental.,1979a;aylortal.,1979a].Contraryo initial heoreticalalculationsased n hydro-dynamicheory Spreitert al., 1970]he onopauseeyondthe terminatordoesnot follow the opticalshadow ine but in-stead emainselatively loseo the surface f thePlanet. hisis shownn Figure , whereonopauseeights replotted safunctionof solarzenith angle SZA) (L. H. Brace,private


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816 SCHUNK AND NAGY: IONOSPHERES OF TERRESTRIAL PLANETS

3800

3000

240018001200

800

0 $0 80 $0 120 150 180SOLAE ZEN I TH ANGLE (DEG)

Fig. 4. The ionopause ltitudesmeasured y the electron emper-ature probe experiment [Brace et al., 1979a] between December 5,1978,and August9, 1979,on PioneerVenus PV). The hatchedareasare inaccessibleo the presentPV orbit. The ionopausemodel in-dicated s a leastsquaresit to the data points from Braceet al., 1980;L. H. Brace,private communication,1980].communication, 1980). The ionopauseheights shown corre-spond o all available nbound and outboundcrossingse-cordedby Braceet al. [1979b]betweenDecember5, 1978,andFebruary 28, 1979.The observed verage onopause ltituderises rom 330 km at the subsolarpoint to 1000 km at the ter-minator. The orbit to orbit changes ary from about 19%at 0 SZA to 36% at the terminator; across he nightside,where or-bital constraints lace serious imits on the determinationofthe average onopauseconfiguration, he variabilities aremuch greater.Measurements f the magnetic ield strength n the Venusionospherey Russell t al. [1979]ndicatehat he ieldstrengthis generallyweak, usually only a few gammas.Since thefield strength ust outside he ionopause s much greater 40-60 ),an ionopause urrentmust flow that effectively hieldsthe ionosphere.Although the magnetic ield strength n theionospheres generallyweak, it can reachmany tensof gam-mas for periodsof the order of seconds. n example s shownin Figure 5, where the high-time-resolutionmagnetic fieldstrength s plotted versusuniversal time. Very large fieldstrengthswere measurednear periapsis in the ionosphere),but only for very brief periodsof time. Further investigation

MAGNETOSHEATH


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SCHUNK NDNAGY: ONOSPHERESFTERRESTRIALLANETS 817and orientation of the bow shockvary with the interplanetarymagnetic ield direction and the solar wind Alfv6n Machnumber. Recently,Formisano 1979]studied he three-dimen-sional characterof the averagepositionand shapeof the bowshock,usingdata obtained rom 2500 shockcrossings y Heos1, Heos 2, and five Imp spacecraft. his studyshowed hat theshocksurface s symmetricwith respect o the ecliptic planeand has an averagenoseradius of 11.9 Re (earth radii) for anaverage olarwind dynamicpressure haracterized y ne= 9.4cm 3 and V = 450 km/s.

Downstream of the bow shock the shocked solar windplasma s divertedaround he hard obstaclen a regioncalledthe magnetosheath. he magnetosheathhicknesss approxi-mately 3 Re near the subsolarpoint but increases apidly inthe downstreamdirection.After being decelerated y the bowshock the heated solar wind plasma is acceleratedagain fromsubsonic o supersonic low. The interaction of this magneto-sheath flow with the geomagnetic ield acts to produce anopen 'magnetic aft' that extendsseveralhundredearth radiiin the antisolar direction.

The boundary layer that separates he confined geomag-netic field from the magnetizedsolarwind plasma n the mag-netosheath s called the magnetopause.The magnetopause sgenerallyvery thin (, 100 kin), and its location s determinedby a balance between solar wind impact pressureand the

ring current is in part the earthward extensionof the plasmasheet. It is composedof trapped energeticprotons and elec-trons which grad-B drift in oppositedirections,causinga ringof current to flow around the earth. During magnetic storms,ring current particlescan transfera substantialamount of en-ergy to the ambient electrons,and this energy is conducteddown along geomagnetic ield lines to the ionosphere,causingelevated electron temperatures.The plasmasphere s a toms-shapedvolume that surroundsthe earth and contains a relatively cool, high-density plasmaof ionospheric rigin. The plasma n this regioncorotateswiththe earth, but it can also flow along geomagnetic ield linesfrom one hemisphere o the other. In the equatorial plane theplasmasphere as a radial extentof about4 Re, and its bound-ary, called the plasmapause, s marked by a large decrease nelectrondensity as one leaves he plasmasphere. he plasma-pause s essentially he boundary between plasma that coro-tates with the earth and plasma that does not.At high latitudes the plasma does not corotate with theearth but instead moves under the action of electric fields ofmagnetospheric rigin. In addition, thermal plasma s capableof escaping rom the topside onosphere long open' geomag-netic field lines, a process ermed the polar wind. This high-latitude region betweenabout 120 and 3000 km and the samealtitude range in the plasmaspheres the region of interest formagnetic ressuref thecompressedeomagneticield.Along this eview.The physicalmechanismsperatingn this1ow-al-the earth-sun ine on the dayside he magnetopauseadial po-sition is typically 9 Re, although t doesvary with solar windimpact pressure.An extensive urrent flows along the magne-topause,which acts to modify the magneticenvironment nthe vicinity of the magnetopause Heikkila, 1972]. On thefront of the magnetopause he current flow is primarily fromdawn to dusk, but it acquiresan increasingmeridional com-

ponent as it flows toward the taft of the magnetosphere.The domain where the earth's magnetic field dominates iscalled the magnetosphere. his large region is populated bythermal plasma and energeticchargedparticlesof both solarwind and ionosphericorigin. On the dayside the solar windplasma n the magnetosheath as direct access o the iono-sphere n the vicinity of the polar cusp (or cleft). At iono-spheric heights the cleft occupiesa narrow latitudinal bandthat is centered near noon but is extended in longitude.Within this band, energeticparticle precipitation affects heionospheric lasma hroughheatingand ionizationprocesses.The mantle, plasma sheet, and ring current are primarilycomposedof magnetosheathplasma that has been injectedinto the magnetosphere y mechanismshat have not yet beenfully established cf. Hill and Wolf, 1977; Burch, 1977]. How-ever, recent satellite observations f energeticO + and He + inthe ring currentand plasmasheet ndicate hat the ionospheremay make an appreciable ontribution o the total populationof the magnetosphere R. G. Johnsonet al., 1974; Sharp et al.,1974, 1976]. The mantle is thought to be an intermediate re-gion through which magnetosheath lasma travelson its wayto the plasma sheet. This region is characterizedby a slowantisunwarddrift of plasma, anisotropic emperaturedistribu-tions, and low densities elative to those ound in the magne-tosheath.The plasma sheet particles have an average energy10 times that found for magnetosheath articlesand a densitythat is lower by a factor of 10-100. A current flows across heplasma sheet rom dawn toward dusk (neutral sheetcurrent)and acts to separate the two regions of oppositely directedmagnetic lux in the magnetosphericaft [Heikkila, 1972].The

titude regime will be discussedn more detail in the para-graphs that follow.Owing to the interactionof the shocked olarwind with thegeomagneticield, an electricpotentialdifference s generatedacross he taft of the magnetosphere,he resultingelectric ieldpointing from dawn to dusk [cf. Stern, 1977]. This cross-taftpotentialdifference s typically 76 kV but can vary from 20 to120 kV dependingon the level of geomagnetic ctivity. Exceptfor isolated regions and brief periods the geomagnetic ieldlines are equipotentials ue to the high electricalconductivityalong field lines. Consequently, his cross-taft otential differ-ence s mapped nto the high-latitude onosphere s an electricfield that is directed perpendicular to the geomagnetic ield.At ionospheric eights his perpendicular or convection)elec-tric field is typically25-50 mV m in the polar cap [BanksandDoupnik,1975]but canbe muchgreater han 100mV m in restricted latitudinal bands [Heelis et al., 1976; $middy etal., 1977;Spiro et al., 1978].Only the high-latitude onosphereis influenced directly by the magnetospheric lectric field,since most of the time the ring current providesan effectiveshield for the plasmawithin the plasmasphere.The effect that the perpendicular electric field has on theionospheredependson attitude, as s shown n Figure 7. At allionospheric ltitudes he electron-neutralollision requencyis much less han the electron cyclotron requency,and hencethe combined effect of the perpendicularelectric field Ex andthe geomagnetic ield B is to induce an electrondrift in the Exx B direction. For the ions, on the other hand, the different

ion-neutral collision frequenciesare greater than correspond-ing cyclotron requencies t low altitudes E region), with theresult that the ions drift in the direction of the perpendicularelectric field. As altitude increases,he ion velocity vectors o-tate toward the Ex x B direction owing to the decreasing on-neutral collision requencies.At F region altitudes >160 km)both ions and electrons drift in the Ex x B direction.A consequenceof the electron and ion motion shown inFigure 7 is that aboveabout 160 km the ionosphere rifts hor-


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818 SCHUNK AND NAGY' IONOSPHERES F TERRESTRIALPLANETS

160km,I!iI'I

i,IR3km -

Fig. 7. Ionosphericon and electrondrift velocities erpendicularto the geomagneticield due to the actionof the magnetosphericlec-tric field (adapted rom Banks 1978]).izontally under the action of the magnetospheric lectric field.The electric field induced drift pattern is a two-cell patternwith antisunward low over the polar cap and sunward low atlower latitudes. When the corotation velocity is added to thistwo-cell pattern, the resultingdrift pattern for the ionosphericplasma is similar to that shown n Figure 8. Also shown inFigure 8 are the locationsof the high-latitude onization hole,the main trough, and the quiet time auroral oval. The ioniza-tion hole is a low-density, low-temperature region that ap-pearsduring quiet geomagnetic ctivity. The auroral oval is aregion where intenseparticle precipitationoccurs,which leadsto the creation of ionization and electron, on, and neutral gasheating. The main trough is a region situated just equa-torward of the nightsideauroral oval that is characterizedbyvery low electron densities. ypically, peak electrondensitiesin the main trough are a factor of 100 lower than those foundon either side of the trough.Another consequence f the electronand ion motion shownin Figure 7 is that the magnetospheric lectric field drives ahorizontal current through the E region owing to the differingelectron and ion drifts. This horizontal current is coupled tothe magnetospherehrough field-aligned Birkeland) currents.The field-alignedcurrent pattern for both quiet and disturbedgeomagnetic onditions s shown n Figure 9 for the northernhemisphere. The field-aligned currents are concentrated ntwo principal areas which encircle the geomagnetic pole[Zrnuda and Armstrong,1974; ijirna and Poternra, 1976].Thepoleward current regions exhibit current flow into the iono-sphere n the morning sectorand away from the ionosphere nthe evening sector, while the equatorward current regionscontain current flows n the oppositedirectionsat a given lo-cal time. The basic field-aligned current flow pattern is thesame during geomagneticallyquiet and active periods, al-though the regionswiden and shift to lower latitudes duringdisturbedperiods.The magnitudesof the currents n the pole-ward and equatorwardregionsare not well known, but it ap-pears that the net current is inward on the morningsideandoutward on the eveningside n the northern hemisphere Iij-irna and Poternra, 1976].

As was noted above, particle precipitation and Joule dis-sipation n the auroral oval produceheat for the neutral gas,and this heat input can significantlyaffect the thermospheric

wind pattern Nagy et aL, 1974].Without auroralheating, hethermospheric ind would blow acrosshe polar cap from thedayside o the nightsidebecauseof solar heating. However,auroralheatingcauseshe thermospheren the vicinity of theoval to expand,as s shownschematicallyn Figure 10. On thedayside this expansion can retard or reverse the thermos-pheric wind, dependingon the auroral heating rate. On thenightside, uroralheatingand momentum orcingby the con-vecting ionosphereact .o produce the so-calledmidnightsurge.This feature,which occurs uringgeomagnetic torms,correspondso very strongequatorwardwindsnear midnight,the wind speed reaching 600 m/s on occasion Sipler andBiondi, 1979].At mid-latitudes,where the geomagneticield lines are in-clined to the vertical direction, the effect of a meridional(north-south)wind is to force the F region onizationup ordown field lines, dependingon the direction of the wind. Anequatorwardwind drives the ionization to higher altitudes,where chemical loss rates are lower, while the reverse is truefor a polewardwind. The equatorwardwind at night thereforeacts o maintain F layer ionization,while the poleward windduring the day acts o depresshe F layer.Another transportprocess hat affects he mid-latitude ion-ospheres the interhemisphericlow of plasmaalonggeomag-netic field lines. Figure 11 shows he direction of the inter-hemispheric low for solstice onditions,as deduced rom dataobtained at Millstone Hill. In the summer hemisphere heflow is upwardand out of the topside onospherehroughoutthe day and night. n the winter hemispherehe flow is up-ward during the day and downward at night. This inter-hemisphericlow will be either a sink or a sourceof F regionionization, dependingon whether the flow is out of or into theionosphere.

At low latitudes he geomagneticield inesare nearlyhori-zontal, and this introducessome unique transport effects.First, thermospheric inds blowing across he equator romthe summer o the winter hemisphere an effectively nducean interhemisphericplasma flow along geomagnetic ield1250 o

18 6

0MLTFig. 8. Schematicllustrationof the earth'spolar cap showingconvectionrajectories f F regionplasma solid ines) n a magneticlocal time (MLT), invariant latitude reference rame. Also shown are

the locations f the high-latitudeonizationhole, the main trough,and the quiet time auroral oval of Feldstein nd Starkov 1967] Thefigure s taken from Brintonet al. [1978].


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SCHUNK AND NAGY: IONOSPHERESOF TERRESTRIAL PLANETS 819

IALI 100 IALI > 100 12' 1214 10

!6 18 6

20

2 220 Currentnto onosphere 0(a) -.-.'..-.v.urrentwayromonosphere {b)

Fig. 9. The distributionnd lowdirectionsf large-scaleield-alignedurrents eterminedrom (a) dataobtainedfrom439 passesf Triad duringweaklydisturbedonditions[AL[< 1007) and b) dataobtainedrom366Triad passesduringactiveperiods [AL > 1007) [from ijirna and Poternra, 978].lines, as is shown schematically n Figure 12. As the iono-sphericplasma iseson the summersideof the equator, t ex-pandsand cools,while on the winter side, t is compressedndheated as it descends.

Another interesting ransport effect at low latitudes s theso-calledequatorial ountain. n the daytime equatorialF re-gion, eastwardelectric fields associatedwith neutral wind in-duced onospheric urrentsdrive a plasmaconvection hat isupward. The plasma ifted in this way then diffusesdown themagnetic ield linesand away from the equatorbecause f theaction of gravity. This combinationof electromagnetic riftand diffusionproduces 'fountainlike'patternof plasmamo-tion, as shown n Figure 13. The resultof this plasma trans-port is that ionizationpeaksare formed n the subtropics n

EXPANSION DUE TO EXPANSION DUE TOHEATING AURORAL HEATING

Fig. 10. Schematic llustration showing he heating region in theearth's auroral zone and antisolar ion convection, which drives a ther-mosphericwind through the Harang discontinuity region near localmidnight [from Babcockand Evans, 1979].

each side of the magnetic equator (often called Appleton'speaks).Mars

At the present ime, very little experimental nformation ex-ists on the particle and magnetic ield environments hat sur-round Mars. Although Mariner 4 [E. J. Smith et al., 1965]crossed he distant bow shock and the Viking mission pro-duced onosphericparticle data [Hansonet al., 1977], he mainsource of experimental information derives from the Sovietorbiter missions Mars 2, 3, and 5, which were conducted dur-ing 1971-1974 [Gringauz, 1976; Dolginov, 1976; Vaisberg,1976].The Sovietobservationslearly ndicate hat Mars possessesa strong,permanent bow shock.Figure 14 showsseveralbowshockcrossingss measured y changesn the ion flux energyspectra Bogdanov nd Vaisberg,1975]. Also shown n Figure14 are the Mariner 4 resultsand a calculated average'shockposition. The measurementsndicate that the bow shock oca-tion is variable, the estimatedstagnationpoint varying fromapproximately he Martian surface 3400 km) to 6000 km. Es-timatesof the upper imit of the bow shock hickness y Bog-danovand Vaisberg 1975] produceda value of about 100 km.

DAY NIGHT

SOLSTICEFig. 11. Schematic llustration showing he direction of the H +

flow along closedgeomagnetic ield lines at mid-latitudes or solsticeconditions from Evans and Holt, 1978]. Reprinted by permissionofPergamon Press.


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Magnetic field line

MYUossinZori zona I--I - uZUe=J + J W$=U os/'sin/'

Fig. 12. Schematic llustration showing he vertical ion drift (W)producedby a horizontalwind component n the magneticmeridian(U o). The dip angle is denoted by I [from Bittencourt nd $ahai,1978]. Reprinted by permissionof PergamonPress.During the time of thesemeasurementshe bow shockdid notshow significantdisplacementswithin 1 day.As the bow shock was crossed and the Soviet satellites Mars2 and 3 approached he planet, the hard magnetosheath pec-trum gradually softenedand becamesimilar to that expectedfor ions with a temperature of about 20 eV. The convectivevelocity of thesehot ions was well below the convectiveveloc-ity in the outer magnetosheath, nd their location coincideswith a region of increasedmagnetic ield of up to 15-30 .Thelocation of these deceleratedhot ions is shown in Figure 15.They appear to be inside the magnetosheath ut just abovethe Martian ionosphere.Owing to the lack of low-altitude measurements,t is notyet possible o determineunambiguouslywhether he magnet-osphereof Mars is associatedwith an induced or an intrinsicmagnetic field. Intn'ligator and Smith [1979] have comparedthe mechanismscontrolling solar wind interactionswith thethree terrestrialplanets.They find that for the earth the inter-action is clearly with the intrinsic magnetic field and forVenus it is the ionosphere which holds off the solar wind.However, they indicate that according o the presently avail-

able data it seems hat for the case of Mars the ionosphericplasma s not sufficientby itself to hold off the mean solarwind pressure-2.5 x 10 9 dyn/cm2), nd thereforeheybe-lieve that the interaction is both ionosphericand magnetic.This would imply that the solar wind interactionat Mars isunique, being ntermediatebetween hat for Venus and thatfor the earth. In this regard it should be noted that data fromthe radio occultation observationsof the Martian ionospheredo not showany evidenceof the variability that would be ex-pected f there were a direct solar wind interactionwith theionosphere.

3. THEORYIn this sectionwe discuss he chemical and physical proc-esseshat affect the structureof the ionospheres. mphasis splaced on describing he fundamental chemical and transportprocesseshat are characteristic f the partially ionized iono-sphericplasmas.Severalof the transportprocesses escribedbelow that were found to be important for the earth's iono-

sphere, uchas supersonic lasma low, diffusion-thermal eatflow, and anomalous esistivity,have not yet been studied orthe Venusian and Martian ionospheres.Also, metastablespe-cieshave not yet been included n chemicalschemesor Marsand Venus. Thus there is still much theoretical work to bedone before the basic physical and chemical processes per-ating in the Venusian and Martian ionospheres re deter-mined to a satisfactorydegree.TransportEquations

The quantitative study of the different flow situations hatare found in the ionospheress normally begun through theuse of conservationequationswhich describe he spatial andtemporal evolution of the concentration,drift velocity, andMAGNETICEQUATOR

0.0 3.0 6.0 g.OIOOO.O

800.0

600.0

400.O200.0

0.0

MAGNETIC LATITUDE (DEGREES)12.0 15.0 18.0 21.0 24.0

Z7.0

FLUX MAGNITUDE SCALE(10 cr2sec-)

Fig. 13. The pattern of plasma drift at low latitudesdue to the combinedaction of an electromagnetic rift acrossmag-netic field lines and plasmadiffusionalong ield lines. The magnetic ield lines are shownevery 200 km above he equator[from Hanson and Moffett, 1966].


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1

50-

4O

::50,

-I.0

,,n.T I I/11C

-202b4b610I 10 1.0 20 3.0 40 501RADIAL OISTANCE RoFig. 14. Martian bow shock rossingsccordingo ion flux measurementsn Mars 2 (circles) nd Mars 3 (squares)from December14, 1971, o May 12, 1972.The shortconnectinginesshow he uncertainty n determination f shock oca-tion due to telemetry equence. he curveshowshe meanshapeof the bow shock alculatedrom the observed rossingsby using he approximation f the parabolic ross ectionn solar-orientedylindrical oordinates.n the lower ight-handcorner he small-scaleigure s given o show he distantcrossingsf the bow shock.Two bow shock rossingsy Mariner4 in 1965 are shownby triangles from Bogdanov nd Vaisberg,1975].temperatureof the different speciesn the plasma. Recently,$chunk [1977] has presenteda general systemof transportequations or flowing neutral gasesand plasmas.This systemof equations, hichwasderived y usingGrads 1958] ormu-lation and Burgers' 1969] collision erms, has the advantageover previoussystemsn that the different species re allowedto have separatedrift velocities nd separate emperatures.Although the system of equations presentedby $chunk

[ 1977]containsa continuity,a momentum,an internal energy,a stress ensor, and a heat flow equation for each species, tsuffices or our purposes o consider ust the first three equa-tions at this point:Ons/Ot . (nsus) Ps'- Ls'ns (5)

nrns-+ Vps V %- nrnsG

xf r - CURVE

CURVE I

RADIAL DISTANCE Ro'Ei. 15. Observationsof the resion of soft ions on Mars 2 andMars 3 satellites thick lines).Bow shockcrossingsre shownby cir-cleswith the samenumbersas the corresponding oft on region ob-

servations.The solid line is a calculated bow shock, and the doubledashed urve epresentshe boundaryof the obstacle from Bogdanovand Vaisberg,1975].

I 1 ] 8Msnses E+--usxB - (6)c 8tDt Ps)ps(. s) . q+ %:us-- + Qs Ls (7)where Ds/Dt = O/Ot + us. is the convective derivative ofspecies , Ps = nkTs s the partial pressure, s s the numberdensity, ms s the mass,es s the charge, Ts s the temperature,us s the drift velocity, q is the heat flow vector,% is the stresstensor, Ps' is the ionization production rate, Ls' is the ioniza-tion loss frequency, Qs is the heating rate, Ls is the coolingrate, G is the accelerationdue to gravity, E is the electric ield,B is the magnetic field, O/Ot s the time derivative, V is thecoordinate-spacegradient, c is the speed of light, and k isBoltzmann'sconstant.The double-dot operator in (7) corre-sponds o the scalar product of the two tensors cf. Chapmanand Cowling, 1970]. The quantities8Ms/St and 8Es/St repre-sent he rate of momentumand energyexchange, espectively,between species and the other species n the plasma.The continuity, momentum, and energy equations (5)-(7)can be applied separately o each species n the plasma, butonly after heat flow and stress ensorexpressions re obtaineddo they constitute a closed set. With the general system oftransport equations presented by $chunk [1977], stressandheat lowale puton an equal ootingwithdensity, rift veloc-


10/40

822 SCHUNK AND NAGY: IONOSPHERESOF TERRESTRIAL PLANETS

TABLE 3. Momentum Transfer Collision Frequencies or Electron-Neutral InteractionsSpecies l nS--N202oHeHCOCO2

2.33X 10-11n(N2)[1 1.21X 10-4Te]Te1.82X 10-1n(O2)[lI-3.6X 10-2Tel/2]Te/28.9X 10-11n(O)[l 5.7X 10-4Te]Te/24.6 X 10-ln(He)Te/24.5 x 10-9n(H)[l 1.35x 10-4Te]T/22.34x 10-n(CO)[T + 165]3.68x 10-8n(CO2)[14.1x 10-1114500--el -93]

Data are taken from Itikawa [1978] and Schunkand Nagy [1978].ity, and temperature,nd it is therefore ecessaryo solveflow equationsn order to obtain the individualcomponentsof the stress ensor and heat flow vector. However, for mostapplications ealingwith the ionospheres,implified tressand heat lowexpressionsanbe adopted, ecausehese ono-spheresre collision ominated. hese implified xpressionsare presented ater.Collision Terms

Only for the simple ive-moment pproximation ave gen-eral collision terms been evaluated which apply to arbitraryinterparticle orce laws, large temperature ifferences, ndlarge relative drifts between he interactingspecies cf.Schunk, 977]. n thisapproximation,tress nd heat low areneglected, nd the behaviorof the plasma s expressedntermsof just the species ensity,drift velocity,and temper-ature. Since he drift velocityhas hree components,herearea total of five parametersdescribing ach speciesn theplasma.With the five-moment pproximationhe appropriate olli-sion terms take the form

8Ms = (8)8E=, nmr,t m + m, [3k(T,- Ts)'tts, m,(us--Ut)2(X)st]9)

where (I)stand st are velocity dependentcorrection actorsand 12sts the momentum ransfercollision requency or gasess and t. The momentum transfercollision requency s definedin a later subsection, nd relevantexpressionsor (I)st nd stare given by Schunk 197]. Here we merelynote that (I)stst = 1 forMaxwellmoleculenteractionsinteractionoten-tial that varies inverselyas the fourth power of the particleseparation) nd arbitrary elativedrift speeds swell as or ar-bitrary interactionpotentials nd low relativedrift speeds.The level of approximation hat includesboth stress nd

heat flow is called the 13-moment approximation. Unfor-tunately, at this level of approximation,collision terms forarbitrary interaction potentials have been derived only forlow-speed elative flows,where the species rift velocitydif-ferences are small in comparison to the species thermalspeeds. n this limit, collision erms have been derived forboth small and large temperaturedifferencesbetween he in-teractingspecies. he former are called Burgers' linear' colli-sion erms,and the latter, Burgers' semilinear'collision erms.For both cases he momentum and energy collision erms aregiven by8t= nsmsPst(Usut)4- 11Sts q qt (10)t

8E_ tn]nt,__,k(TTt) (11)t ms+ mtwhereps= n]n is the massdensity, t = msmt/(ms mt) s thereducedmass,and Tst= (mtTs 4- mTt)/(ms + mt) is the re-duced emperature.The quantity zt is a pure number hat isdifferent for different combinationsof speciess and t; repre-sentativevalues are given by $chunk [1977].The heat flow terms hat appear n the momentumcollisionterm (10) account or thermaldiffusion nd the effectof heatflow on ordinarydiffusion. n the 13-moment eat flow colli-sion erm thereare drift velocity erms cf. Schunk,1977],andthey account or diffusion-thermalffects.The 13-moment ystem f transport quationss capableofdescribing rdinarydiffusion,hermaldiffusion,hermalcon-duction, iscosity, iffusion-thermal,nd thermoelectricrans-portprocessest a levelof approximationhat s equivalentoChapman nd Cowling's1970] irst and second pproxima-tions,depending pon the process. owever, he 13-momentapproach asan advantage ver he Chapman nd Cowlingmethod in that the different components f the gas mixturecan have separate temperatures.CollisionFrequencies

Since the ionospheres re partially ionized plasmas, herelevant collision processesnclude Coulomb interactions,nonresonant ion-neutral interactions, resonant charge ex-change,and electron-neutralnteractions. he appropriatemomentum ransfercollision requenciesor two-body,elasticelectron-neutral nteractionsare given in Table 3.For Coulomb interactions the momentum transfer collisionfrequency akes he form

16(r)/2 ntmI -[st3/2es2etlnA (12)12st 3 m nt2-t ]-stTABLE 4. The Collision Frequency CoefficientsBst or Ion-Ion Interactions

H + 0.90 1.14 1.22 1.23 1.23 1.25 1.25 1.25 1.25 1.26He + 0.28 0.45 0.55 0.56 0.57 0.59 0.59 0.60 0.60 0.61C + 0.102 0.18 0.26 0.27 0.28 0.31 0.31 0.31 0.31 0.32N + 0.088 0.16 0.23 0.24 0.25 0.28 0.28 0.28 0.28 0.30O + 0.077 0.14 0.21 0.22 0.22 0.25 0.25 0.26 0.26 0.27CO + 0.045 0.085 0.13 0.14 0.15 0.17 0.17 0.17 0.18 0.19N2+ 0.045 0.085 0.13 0.14 0.15 0.17 0.17 0.!7 0.18 0.19NO + 0.042 0.080 0.12 0.13 0.14 0.16 0.16 0.16 0.17 0.1802 + 0.039 0.075 0.12 0.12 0.13 0.15 0.15 0.16 0.16 0.17CO2+ 0.029 0.055 0.09 0.09 0.10 0.12 0.12 0.12 0.12 0.14

t

s H + He + C + N + O+ CO + N2+ NO + 02 + CO2+


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SCHUNK AND NAGY: IONOSPHERES OF TERRESTRIAL PLANETS 823

TABLE 5. Momentum Transfer Collision Frequencies or Resonant on-Neutral InteractionsSpecies T, K Vin, --I

H +, H >50He + He >50N +, N >275O +, O >235N2 +, N2 > 170O2+, O2 >800H +, O >300CO + CO >525CO2+, CO2 >850

2.65X 10-ln(H)Tr /2 1 - 0.0831Oglo r)8.73x 10-lln(He)Tr1/2 1 -0.093 1Oglo r)3.83X 10-11n(N)Tr /2 1 -0.063 1OglOr)3.67X 10-11n(O)Tr/2 1 - 0.064 ogloTr)5.14x 10-n(N2)T,l/2 (1 - 0.069 Oglo r)2.59X 10-11n(O2)Tr/2 1 - 0.073 ogloTr)6.61X 10-n(O)Ti /2 1 - 0.0471Oglo /)3.42X 10-11n(eO)Tr/2 1 -- 0.0851OgOr)2.85X 10-11n(eO2)Tr/2 1 - 0.0831Oglo r)

Tr -- (Ti + Tn)/2. Data are taken from Banksand Kockarts 1973] and Butler [1975].The CO + andCO2+ collision frequencieswere calculated,not measured.where n A is the Coulomb ogarithm cf. Burgers,1969].Forthe ionospheres, n A 15, and the Coulomb collision fre-quencycan be approximated umericallyby

lSt' 1.27S2Zt2Astl/2tt Tst32 (13)where As is the particle mass n atomic massunits, As, s thereducedmass n atomic massunits, zs and zt are the particlecharge umbers, t s n cm 3, and Ts, s n degrees elvin. Forion-ion interactions his expressioneduces urther to

a tlst--'stst3/2 (14)where Bs, s a numericalcoefficient; aluesare given n Table4 for the ion speciesound n the ionospheres.quation 13)also reduces further for electron-electron and electron-ion in-teractions:

Pei54.5 zfiTe3/2 (15)54.5 neVee--- 1/2e3/2 (16)

where subscript denoteselectronsand subscript denotesions.Ion-neutral interactions can be either resonant or non-

resonant.Resonantchargeexchangeoccurswhen an ion col-lideswith ts parentneutral,or it canoccur ccidentally,s nthe case of the reaction H + + O H + O +. The relevant reso-nant ion-neutralmomentum ransfer ollision requencies regiven Table 5. Nonresonant ion-neutral teractions occurbetweenunle ions and neutrals,and they coespond o along-range olaation attraction oupledwith a sho-range

repulsion. n this case he ion-neutral momentum transfer col-lision frequency akes the form [Dalgarno et al., 1958]= (17)mi + m n

wheresubscript correspondso neutralsand n is the neutralatom polarizability.For a given on-neutralpair, (17) takesaparticularly simple form:

Pin-' Cinnn (18)where Cn s a numerical coefficient; aluesare given in Table6 for the different ion-neutral combinations found in the iono-spheres.

It should be noted that the momentum transfer collisionfrequencies re not symmetricwith respect o a changeof in-dices but satisfy the relation

nsmsPst ntmtPts (19)Ambipolar Diffusion

Recently, Conrad and Schunk [1979] have derived an am-bipolar diffusion equation and ion and neutral heat flowequations rom the 13-momentsystemof transportequations,taking into account he possibilityof large temperaturediffer-ences between the interacting species.These equations werederived by assuming hat the plasma was in a steady state,that collision-dominatedconditionsprevailed, that the speciesdrift velocitieswere much less han the species hermal speeds(low Mach numbers), and that charge conservation andcharge neutrality were satisfied. With these assumptions,stress nd inertia terms are negligible,and the 13-momentsys-tem of transportequations educedsignificantly.For the case of a three-componentplasma composedofelectrons subscripte), ions (subscript ), and neutrals (sub-TABLE 6. The CollisionFrequency oefficients inX 10 or Nonresonanton-Neutral nteractions

n

i H He N O CO N2 O2 CO2H + R 10.6 26.1 R 35.6 33.6 32.0 41.4He + 4.71 R 11.9 10.1 16.9 16.0 15.3 20.0C + 1.69 1.71 5.73 4.94 8.74 8.26 8.01 10.7N + 1.45 1.49 R 4.42 7.90 7.47 7.25 9.73O + R 1.32 4.62 R 7.22 6.82 6.64 8.95CO + 0.74 0.79 2.95 2.58 R 4.24 4.49 6.18N2+ 0.74 0.79 2.95 2.58 4.84 R 4.49 6.18NO + 0.69 0.74 2.79 2.44 4.59 4.34 4.27 5.8902 + 0.65 0.70 2.64 2.31 4.37 4.13 R 5.63CO2+ 0.47 0.51 2.00 1.76 3.40 3.22 3.18 RR means that the collisional interaction is resonant.


12/40


2.6

2.4

2.0

1.81.6

1.4

1.2

tl 1.00.8

0.6

0.4

0.2

0.0

(3)

-- ()

0/3)i0- i0- i0 o I0 I0

ni/nFig. 16a. Thermaldiffusion oefficient ij as a functionof ni/nfor zi/zj = 1, mi/mj= 4 and or several i/Tj ratios. he temperatureratios regi;enn parenthesesext o the curvesfromConrad ndSchunk, 1979].

scriptn) the on and neutralheat lowexpressionsnd the am-bipolar diffusionequation are given byq, =-K.[VTi- Ki.VT.+ R,.(u,- u.)q.= -K.V T, - K,.V T.- R.(u, - u.u,u.-DaVn, m,GV(TiTe), k(T,+ Te) (T,

VT. +2o;* VT,+2wt,+ ( YeJwhere the ambipolar diffusioncoefficient s defined as

c(Te + T,) 1Oa --- mr',. (1 -

1.4

3.23.02.82.62.42.22.OI.$

1.2I.O

0.80.60.40.20.0

-0.210-2

_ (:5)

-- (io) I I I ill I I 1--I0-1 I0 0 I01 I0 2ni/n

Fig. 16b. Thermaldiffusion oefficient%* as a functionof nJnjfor z/zj = 1, m/rn= 4 and or several /Tj ratios.The temperatureratios are given in parentheses ext to the curves from Conrad andSchunk, 1979].

1.2

I.O

0.8

O.G

0.4

0.2

mi/rnj =4 (IO)

I I I I

(1/IO)(1/3)(I/2)

(i)

0.0 10-2 I0- I0 0 I0 102ni/n 1

Fig. 16. Ordinary iffusion orrectionactor&ijasa function fn/nj or z/zj = 1, m/mj= 4 and or several /Tj ratios. he temper-ature ratios are given in parentheses ext to the curves from Conradnd Schunk, 1979].

and where w and w* are thermal diffusion coefficients, the Kare thermal conductivities, the R are diffusion-thermal coeffi-cients, and A,, is a diffusion correction actor. Complete ex-pressionsor thesecoefficients nd conductivities re given byConrad and Schunk [1979]. Here we merely note that in thelimit of equal ion, neutral, and electron temperatures he am-bipolar diffusionequation (22) reduces o the diffusionequa-tion derived by Schunkand Walker [1970a, b] from the multi-component ormulation of Hirschfelder t al. [1964],with (w +w*) correspondingo Schunk and Walker's thermal diffusion(20) factor.

(21) It is evidentrom he formof theheat lowequations20)and (21) that a temperaturegradient n either the ion or theneutral gas causesheat to flow in both gases.Also, a relativeion-neutral drift inducesa heat flow in both gases diffusion-thermal effect). From the form of the ambipolar diffusion(22) equation22) it is apparenthat a temperatureradientn ei-ther the ion or the neutral gascauses hermal ambipolar diffu-sion.

Equations 20)-(22) were derivedwithout allowance or the(23) effectsof an external magnetic field. Consequently, hey areapplicable to planetary ionospheres t altitudes where therelevant collision frequencies are much greater than corre-sponding yclotron requencies, nd they are applicablealongmagnetic ield lines for planetary ionosphereswith strong n-trinsic magnetic ields.To date, the possible mportanceof thermal diffusion,cor-rections o ordinary diffusion,and diffusion-thermalheat flowin a partially ionized plasma has been evaluated only for theearth's F region ionosphere Conrad and Schunk, 1979]. t wasfound that the correction to ordinary ambipolar diffusion isnot important in the earth's F region ionosphere. t was alsofound that thermal ambipolar diffusion s not very significant.Although o was found to be large for Ti .-. T., it multipliesVT., which is generally small in the F region. On the otherhand, V Ti can be large throughout he daytime F region, buto* was found to be small for all T,./T ratios.

As far as the ion and neutral heat flow equationsare con-cerned,a comparison f terms ndicated hat the familiar VTterm is the dominant temperature gradient term in the ex-pression or q. For qi the 7T and VT terms nduce com-parable heat flows f the two temperaturegradientsare corn-


13/40


parable; owever, inceV Tn s small hroughouthe F region,the V Ti term should dominate.It was also found by Conradand $chunk 1979] that O+-Ofield-aligned elativedriftsgreater han several ilometers ersecond re needed or the diffusion-thermal rocesso inducea qn equivalent o a 1K/km neutral temperaturegradient;therefore his processs not important for the neutral gas.However, depending on altitude and Ti/T, O+-O relativedrifts between 1 and 300 m?s induce O + heat flows in the F re-

gion that are equivalent o a 1K/kin O+ ion temperature ra-dient. Since he relativedrift needed o matcha givenO+ tem-perature gradient decreases as Ti/T,increases, diffusion-thermal heat flow could influence the O + thermal structure inregionsof rapid convection,where frictional heating acts toincreasesignificantly he ratio T/T,. In this regard, t shouldbe noted that field-alignedO + drifts of the order of 300 m/shave beendetected ecently n the high-latitude region J.R. Doupnik, personalcommunication, 980).

:DZ-r-', -I

AL'I'I'I'UDE

Diffusionna Fullyonizedlasma Fig.17. IonMachumberiagramhowinghe ransitionromsubsonic o supersonic low for both outward and inward solutionsConradnd chunk1979]ave lso erivediffusionnd [from anksnd ockarts,973].eprintedypermissionfAca-heat flow equations for a three-component ully ionizedplasma composed f two major ions (subscripts and j) andelectrons subscripte). These equations,which were derivedby usingassumptionsimilar o those istedabove or the par-tially ionizedplasma,are givenbyI m,G,= j-D,1Vn,--- VTiz,i ii Te/Tirtee

demic Press.

kT 1 (30)D----jvj1 -Aand where the electron density and drift velocity are deter-mined by the requirementsof charge neutrality,and chargeconservation, espectively,

(z,-7,)VTe nT n, + nj ol ol*Tj},VT,-Tv1j=ui--OJj kTj j zjTelTjnee+ (zj+j)Te- n---L--Tj ni + nj

(24)

Yjj T,- V

where

qe= --eVTe 'j- ei(Ue- Ui) J- j(Ue- Uj) (26)q, = --Kj,'VT,- K,,VT,. Ro(u- uj) (27)q/= -Kj, VT,- Ko'VT - Rj,(u,- uj) (28)

kT 1D, -- (29)mivij 1 -- A3TABLE 7. Values of ,, for Various Ions in Mixtures of O + and H +

n(O+)/n(H+) He+ N + 02+0.001 1.67 2.34 9.60 9.470.01 1.62 2.29 9.45 9.310.05 1.44 2.11 8.82 8.670.10 1.25 1.92 8.17 8.010.125 1.16 1.84 7.88 7.720.25 0.82 1.51 6.76 6.570.5 0.38 1.09 5.40 5.161.0 -0.07 0.68 4.07 3.772.0 -0.45 0.35 3.04 2.674.0 -0.72 0.12 2.36 1.938.0 -0.89 -0.02 1.97 1.49

16.0 - 1.00 -0.09 1.75 1.25100.0 - 1.08 -0.16 1.56 1.03

1000.0 - 1.09 -0.17 1.53 1.00Data are taken from Schunkand Walker [1969].

rt = nz,+ rtjzj (31)hen ---- ziu, ttjzjuj (32)

In (24)-(30),au,%*, , nd jare thermaldiffusion oeffi-cients; the K and he are thermal conductivities;and the R and8 are diffusion-thermalcoefficients. xpressionsor thesecoef-ficientsand conductivities hat are valid when there are largetemperature differencesbetween the interacting speciesaregiven by Conradand $chunk 1979]and are not repeatedhere.We do, however, ote hat the coefficientsijOij*and Aijex-hibit significant ariationwith the on temperatureatio T/Tj,as is shown n Figures 16a- 16c.Several important features should be noted about the diffu-sion and heat flow equations.First, it is apparent that a flowof heat through both major ion gases esults rom a temper-ature gradient in either gas or from a relative drift betweenthe major ion gases. The latter effect is important in theearth's topside ionosphere at mid-latitudes [St-Maurice and$chunk, 1977; Bailey and Moffett, 1978], in the polar wind[$chunk et al., 1978],and in the topsideVenusian ionosphere[$chunk and St-Maurice, 1977], but it was found to be unim-portant in the earth's low-latitude ionosphere [Nagy et al.,1977].Next, it shouldbe noted that a temperaturegradient neither of the major ion gasesor in the electron gas causes her-mal diffusion in both major ion gases.Thermal diffusion isimportant in the earth's topside onosphereat mid-latitudes[Walker, 1967;$chunkand Walker, 1969, 1970a], n the polarwind [Bankset al., 1974b], nd in the Venus onosphere$chunkand St-Maurice, 1977;Bauer et al., 1979]. Finally, it should benoted that the quantity Au, which appears n the expressionsfor the ordinary diffusion coefficients,accounts or the effectof heat flow on ordinary diffusion. This effect can increase heordinary diffusion coefficientsby more than a factor of 5 forlight minor ions such as He+ and H + [Schunkand Walker,


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826 SCHUNKAND NAGY: IONOSPHERES F TERRESTRIAL LANETS

1970b; St-Maurice and Schunk, 1977; Conrad and Schunk,19791.The diffusion and heat flow equationsderived by Conradand Schunk 1979] are similar in form to thosederivedby St-Maurice and Schunk [1977], but the Conrad and Schunktransport coefficientsare valid for arbitrarily large temper-ature differencesbetween the interacting species. n the limitof small temperaturedifferenceshey reduce o thosederivedby St-Mauriceand Schunk 1977].Also, when he relativedriftbetween he major ions s neglected nd when the major ionsare assumed to have a common temperature, the diffusionequations 24) and (25) reduce o the diffusiveequilibriumequationsof Schunkand Walker [1969], % - ao* agreeingwith their major ion thermal diffusioncoefficient.St-Maurice and Schunk [1977] and Schunk and Walker[1969, 1970a, b] have also derived diffusion equations or aminor ion in a mixture of two major ions and electrons.Forthe caseof diffusiveequilibrium with a common on temper-ature for all ion specieshe minor ion equation s given byI Vnx mxG Tel i (1 fix)... Zx Vne--nx kTi ne Ti

_ (Zx-Dx)WTe (33)T,where subscript denotes he minor ion, Dx and fix are ther-mal diffusion coefficients, nd ix is the minor ion charge num-ber.

The thermal diffusioncoefficientix depends n the massandcharge atiosof the minor ion with both major ions as well ason the major ion density ratio. Values of fix appropriate tothe earth's opside onosphere re given n Table 7. The otherthermal diffusioncoefficientDx depends n the charges f thethree ons as well as on the major ion density atio. When allthree charges re the same,Dx is zero. For multiply chargedminor ions in a mixture of singly chargedmajor ions, Dx =0.8ix(ix - 1). The completeexpressionsor fix and Dx aregiven by Schunkand Walker [1969].Supersonic lasmaFlow

The diffusion quations hat werepresentedn the previoussubsectionsare not valid if the species drift velocity ap-proaches r exceedshe specieshermalspeed,owing o theneglect of the inertial and stress erms in the momentumequation6). The H + escape elocity anbe comparableo orgreater han the H + thermalspeed or polarwind flow,andconsequently, + stress nd inertia mustbe consideredn or-der to describe igh-speedH + outflowcorrectly.With allow-ance or theseprocesseshe H + momentum quation or high-speed low becomesRaitt et al., 1975]

I du(H) drH+)/dSu(H+ +u(H+)v(H) dS n(H+)m(H)I TelT(H+)ndn(H) +---D(H+)n(fi ) dS n dS1 dm(H+)gll [T(H)+ TelkT(H +) T(H+) dS

a(H+)T(H+)(H +) dS

whereD(H +) -- k T(H+)/m(H+)v(H +) (35)

v(H+) = v(H+, N:) + v(H+, O:) + v(H+, O)(H +, O)+ v(H+, O+)(H +, O+)/2.15 (36)

and where S is the coordinate along the geomagnetic ield,a(H +) is the thermal diffusion coefficient, and v(H +) =?(H+):BB/B s the parallelcomponent f the H + stressensor.To emphasize he high-speednature of the H + momentumequation (34), it is useful to introducea local H +Mach num-ber M, defined asM = u(H+)/C(H +) (37)

whereC(H +) = [kT(H+)/m(H+)] /2 s the H + isothermal oundspeed. n terms of the local Mach number the H + momentumequation 34) becomesI dM M:- 1)M dS d(H+)/dSn(H+)m(H+)Ce(H)

gll v(H+)M I dQ(H+)Ce(H ) C(H+) Q(H+) dSI dT(H+)M: l)- Te/T(H)dn2T(H +) dS ne dSI dTe a(H +) dT(H +)+ (38)T(H +) dS T(H +) dS

where the H + pressuregradient term has been eliminated withthe aid of the H + continuityequationand whereQ(H +) = [P'(H+) - L'(H+)n(H+)] dS (39)

is the net ion flux flowing through the magnetic ield tube.In the initial studiesof the polar wind by Banksand Holier[1968, 1969]and Marubashi 1970] he effectof H + stresswasneglected.Consequently,when H + was a minor ion, the H +momentum equation (38) possessedingularitiesat M = +1,that is, at the point of transition rom subsonic o supersonicflow. For such a case the different solutions to the Mach num-ber equationare shownschematicallyn Figure 17. For M >0, corresponding o polar wind outflow, there are both sub-sonic M < 1) and supersonicM > 1) regions.All of the solu-tions that remain subsonic t all altitudesare valid, physicalsolutions.However, for supersonic utflow, only the criticalsolutionA is a physical olution. or thi solution he H + flowis subsonic t low altitudes,passeshrough he singularpointM = 1, and then is supersonic t high altitudes.In addition to the neglectof stress ffects he initial polarwind studies id not include he energybalanceequation;anisothermal plasma was assumed.To include stress ffectsandthe energybalance, t is necessaryo have expressionsor theH + stressensorand heat flow vector.These expressionsouldbe obtained from the 13-moment stress tensor and heat flowequations.Unfortunately, as was noted earlier, general colli-sion terms for the 13-momentsystemof transportequationshave been derived only for low-speed low.Instead of neglecting H + stressand the energy balance

(34) equation,aitt tal. 1975]sedheow-speedollisionermsand derived the following expressions or the stress ensor


15/40

SCHUNKAND NAGY: IONOSPHERES F TERRESTRIALPLANETS 827

component nd the thermalconductivityn the limit of a col-lision-dominated plasma:10n(H+)kT(H)du(H) (40)r(n+)- 9 v'(n) dS

K(H)= 3.1 10n(n+)5/2(H+)--!I 10 (n, 2) 0 (n, 2) 7(n+,) (41)3 (n ,O )+ -i v(n ,O )+ v(',,

wherev'(H+) -- v(H+, H +) + 1.04 v(H+, O+) + 1.02 v(H+,

+ 1.02v(H+, 02) + 0.88 v(H+, O) (42)and where he units for the thermalconductivity re eV K -1cm-I

The effect of H + stresson the density and temperaturestructure f polar wind flow has beenstudiedby Raitt et al.[1975].From the mathematical oint of view, H + stressn-troduces term in the H + momentumequation hat is propor-tional to d2M/dS2 and, consequently,liminateshe singular-ity thatappearedn theoriginal tudiesf thepolarwind.The theoreticalmodelsof polar wind outflowpresentednthis subsectionare valid at the altitudes where the H + gas iseffectivelyollision ominated. s a roughguide, he H+ gasis effectively ollisiondominatedwhen

u(H+)/H v(H+, O+)


16/40

828 SCHUNK NDNAGY:ONOSPHERESFTERRESTRIALLANETS

O

.,-

O.,-

O

OO

O


17/40


ity, parallel electron hermal conductivity, nd perpendicularion thermal conductivityare affectedby the ion-cyclotron ur-bulence, and these coefficients re given byll 0'3(0i/i)/2Es3/4) - (51)

fell= 2 (fel )classical (52)ean,2k ,Ki.= m?,-'"-'-s (53)

where Opes the electronplasma requency,ees the electron-electron Coulomb collision frequency,and k is Boltzmann'sconstant.

The saturated evel of turbulence s occurs n two stages.The first stage s completed n a few ion Larmor periods andlastsonly seconds, fter which a second evel of turbulence sestablished,which then lasts hroughout he length of the dis-turbance lionson et al., 1976, 1979]. For the second evel ofelectrostatic on-cyclotron turbulence there is an anomalousincrease n field-aligned electrical resistivity and cross-fieldion thermal conductivityas well as a decrease n electron ther-mal conductivityalong the geomagnetic ield. In addition, tur-bulent heating leads to a significant ion temperature ani-sotropy.lhotochemistry

Solar extreme ultraviolet radiation is the main source of io-nization in the ionospheres. he photoionizationof the neu-tral constituents f the planetary atmospheres roduces reeelectronsand ions, which can then undergo chemical reac-tions or be transportedby diffusionand atmosphericwinds toother regionsof the ionospheres.The calculation of the photoionization rates requires aknowledgeof the number densitiesof the neutral constituents,nn,as a functionof altitudez, the absorptionOn(a)(X)nd ioni-zation on{(2,)ross ections f theseconstituentss a functionof wavelengthX, the branching ratios of the various excitedion statespn(h, E), and the spectrumof solar radiation in-cidentupon he top of the atmosphereoo(X).n termsof thesequantities he photoelectronproduction ate is given byPe'(E,) n,,(z)o d2d(X){(X)P(X')n

exp [-,(X, z)] (54)where the optical depth, is given by

and where X,z) = o?)(X)n-/ h R,X) (55)

H,, = kT,,/m,,g (56)= (n + z)/H (57)

In (54)-(57), E = Ex - Et, Ex is the energy corresponding owavelengthX, Et is the ionizationenergyof a given excited onstate , R is the planetary radius, X is the solar zenith angle,and ch (Rn, X) is the Chapman grazing incidence function[Chapman, 1931]. The Chapman function has been tabulatedby Wilkes [1954], and approximateexpressions alid for bothlarge and small solar zenith angles have been presentedbyF. L. Smith and C. Smith [1972]. For X < 80 the Chapman

functioncan be replacedby secX in every erm in the summa-tion in (55).As was noted above, the calculation of the photoionizationrate requiresa knowledgeof the neutral densities, he absorp-tion and ionization cross sections, and the incident solar flux.The neutral compositions f the terrestrial planets are given inthe next section, dealing with observations.The absorptionand ionization cross sections for the dominant neutral constit-uents of the planetary atmospheres re given in Table 8. Thesolar radiation spectrumhas been measured ecently by spec-trophotometers arried aboard the AtmosphereExplorer (AE)satellitesC, D, and E [Hintereggeret al., 1973, 1977]. A refer-ence spectrum,basedon thesemeasurements nd appropriatefor low solar cycle conditions,was compiled by H. E. Hinte-regger or use by the AtmosphereExplorer investigator eam.This spectrum,which is appropriate or the earth, is given inour previous review [Schunk and Nagy, 1978] and is not re-peatedhere. Extreme ultraviolet (EUV) spectra,measuredbyHintereggerduring solar cycle maximum conditions,were re-cently publishedby M. R. Torr et al. [1979].To be applicableto Venus and Mars, the solar EUV fluxes n thesespectramustundergoan r scaling,wherer is the distance rom the sun tothe planet.There are a myriad of chemical reactions hat are possiblein the ionospheres, s shown n Tables 9 and 10. Also given inTables 9 and 10 are the chemical reaction rates, except forthose involving O + in reactionswith N2, 02, and NO, whichare given byk, -- 1.533X 10 '2- 5.92 X 10-'3(T/300)+ 8.60

X 10-'4(T/300)2 300 _


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TABLE 9. Ion-Neutral Reaction Rates

ReactionNumber Reaction Rate Constant, m3 s-l(R1) CO_ + O--> O+ + CO_ 9.6 X l0 -11(R2) CO2 + O--> 02+ + CO 1.64X l0 -10(R3) CO2+ + O2 -> 02 + + CO2 6.4 X l0 -11(R4) CO2 + NO--> NO + + CO2 1.2X l0 -10(R5) CO_ + H_->CHO_ + H 1.4x 10 9(R6) CO2 + H -->CHO+ + O 5.0 x 10 l(R7) CO2 + H- H + + CO2 1.0X l0 10(R8) CO_ + N -->products _ O+ + CO 1.4X l0 10(R10) CO+ + O_ O_ + CO 1.2X l0 -10(R11) CO+ + CO2 ->CO2 + CO 1.0x 10 9(R12) CO+ + NO--> NO + + CO 3.3 X 10 10(R13) CO+ + H_-->COH+ + H 1.8x 10 9(R14) CO+ + N--> NO + + C _ NO + + N2 3.3 X l0 -10(R20) N2+ + O--> NO + + N 1.4x 10-1(300/T)'445.2 X 10-11(T/300)'2(R21) N2+ + O--> O+ + N2 1 X 10-11(300/T)0'233.6 X 10-12(T/300).hI(R22) N2+ + 02 --> 02 + + N2 5 X 10-11(300/T)(R23) C+ + CO2 ->CO+ + CO 1.1x 10 9(R24) O + + N 2 --> NO + + O k (equation 58))(R25) O+ + 02 --> 02 + + O k2 (equation 59))(R26) O+ + NO--> NO + + O k3 (equation 60))(R27) O+ + CO2 ->02+ + CO 9.4 X 10 10(R28) O+ + H2 -> HO+ + H 1.7x 10 9(R29) O+ + H--> H + + O 2.5 X 10-llrnl/2(R30) N + + CO--> CO+ + N 4.0 x 10 l(R31) N + + CO--> NO + + C 5.0 x 10 ll(R32) N + + NO--> NO + + N 9.0 x 10 l(R33) N + + CO2-->CO+ + NO 2.5 x 10 l(R34) N + + CO2-->CO2 + N 7.5 x 10 l(R35) N + + 02 -->NO+ + O 2.6 x 10 l(R36) N + + 02--> 02+ + N 3.1 x 10 l(R37) N + + O2--> O+ + NO 3.7 x 10 ll(R38) N + + NO--> NO + + N 2 X 10 II(R39) He+ + CO2- CO+ + O + He 8.7x 10 l(R40) He+ + CO2 ->CO2 + He 1.2x 10 l(R41) He+ + CO2 ->O+ + CO + He 1.0x 10 l(R42) He+ + CO--> products 1.68x 10 9(R43) He+ + CO- C+ + He + O 1.4x 10 9(R44) He+ + NO- N + + He + O 1.25x 10 9(R45) He+ + N2 > N + + He + N 9.6 x 10 l(R46) He+ + N2-->N2+ + He 6.4 x 10 l(R47) He+ + 02 -->O+ + He + O 1.1x 10 9(R48) H + + CO2 ->CHO+ + O 3.0 x 10 9(R49) H + + O--> O+ + H 2.2 X 10-llri 1/2(R50) H + + NO--> NO+ + H 1.9x 10 9

MajorReactionon

Venus (V),Earth (E),or

Mars (M)

T_< 1500KT> 1500K

T_< 1500KT> 1500K

V,MV,M

V,M

V,MV,M

V,E,MV,E,MV,MEEV,MV,E,MV,MV,MV,MV,M

V,MV,M

V,MV,E,M

Data are taken from 4lbritton [1978], D. G. Torr and M. R. Torr [1978], and Schunk and Raitt [1980].with electronsand other neutral particlesand by radiation.Photoionizationproducesenergeticphotoelectrons, ince theenergycarried by the ionizing photonsexceedshe energy e-quired for ionization. The initial photoelectron nergy de-pendsnot only on the energyof the ionizingphoton and theidentity of the neutral species ut also on the ionization stateof the photoion. Typically, photoionizationproducesphoto-electronswith initial energiesof some ens of electronvolts.Only a relatively modestamount of the initial photoelec-tron energy s depositeddirectly in the ambient electrongas.

Most of the excess inetic energy s lost in both elasticand in-elastic collisionswith the neutral particles and in Coulombcollisionsith the ambientons. f the photoelectronsosetheir energyat an altitude near where they are produced, heheating s said to be 'local,' while if the photoelectronsosetheir energy over a distance greater than a neutral scaleheight, the heating s termed nonlocal.'Nonlocal heating ef-fects occur mainly at high altitudes,where ambient densitiesare low, and at high photoelectronenergies.The heat gainedby the ambientelectrongas rom the pho-


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toelectronsand from superelastic ollisionswith the neutralsacts to raise the electron temperature above the ion and neu-tral temperatures.The hot ambient electrons hen lose energyin Coulomb collisions with the ambient ions and in elastic andinelastic collisionswith the neutral atmosphere.The energygained by the ion gas is generally sufficient to raise the iontemperature above the neutral gas temperature. The extent towhich the electron and ion temperatures re elevated dependson the relative mportanceof the variousheating,cooling,andenergy ransportprocesses,nd this in turn is dependentuponaltitude, latitude, local time, season,and solar activity.There are certain regionsof the ionosphereswhere the elec-tron and ion energy balance s more complicated,owing pri-marily to the presenceof additional heat sources.For ex-ample, in the earth's dayside cleft and the nocturnal auroraloval, precipitating energetic electronsare an important heatsource or the ambient electron gas. At the earth's plasma-pauseand the Venusian ionopause,heat conducteddown intothe ionosphere an be the dominant energysource or the ion-ospheric plasma. Throughout the earth's polar cap, con-vection electric fields are responsible or a significantamountof Joule dissipation. oule heating may also play a role in theionospheres f Venus and Mars. Although the ambient elec-trons receive only a small fraction of this heat directly, theheat gained by the ions can result n higher ion temperaturesthan electron emperatures hroughoutmost of the F region,thereby turning the ion gas nto a heat source or the electronsinsteadof a heat sink. In addition, field-alignedelectroncur-rents, which are present in the earth's high-latitude iono-sphere, further complicate he electron energy balance be-causeof the need o considerhermoelectric nergy ransport.Becausewe have recentlypresenteda detailed discussion fthe electron heating and cooling rates for the earth's iono-sphere Schunkand Nagy, 1978],we will only list the relevantprocesses nd provide expressionsor the energy exchangerates that are important for Mars and Venus but that were notincluded in our previous review. The heat sources or the elec-tron components f the ionospheresre (1) photoelectrons,2)precipitatingenergeticcharged particles, 3) deactivationofexcited neutral and ion species, 4) dissociative ecombina-tion, and (5) Joule dissipation.The cooling processesor theelectron gas are (1) rotational excitation of molecular neutralspecies,2) vibrationalexcitationof molecularneutral species,(3) fine structureexcitationof atomic oxygen, 4) electronicexcitation f atomicand molecular eutralspecies,5) elasticelectron-neutralnteractions, nd (6) Coulomb collisionswithions.

The electronenergy exchange ates that are important forVenus and Mars but that were not included n our previousreview are given in the appendix for completeness.TABLE 10. Dissociative Recombination Rates

Reaction Rate Constam,cm3 s-lN:z + e-- N + N02 + + e-- O + ONO + + e-- N + OCO: + + e-- CO + OCO + + e-- C + O

1.8X 10-7(300/Te)-391.6X 10-7(300/Te)-554.2 X 10-7(300/Te)'853.8 x 10-7(300/Te)---2.0 x 10 7Data are taken from D. G. Torr and M. R. Torr [1978], Weller andBiondi 1967],and M. A. Biondi privatecommunication, 979).Thetemperaturedependence f the CO: + recombination ate was onlymeasuredover a very limited temperature ange. Some of these re-combination atesdependon the vibrationalstateof the ion, and thisdependences not included.

260-

240

220

200

180

160'

14(

,o(era 3)Fig. 18a. Preliminary ambient gas density profiles obtained byPioneerVenus rom the descending art of the orbit 17 periapsis ass.The solar zenith angle was 88 at periapsisnear the evening termi-nator. The solid curves epresent inear fits to the data points [fromNiemann et al., 1979a].Copyright 1979 by the American Associationfor the Advancement of Science.The primary heat source for the ion gases n the iono-spheres is the ambient electron gas. Additional ion heatsourcesexist, such as electric field heating, heating by exo-thermic chemical eactions,and frictional heating by meansofneutral winds. However, theseheat sources re generally char-acteristicof certain regionsof the ionospheres nd are seldomthe primary heat source or the ions.The heat gained by the ions from the electrons s given by(11), and the ion-electron collision frequency by (12). If thedifferent ion species n the ionosphericplasmashave separatetemperatures, he energy exchangebetween the different iongases s also given by (11) and (12).If an ion speciesn the ionosphericplasmas s flowing in re-lation to the other ion species r to the neutral gas,suchas oc-curs in the earth's polar wind, frictional heating occursthrough collisionsas energy of directed motion is convertedinto random thermal energy. Likewise, if a thermosphericwind blows hrough a stationary on gas, he ion gascan againbe frictionally heated owing to ion-neutral collisions. Thistype of frictional heating is not describedby the linear colli-

I I I I

a --I00 **l ' I I ' ,o"AMBIENT ENSITY c $)Fig. 18b. Preliminary ambient gas densityprofilesat a solar ze-nith angleof 180 obtainedwith the aid of an empiricalmodel it toPioneerVenusdata from 60 selected rbits.The solidcurves epresentthe fits o the actualdata [from Niemannet al., 1979b].Copyright 1979by the American Association or the Advancement of Science.


20/40

832 SCHUNKNDNAGY:ONOSPHERESFTERRESTRIALLANETS

DENSITYcm$) DENSITYcm$Fig. 19a Fig. 19b

Fig. 19. Altitudeprofiles f theearth's eutralgasdensitiest a mid-latitudeocationor (a) daytime nd b) nighttime[from Hedin et al., 1977a].sion term (11). When frictional heating is important, this lin-ear term shouldbe replacedby the five-momentcollision erm(9), which takesaccountof the frictional heating hat arisesasa result of a relative flow between nteractinggases.The ap-propriate collision requencies re given n Table 4 for ion-ioninteractions, Table 5 for resonant ion-neutral interactions, andTable 6 for nonresonant ion-neutral interactions.

For the caseof horizontal plasma convection n the earth'shigh-latitude ionosphere he frictional heating of the ion gasdue to the relative ion-neutral flow is simply related to theconvectionelectric field by [Schunk et al., 1975]Q nimimnpinEftc/B) (61)n mi + mn 1 ' i2/i 2

where Eft is the effectiveelectric field, given byE'= E + 1/C(UnX B) (62)

and where E is the convection lectric ield, , s the total ion-neutral collision requency, 2 s the ion cyclotron requency,and Un s the neutral wind velocity.The neutral atmospheresprovide the main cooling of theion gases n the ionospheres.The energy exchange betweenthe ion and neutral gases s given by (11), and the collisionfrequenciesare given by Table 5 for resonant on-neutral in-teractions and Table 6 for nonresonant ion-neutral inter-actions.

4. OBSERVATIONSNeutral AtmosphericComposition

The neutral massspectrometers n the Pioneer Venus Or-biter [Niemannet at., 1979a]and Bus [yonZahn et at., 1979]are providingcomprehensive ata on the composition f themajor neutral gas constituents n the Venus upper atmo-sphere.Figures 18a and 18b showrepresentative aytime andnighttimealtitude profilesof the major gases.A model atmo-sphereof Venus, basedon Pioneer Venus data, is under devel-opment at this time (A. E. Hedin, private communication,1979).The great wealth of terrestrialcomposition ata gath-ered by satellite-borne nd ground-basednstrumentshas ledto numerousatmosphericmodelsover the years.Figures 19aand 19bshow ypical quiet time altitudeprofilesof the majoratmospheric pecies, btained rom the massspectrometern-coherentscatter MSIS) model [Hedin et al., 1977a].The Vi-king 1 and 2 landerscarded massspectrometerso measurethe daytime compositionof Mars' upper atmosphere bove100 km [Nier and McElroy, 1977].A model atmosphere, on-structedby Chen et al. [1978], using this Viking massspec-trometer data along with atomic oxygenvaluesdeduced romion compositionnformation [Hansonet al., 1977], s shown nFigure 20.The major atmosphericconstituentof Venus and Mars isCO2, and while their surfacepressure iffersby more than 4ordersof magnitude, he densities f thesegases ecomevery

340 - _0

300 _

' 260 -

- 220 _

18C) -O14C) -ioo I [ I I

I0 2 IO 10 10 10 107 108 10 i010 i011 1012NUMBERDENSITY {crff3)Fig. 0. Altituderofiles'ofheMartianeutralas ensitiesordaytimeonditionsfrom hentaL, 978].


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SCHUNKAND NAGY: ONOSPHERESF TERRESTRIALLANETS 833400

350

3002502OO

150 COiH+N++_ 10 10 10 10 10" 10=IONS/cm 10"Fi':. 1. Ioncompositionf thedaysideonospheref Venus[from Bauer et al., 1979]. Copyright 1979 by the American Associa-tion for the Advancement of Science.similar by an altitude of about 120 km becauseof the differenttemperaturestructures t lower altitudes.Atomic oxygenbe-comes he major neutral gasspecies t about 150 km on Venusand 180 km on the earth; no direct atomic oxygen measure-mentshave yet beenmade on Mars, but ion composition ataindicate [Hanson et al., 1977] that atomic oxygen becomesdominant near 200 km. Information on the abundance ofatomic hydrogen s obtained rom airglow and ion composi-tion data. The exobase eutral hydrogendensities educed orVenus, the earth, and Mars are of the order of 105 D. E. An-derson,1974, 1976;Brinton et al., 1975].Ion Composition

The ion composition rofilesshown n Figure 21 were mea-suredby the PioneerVenus on massspectrometerBaueretal., 1979]and are representativef late afternoonquiet condi-tions. Figure 22 givesan indicationof the changesn majorion composition rom daytime to nighttime conditions onVenus [Taylor et al., 1979b];however, t is important to re-member hat the nighttime onospheres highlyvariable seesection5). The mid-latitude on composition rofilesof theterrestrial onosphere, hown n Figures23a and 23b, were ob-tained by averaging yearsof data gatheredby a variety ofinstrumentson AtmosphereExplorer C (D. G. Tort, private

200

190

._. 180-Etu 170 -

I- 160 -

150 -

14010o

PIONEER VENUS OIMS OUTBOUND -"- ORBIT 12ORBIT 59I I I 1 i I I .,// Io+/ \\]/ \ //x/ X / \

iI/

; I I I I I I I I I I I , I , 101 102 103 104 105 10sIONS/cm3

Fig. 22. A comparisonof representativeon densityprofiles romPioneer Venus orbits 12 (dayside,predusk)and 59 (nightside),withsolar zenith anglesof 80 and 150, respectively from Taylor et al.,1979b]. Copyright 1979 by the American Association for the Ad-vancement of Science.

475 i i I

\_' o+ \'- o+',x.,25 N

I0 I03 I04 I05Fig. 23a. Ion density profiles for the eah's mid-latitude iono-sphere t a solarzeth angleof 30 (from D. G. Tort, privatecom-munication, 1979).

communication, 1980). The two figurescorrespond o 30 and107 solar zenith angles, espectively.No direct measurementsof the ion composition of the Martian ionospherehave beenmade so far; however, the retarding potential analyzers,car-ried on the Viking 1 and 2 landers Hansonet al., 1977], pro-vided information on the daytime major ion distributions,shown in Figure 24.The major daytime ionic species n both Venus and Mars is02 +, with O + becoming he dominant constituent t higher al-titudes. The transition altitude is near 200 km for Venus; thevery limited experimental data available for Mars indicatethat O + and 02 + becomecomparablenear 300 km during thedaytime. In the terrestrial daytime ionosphere he molecularions 02+ and NO + dominate n the E region -100-150 km),and O + becomes he major ion above about 200 km. On theearth, H + becomes he major ion at very high altitudes, butthe transitionheight s extremelyvariable, dependingon manyfactors, such as local time, season,and latitude. A large vari-ety of minor ions have been measuredand/or deduced o bepresent n the ionospheres f both Venus and the earth. Thereis a great deal of experimentalevidenceavailable on the pres-ence of excited ion species n the terrestrial ionosphere,andtheir presenceand relative importance s now emerging forVenus. Only fragmentary information regarding minor and475 I I

425 \ X=107'37525.275 -

175 - ,----' "o /o/125io o o io oDENSITYcm 3 )Fig. 23b. Ion density profiles for the earth's mid-latitude iono-sphereat a solarzenith angle of 107 (from D. G. Tort, private com-munication, 1979).


22/40


I0 I0z I03 K) IONUMBER DENSITY (rn-3)

Fig. 24. Ion densityprofiles or the daytimeMartian ionosphere.The solid curvesare calculatedprofiles.The circlesare 02 + data, thetrianglesare O+ data, and the crosses re CO2+ data, all being romthe Viking I retarding potential analyzer experiment Hansonet at.,1977].The figure s taken rom Chenet at. [1978].excited on species s available from Mars, but these speciesare undoubtedly present and play an important role in theoverall ion chemistry.Neutral Gas Temperatures

The altitude profile of the neutral gas temperature n theVenus thermospherewas obtained from the CO2 profile mea-sured by the neutral massspectrometer arried on board thePioneer Venus Bus [yon Zahn et al., 1979]. This temperatureprofile, shown in Figure 25, correspondso a solar zenithangle of 60 The temperaturebecomesnearly isothermalabove about 180 kin, and thus by using either data from (1)the orbiter neutral mass spectrometer Niemann et al., 1979a,b] or (2) the satellite orbit perturbationsdue to atmosphericdrag [Keatingei al., 1979, 1980] he changes n the exospherictemperature,as a function of solar zenith angle/local time,can be established. igure 26 shows he diurnal variationof exo-spheric temperature deduced from the satellite drag data[Keating et al., 1980]. The thermospheric emperaturevaria-tion in the terrestrial atmospherehas been studiedby a widevariety of satelliteand ground-based echniques or about twodecades cf. Hedin et al., 1977b].Figure 27 shows ypical alti-tude and diurnal variations obtained over a mid-latitude ra-dar backscatterstation [cf. $tolarski et al., 1975]. The avail-able thermospheric emperatu?enformation on Mars comesfrom the scaleheightsmeasuredby the massspectrometers nthe Viking landers [Nier and McElroy, 1977]. The deduced

I I I I200 -

E 180- 16o -

140 -

I I I I220 24.0 260 280 300TEMPERATURE K

Fig. 25. Altitude profile of the neutral gas temperaturen thedaytimeVenus hermosphere,lotted rom datagivenby yonZahn etat. [1979].Copyright1979by the AmericanAssociationor the Ad-vancement of Science.

DIURNAL VARIATION OF VENUS EXOSR"IERIC TEMPERATURE

SON-RIDLEY L, 1977

250 'EMPERATURE (K) ! -OBSERVATIONSIO0 . ! I I I I I ! I INOON 4 8 MIDNIGHT 4 8 NOON

LOCAL SOAR TM HRS)Fig. 26. Diurnal variation of the Venusianexosphericemperatureas deduced rom satellitedrag data [from KcatingtaL, 1980].exospheric emperatures are approximately 175 and 120Kfor Viking 1 and 2, respectively.The neutral gas temperaturevariations in the terrestrial thermosphereare fairly well un-derstoodat this time, but the extremely ow nighttime temper-aturesobservedon Venus are very difficult to explain [cf. Nie-mann et aL, 1979b]. The two Viking measurements ofexospheric emperaturewere made about 2 months apart, un-der similar solar zenith angles,and no suggestions ave beenoffered or the different temperatures btained.Ion Temperatures

Daytime and nighttime ion temperatureprofilesmeasuredby the PioneerVenus retardingpotential analyzer [Knudsen taL, 1979a, b] are shown n Figures 28a and 28b. Representa-tive terrestrialmid-latitude ion temperatures, s a function ofaltitude and local time, obtained by the Millstone Hill radarfacility [Evans, 1967], are shown n Figure 29. The two day-time ion temperatureprofiles measuredby the Viking-borneretarding potential analyzers Hansonet aL, 1977]are given inFigure 30.The mid-latitude daytime ion temperatureson earth, typi-cally between 1000 and 1500K, are controlled to a majordegree by the energy gained from the thermal electronsandthe energy lost to the neutral atmosphere. The relativelyhigher daytime ion temperatures measured on Venus andMars, planetswith small or no intrinsic magnetic fields o holdoff the solar wind, indicate that heating and/or transport

NEUTRALTEMPERATURE,. (Z)

400o9 1200K

300 o ooiooo

20

0 2 4 6 8 I0 12 14 16 18 20 22 24LOCAL TIMEFig.27. Diurnal ariationf theneutralasemperatureverMill-stone Hill on March 24, 1970 [from $tolarski et al., 1975].


23/40

SCHUNKNDNAGY:ONOSPHERESFTERRESTRIALLANETS 835500

400

_ 3OO

.. T.toI Io' 2000 4000 6000TEMPERATUREK)Fig. 28a. Daytimeelectron nd ion temperature rofilesmea-sured uring ixof the irst10Pioneer enus rbits fromKnudsental. [1979].Copyright 979by the American ssociationor the Ad-

vancement of Science.

mechanismsdifferent from the terrestrial ones must play animportant ole. On the earth the mid-latitudeon temper-aturesdecreaset night,emphasizinghe fact that the overallenergybalance s largelycontrolled y solarEUV heating.However,on Venus the nighttime on temperatures re 2-3times arger han the daytimeones, mphasizinghe impor-tance of 'nonconventional'heat sources.Unfortunately, thereis no information available on the nighttime on temperatureson Mars, which are likely to be more like those found onVenus than on the earth.

80O

7oa-

600-

,./ 500-

400-

300-

200-

100

SZA = 150 +.t 20

ORBIT IN OUT51 o 57 [3 65' A MODEL

O

!

I I I , , lill I I I I ! lllJoo I ooo I o,oooION TEMPERATURE K)Fig. 28b. Nighttimeon temperaturesrom hreeselected ioneerVenus orbits.The ion temperatures re comparable o the electron

temperatures.he solidcurve s a calculatedon temperature rofile[fromKnudsent al., 1979b]. opyright 979by theAmerican ssoci-ation for the Advancement of Science.

700 APRIL 964 T. \/ I'iJ'" , \ r IIIIIII II I Ill t800t- i111111' ' N,oo" Illlilt I',,,,F ,oo T .F ooo - '

200 0 6 2 18 24EST

ig. 29. id-latitude ion temperature, s a [unctiono[ altitudeand local time, measuredby the iUstone iU radar [acility [[romwm, 1967].Repented by pcissionElectron Temperatures

The variationsof the electron emperaturemeasuredby thePioneerVenusLangmuirprobe Theiset al., 1980]are shownas a function of altitude and local time in Figure 31. Repre-sentative altitude and local time variations of the mid-latitudeterrestrial lectron emperature re shown n Figure 32; theseresultswere obtained at Millstone Hill [Evans, 1967] simulta-neouslywith the ion temperature ata shown n Figure 29.There have beenno electron emperaturemeasurements adein the Martian ionosphere;hereforeonly indirectand modelpredictions re available seesection ).The measured lectron emperatures n both Venusand theearth exceed he neutral temperatures uring the daytime, n-dicating he importance f selective eat sources nd heat

300

250 -

200-

150 -

I00 I00 200 400 600 I000 20 0 3000ION TEMPERATURE (K)

Fig. 30. Ion temperaturerofilesor theMartian onosphere,smeasuredy the Viking-borneetarding otential nalyzersfromHanson et al., 1977].


24/40

836 $CHUNKNDNAGY.'ONOSPHERESFTERRESTRIALLANETS1200 I I I Iiooo Te8OO600

400

200

o I I Io so eo so 12o 15o eoSOLAR ZENITH ANGLE (BEG)

Fig. 31. Electron emperaturen the Venus onosphere s a functionof altitudeand local time [from Theis t al., 1980].

z,zo

"' 00 00

MAIN PEAK"LOWER PEAK

e e

80 90 ]00 ] 0 ]20 ]30 ]40 ]SOLAR ZENITH ANGLE. X. deq

Fig. 33. Peakelectron ensityn theVenusonospheresa func-tionof solar enith nglefromKliore tal., 1979]. opyright979 ythe American Association for the Advancement of Science.

I

160 170

flows nto the electrongas.Downward heat flows nto the ion-osphere re due mainly to processesuchas photoelectronheating and wave-particle nteractions or the earth, whileLandau dampingof ion cyclotronwavesappears o be of ma-jor importanceor Venus.Mid-latitudeelectron emperaturesdecrease uring he night on the earthas solarEUV heatingsubsidesnd the energystored n the high-altitude lasma-sphere s depleted.Conditionson Venus are different, as thenighttime lectronemperatureso not varysignificantlyromthe daytimeonesand are close o the on temperature alues.Electron Density

The in situ plasma nstruments s well as radio occultationobservationsrom the PioneerVenusOrbiter PVO) are pro-vidingextensivenformation n the total electron ensity ar-iations.Figure 33 shows he observed ariationsn the peakelectrondensity rom the radio occultation ata [Klioreet al.,1979].The altitude of the peak remains airly constantat avaluenear 142kin. The terrestrial lectron ensity ariations,measuredt MillstoneHill at the same imeas he previouslyshownon andelectronemperaturealues, reshownn Fig-ure 34. The Mariner 9 Orbiter providedan opportunity omeasure, sing adiooccultationechniques,he Martianday-

time electron ensity rofiles veran extended eriod Klioreet al., 1973].The variationsof the measuredpeak electrondensity ersus olar enithangle reshownn Figure35a.Thealtitude f thepeakdensity, orrespondingo thesame eriod,is shown n Figure 35b. The peak altitudevaried between120and 140km; hisvariations believedo be related o temper-ature changes Kliore et al., 1973].No direct in situ measure-mentsare available rom the nightside.Mars is known to havea nightsideonosphere;t was detected, sing adio occulta-tion techniques,y the Mars

Ionospheres of the Terrestrial Planets -Schunk 1980

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