Dec 30, 1980 - from the Venera 9 and Venera 10 orbiters [Ivanov-Kholodnyi .... 13. 11. Dec. 15. 12. Dec. 16. 14. Dec. 18. 17. Dec. 21. 18. Dec. 22. 63.3. 530.
JOURNAL OF GEOPHYSICALRESEARCH,VOL. 85, NO. AI3, PAGES7642-7650,DECEMBER 30, 1980
le
An Empirical Model of the VenusianOuter Environment The Shapeof the DaysideSolarWind-AtmosphereInterface O. L. VAISBERG
SpaceResearchInstitute, USSR Academyof Sciences, Moscow,USSR D.
S. INTRILIGATOR
Departmentof Physics,Universityof SouthernCalifornia,LosAngeles,California90007 V.
1NI. SMIRNOV
SpaceResearchInstitute,USSR Academyof Sciences, Moscow,USSR
Solarwind plasmaand magneticfield data and ionospheric dataobtainedfrom the PioneerVenusorbiter are considered.It is shownthat the variationof the magneticfield pressurewithin the magneticbarrier is similarto that expectedfor the solarwind pressurevariationsalongan obstacle's boundarywhena more realisticapproximationof the shapeof the ionosphereis included.Simultaneoussolarwind pressure,ionospheric pressure, and magneticbarrierpressuredatashowthat the ionospheric pressurebelow the ionopause is approximately equalto the solarwind pressure. The magneticbarrierpressure is equal to approximately 2/3 to 3/4 of boththe solarwind pressure and the ionopsheric pressure. Estimatesof the hot plasmapressurecontributionto the total pressurewithin the magneticbarriervary from 1/4 to 1/3. Justbelowthe ionopausethe ionosphericpressuredeviationsfrom the mean ionosphericpressure
aresignificant especially below~400 km andappearto beindicativeof theadjustment of the ionospheric structureto changingsolarwind conditions.A first-ordermodelof the ionopausepressurevariationsas a functionof heightand solar-zenithangleis suggested. This modelprovidesan estimateof an 'instant' ionopause profilefor thegivensolarwindconditions. The meanand'instant'shapes of theionopause do not appearto correspond to theestimates obtainedfrom ionospheric pressure equilibriumfor a givenH/ ro.
INTRODUCTION
The first measurementsnear Venus of the magnetic field and plasma revealedthat the planetary magneticfield is too weak to prevent the direct interaction of the solar wind plasma with the Venusian atmosphere[Smithet al., 1965;Dolginovet al., 1968].The existenceof a bow shockdemonstrated that most of the oncomingsolar wind plasma is deflectedby the atmosphereof the planet. Several models of the interaction have been proposed[Johnsonand Midgley, 1969;Cloutier et al., 1969;Michel, 1971; Wallis, 1972]. Previousdata on the height of the Venusianionopausewere obtained by radio occultationfrom the Mariner 5 and Mariner 10 flyby's [Fjeldboet al., 1969;Howard et al., 1974] and from the Venera 9 and Venera 10 orbiters [Ivanov-Kholodnyi et al., 1977, 1978] as well as from measurementsof the location of the boundaryjust beyond the terminator [Romanovet
Gringauzet aL, 1976; Vaisberget aL, 1976; Russell,1977; Slavin et aL, 1979].Recently,the PioneerVenus orbiter has
made detailed measurementsof the solar wind plasma interaction and the structureof the Venusian ionosphereand atmosphere[Science,203, No. 4382, 1979; Science,205, No. 4401, 1979]. The PioneerVenus orbiter data analyseslead to two important conclusionsfrom the point of view of the solar wind interaction with the planet: 1. Above 200 km, the daysideionosphereis in a state of diffusiveequilibriumwith the O+ ion as the main constituent. The elevatedelectronand ion temperaturesin the ionosphere together with the number density below the ionopauseprovide enoughpressure,in mostcases,to sustainthe externalsolar wind pressure.The height of the ionopauseis stronglyinfluencedby the solar wind pressure[Baueret aL, 1979;Kliore al., 1978].The resultsof Romanovet al. [1978]on the heightof et aL, 1979a;Knudsenet aL, 1979a; Taylor et aL, 1979a, b]. 2. The magneticfield insidethe ionosphereis too weak, in the ionopausenear the terminator give the ionopauseas it was most cases,to make a significantcontribution to the pressure understoodin the gas dynamic concept [Sprieteret al., 1970]; i.e., as the lower boundary of an external flow. Recent results below the ionopause.Above the ionopausethere is a region of from the PioneerVenus orbiter (PVO) indicate[Intriligatoret increasedmagneticfield wherethe magneticfield pressureapal., 1979;Spenneret al., 1979]that there is a gap betweenthe proximatelyequalsthe solarwind pressure.This regionof inionopauseas defined by the upper boundary of the topside creasedmagneticfield presumablytransferssolar wind presionosphereand the ionopauseas definedby the lower bound- sure to the ionosphere[Russellet aL, 1979a, b; Knudsenet aL, ary of an external flow. A consistentresult was obtainedby 1979a;Brace et aL, 1979a;Elphic et aL, 1979]. It seemsapproVaisberget al. [1976],who reporteda low-energyplasmapop- priate to call this region the 'magneticbarrier' becauseof its ulation in the tail just behind the terminator (internal flow). similarity to the earlier proposedmodel [Michel, 1971]. Recently,a model of the ionosphereof Venus wasproposed Several attemptswere made to infer the subsolarionopause in order to explain the variations in the height of the ionoattitude from the shape and location of the bow shock [e.g., pauseas determinedfrom radio occultationdata in response to changesin the solarwind pressure[Wolffet al., 1979].VariCopyright¸ 1980by the AmericanGeophysicalUnion. Paper number 80A 1141. 0148-0227/80/080A-
1141501.00
7642
VAISBERG ET AL.' VENUS SOLAR WIND=ATMOSPHERE INTERFACE
•2oo• • i •i•111 i i •i•ml• I leo
(o) _
I
I
I I IIIll
I
I
-
7643
I I III
(b) -
ß 0ø < ½,63 ø for data on the ionosphereand on the near-planetary magnetic field that can be consideredin comparisonwith the solar wind data. Figure 2 showsthe behaviorwith $ZA of the magneticbarrier pressure(determined from the peak magnetic field), accordingto the data of Russellet al., [1979a,b] and Elphicet al. [ 1979]. The mean values and the standard deviations of the
IO-SO10 20 30 40 50 60 70 80 90 SOLAR ZENITH ANGLE,•ø
Fig. 2. The dependenceon solar zenith angle of the magnetic field pressurein the magnetic barrier region. Closed circlesare from Russellet al. [1979b],open circlesare from E!phic et al. [1979]. Trianglesare the values of the magneticfield pressurefor orbits with
availablesolarwinddatanormalizedto thesolarwindpressure of 5.2
pressures withinthemagnetic barrier areshown alongwith x 10-8dynes/cm •.Curve 1isCos • q?;curve 2isCos • XøwithXequal theindividual measurements forseveral orbits where thesolar totheexpected angle between thenormal totheionopause andthesolar wind flow direction;and curve3 is the gasdynamicpressurevariawinddataareavailable. Curve1inFigure 2 shows theCos 2q•o tionalong theboundary [$preiter eta!.,1968]. Thecurves arenormaldependence,where q•ois the $ZA. This dependenceis fre-
izedtothesubsolar magnetic pressure valueof3.7x 10-8dynes/cm •.
7644
VAISBERG ET AL.: VENUS SOLAR WIND-ATMOSPHERE INTERFACE
TABLE 1. PVO SolarWind, Magnetic,and IonosphericData Relatedto the DaysideIonopause nk(T•
Orbit I 2
Day, 1978 Dec. Dec. Dec. Dec. Dec. Dec. Dec. Dec. Dec. Dec.
3 4 5 6 9 11 12 14 17 18
SZA,q/o
5 6 7 8 9 10 13 15 16 18
Dec. 21 Dec. 22
63.3 64.9 66.2 67.8 69.4 71.0 75.7 78.8 80.3 83.5 88.2 89.8
h,, km 530 830 460-570 930-1080 1700-1750 750 280-360
>400
Pswo
P•
Pstz
+ T•)
4.3
1.5
5.6 2.1
1.8 0.65
1.5 0.62 0.92 0.41
1.4 1.1 1.3 0.72 0.14
3.8 15.9 23.6 2.4 8.5 4.9
1.07 4.0 5.2 0.52 1.7 0.85
450-500
B?/8r .•O.6 70ø. The slightdecreaseof the
the solar wind pressurefor the time intervals under consideration, these ionopauseprofiles do not necessarilyrepresent
VAISBERG ET AL.: VENUS SOLAR WIND-ATMOSPHERE INTERFACE
ionopauseheight at SZA > 70ø may be real and suggeststhe existenceof a daysideionosphericbulge. The lower height of the near-terminator ionopausemay be connectedto significant negative ionosphericdensity gradients near the terminator [Brace et al., 1979b].The strong ionosphericdrifts observed with PVO instrumentation [Taylor et al., 1979b] and implied by the 'fiat' ionosphericprofiles of the near-terminator ionosphere[Kliore et al., 1979a] also may significantly contribute to the low ionopauseheight near the terminator. There is other experimental evidenceof ionosphericmotions near the terminator [Taylor et al., 1979b; Nagy et al., 1979; Bauer et al., 1979].The important influenceof the solar wind drag on ionosphericdynamics was stressedby Knudsenet al. [1979b], Perez-de-Tejada and Dryer [1976], and Perez-de-Tejada et al. [1977].Further analysisof the data and the dynamics of the ionosphereare needed to understandwhat specific processes of plasmaand heat transportare responsiblefor the existenceof a nearly fiat or declining ionopauseprofile near the terminator.
The ionopauseprofile model obtained above can be used for comparison with hydrodynamic analogy models. The mean ratio of the terminator
to subsolar altitudes of the iono-
pause correspondsto a changein H/ro from
