Nov 30, 1982 - JAMES A. SLAVIN AND ROBERT E. HOLZER. Institute of ..... The initial reports of the Mariner 4 experimenters [Van Allen. P (dynes/cm 2) = 1 x ...
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 87, NO. B12, PAGES 10,285-10,296, NOVEMBER 30, 1982
The Solar Wind Interaction With Mars Revisited JAMES A. SLAVIN AND ROBERT E. HOLZER
Instituteof Geophysics and PlanetaryPhysics,Universityof California, Los Angeles,California 90024 Owingto a paucityof observational data,no clearconsensus hasbeenreachedconcerningthegeneralnatureof the solarwind interactionwith Mars. In particular,thepreviousanalysesare still at oddsregardingtheexistenceof a smallintrinsicfield magnetosphere at Mars asopposedto a Venus-typeionospheric interaction(e.g., Russell, 1978a,b;Dolginov, 1978b,c). This studycontributesto the resolutionof this questionin threeways. First, an improveddetermination of effectiveobstaclealtitudeandshapeis obtainedfrom the Mars 2, 3, and5 bow shock encounters throughthe useof a recentlypublishedcatalogof gasdynamicflow solutions(Spreiterand Stahara, 1980a,b).Second,buildinguponthePioneerVenusfindingsat a field-freeplanet(Braceet al., 1980;Elphicet al., 1980), it is shownthat the Martian ionospherecannotsupporta Venus-typeionopauseat the obstaclealtitudes inferredthroughour modelingof thebow wave observations evenwhenmaximalinducedionospheric magnetic fieldsandsolarmaximumEUV levelsareassumed.Theseresultsallow aneffectiveMars magneticdipolemoment
of1.4(_ 0.6)x 1022 G-cm 3tobedetermined thatstands offthesolar windoverthedayside hemisphere ataltitudes rangingfrom,• 500kmat thesubsolar pointto•-1000km neartheterminator withnodirectaidfromtheionosphere underaveragesolar wind/magnetospheric conditions.Third, a searchof publishedMars and Mariner radio occultationmeasurements producedno evidencefor theexistenceof an ionopause at Mars in agreementwith the Viking studyof Lindal et al. (1979). Rather,the electrondensityaltitudeprofilesappearqualitativelyconsistent withtheMartianionosphere terminatingin a chemopause associated with theeffectsof magnetospheric convection as first proposedby Bauer and Hartle (1973). After a review of the variousargumentsin the literature, as supplemented by theresultsof thisstudy,we concludethatMars mostprobablypossesses a smallintrinsicfield magnetosphere.
willbeabletopenetrate toheights -1RMs (1RMs= in theionosphere areconsidered, do theVikingdescent measure- 3390 km). Solid line segmentsspecifyintervalsover which the ments [Hansonet al., 1977] indicatethe need for an intrinsic transitions betweenshockedand free streamingsolarwind took magnetic fieldto stand-off thesolarwind;andif so,howlargemust place,whilein oneinstance dashes connectmultipleencounters by the planetarymagneticmomentbe? Mars 3 on the samepass.Fittingthe meanshockpositionsper 3. On the basis of both the various radio occulation measureinboundor outbound passwitha 2ndordercurvesymmetricabout mentsandtheViking descentobservations, at whataltitudedoesthe thex' axisproduces the ellipseof eccentricity0.94 andsemi-latus upperboundary of theMartianionosphere lie andis it formedbythe rectum1.94Ruswithitsfocusatx' - 0.5Russhownasa solidcurve actionof magnetospheric convection (i.e., a chemopause) or solar in Figure1. In contrast toearliermodelsof thistype[Bogdanov and wind ram pressure(i.e., an ionopause)? Vaisberg,1975;Russell,1977]wehaveobtainedsuperior fitstothe We foundin thisstudythatall availableobservations areconsis- observations by allowingtheconicfocusto lie anywherealongthe tent with the solarwind beingdivertedaboutMars by a modest symmetryaxis as opposedto beingfixed on the origin (seealso intrinsicmagneticfield withoutany directinvolvement by the Slavin and Holzer [1981]). In addition,consideration has been ionosphere duringaveragesolarwind/magnetospheric conditions. restrictedto only the shockcrossings forwardof x' - -1R•s However,duringintervals ofenhanced solarwinddynamic pressure becauseof both the poorersamplingfartherdownstreamand the and/ordaysidemagneticmergingit is possiblethatthe solarwind desireto investigate thedaysideflow thatis linkedby characteristic Copyright 1982by the American GeophysicalUnion. Paper number 1A 1202. 0148-0227/82/001 A- 1202505.00
linesof the gasdynamicsolutionsto thisforwardregionof the bow wave [Spreiteret al., 1966]. For comparison,also displayedin the first figure are the mean observed bow shock locations at Venus obtained from the Venera 9
10,285
10,286
SLAVINANDHOLZER: MARSSOLAR WINDINTERACTION REVISTED
o. 4 ,•'[ Pioneer Venus
MARS BOW SHOCK ]•N• /
t •
,•
½S•ovin eto/., 1980)
,•
.... •Mors
>2
(Verigin
Russell, 1980], while a sonicMach numberof 7.4 is about20% lowerthantheexpectedmeanat 1.5 AU. However, hypersonic flow
in the nearfield (i.e., x'•> -1RMs) is relativelyinsensitive to changingMach numberbeyondM = 5 so that the useofM = 7.4, becauseof its availability[$preiterand $tahara, 1980b], is not a significantsourceof errorin the model. Hence, in responseto the first questionposedby the introductionthe model in Figure 2 indicatesthat the averageeffective Martian obstacleto the solar wind rangesfrom 510 km (+20%) at sun-planet-satellite anglesof 0ø to nearly 1000 km at 90ø. This modelingof the Mars bow wave with a completegasdynamicflow field representsan important advanceovermanyof theearlierstudies[e.g.,Russell,1977]which haveexaminedshockcrossings to determinethe subsolarheightof thebow wave andthenassumedtheMars magnetosheath to havethe same relative thickness as that of the earth in order to infer obstacle
altitude. In fact, the width of the magnetosheath is a functionof obstacleshapewith thickermagnetosheaths beingnecessary when redirectingflow aboutblunterobstacles[FanDyke, 1958;$preiter that Fig. 1. Bowshock crossing locations during14passes byM•s 2, 3, and5 and$tahara, 1980b]. The modelin Figure2 showsan obstacle (O. L. VaisbergandV. N. Smi•ov, privatecommunication, 1979)have is slightlylessbluntthanthe terrestrialanalogueand indicatesthat beenplottedin abe•atedeclipticcoordinates with solidline segments studiesassumingthe samerelative magnetosheath thicknessas at spanning theintervals in theobse•ations overwhichtheentiretransition obstaclealtitudeby about20%. This variation tookplace.A bestfit 2ndordercurvethrough theM•s bowwaveencounters earthunderestimate is shownas a solidline andcomp•ed with the meanshocksurfaces from in obstacleshapebetweenearthand Mars could be due to many Venera 9 and 10 and PVO at solarminimum and maximum, respectively. causesincluding differencesin their intrinsic field strengthand Notetheapparent solarcyclev•ation intheCytherean bowwaveposition. multipolarcomposition,the distributionof plasmabehindthe obstacle,and internalcurrentsystems.By comparisonwith the other
and 10 orbiters[Veriginet al., 1978]andPioneerVenus[Slavinet al., 1980] in 1975-6 and 1978-9, respectively.In the past, great emphasishasbeenplacedon the relativelocationsof the Mars and Venusbow waveson the premisethat if the Martian bow shockis significantlymoredistant,thenthe causeis an intrinsicmagnetic field standing-off thesolarwindat altitudeshigherthanarepossible at 'field-free" planetssuchas Venus. Accordingly,prior to the PioneerVenusmissionthe largedifferencein altitudebetweenthe Martian bow wave definedby the Mars orbitersand that of Venus from the Venera satellites(the dashedline in Figure 1) was interpretedas strongevidencefor the solarwind beingdeflectedabout Mars predominantlythroughan interactionwith magneticfields intrinsicto the planet[Veriginet al., 1978;Breusand Gringauz, 1980]. However,as alsoshownin the first figure, PVO hasfound the Venus bow wave to be more distantthan was reportedby the earlierVeneramissions [Slavinetal., 1979b,1980].Thisdiscovery hasbeenconsideredby Slavin et al. as evidencefor a solarcycle variation
in Venus-solar
wind interaction
worksthat have usedgasdynamic flow calculations[Dryer and Heckman, 1967;$preiterandRizzi, 1972;Gringauzet al., 1973; Bogdanovand Vaisberg,1975;Gringauzet al., 1976;Dolginovet al., 1976a,b] thisstudyobtainsa betterrepresentation of theobservationsduelargelyto thegreaternumberof completeflow models that have becomeavailablethroughthe efforts of Spreiterand
GASDYNAMIC MODEL OF SOLAR[ WIND FLOW ABOUT MARS •13
Mean Shock
GD Best F•t BS + and Obstacle--MP Locahons ? ß
CPLocations ?ß
-
with the distance to the
shockobservedby PVO beingcomparableto thatof Mars nearsolar maximum, but much smallerat solarminimum as seenby V9 and V 10 despitethe similar solarwind Mach numbersduringthe two periods. Accordingly,simplecomparisons betweenthe VenusandMars bow wavesno longerappearadequatefor determiningthe type of obstacleMars presentsto thesolarwind. For thisreasonin Figure2 a gasdynamicmodel of the solar wind flow aboutMars has been constructedby taking advantageof the one-to-onerelationship betweenshockshapeandobstacleshapefor a givensetof upstream conditions[Fan Dyke, 1958]. A bestfit shockwas selectedfrom amongthe many publishedhypersonicgasdynamicsolutionsfor flow aboutobstaclesof variousshapesthat have recentlybecome available[Spreiterand $tahara, 1980a,b; $tahara et al., 1980]. Thissolutionusing7 = 2 andM = 7.4 hasbeenplottedasplussigns in Figure2 with the corresponding obstacledisplayedas a dashed curve. The choiceof 7 = 2, the adiabaticexponent,is consistent with the earth observations[e.g., Fairfield, 1971; Zhuang and
,
15
0'5
0
-0.5
-t
X' (RMs) Fig.2. ThemeanMarsshocksurface fromthefirstfigureismodeled with a theoretical gasdynamic flow calculation (shownwithplussignsmarking theshockanda dashed curveindicating thecorresponding obstacle) selected fromtheliteratureto matchtheshapeof thebowwave(i.e., 7 = 2, M = 7.4, anda shapeparameter of H/R o = 0.03;SpreiterandStahara[1980b]).As shownthefit to themeanempiricalshockis quitegood.Alsodisplayed are twopurported entriesandexitsfromtheMartianmagnetosphere byMars2 and3 indicatedby connected triangles.The threeoccassions on whichthe topof theionosphere mayhavebeenobserved arethenshownwithsquares. Hence,the high altitudetrianglesand muchlower squaresbracketthe inferredmeanobstaclesurfaceandserveto setupperandlowerlimitsonthe observedheightof the magnetopause.
SLAVINANDHOLZER: MARSSOLARWIND INTERACTION REVISTED
10,287
co-workers andtheuseof shockcrossings fromonlytheadequately sampled portionof thebowwavesunward ofx' = - 1RMs (seeSlavin andHolzer [1981] for a discussion of spatialbiasesin the downstreamobservations associated with the Mars 2, 3, and5 trajecto-
Mars to supportthe solar wind pressurewere'also raised by Gringauzet al. [1974],WhittenandColin [1974],Wallis[1975], Gringauz [1976], Breus and Gringauz [1980], and Gringauz
ries).
lackof ionopause signatures andweaknesses of theionosphere as
[1980]. However,only Gringauz,Wallis, andBreushaveusedthe
evidencefor theexistenceof a significantintrinsicmagneticfield. As a resultof the retardingpotentialanalyzermeasurements duringViking 1 and2's descent,the ability of the MartianionoThe Mariner4 and5 fly-by missions to MarsandVenus,respec- sphereto standoff the solarwindwithoutaid from intrinsicmagtively, providedthefirstdetailedexamination of theirionospheres neticfieldshasagainbeencalledintoquestionbecause the maxiby meansof radiooccultationexperiments[Klioreet al., 1965; mumionpressure observed in theionosphere wasfoundto be only BALANCING SOLAR WIND DYNAMIC PRESSURE
1967;FjeldboandEshlernan, 1968,1969].In thecaseof Venusa •2 x 10-9 dynes/cm 2 [Hanson etal., 1977].Forthetheoretically denseatmosphere wasconfirmed(seereviewsby WhittenandColin predictedratio of electronto ion temperature nearthesealtitudes [ 1974]andSchunkandNagy [ 1980])with theionosphere standing (i.e.,•-,250-300km) of 1.5 [Rohrbaugh et al., 1979]a maximum off the solarwind throughthe formationof an ionopause at an totalplasma pressure of onlyabout5 x 10-9 dynes/cm 2isimplied altitude,on thatoccasion,near500 km at a solarzenithangleof 33ø whichis a factorof 2 belowthe expected longtermaveragesolar [Fjeldboand Eshleman,1969]. More recentlythe PioneerVenus winddynamic pressure at 1.5AU of•-,1 x 10-8 dynes/cm 2 [e.g., orbiterhasunequivocally established the ionospheric natureof the Slavinand Holzer, 1981]. .• Cythereansolarwind interactionby meansof in situobservations Thislackof sufficient ionospheric pressure wasdemonstrated by suchas the electrondensityprofilein Figure3 showinga typical IntriligatorandSmith[1979]andBreusandGringauz[1980]who ionopausecrossing[Theiset al., 1980]. However, at Mars the then attributedthe ability of Mars to deflectthe solarwind to the atmosphereis far thinner than at Venus and earth with a corre- existenceof an intrinsicmagneticfield. However,they did not spondinglyweak ionosphere[Kliore et al., 1965]. No ionopause examinethepossible roleof inductiveionospheric magneticfields. was detectedby Mariner 4 [Fjeldboand Eshleman,1969] even In fact,thePioneerVenusmission hasobserved ionospheric magthoughSpreiteret al. [1970]calculated thatfor averagesolarwind neticfieldsof interplanetary origin(i.e., Venushasbeenfoundto dynamicpressureconditionstheMartianionosphere mightbe able haveno measureable intrinsicfield, Dolginovet al. [1969] and
to stand-off the solarwindat heightsof 155-175 km overthe subsolar pointwhichis justabovethepeakionization layernear 120-150km [e.g.,Klioreet al., 1973].Similardoubts aboutthe presence of sufficient plasmapressure withintheionosphere of Mars
Venus
h (kin)
h (kin)
•Pioneer
VenusOETP
it177 SVS =58 ø_37o 300
-
•4
-
f57
Russell etal., 1980a]withintensities of order10-102nT onabout 30%of theorbitswithsolarzenithangles lessthan50ø [Elphicet al., 1980b; Luhrnann etal., 1980].However,whiletheCytherean ionosphere cansometimes become magnetized through the'capture' of interplanetaryfield lines, statisticalstudiesof pressure balanceacrossthe ionopausewhich neglecttheseinducedfields haveobtainedgoodresults[Elphicet al., 1980a;Braceet al., 1980; Slavinet al., 1980]. Thesefindingssuggest that, on average,these magnetic fields make only a minor contributionto the total ionospheric pressure, Ptotal, standingoff thesolarwind. Still, for the purposeof estimating theirgreatestpossiblecontribution to supporting the solarwind at Mars we will assumethat inductivemagnetic fieldsarealwayspresentandcanbe solargeasto produceequality betweenparticleandfield energydensitiesin the upperionosphere so that
13-- 8,rnk(T i + Te)/B2 • 1
(1)
•50
psis) 200
Such a limit is reasonablein that the primary sourcesof field compression are the solarwind dynamicand ionospheric thermal pressures whicharein equilibriumonaverage.Expressed in another form
B % (8rrnk(T i + Te))•
f50
(2)
we are suggesting that an upperlimit on the magneticflux density that may be acquiredby an ionospherein suchinteractionsat a lO'1 1o2 lO3 104 fo5 106 planet,or evena comet,is determinedby the ionopauseplasma pressure. The conditionin (2), whichis generallysatisfiedin the Ne(cm-$) upperionosphere of Venus [Elphicet al., 1980a,b], then setsa Fig.3. Electron density isplotted asafunction of spacecraft heightacross pressureavailableto balancesolar anionopause atVenusfromthePVOobservations [Theis etal., 1980]and rangeon the total ionospheric compared withtheViking2 measurements [Hanson etal., 1977]showing a wind dynamicpressurewith similarappearing boundary at Mars.However,if it weredisplayed, the earth'splasmapause wouldalsolookmuchliketheseboundaries in interms (3) nk(Ti + T•) 350km) (4) etal., 1965; O'Gallagher andSimpson, 1965; Smith etal., 1965' whereanupperionospheric Ti = 3000øKandTe/Ti - 1.3 (i.e., forh
• 350km;P isoverestimated by(4) belowthislevel)istakenfrom theionospheric modelofRohrbaugh etal. [ 1979].Hence,fromthis expression we seethatevenassuming thatPtota I is twicethisamount due to the presenceof inductivemagneticfields, the available pressure at the500 km obstacle heightin Figure2 is onlyabout6 x
Lazaruset al., 1967]indicatedthatwitha closestapproachdistance of 3.9RMsthebowshockwasnotcrossed. Upperlimitsof 2.4 - 8 x
1022G-cm3weretheninferredfromthisnegative result.However,a later interpretationof the magneticfield data showeda pair of bow wave crossingswhich were then usedby Smith [1967, 1969] to
estimate a momentof 0.8 x 1022G-cm3 by scalingtheterrestrial analogueto theMars observations. In addition,DryerandHeckman
10-13dynes/cm 2 whichis 4-1/2 ordersof magnitude belowthe [1967] utilized an earth magnetopauseshaped obstacle and
mean Psw. Inthecase ofsolar maximum condition a 1-1/2orders of gasdynamic model toobtain anintrinsic moment of 1.7x 1022G magnitude pressure deficit stillresults when ionospheric density,cm3fromtheMariner 4 observations. However, Spreiter andRizzi temperature, andscaleheightareall doubledin (4). Thusevenat solarmaximum(4) whensetequalto theaveragedynamicpressure, ---1 x 10-8 dynes/cm 2, showsthattheaveragesubsolar heightof a Martianionopause nearthe subsolar pointwouldbe no morethan 350 km as contrasted with the 510 km requiredby the bow shock
[ 1972]reducedthatvalueto anupperlimit by showingthattheoretical ionosphericobstaclescould also be used to accountfor the positionof theMariner4 shockcrossings. The maindifficultyin the interpretationof the Mariner 4 shock observationsis that they occurredfar downstream(i.e., - 12RMs< x' < - 2RMs)sothattheir
observations. Accordingly, wefindananswer tothefirstpartof position is moredependent upontheambient solarwindflow question 2fromtheintroduction thattheViking observations ofthe direction andMach number than upon obstacle height. TheSoviet weaknessof the Martian ionospheredo indicatethe needfor an analysesbasedupon their lowest altitudeand near wake orbiter intrinsic field to create the obstacle to the solar wind inferred in from measurements are mostlysupportiveof a smallintrinsicfield mag-
bow shock location.
netosphere, 1.2 - 2.6 x 1022G-cm3, andarediscussed in a later
With thisknowledgeandthesubsolar obstaclealtitudefromthe sectionas are the studiesof Russell.Intriligator and Smith [ 1979] precedingbow waveanalysisan effectiveintrinsicmagneticmo- arrived at a moment of 0.8 x 1022G-cm 2 from their estimate of the mentmaybe calculated fromtheclassical pressure balancecondi- magneticfield magnitudenecessary to makeup the deficitbetween tionfora magnetic dipolein thesolarwindgivingrisetoa terrestrial the solarwind and Viking epochionosphericpressures.Thus our typemagnetosphere [e.g., Spreiteret al., 1966] momentof 1.4(_+0.6)x 1022G-cm3 is an intermediate valuewith respectto the previousstudies.Given our estimateduncertainties, M = (2rrPswR•6k/Je)'/' (5) only the most extrememomentsof Russelland Dolginov appear inconsistentwith this analysis.
wherek is half the obstacledragcoefficienttakento be the earth valueof 0.88,f is half theenhancement factorof the subsolarpoint magnetic flux densityalsoassumed tohavetheterrestrial magnitude
VERTICAL EXTENT OF THE MARTIAN
IONOSPHERE
Anotheraspectof theMarsenvironment with a greatbearingon of 1.22,Rsisthesubsolar stand-off distance of 510 + 3390= 3900 km, andthemeansolarwinddynamicpressure for theshockcross- the solarwind interactionproblemis the verticalextentof the and the natureof its high altitudetermination.In the ingsinFigure1was•l.5 x 10-8 dynes/cm 2(Dolginov [1976];4% ionosphere
SLAVINANDHOLZER:MARS SOLARWIND INTERACTION REVISTED
TABLE 1.
Spacecraft Mariner
Study
Basis
•
