Dec 30, 1980 - not uniform over the daysid. e atmosphere but was calculated to be greatest near the terminator. The ef- fect that these penetrating solar wind ...
JOURNAL OF GEOPHYSICALRESEARCH,VOL. 85, NO. AI3, PAGES7747-7753,DECEMBER30, 1980
SolarWind Absorptionby Venus T. I. GOMBOSI,• T. E. CRAVENS,AND A. F. NAGY SpacePhysics Research Laboratory, Department ofAtmospheric andOceanicScience University of Michigan,AnnArbor,Michigan48109 R. C. ELPHIC AND C. T. RUSSELL
Instituteof Geophysics andPlanetary Physics, University of California LosAngeles,California90024
Thefraction of solarwindwhichinteracts withthedayside ionosphere andatmosphere of Venuswas calculated assuming thepresence of magnetic fieldfluctuations in the, • ionosheath regionandtakinginto consideration theinteraction withtheupperneutralatmosphere extending abovetheionopause. The MonteCarlocalculations indicatedthatfluctuations with Brms/(B)of 0.3, a not unreasonable valueac-
cording to recent observations, implies thatabout0.3%of solarwindmaybeabsorbed ontheaverage. Interactions withtheupperneutralatmosphere dueto chargeexchange candecrease theshocked solar
windfluxby 1-7%,depending ontheionopause altitude. Mostof thefastH atoms produced by charge
exchange interactions with the atmosphere escape, andonlya negligible fractioncanenterthe atmosphere. Theprecipitation of solarwindionsintotheatmosphere because of magnetic fieldfluctuations is notuniformoverthedaysid. e atmosphere butwascalculated to begreatest neartheterminator. The effectthatthesepenetrating solarwindparticles haveonthestructure of theionosphere wascalculated. It wasfoundthatwhilesomeof theenergy deposition processes leadto uniqueandobservable signatures (e.g.,a narrowLymana emission region), thesepenetrating solarwindparticles donotappearto playa significant rolein controlling thephysical and/orchemical structure of thedaytime Venusionosphere.
1.
INTRODUCTION
later, from theseparametersthe magneticfield can alsobe obThe mostextensivelystudiedsolarwind-planetaryinter- tained; in this model the magneticfield is frozen into the
the daysidemagnetoionosphere actionregionotherthanthe earth'sis that of Venus.During plasma.The modeldescribes et al., 1976;Wo/fe the lastfew years,orbitersand entryprobescarriedout mea- ratherwell [Gringauzet al., 1976;Yaisberg in describing nightsurementsin the vicinity of Venus.Thesemeasurements led to et al., 1979],althoughit is lesssuccessful side conditions [Romanov et al., 1978; Verigin et al., 1978; some improvementsin the relevant theoretical models, thus increasingour understanding of the large-scalepropertiesof Braceet al., 1979a;Intriligatoret al., 1979]. The extent to which the shocked solar wind interacts with the Venus interactionregion.The comprehensive data base and is 'absorbed'by the planetis still a centralquestionfor being acquiredby instrumentsaboardthe PioneerVenus orthe planet Venus.An importantaspectof this questionis the biteris nowprovidingan opportunityto testand improveour presentunderstandingof the Venus solar wind-ionosheath- role this absorbedsolarwind playsin determiningthe basic physicalparameters of the ionosphere. Analyzingthe Mariner ionosphereregion. 10 radio occultation data, Bauer and Hartle [1974]havesugMore than a decadeagoseveralauthorssuggested that hygested that a significant fraction of solar wind can interact didrodynamicformulations are appropriatefor an approximate DanJelland Cloutier[1977]calcudescription of plasmaflowsaround'magnetic'planets[e.g., rectlywith the atmosphere. currentsinducedby the Dryer and Faye-Petersen, 1966;Spreiteret al., 1966].Most of latedthe distributionof ionospheric solar wind interaction with Venus and concluded that the ionthe recenttheoreticalmodelsarebasedon the quasi-magnetoispracticallyimpenetrable to solarwindparticles; they hydrodynamical calculations of Spreiteret al. [1970],which opause extendedthe theoryto a 'nonmagnetic' planet.The solarwind findan upperlimit for thetotalparticleinfluxof 0.1g/s (~2 x
cm-2 s-l). By examining thebowshockposition, is considered to be a steady,dissipationless, perfectgasflow- I(P particles Russell [1977] concluded that about 30% of the solar wind can ingaroundan assumed shapefor theeffectiveboundaryof the planet.One consequence of the omissionof dissipative terms enter into the daysideatmosphere.This conclusionwas later is that a bow shockdiscontinuitysurfacedevelopsin the flow; confirmedfor an individualorbit of Venera9 by Romanovet acrossthisdiscontinuity the mass,momentum,magneticflux, a/. [1978]. On the other hand, from 33 Yenera 9 and 10 bow and energymustbe conserved. As a resultof the largeMach shockcrossings,Veriginet al. [1978]and Breus[1979] connumbersof the incomingsolarwindflowandthe assumption cludedthat the upperlimit for the solarwind precipitationin is about 1%.Recently,on of B ]] V in theflow,certainmathematical simplifications can the Venusiandaysideatmosphere
bemade, which leadtothedecoupling ofthose equations de- thebasis of preliminary analysis of 86Pioneer Venus bow scribing theplasma flowandthose determining themagnetic •shock crossings, Slavin etal.[1979] pointed outthatthestanfield(fordetails see$preiter etal.[1970]). Using thismodel,darddeviation in thedistribution of shock positions was theaverage flowparameters canfirstbedetermined, andthen, about twice aslarge during thefirst65orbits ofPioneer Venus orbiter as during the Venera 9 and 10 missionand that the
•Permanent address: Central Research Institute forPhysics, Hun-bowshock atVenus issomewhat closer totheplanet than
garian Academy ofSciences, Budapest, Hungary 1525.
Copyright ¸ 1980bytheAmerican Geophysical Union. Paper number 80A0457. 0148-0227/80/080A-0457501.00
would beexpected from quasi-magnetohydrodynamic modelirig.The authorsconcludethat significantabsorptionof
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7748
GOMBOSI ET AL.: SOLAR WIND ABSORPTIONBY VENUS
The large-scaleflow distributionand the orientationof the magneticfield lines were simplifiedversionsof those calculated by $tahara et al. [1977] for H/ro -- 0.01, M• -- 8.0, and ¾ -- 5/3 values(H is the constantionosphericscaleheight, ro is the distancebetweenthe centerof the planet and the noseof the ionopause,M• is the free stream Mach number, and ¾ is the specificheat ratio). We assumedthat the averageplasma flow velocity along the streamlines and the averagemagnetic field strengthalong field lineswere constant.The flow velocity F'bu•k in the ionosheathregion(the regionboundedby the bow shockand the ionopause)was assumedto be 250 km/s for the resultspresentedhere. We repeatedthe calculationsfor bulk
IONOPAUSE• BOW SHOCK
20
i
0
8
16
R{105km)
velocities of 150 and 350 km/s and found the results to be
within 50% of the values presentedhere; the degreeof solar wind penetrationwas found to be an inversefunction of the Fig. 1. The averagemagneticfield strengthfor a field line interbulk velocity. The averagemagnetic field lines were assumed sectingthe terminator at a radial distancer. to be curvedlines in the x, y plane following the shapeof the magnetosheath plasmamaytakeplace,at leastduringlimited daysideionopause.The coordinatesystemis the following:x periods. is pointingtoward the sunalongthe Archimedeanspiralof inThe penetrationof solarwind into the nightsideVenusian terplanetarymagneticfield, z is perpendicularto the ecliptic atmospherewasobservedby F'eriginet al. [1978],Romanovet plane and pointsnorth, while y is suchas to make the coordial. [1978],and Intriligatoret al. [1979].The penetrationof so- nate systemright-handed. lar wind particlesinto the wake was considered to be a conThe assumedvalue of the averagemagneticfield strength, sequence of their interactionwith fluctuatingmagneticfields for a field line intersectingthe terminator at a radial distance in this regionby Gombosiet al. [1979]and wasconsidered by R, is shown in Figure 1. This simplifiedmagneticfield was Perez-de-Tejada[1979] to be due to a thin viscouslayer near chosenon the basisof early resultsfrom the PioneerVenus orthe ionopause.Penetratingelectronfluxesappear to play a biter magnetometer(orbit 18 from Russellet al. [1979]). significantrole in the formationof the nighttimeionospheric We furtherassumedthat in the ionosheath(aswell asin the ß
peak [Gringauzet al., 1977;Chenand Nagy, 1978;Cravenset interplanetary medium)thereare magneticfieldfluctuations al., 1979;Kliore et al., 1979b],but they do not composea sig- movingtogether with the averageplasmaflow.The magnetic nificant fraction of the total solar wind flux. The total fraction field wasconsidered to be steady(DB/Dt -- O) in the moving of the solar wind absorbedby the nighttime atmosphereis frame of reference.This impliesthat the characteristic time negligible,althoughthe consequences are not. scaleof magneticfieldvariationsis assumed to be largerthan
Thepurpose of thispaperisto estimate (1) thefraction of thetypical lifetimeof anindividual particle in theionosheath solarwind particlesremovedfrom the ionosheathflow by ei- (--10 s). A furtherassumption wasthat the fluctuations were ther chargeexchangewith the neutral atmosphereor scatter- perpendicular to the averagemagnetic fieldlinesandhad at ing by magneticfluctuations,(2) the fraction of solar wind every point two independent,orthogonalrandom comparticlesreachingthe daysideionosphereof Venus, and (3) ponents. the various ionosphericeffects of these penetrating proton The magneticfieldin the daysideionosheath is represented fluxes. The solution of the decoupled magnetohydro- by 50fieldlines,onwhichtheassumed spatialfluctuations are dynamical equations describingthe behavior of a steady superimposed. The normalized fluctuation value8(R, 8) -dissipationless, perfectgasis alreadyextremelydifficult,but to B,uct/(B)(whereR and 8 are polarcoordinates in the x, y includeadditionalphysicalprocesses (e.g.,magneticfield fluc- plane)wasassumed to bea function of onlyR and8 andindetuations)makesthe problem prohibitiveto all practicalpur- pendent ofz. The50independent fieldlinesweregenerated in poses.In this work the large-scalepropertiesof the plasma the x, y planeby usinga newstatistical techniquedeveloped flow are consideredto be describedby the quasi-magneto- by Owens [1978].The algorithm generates therandom8(R, 8) hydrodynamicalmodel,and the additionalphysicalprocesses functionfrom the powerspectrumof the magneticfield flucare assumedto be perturbations. tuations,assumingthat the fluctuationsare spatialand not In the next section we describe the method used to estimate
temporalones,so that the individualfluctuations are inde-
the degreeof solarwind penetrationdue to magneticfluctua- pendentandtheirpowerspectrum is identicalwith the one tions, in section3 we calculate the interaction with the neutral used in the algorithm: upperatmosphere,and in section4 we discuss the ionospheric •4' Brms 2' 1 implicationsof the absorbedsolarwind flux. 2.
CALCULATION OF SOLAR WIND PENETRATION DUE TO MAGNETIC FIELD FLUCTUATIONS
The calculationsdescribedhere are based on the assumption of a large-scalebackgroundfield, with superimposed spatial fluctuationsand particle trajectorycalculationsusingstatistical consideration.The viscousboundary layer suggested
P(k) = 1+(k.l)•
(1)
where k is the wave number, Brms is the rms fluctuation of
magneticfieldalonga fieldline,A is a dimensionless constant whosevalue is near unity, I is the correlationlength,and q is
the spectralindex.This form of the powerspectrumis commonlyusedfor fluctuations of variousphysicalquantities in a in by Perez-de-Tejadaand Dryer [1976] and Perez-de-Tejada varietyof physicalproblemssuchasmagneticfluctuations medium[c.f.Hedgecock,1975].From Ven[1979]might be due to fluctuationssimilarto thosediscussed the interplanetary
here.
era 9 and 10 measurements we estimatedq to be about 1.5 and
GOMBOSI ET AL.: SOLAR WIND ABSORPTIONBY VENUS
I to be • 108cm, which are consistentwith the Pioneer Venus measurements.The value of Brms(preciselyBrmJ(B)) was a free parameterof the calculations. We generatedthe 8(R, • functionat 256 pointsfor each of the 50 field lines using Owens' algorithm. The method enabled us to generatea model of the daysideionosheathmagnetic field using about 3 s on the CRAY-I computer of the National Center for AtmosphericResearch. The daysideionosheathis not filled with cold plasmaflowing uniformly along streamlines but is insteada hot ionized gas.Venera 9 and 10 observeda typical temperatureof about
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