Pioneer Venus Observations of Bow Shock Location and Structure

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 85, NO. AI3, PAGES 7625-7641, DECEMBER 30, 1980

The Solar Wind Interaction With Venus: Pioneer Venus Observations of Bow Shock Location and Structure

J. A. SLAVIN,• R. C. ELPHIC,• C. T. RUSSELL,• F. L. SCARF,2 J. H. WOLFE,3 J. D. MIHALOV,3 D. S. INTRILIGATOR,4 L. H. BRACE,5 H. A. TAYLOR, JR.,5 AND R. E. DANIELL, JR.5

PioneerVenus observations are usedin conductinga studyof the locationand structureof the Venus bow shock.The trace of the shockin the solar wind aberratedterminator plane is nearly circular at an altitudeof 1.38R•, independentof interplanetarymagnetic field orientation with an extrapolatedsub- solarheight of 0.38 R•,. Gas dynamicrelations and scalingof the terrestrialanalogue are usedto deter- minethe effectiveimpenetrable obstacle altitude from the meanshock surface with the conclusionthat it liesbeneath the observedheight of the ionopause.The short-termvariability in shockposition is similar to that found at the earth, while over the long-term bow shock,altitude varies by up to ,-,35%in phase with the solarcycle owing to causesother than changingsolar wind Mach number.In contrastto ionopauseposition, which is shownto be well determinedby externalpressure measurements, bow shock altitude is found to be only weakly dependentupon ionopauseheight and solar wind dynamic pressure. Theseresults are interpretedin termsof interactionswith exosphericneutrals and/or lack of complete deflectionof the incident solar wind by currentsinduced in the ionospheremodifying the flow about Venusfrom that associatedwith a tangentialdiscontinuity obstacle of nearly constantradius. The downstreambow shockis smallerin diameter than that of terrestrialcase despite the larger Mach coneangle at 0.72 AU mostprobably due to the smallerrelative size of the Venusmagnetotail. A brief surveyof shock structurewith PioneerVenus instrumentation shows general agreement as to the time and locationof the shockcrossings with a transitionlayer thickness of the orderof the ion inertiallength scale. The observed variation in bow shockstructure and the foreshockwith upstreamparameters was similar to that seenat the earth.

1. INTRODUCTION ings with an emphasison their timing relative to the solar cycle,in relationshipto which PVO (1978-) and M5 (1967) PioneerVenus orbiter (PVO) magnetometerobservations at observationsare taken near maximum and M10 (1974) and low altitudesin the nightsideionosphere have been used by V9/V10 (1975-1976) closeto minimum. Russellet al. [this issue]to set an upper limit on the intrinsic General characteristics of the Pioneer Venus orbiter are magneticmoment of the planet that is small enoughto pre- presentedby Colin [1979]. On December4, 1978, PVO was clude its playing any role in the solarwind interactionwith placed into a 24-hour-periodorbit with an inclinationof Venus.Hence the magnetosphereof Venus (i.e., thoseregions about 105.6ø, an apoapsisof 12Rv, and a periapsisaltitude near the planet and its wake in which magneticfields domi- and latitude of-• 140-250 km and 17.0ø, respectively.Figure 1 nate chargedparticle motion) is totally inductivein nature. displaystraces of severalorbits in the two-cylindricalcoordi- Early theoriesof the solarwind interactionwith a 'field-free' nates X and (ya + Z2)•/2, where hereafter all unprimed planet[see Michel, 1971;Bauer, 1976] differ mostfundamen- coordinateswill refer to the Venus-centeredsolar ecliptic sys- tally in the extentto which the flow into and about the exo- tem. Primes will be used when the X, Y axes have been aber- sphere/ionosphereis influencedby chargeexchange and mass rated for the effectsof Venus orbital motion so that a radially accretioninvolving the neutral atmosphereand the amount of outward solar wind flow velocity in the planetary rest frame the incidentplasma which is absorbedin the processof pow- will be antiparallelto the X' axis.Orbits 71 and 184have peri- ering the ionosphericcurrents producing the magneticfields apsisnear localmidnight and noon,respectively, while that of which deflect the remainder of the solar wind [e.g., $onett and orbit 130 is near the dawn terminator. A curve representing Colburn,1968]. Pre-PVO observationalknowledge of the solar the mean bow shocklocation at Venus observedby PVO from wind interaction with Venus is reviewed by Russell [1979], Slavinet al. [1979a]is alsodepicted. The directionof satellite Breus[1979], and $iscoeand Slavin [1979].In particular,the motionin Figure 1 is towardthe planetfor (Y• + Za)•/2 less radio occultation and bow shock measurementsby Mariner 5 than 2 R• for orbits 71 and 184. The main advantagesof (M5), Mariner 10 (M 10),Venera 9 (V9), and Venera 10 (V 10) PVO's orbit over orbits of the earlier Venera 9 and 10 are its will be discussedhere in comparisonwith PioneerVenus find- low-altitude periapsisand daily sampling of the Cytherean ionosphere-ionosheath-bowshock regions and the solarwind • Instituteof Geophysicsand Planetary Physics, University of Cali- for all but the few tail orbits suchas 184 that may or may not fornia,Los Angeles, California 90024. passout into the unshocked solar wind. Disadvantages forthe Group,2Space RedondoSciences Beach,Department, California TRW 90278.Defense andSpace Systems investigation ofthe solar wind interaction, butnot of course 3Space Science Division, NASA Ames Research Center, Moffett for other aspects of the mission, are a lackof coverageof the Field, California94035. shock and ionosheathfor small sun-Venus-satellite(SVS) an- 4Physics Department, University ofSouthern California, Los An- glesand a paucityof orbitsthrough the magnetotail at high geles,California 90007. altitudesdue to PVO'shigh orbital inclination. 5 Laboratoryfor PlanetaryAtmospheres, NASA GoddardSpace FlightCenter, Greenbelt, Maryland 20771. Figure2 contains a plot of the magnitude ofthe magnetic field [seeRussell et al., 1979a,b, c] versustime for orbit 164. Copyright¸ 1980 by the American Geophysical Union. As is indicated,a seriesof bowshock crossings take place at

Paper number 80A0609. 7625 0148-0227/ 80/080A-0609501.00 7626 SLAVIN ET AL.: VENUS BOW SHOCK

vations in the manner used by Elphic et al. [1980] have been plotted against SVS angle in Figure 3 for orbits 130-212. Shown for comparisonare the ionopauseheights inferred from radio occultation measurementson Mariner 5 [Kliore et al., 1967;Mariner Stanford Group, 1967], Mariner 10 [Fjeldbo et al., 1975], and Venera 9 and 10 [Ivanov-Kholodnyet al.,

12 8 4 0 -4 -8 -12 1979].Prior to the direct observationsof the ionopausethe M5 X altitude recorded in 1967 has sometimes been regarded as somehow anomalous with the smooth variation with $VS Fig. 1. PioneerVenus orbiter'spath during orbits 71, 130, and 184 shown in Venus-centeredsolar ecliptic coordinateswith the mean anglein the solarminimum positionsfrom M 10, V9, and V 10 bow shock from Slavin et al. [1979a] also displayed. representedby a solid line in Figure 3 taken from Ivanov- Kholodny et al. being interpretedas supportfor a simplehard •7 Rv downstreamfrom Venus, after which a disturbed iono- impenetrablemodel of the solarwind interactionwith Venus sheathis observeddown to an altitudejust above 1 R v, where [Breus, 1979]. However, Pioneer Venus ionopausecrossings the magnetic flux density increasesdramatically until the ion- indicatedin Figure 3 with plus signsshow large variationsrel- osphereis entered,where it dropsto a value below that of the ative to the M10, V9, and VI0 points predominately in re- interplanetarymedium. The outboundpassage is similar but sponseto changingsolar wind dynamicpressure [Elphic et al., with a much shorterinterval spentin the ionosheaththan ex- 1980]with ionosphericparticle measurementsimplying an en- pectedfrom viewingFigure 1. Elphicet al. [1980]have estab- ergy input from the solarwind of the order of 10-[ erg/cm2 s lishedthat in generalthere is a significantcorrelation between [Knudsenet al., 1979],which correspondsto a 10%absorption the B2/8•r maximumjust exteriorto the ionopauseand the al- of theincident solar wind kinetic energy flux (i.e.,«nm•, F'sw 3 • titude of that boundary, as would be consistentwith approxi- 10ø erg/cm2 s) by suchmeans as joule heatingwith iono- mate pressurebalance between an isothermal homogeneous spheric currents [Elphic et al., this issue]or the damping of ionosphere,the exterior magnetic field, and solar wind dy- whistlermode wavesof bow shockand ionosheathorigin after namic pressure.By comparison,diversion of the solar wind they propagate into the ionosphere[W. W. L. Taylor et al., about Venus through massloading with photo-ions[Cloutier 1979].For the purposeof understandingbow shocklocation it et al., 1969] and charge exchange [Wallis, 1973] produces is important to note that while the altitude of the ionopauseis much slighter enhancementsin the total magnetic field mag- quite variable during the current epoch,the spreadin shock nitude owing to the deflection'sbeing more gradual and oc- position due to suchvariability in the context of an impene- curring over much longer length scales. Accordingly, the trable obstaclemodel is only • 1-2% about the mean because study by Elphic et al. [1980] suggeststhat on the averageonly of the large radius of Venus relative to the thicknessof the a minor role may be played by the neutral atmospherein the ionosphereand its changes.Ionopause structure is studiedin final shielding of the ionospherefrom the solar plasma at the detail by a number of authorssuch as Brace et al. [this issue], ionopause.Daniell and Cloutier[1977] have modeledthe iono- Elphic et al. [this issue],$penner et al. [this issue],and H. •1. sphericcurrents associated with sucha 'magneticbarrier' in- Taylor, Jr., et al. [this issue]. teraction [Johnsonand Midgley, 1968] on the assumptionof flow speedinto the ionospheremuch lessthan the ionosheath 2. NEAR-PLANET BOW SHOCK flow speed tangential to the ionopauseand given an iono- The solar wind is a collisionless anisotropic multi- spheric conductivity profile based upon an assumption of componentmagnetized plasma that expandsoutward from negligiblepenetration of the ionosphereby the compressedin- the sunwith a typicalratio. of bulk flow speedto the medium's terplanetarymagnetic field (IMF) in the ionosheath.Consis- Alfv6n and sonicwave group velocitiesof about 6-7 at 0.72 tently with the last assumption,they concludedthat the iono- AU (seeTables I and 2) givingrise to a bow shockupstream sphere of Venus presentedan impenetrableobstacle to the solar wind, the ionosphereabsorbing less than 0.1% of the in- ' I ' I ! J cident plasma. However, large magnetic fields have been found by PVO within the ionosphereboth over short-distance Orbit 164 Periapsis 21:19:46 scalesin the form of 'flux ropes' [Russelland Elphic, 1979] and May 17, 1979 over large regions of the ionospheredown to periapsisalti- 120 tudes following suddenlarge enhancementsof solar wind dy- E 100 namic pressuresuch as accompanyinterplanetary shocks [El- phic et al., this issue]. In addition, depletion of energetic electrons(> 102eV) in the ionosheathobserved by Mariner 10 ._e o 60 [Bridgeet al., 1976] and the closenessof the bow shockto the planet [Russell, 1977; Romanov et al., 1978; Slavin et al., • 40 1979a] have been interpreted as evidencefor partial absorption of the solar wind by the Cytherean ionosphere.Finally, • 20 model computationsby Gombosiet al. [this issue]using PVO- 0 [ J I measuredionosheath and neutral atmosphereprofiles indicate UT 1450 1750 2050 2350 2650 AIt(Km) 55961 39151 6310 30902 51488 that a variable portion of the incoming solar wind protons of SZA(deg)146 148 114 115 129 the order of 10-• by number may undergocharge exchange Fig. 2. Total magnitudeof the magneticfield observedduring or- with neutrals at ionopausealtitudes, thus creating an energy bit 164with approximatebow shockand ionopausecrossing locations and momentum sink for the ionosheath flow. indicatedplotted againstuniversal time, altitude, and solar zenith Dayside ionopauselocations determined with PVO obser- angle. SLAVIN ET AL.: VENUS BOW SHOCK 7627 of deflecting obstaclessuch as the planets. Theoretical ap- TABLE 1. Typical Interplanetary ParametersMeasured at I AU proachesto modelingthe hypermagnetosonicflow of plasma and Scaled to 0.72 AU past thesebodies have centeredon the gas dynamicdescrip- Parameter 1 AU 0.72 AU Units tion to which the dissipationlesssingle fluid MHD problem is 7.0 13.5 cm -3 thoughtto reducefor Alfv6nicMach numbers MA in excessof n 4.24 8.2 nT •10 [Spreiterand Rizzi, 1974; $preiter and $tahara, [this is- B• 4.24 5.9 nT sue].In the vicinity of the planet, bow shockshape and 1oca- B 6.0 10.1 nT tion are determined by the upstream flow parameters and 430 430 km s-• boundaryconditions on theobstacle surface. Examination of 7 x 104 9.7 x 104 øK characteristiclinesproduced bygas dynamic calculations of 1.5 x l0 s 2.5 x 105 øK flow about the earth'smagnetopause [e.g., $preiteret al., 1966] showsthat only the portionof the shocksurface forward of quitestable in location,as was the dayside ionopause altitude about one stagnation point radius behind the center of the inferred from their radio occultationmeasurements in Figure planet possessesdirect knowledgeof the dayside deflecting 3. Its closeproximity to the planet, at least in some cases,was boundary. Far downstreamthe obstaclestarts to resemble a difficult to reconcile with the expectationsof gas dynamics point sourcewith the distant shockwave weakeningas it [Romanov et al., 1978]. The Mariner 10 shock encounter asymptoticallyapproaches the 'Mach cone'of haft angle [Bridgeet al., 1974;Russell, 1977] was in goodagreement with the mean V9 and V I0 boundary position,while the Mariner 5 crossing[Bridge et al., 1967;Russell, 1977] in 1967 was more • --sin-•IM• :+MAM•2M'•2 2-1 1 -•/• (1) distantfrom the planet. For both the M5 and M10 flybys the oriented symmetricallywith respectto the solar wind velocity upstreamflow Mach numberswere similar to M• and MA val- vector in the planetary rest frame, where M• is the free stream ues of 6-7 [Sheferet aL, 1979;Lepping and Behannon,1978]. sonicmach number [Dryer and Heckman, 1967].However, the Assumingthe SVS angle dependenceof ionopausealtitude flow about the earth and to a lesser extent Venus differs from from the Venera observationsdisplayed as a solidline in Fig- that about a simple hard impenetrablesphere in that the mag- ure 3, the subsolarheights for M5 and M I0 were about 500 netotail region effectivelyexcludes inflow and in axial diame- km and 280 km, respectively,the difference being attributed ter is as large as or larger than the forward deflectingsurface to changingsolar wind dynamicpressures and the ionospheric in the terminator plane, as is discussedfurther in section4. effectsof solarEUV radiationin eachcase [ Wolffet al., 1979]. Accordingly,for the purposeof studyingthe solar wind in- As was discussedearlier, ionopausealtitude variations should teractionwith the Cytherean ionospherethis sectionconsiders have little effect on obstacleradius and hence shock altitude, in Figures 4-8 only those bow shock crossingsrecorded by suchas in this case,where the simpletangential discontinuity PVO and other spacecraftwith X' _>-1 Rv. Figure 7 displays obstaclemodel of the interactionwould predict the M5 shock as line segmentsthe portions of the Venera 9 and 10 orbits crossingto be more distantby a factor of (6050 + 500)/(6050 over which the transition between shocked and unshocked so- + 280) = 1.03 than that of MI0 after the SVS angle depen- lar wind plasma occurredin the wide-angle plasma analyzer dence of shockshape is taken into account.However, the ob- [Verigin et al., 1978]. As is shown, the near-planet bow shock servedratio in the terminator plane is • 1.3, as is evident in observedby the two Soviet orbiters in 1975-1976 appeared the figure. A possibleexplanation that was first suggestedin referenceto the solarwind interactionwith the moon [Sonerr and Colburn, 1968] and later invoked by ,Russell[1977] at 1200- IonopauseAltitude vs. SVS ø + Venusis that the exosphere/ionospherecan absorbpart of the Mariner 5 (1967)..... incident flow and can thereby presentan effectiveobstacle to Manner I0 0974)..... the solar wind that is smallerthan the physicaldimensions of I000 _ Venera 9 • I0 (1975-6) PmoneerVenus (1979)--+ the planet. In this connectionit is of interest to note that not

+ + + only upstream solar wind parameters but also solar EUV emissionsmay exert somecontrol over the ability of the upper 800 - + + ionosphereto shielditself from the solarwind as suggestedby +

i Slavin et al. [1979b]. Solar EUV flux, which plays an impor- + + + 0 tant role in ionosphericphotochemistry [e.g., Chenand Nagy, 600- + + + + + + ++ &+ + /+ + ++ + TABLE 2. Derived Quantities From Table I Relevant to the Bow 4. + + +.•. + /•) •00 + + •+++++++- Shocks of Venus and the Earth ++ + ++•++ +•.• - •+ + ++ + + .,..,.++ + O++_+....-O"•v+ +++ Derived +c-,-o;>-+••++ +-•++z--/ Quantity 1 AU 0.72 AU Units 200 + ++ • + +/ -- Venerag-IO•' IMF spiral angle 45 36 deg (Ivanov-Kholodn¾ %,, 24 33 kHz , I • I , I , I %,i 556 772 Hz 0o 200 400 600 800 c/%,• 89 64 km SVS(Degrees) P = nks(T•+ T•) 2.1x 10-•ø 6.5x 10-•ø dynescm -• B:/8•r 1.4x 10-•ø 4.1 x 10-•ø dynescm -• Fig. 3. Ionopauseheight determined from magnetometerdata for fi = 8•rP/B• 1.5 1.6 ..- orbits 130-212 plotted as plus signsversus SVS angle along with radio g•M, 55, 7.9 69, 6.2 km s-• -.. occultationmeasured altitudes from Mariner 5 and 10 and Venera 9 V•,M• 50, 8.7 60, 7.2 km s-, ..- and10 as indicated (see Brace et al. [this issue] for model fit to PVO Msts 5.8 4.7 -.. surface). 7628 SLAVIN ET AL.: VENUS BOW SHOCK

PioneerVenus T field, 1971]. At Venus the small contribution of ionosphere BowShock Crossings thicknessto obstacleradius and the exponential dependence of ionosphericpressure on altitude limit the effectsof varying "• ,+ +++ +•+•+ external pressureon shock altitude to -• 1-2% comparedwith -•10% at the earth [Holzer and Slavin, 1978; Slavin et al., + 1979a]. In the earlier study, bow shock crossingsrecorded during the first 65 orbits of PVO were modeled with a simple polar form conic whosefocus was centeredon the planet [Holzer et • c•'.•+ ] xo= 02 Rv al., 1972], '<•½ + / e: 0.80 •-D Normal RMS = 0.17 Rv r-- L/(1 + ß cos(SVS)) (2) wherer = (X '•-+ Y'•-+ Z'•-)•/•-, L is the semi-latusrectum, and ß is the eccentricity.Thus aberration due to the orbital motion of Venus is taken into account, but the actual solar wind ve- Ionop locity direction at the time of the crossingsis not. However, this latter effect is small owing to the strongtendency of the solar wind velocity to be oriented radially outward from the sun [Wolfe, 1972]. A best least square fit to the near planet I 0 -I crossingsfrom the first year of magnetometerdata in Figure 4 x'(R•) producedß -- 0.73 and L -- 2.39 R•, with a root-mean-square Fig. 4. One hundred and seventy-twoPioneer Venus sh•k cross- deviation normal to the curve for all 172 points of 0.18 R•, in •gs plotted • solar •d a•atcd solar ccUptic•rd•atcs along agreementwith the earlier ß -- 0.80 and L -- 2.44 R•, [Slavinet with a mcan fo•ard ionopauscassumed spherical at an altitude of al., 1979a]. In order to further enhancethe goodnessof fit the 3• •. focus was allowed to range along the X' axis with the opti- mum fit found to occur with the focus at Xo = +0.2 R•, for 1978], and hence perhaps electrical conductivity, varies with which ß -- 0.80 and L = 2.22 R•, with a slightlysmaller rms de- solar cycle [Hinteregger,1979] and has been estimatedto have viation of 0.17 R •,. It was further noted that this fit as shown in been -30% greater for the Mariner 5-Venus encounter in Figure 4 appearsa little too far from the planet for the small- 1967 as solar maximum approachedthan for Mariner 10 and est SVS angles and too close at the largest angles, suggest- Venera 9 and 10 near minimum [Wolffet al., 1979]. By com- ing that a better fit (i.e., smallerrms deviation) is possiblewith parison,dynamic pressurewas twice as high for M 10 than for a larger eccentricity. Accordingly, the fit in Figure 4 was M5 [Leppingand Behannon,1978; Shefer et al., 1979], while iterated upon by binning the points by equal intervals in cos the upstream interplanetary magnetic field was less aligned (SVS) normal to the curveand fitting the mediansas shownin with the X axis and less steady for M5 [Bridge et al., 1967] Figure 5, which effectivelyweights the data to create a uni- than was the situation for M10 [Nesset al., 1974].In addition, form samplingwith cos (SVS) to be fit. In the coordinates1/r the Mariner 10 Lyman alpha observations indicated a de- and cos (SVS), conic sectioncurves are straightlines, as can creasein exosphericneutral hydrogen by at least a factor of 2 be seenby inverting (2): from the Mariner 5 levels near solar maximum [Broadfoot et al., 1974]. Thus the larger effectiveobstacle to the solar wind 1/r -- ß cos (SVS)/L + 1/L (3) inferred from the M5 measurements could be associated with The best least square fit to the points in Figure 5 does give a higher solar EUV flux, a denserneutral exosphere,a lower ex- larger eccentricityof 0.88 with nearly the sameL -- 2.21 R •, to ternal dynamic pressure,and/or the effects of IMF orienta- producea better fit with an rms deviation for 172 crossingsof tion and its variations on the ionosphere.Studies to be per- 0.16 R•,. As it is displayed, this curve is slightly higher than formed in the future as well as those discussed in this article are aimed at gaining an understandingof the variation in the Venus interaction with the solar wind associated with these PioneerVenus Bow Shock Crossing Medians (Binnedby Cos (SVS) Normal to parametersand others. CurveX o = +0.2 Rv. cc= 030, L= 2.22Rv) _ Figure 4 showsthe location of 172 bow shockcrossings by PVO during its first Venus year in orbit for which X' _>-1 R•,. Multiple encounterswere relatively rare for Pioneer Venus dayside boundary crossings,but when present, the average bow shock position per inbound or outbound passwas used. All crossingidentifications were made with magnetometerob- Xo = +0• Rv • = 0.88 servationsas available, crossingsat small anglesto the shock L = 2.21R v surfacesuch as occur at smaller and larger SVS anglesbeing 0.2- 2-D NormalRMS (N=172) = 0 16 Rv excluded to avoid biasing the surface mapping with nonuni- form spatialcoverage. As was found by Slavinet al. [1979a] in a preliminary study, the Venusian bow shock surfaceis not i I i I only very similar to its terrestrialcounterpart in shapebut also -0.5 0 0.5 in variation about its mean position, which at the earth is due Cos (SVS) to the sixth root dependenceof magnetopauseheight on solar Fig. 5. Mediansin shockposition as describedin the text displayed wind dynamic pressure[Binsack and Vasyliunas,1968; Fair- in conic coordinatestogether with a best fit (seeTable 3). SLAVIN ET AL.' VENUS BOW SHOCK 7629

the medians at both ends but lower in the center, so that no further improvement can be expectedwith a different eccen- •/y2 +Z2 (lO3km ) tricity. These results are summarizedin Table 3. Using the best mean surface, all of the crossingsin Figure 4 have been mapped into the aberratedterminator plane and plotted as a histogramin Figure 6. As is shown,the mean of the distribution is 2.39 R•, with a standard deviation about the mean of nearly 10%, which as mentioned earlier is similar to that ob- (Verigino/o/, 1978) served for the terrestrial shock [Fairfield, 1971], where the causeis a nearly impenetrable,but compressible,obstacle. ß The average bow shock from the Pioneer Venus observa- /// J t Mariner5 (1967) n tions described above is compared with M5, M10, V9, and .•' '// • Moriner10(974) • V 10 in Figure 7. While in good agreementwith M5 near solar maximum in 1967, the shockin 1978-1979 probed by PVO is seen to be ,-,35 percent more distant from the center of the planet in the X' = 0 plane than was the casefor M 10, V9, and V10 in 1974-1976. The possibilityof this increasein shock altitude being due to a spatial asymmetryin shock shape such 8 6 4 2 0 2 4 6 8 I0 as that proposedby Cloutier[1976] sampledat differing loca- X (103km) tions by the high-inclination Pioneer Venus orbiter and the lower-inclinationVenera satelliteswas discountedby Slavinet Fig. ?. The mean •w sh•k location dete•ined by PVO •m- pared with that found by Vanera 9 and 10 • 1975-1976 [from Verigin al. [1979b] after an examination of the dependenceof shock et al., 1978] •d Manner 5 and 10. altitude on the IMF orientation failed to detect any significant asymmetry such as had been reported by Romanov et al. altitude in 20 ø bins as a function of upstream IMF orientation [1978] and set an upper limit of •12% on the contribution is displayedas was done by Slavinet al. [1979b].The standard from this sourceto the total change evident in Figure 7. The deviation of the means is only 0.06 Rv, and hence the most study of Slavin et al. is extended here with the 172 shock probable upper limit on any asymmetrywith respectto IMF crossingsin Figure 4 as displayedin Figure 8. The angle mea- directionis • 102km. Accordingly,it doesnot appearthat the sured counterclockwise as viewed from the sun between the different inclinations of PVO and Venera orbiters could con- upstreamIMF vector projectedinto the terminator plane and tribute as much as 1 percent to the •35 percent growth in the shock position vector in that same plane is termed Axs. shock altitude from 1975 to 1979. Further, it is also concluded Thus an interplanetary field parallel to the +Z' axis at the that at Venus the shock position in the terminator plane is time of a shock crossingin the Y' = 0 plane with Z' > 0 corre- more symmetricthan has been reported for the earth by some spondsto Axs -- 0 ø, while if the IMF had been orientedparal- studies[e.g., Formisano, 1979], possibly owing to the intrinsic lel to the + Y' axis for that samecrossing, then Axs -- 90ø. The nature of an ionosphericobstacle as opposedto a dipole with near-polar nature of the PVO orbit and the tendency of the variable, but approximately perpendicular, orientation with IMF to lie parallel to the plane of the ecliptic thus produce respect to the solar wind. It should also be noted that this more PVO shockencounters in the regionsabout Axs = 90ø, growth in shock height cannot be attributed solely to the 270ø comparedto Axs = 0 ø, 180ø as is evidentin the top por- higher ionopausealtitude in 1978-1979, which may be due to tion of Figure 8. In the lower part of the figure, mean shock depressedsolar wind dynamic pressureas occurredat last solar maximum [e.g., Fairfield, 1979], over 1974-1976 shown in Figure 3. The •270-km subsolarionopause height suggested by the Venera and M 10 radio occultationdata would require Mean= 2 39 Rv •-021RV •-0 21RV a subsolaraltitude during the current epoch of [1.35(6050 +

4O 270) - 6050) • 2480 km to explain the growth in shock altitude in terms of a simple tangential ionospheric obstacle I I ', model of the interaction without any 'cometary' processesin-

3O - i i - , ,, 20• TotalNo.Events= f72

, i', '• 20

i i

•0 - i - i

i II I

• i i

t.0 z0 z z 2.4 2.6 z.0 3.0 3.2 3.4 cn'l , I , I t I I , , 0 40 80 f20 f60 200 240 280 $20 $60 R (x'= 0) (%) AXB(degrees) Fig. 6. Bow shockcrossings from Figure 4 mapped into the aber- Fig. 8. Averagesh•k distan• • the a•atcd testator plane rated terminatorplane and plotted as a histogramwith the mean and by 20ø b•s as a functionof Axs for the 172cross•gs • Figure 4 •th standard deviation shown. the numar of events• each b• displayed• •e top panel. 7630 SLAVIN ET AL..' VENUS BOW SHOCK

TABLE 3. Summaryof the Resultsfor Fitting the 172 PioneerVenus ShockCrossings in Figure 4 Xo, Rv e L, Rv Nose, Rv RMS, Rv Fitting 0 0.73 2.39 1.38 0.18 all points +0.2 0.80 2.22 1.43 0.17 all points +0.2 0.88 2.21 1.38 0.16 medians volving the neutral atmosphereas have already been men- extrapolatedto the subsolarpoint givesan altitude of ~2270 tioned. The actual growth in the ionopauseheight with the km for the nose of the bow shock which for d/D -- 0.39 re- approachof solarmaximum is closerto ~50 km withpart of it quiresan effectiveobstacle of ~60 km below the surfaceof the probablyalso due to the previouslycited solarcycle variation planet. in solar EUV flux. The contention of Slavin et al. [1979b] An estimateof the uncertainty in the determinationof the basedupon terrestrialsolar wind and shockstudies that there subsolarshock height can be madedirectly from (2), was no large change in solar wind Mach numbers between 1974 and 1979 is further strengthenedin section4, where the Ar(SVS= 0ø)/r(SVS= 0ø) = IAL/LI• + IA•/(1 + •)1,- (5) downstreamPVO shockcrossings are shownto lie no farther whereAr, AL, and Ae signifythe uncertaintiesassociated with from the X' axis than were those of Venera implying similar the shockdistance, semi-latus rectum, and eccentricity.The Mach coneangles, and henceMach numbersvia equation(1), uncertaintyin L is quite small owing to the large number of at the time of solar minimum and maximum. Further, the shock crossingsnear SVS -- 90ø, so that AL/L ~ 5 x 10-3, Mach numbersto be displayedin later portionsof this article whereaswithout observationsfor small SVS anglesthe eccen- computedfrom PVO measurementsdo not seeminconsistent tricity cannotbe determinedto betterthan Ae/(1 + •) ~ 2 x with the expectationsbased upon averagevalues at 1 AU 10-2. Hence for d/D -- 0.39 the best determinationof effective scaled to the orbit of Venus as shown in Tables 1 and 2. Thus subsolar obstacle height is -60 + 150 km, or more con- the causeof the increasein shockaltitude evident in Figure 7 servatively,using d/D -- 0.33, it is +210 +_ 160 km, both of appearsto lie with a solarcycle modulation in the solarwind which lie below the typical ionopausealtitudes. interactionwith the planet possiblyassociated with the effects Accordingly,the bow shockencounters presented in Fig- of changing EUV flux on the upper ionosphereand exo- ures 4-8 are often too closeto the planet to be compatible sphere.In the formercase, any variationin ionosphericelec- with the gasdynamic models assuming no upstreamsources trical conductivitywould influenceits ability to deflectthe so- or sinks and a nonabsorbingobstacle at the observediono- lar wind as well as the rate of solar wind energy deposition pausealtitudes. This finding togetherwith the variability in throughjoule heating, while in the latter case,altering the shocklocation and its apparentsolar cycle dependence are in- neutral particleprofile at ionopauseand higher altitudeswill dicative of a dynamic interactionbetween Venus and the solar affect the rates of chargeexchange and photo-ionpickup and wind possiblyas has been proposedby Cloutieret al. [1969], possiblymodify the ionosheathflow field. In addition,the Bauer and Hartle [1974], and Russell[1977]. More recently, large orbit to orbit variability in shock height already dis- cussedis preliminary evidencefor shorter-termmodulations 3.0 of these types. In the gasdynamic approximation a theoreticalrelationship TerrestrialAnalague (Fairfield, fgTf) has been establishedby $preiter et al. [1966] betweenthe nose distance of the obstacle, D, and the shock standoff distance, d, such that ..' +++...... ---'• did--(1.1/Ms2)((¾ - 1)Ms2 + 2)/(7 + 1) (4) + + + where ¾is the ratio of the specificheats of the gasat constant + pressureand volume.At the earth the observationallydeter- (X•0)=4.15['Ionopause RadiusJ-l. a7R v mined value of did is 0.33 [Fairfield, 1971]for a measuredMs 2.0 Correlation Coefficient= 0.4 ~ 7 implying¾ ~ 1.9,which is between2 and 5/3, correspond- ing to an ideal monotonicgas with 2 degreesand 3 degreesof $.0 freedom, respectively.In the absenceof theoreticalor empirical meansof obtainingvalues of ¾from basicplasma parame- TerrestrialAnalague (Fairfield, fg7f) ters (see, for example, Chao and Wiskerchen[1974]) it is not possibleto scale¾ with distancefrom the sun as was done for • + +,+ other relevant quantities in Tables 1 and 2. However, in the C) ..--'"' ,2' 2.5 high Ms limit the jump conditionsacross a shockgo as (7 +

1)/(7 - 1) [e.g.,Spreiter et al., 1966],and thus the preliminary ...• study by Russellet al. [1979d], indicating that the bow shock O• +++++ + + of Venus may be weaker than that of the earth, as well as the (X60)=2.62lianapause Radius]-O.35 Rv study of plasmajump conditionsby Mihalov et al. [this issue] CorrelationCoefficient = 0.3 2.0 may imply that a slightly larger than terrestrialvalue of ¾ f.00 f.02 1.04 f.06 1.08 f. fO should be used at Venus, as was suggestedby Slavin et al. Rzp(RV) [1979a].Using ¾ = 2 and Ms -- 6 from Table 2 in (4) yieldsd/ Fig. 9. The distance to the shock in the aberrated terminator D -- 0.39, which is ~20 percentlarger than that found for the plane as a functionof ionopauseradius is shown(a) without and (b) earth. The best fit to the PVO shock crossingsfrom Table 3 with correction for the effect of Mach number on shock location. SLAVIN ET AL.: VENUS BOW SHOCK 7631

Gombosiet al. [this issue] have suggestedfrom theoretical 2.8 modeling that up to 10%of the incident solar plasma may be R•S(X/=O)=-l.SnmpV2+ 2.5 absorbednear the ionopause(i.e., charge exchangewith neu- CorrelationCoefficient: 0.2 trals), thus influencingshock location. In this event the elec- 2.6 -•r + tric potential imposed by the solar wind acrossthe planet could be of the order of 10 percent the total available, or *-4 . + + • + + + kV, as opposedto the small 40 V implied by the near tangential discontinuity model of the ionopause [DanJell and + Cloutier, 1977]. Just as dayside magnetic reconnection is + + + + + thoughtto control the potential acrossthe terrestrialmagnet- + osphere,the amount of ionosphericabsorption determined by 2.2 + solar wind conditions and possibly solar radiation sets the electric potential drop which powers the magnetosphereof Venus. The larger potential suggestedhere on the basis of •.0 I I I I I I I bow shock-inferred knowledge of the ionosheath flow field 2 • 6 8 could provide the ultimate sourceof energy for the 1- to 3- nmpV2(10 -8 dynes/cm 2) keVions observed in themagneto•ail plasma sheet by l/erigin Fig. 11. Distanceto the bow shockin the aberratedterminator plane et aL [1978] as well as during periods of changing tail mag- as a function of solar wind dynamic pressure. netic configuration referred to as 'substorms'by Romanov et al. [1978] when inductive mechanismsshould also produce els, with the bottom one to be discussedlater, is the expected chargedparticle acceleration. mean terrestrial dependencealready considered

3. INFLUENCE OF IONOPAUSE ALTITUDE AND SOLAR Res(X' - 0) -- 1.33(subsolarobstacle radius) WIND CONDITIONS x (terrestrialvalue of Res(SVS= 90ø)/Res(SVS= 0ø)) To further investigatethe large variation observedby PVO = 2.4R,,p (6) in bow shock position, the ionopause crossingsin Figure 3 with SVS _< 50ø, for which a shock crossingrecorded on the In agreementwith the conclusionreached in the previoussec- same inbound or outbound passis available in Figure 4, were tion that the shock is generally closer to the planet than is identified. Crossingsof thesetwo boundarieson the same leg found by scalingthe earth observationsdespite the slightly of the orbit were required to minimize the effectsof temporal smaller Mach numbers anticipated at 0.72, 13 of the 18 cases variability and provide the closestapproximation to a 'snap- in the top panel are at lower altitudesthan the terrestrialana- shot' possiblewith Pioneer Venus. Ionopauseencounters with logue. Further, the correlationcoefficient between Res and SVS _< 50 ø were chosen both because it is the forward ob- R•p is not great, suggestingthe presenceof more important stacle surface that is expected to influence the bow shock in controllingfactors to be identified.A trend toward increasing the region consideredin Figure 4 and becausefor those sun- Res with larger R•p is evident, but as the shock altitude in- ward crossingsthe ionopauseappears approximately spheri- creaseswith decreasingMach number and ionopausedistance cal, so that the observedboundary distancecan be assumedto growswith smallerdynamic pressure, both of whichfollow so- approximatelyequal the subsolar distance for thepurposes of lar windspeed, the effects of thesetwo parameters must be ex- making comparisonsamong crossingsat various SVS angles. amined separatelybefore a causalrelationship between Res Of the 23 events meeting those two criteria, solar wind ion and R•p is drawn. number density and flow speedwere available for 18 which To this end, Figure 10 investigatesthe dependenceof Res are consideredin Figures 9 through 13. on the upstreamAlfv6n Mach number.A significantcorrela- In the upper portion of Figure 9 the distancefrom the cen- tion is presentwith decreasingMach number resultingin ter of the planet to the bow shock,Res, in the aberrated termi- more distant shock position in the terminator plane, as has nator plane is plotted as a function of ionopausedistance from the center of Venus, Rtp. Shown as a dashedline in both pan- -BM2/8•r=O.76 (O.88nmpV 2cos2 SZA)+ 7.3x lO -9 CorrelationCoefficient=0.8 + $.0- Res(X/--o)=-o.t15M A+ 3.f5, cc = 0.7

o 2.5 I! + +

I i + I I I I I 2 4 6 8 2.0 I I I I I 0.88nrnpV2CO?SZA(10- 8 dynes/crn 2 ) Fig. 12. Comparisonof external pressureon the ionopauseat Fig. 10. Distanceto the bow shockin the aberratedterminator plane given locationsdetermined by magneticpressure in the barrier and as a function of Alfv6nic Mach number. upstreamdynamic pressure with simplemodel assumptions. 7632 SLAVIN ET AL.: VENUS BOW SHOCK been found to be the caseat the earth [Formisano,1979]. Simi- whether or not the effective obstacle that the shock senses lar resultsare obtained when electron temperature,which in continuesto shrink with increasingpressure or approachesa general is not measured,is assumedand the magnetosonic minimum altitude as can be shown to be the case for the iono- Mach number used in place of MA. The only completeMHD pauseitself. solutionfor the flow of magnetizedplasma about a blunt body As is noted in the introduction,the relationshipbetween is that of Spreiterand Rizzi [1974]for the specialcase of paral- ionopausealtitude and external pressurehas been examined lel IMF and solar wind velocity vectors,in which event meth- by Elphic et al. [1980]. It is pointed out in their equation (4) ods exist to reducethe governingequations to thoseof gasdy- that for a homogeneousisothermal ionosphere in equilibrium namics as also occursin the high MA limit [Spreiter and with its exteriorenvironment without significant intermixing a Stahara, this issue].While in generalV and B are not parallel, linear relationshipexists between Rzp and the logarithmof ex- the location of the bow shockin the terminator plane for Ms = ternal pressurewith the slopeand interceptrelated to the ion- 10 and ¾-- 5/3 as a functionof M• from the modelof Spreiter osphericscale height and referencelevel pressure.Elphic et al. and Rizzi is shown with trianglesin Figure 10 in the absence alsodetermined that to a high degreethe maximum magnetic of any theoreticalmodel allowingarbitrary V and B. The ab- energydensity just outsidethe ionopausewas approximately solute values of RBshave been scaledby the ratio of subsolar balanced by the ionosphericpressure inside the ionopause, obstacleradii at earth and Venus for the purposesof com- suggestingthe presenceof very little plasmawithin the 'mag- parison.For larger M• the alignedflow predictionsapproach netic barrier.' As is discussedby Siscoeet al. [1968] and Sprei- that of the gasdynamic modelswhich neglectB and for which ter and Stahara [this issue],the pressuredistribution along the Ms -- 10 produces shock surfacestoo close to the planet at forward portion of a blunt obstaclein a high Mach gas flow earth [e.g., Fairfield, 1971] where Ms -- 8 is more typical as should be near that predicted by the Newtonian approxima- well as at Venus where Ms -- 6 should be close to the mean. tion, in which the pressureaway from the stagnationpoint However, the slopeof the linear fit displayedover the range in varies with the square of the cosineof the angle betweenthe M• of 4 to 8 doesnot appear inconsistentwith the model pre- upstream flow direction and the normal vector from the ob- dictions of Spreiter and Rizzi. stacle surface. For the ionopausecrossings considered here, In the bottom panel of Figure 9, RBs' is the shock distance this boundary surfaceis nearly sphericalso that the external scaled by the linear relationship in Figure 10 to the mean pressurewill be assumedgiven by 0.88nmpV• cos 2 (SZA), value of M• for the 18 crossingsof 6.4. While the correlation wherethe factorof 0.88 relatesdynamic pressure to stagnation is still not strong,the slope of the linear regressionto the MA pressureon the obstacleand SZA is the solar zenith angle correctedpoints is lower than in the upper panel as expected which is equivalent to SVS. Figure 12 plots the maximum owing to the additive effects of Mach number and dynamic magneticpressure in the magneticbarrier as usedby Elphic et pressureas solar wind speedchanges. Hence while the spread al. againstthe predictedexternal pressurefrom the PVO up- is large and the Cytherean shockis at smallerrelative heights streammeasurements. The agreementis good especiallycon- than the terrestrial model, the relationship between R•s' and sidering the neglect of helium ions in the solar wind, which R•p doestend to parallel that of the earth's bow shock/magne- would probably increasethe 0.88 coefficientto a value near 1 topauseon the average. and make the linear regressionintercept closerto the origin Figure 11 further contrasts the differences between the and the variable time lag betweenthe period of solarwind ob- earth and Venus in the bow shockobservations by plotting the servations and the ionopause crossings,which for the 18 MA-corrected shock distance in the aberrated terminator eventshere is the order of half an hour. Ionopausestructure plane against solar wind dynamic pressure.In doing so, it is and the transmissionof solarwind pressurethrough the iono- assumedin the absence of any information on the helium sheathto the ionosphereare further examinedby Elphic et al. abundancein the solar wind plasma that the ions are all pro- [this issue]and Brace et al. [this issue]. tons. The actual helium contribution to the solar wind plasma In Figure 13 the dependenceof R•p on both the external is variable both on short time scalessuch as days and over pressureinferred from upstreammeasurements and magnetic longer periodswith the solar cycle with the mean near 4% by field observationsnear the ionopauseis investigatedby re- number density and the range of the order of 1-10% [Hir- gressingboundary crossing distance on the logarithmof pres- shberg,1973]. Hence as will be noted againin later figures,the sureas suggestedby Elphicet al. [1980].While the number of useof nmpV 2 for solarwind dynamic pressure is on the aver- eventsconsidered in thisstudy is not great, the superior order- age an underestimateby a factorof 1.16.Future studies at ing of R,, by log (B•?/8vr)as comparedwith 1og(0.88nmpV2 Venusfor whichthe absolutevalue of the pressureis impor- cos2 SZA) stronglysuggests that thetemporal variations in the tant should endeavor to obtain actual numbers for the ionized solar wind during the time required for PVO to transversethe helium componentof the solarwind duringthe currentepoch ionosheathare significantin many cases.The slopesand inter- near solarcycle maximum. At the earth wheremagnetopause cepts in both panelsare generallysimilar to thoseobtained by positionis largely determinedby the externaldynamic pres- Elphic et al. despitethe limited numberof eventsused in this sure,a factorof 4 changein this parametersuch as in Figure analysisand hencecorrespond to physicallyreasonable iono- 11 would producea variationin shockdistance by a factorof sphericpressures and scaleheights as discussedin that paper. (4)1/6 = 1.26 with R•s' decreasingwith increasingpressure As solarwind pressureincreases the percentagedecrease in [Binsackand Vasyliunas,1968; Fairfield, 1971].In Figure 11 ionopausedistance is similarto that found for the bow shock the dependenceof R•s' on pressureat Venus is weak with in overallmagnitude. However, in contrastto the shockobser- onlya ---5%decrease in shockdistance as nmvV 2 variesfrom 2 vationsin Figure 11 a lowerlimit on ionopauseheight near x 10-8 to 8 x 10-8 dynes/cm2, consistentwith the stiffnessex- 200 km is evidentas has also been noted by Wolffetal. [1979], pectedof an ionosphericobstacle with an exponentialdepen- Vaisberget al., [1980],and Braceet al. [thisissue] and is simi- denceof pressurewith altitude.However, the smallnumber of larly apparentin Figure 3. Thesestudies have suggested the eventsavailable at thistime do not permitit to be determined causeof thislower bound to be simplythe exponentialdepen- SLAVIN ET AL.: VENUS BOW SHOCK 7633

denceof pressureon altitudewithin the ionosphere,enhanced quired for pressurebalance with the solarwind for a magneto- ionosphericheating from the solarwind for lower ionopause tail that flares out into the flow less than that of the earth and altitudes,and possiblythe effectsof increasedion-neutral col- experiencesless aerodynamic stress normal to its outer bound- lisionsjust beneaththe ionopauseon the transportprocesses ary [e.g., Mihalov et al., 1970]. Eroshenko[1979] has used Ven- suchas diffusion,convection, and electricalconductivity. The era orbiter magnetic field data at 1500- to 2000-km altitudes studyby Gombosiet al. [this issue]suggests an additional fac- on the nightsideto showthat as expectedfor an induced mag- tor limiting the minimum ionopauseheight in the form of the netosphere,the orientation of the current sheetseparating the neutral atmosphere supporting the solar wind pressure two lobesof the tail is perpendicularto the vector component throughincreased charge exchange at low altitudesremoving of the interplanetarymagnetic field transverseto the upstream hot H + from the ionosheathand replacingit by cool ions,the solar wind velocity vector with a time lag of up to -30 rain for differencein energyand momentumbeing carried away by changing IMF direction. By utilizing the same type of argu- hot neutralhydrogen as is thoughtto occurextensively in the ments as have been applied to the terrestrial magnetotail solar wind interactions with comets. [Dungey, 1965] a magnetotail length at Venus up to •'F'sw• 150 R•, is implied by the findingsof Eroshenkoassuming the 4. DISTANT BOW SHOCK tail field lines to be connectedto the IMF as opposedto At the earth the solar wind interaction with the geomag- 'closed.'The occasionallylarge ionosphericmagnetic fields re- netic field pulls back magnetic field lines anchored in the ported by Elphic et al. [this issue]also suggest the possibilityof planet to form a magnetotail [Ness, 1965]. It is composedof a highly variable magnetotail with its length and magnetic two lobescontaining oppositely directed magnetic fields sepa- flux contentcontrolled by the daysideinteraction as has long rated by a current layer embeddedin a thicker plasma sheet been envisioned for comets [e.g., Alfv•n, 1957]. In fact, Mari- of averagedensity of •10-' cm-3 and mean particleenergies ner 10 observed plasma electron and magnetic field per- of • 103eV. In addition,the tail regionis subjectto large spa- turbations believed to be associated with the VenusJan wake tial and temporal variationsincluding bulk plasma flows dur- regionup to • 102R•, downstream,although the magnetotail ing periodsof enhanceddawn-dusk electric fields and energy proper is not thought to have been penetrated[Yeates et al., input from the solar wind termed geomagneticstorms and 1978; Lepping and Behannon, 1978]. It is also of interest to substorms[e.g., ½aan et al., 1979]. While the magnetotail has note that the delay time of •30 rain in the responseof the not been well studiedeither experimentallyor in theory past Venus magnetotail to the interplanetary electric field varia- the oribt of the moon, •60 Re, it has been reportedly observed tions would suggesta crossmagnetospheric electric potential as far as 3100 Re downstreamfrom the earth by Pioneer 7 [In- of •1 kV by scaling the terrestrial value using the tail flux triligator et al., 1979a]. In Figure 14 the earth's distant bow convectioncycle time of $iscoeet al. [1975] for intrinsic field shockand magnetotail from Fairfield [1971] have been plotted magnetosphereswhich may have applicability at Venus if the in relation to Venus with the scalingof 10.8 Re-- 1 R•, which feet of the tail field lines observedby Eroshenko[1979] were in correspondsto making the near-planet shock observed by fact actually convectingthrough the ionosphereas for ex- PVO approximately coincide with that of the earth and is ample is shownby Brace et al. [this issue,Figure 13]. roughly equivalent to scalingby the subsolarradii at each Plasma instruments on Venera 9 and 10 have in addition to planet. Past -6 Rv (i.e., •65 Re) no magnetopauseand a detectingthe plasma sheet also identified a broad boundary dashedbow shockcurve is shownowing to the poor sampling layer analogousto the plasmamantle of the terrestrialmagne- available farther downstreamonly by Explorer 33 [Howe and totaft [e.g., F'aisberget al., 1976; Gringauz et al., 1976; Roma- Binsack, 1972; Mihalov, 1974] and Pioneers 7 and 8 [Scarf, nov et al., 1978]. These observationsinclude evidence for mix- 1979,and referencestherein] as mentionedabove. Near -6 ing of plasmasof ionosphericand solar wind origin [Vaisberg in the figure the maximum spread in magnetopauselocation et al., 1976] and hence some direct flow of solar wind behind (y2 ..[_Z2)i/2 scales to about+_1.4 R•,[Howe and Binsack,1972] the planet, as has been suggestedwould be the consequencein and +-1.1 Rv for the earth's shock [Mihalov, 1974]. fluid mechanics of a 'viscous' interaction between the solar Venera 9 and 10 observations have shown that Venus also wind and the ionosphere [Perez-de-Tejada, 1979]. Pioneer possessesa magnetotaft with two lobessimilar enoughto that Venus orbiter plasma analyzer measurements[Wolfe et al., of the earth to have thesefields at one time partially attributed 1979] have also found evidence of ionosheath flow with a to the existenceof a significantplanetary magnetic moment component in toward the X' axis behind the planet [In- whose magnitude was then estimatedfrom the tail magnetic triligator et al., 1979b]. flux density [Dolginovet al., 1978]. However, as was correctly An additional source of information on the ionosheath flow concludedby Dubinin et al. [1978] on the basisof laboratory field in the neighborhood of the wake is the location of the experimentsand confirmed by Pioneer Venus magnetometer distant downstreambow shock surface. For the purposesof measurements[Russell et al., this issue]the presenceof a mag- conductinga preliminary surveyof the high-altitude nightside netotaft is not a sufficient condition upon which to conclude shockcrossings with PVO the magnetic field data from orbits that the central body has a net magneticmoment. The diame- 165 to 180 (see Figure 1) were examined as available. Pioneer ter of the Cythereantail as determinedby V9 and V 10 magne- Venus bow shockcrossings are shown as plus signsin Figure tometer observationsvaries from -• 1.3 R•, at X' -- -1.3 R•, to 14 with the initial and final encounterson a given pass dis- •2.3 R•, at X' -- -5.6 R•, [Dolginovet al., 1978] and thus tends played and connectedwith a solid line segmentin the event of to lie just within the optical shadowof the planet at lower alti- multiple encounters. Shock locations from the M5 flyby tudes and outside the shadow for distancesof more than 6 [Bridgeet al., 1967;Russell, 1977], the Venera 4 and 6 probes downstream. In addition, Venera observations[Dolginov et [Gringauz et al., 1970], and the Venera 9 and 10 satellites al., 1978, and referencestherein] have also indicated that in- [$mirnov et al., this issue] are also shown for comparison side the tail lobesas at the earth there is little plasma, fi << 1, as indicated in the figure. In the region -6 R•, < X' < -1.5 and the magnetic flux density is 15-20 nT, as would be re- Re the Venus shock crossingsfrom all sourcesstraddle the 7634 SLAVIN ET AL.: VENUS BOW SHOCK

1(•12 1.1øf Goddard OIMS Orbit 17 f.oet + + + a f.o•'t +• +'••+• ld 13 nmAV+O.e 103 - Goddard OETP , , , , , , , . ',,, '_ ,' - ,', ,, - "_,," , Z 102 :;,:,':'_,,'L , c,_,_,/:,..,_,_ , _L'- - - ,-- -_ - _ _- ._,._ - • f.f0• (fO-edynes/cm•) _ __ ,.0,V 101

• • •; 10-4

10'6 -100 Hz ._ - UC•LAOtMAGI I , I I I• .• •. 40 f.00 • • 2O 0 B•/8• UT 1602 1604 1606 1608 1610 (f•edynes/cm2) AIt(km) 6374 5836 5296 4757 4221 SZA(deg) 77 77 76 76 76 FJS.]3. ]onopau• dJst• • a functionof externalpressure determined by two diCerent setsof me•urements. Fig. 15. Pioneer Venus ion mass spectrometer,electron temperature probe, electric field detector, and magnetometer measurements acrossthe bow shockon orbit 17 (note that the lack of OIMS data be- scaled terrestrial shock as is expected ow•s to the pre- fore 1604 and OETP data after 1607 is due to plotter limitations and viously d•cussedsimila•y • shape•tweeo the not the instruments.) nod te•estfia] oear-p]aoet shock su•a•s nod the sc•s used • coastsetas Fisure 14. However, at Sreater dis- the oppositeeffect is seenin Figure 14 with the Venus shock taoces dowostream the Veous shock as sampled by both of •1-1.5 R• closer to the X' axis for -11 R• < X' < -7 R• PVO nod Soviet orbiters lies closer to the X' axis tbao than the scaledterrestrial shock surface. The probable causeis does the shock at the each. From equatioo (]) nod the small volume occupiedby the Cythereanmagnetotail rela- hie 2 the asymptotic Macb cooe noS]es at I AU tive to that of the earth from which ionosheath flow is be- 0.72 AU are expected to • •10 ø nod •12 ø, respectively, lieved to be largely excluded. At the distancesdownstream oo the averaSe. Thus with the two shock waves shown the shockwave has a slope twice as great as the even- close tosetber for • • -4 •r it would be expemedthat tual Mach cone angle so that it is still being influencedby the the shock surface at 0.72 AU would move outward from the flow field adjacent to the obstaclewhich at Venus and the axis of sy•et• more rapidly with decreases • th• the earth is of different shape downstream of the terminator shockat ] AU, ow•s to its JarsetMach cooe aoS]e. However, plane. In addition, if there is enough charge exchangeand

10- Distant Bow Shock of Venus and Earth (10.8RE'e.i.O Rv)

TerrestrialBow Shock (Fairfield,1971)'• • + ^+•' + V«o

PioneerVenus (1979) Mariner 5 (1967) Venera4 (1967) Venera6 (1969) Terr•pause Venera 9, I0 (1975-6) Venus Shadow

• I t I I 0 -;2 -4 -6 -$ -I0 -I;2 -14 -16 X(Rv) Fig. 14. Distant bow shockof Venus as observedby Mariner 5, Venera 4, Venera 6, Venera 9, Venera 10, and Pioneer Venus comparedto that of the earth by means of the scaling 10.8 R• = 1 R•. SLAVIN ET AL..' VENUS BOW SHOCK 7635 photo-ionpickup near the planet,the ionosheathflow could I I I I I I Orbii 1 be altered at Venus relative to the earth as would also be the - 30K• caseif a differentvalue of ¾were more appropriateas has already been discussed.Studies to be conductedwill map the entireVenus ionosheath region for X' > -12 R•, usingPioneer 5.4 KHz Venus measurements and examine the far field flow about Venus and its wake in a more definitive manner than has been possiblein this very preliminarysurvey of shockposition. As such,it will be equivalentby the scalingdescribed to studying 730 Hz the terrestrialmagnetosheath up to 130Re behindthe planet, O 10'6 whichhas only rarelybeen possible, and perhapsyield knowl- .t

_ - edgethat will be of usein the comingdecade when the first 100 Hz cometarymissions are conducted. 10-4

5. SHOCK STRUCTURE I I I I I I I In thepreceding sections the Venus bow shock has been ex- I I I I I I I

amined in the context of MHD and gas dynamic continuum 20 modelsfor the purposeof studyingthe flow of solar wind plasmaabout the planet.Owing to the dissipationlessnature of thesefluid approachesthe shockis treatedas a discontinuity surfaceof vanishingthickness across which the flow pa- rametersjump in accordancewith the MHD Rankine-Hugo- niot relations[de Hoffman and Teller, 1950]. Observations made at the terrestrial bow shock have shown the shock transitionlayer to accomplishthese changes in theplasma param- etersthrough a hostof dissipativemechanisms including electrical resistivity, various plasma instabilities, and wave •; -20 particleinteractions, upstream solar wind conditionsdeter- 4O

o mining the dominantprocess at a given time and location i,- 20 [e.g.,Greenstadt and Fredricks,1979]. In addition,there exist o I I I I I I I ahead of the bow shock regions of upstream waves and 1622 1624 1626 1628 backstreamingparticles collectively termed the 'foreshock' AIt(Km) 15729 16165 16596 17023 17446 which are alsoof greatinterest [e.g., Greenstadt et al., 1979]. SZA(deg) 108 108 109 109 109 Table 1 scales some relevant interplanetary parameters: Fig. 17. PioneerVenus electric and magneticfield observations acrossthe bow shock on orbit I outbound. 1612 Goddard OIMS ' Orbit 18 densityn, radialcomponent of IMF, Br, azimuthalcomponent of IMF, B, the total magnitudeof the IMF, B, solarwind velocity, V,•, plasmaion temperatureTi, and plasmaelectron temperatureTe from the orbitof the earthto that of Venusus- ingtypical 1-AU valuesand assuming a 'Parker spiral' configuration for the IMF with r-2, rø, r -z, and r -•'5 dependencesfor n, V•w,Ti, and Te, respectively(see, for example,Greenstadt Goddard OETP and Fredricks[1979] and referencestherein). The mostuncer- - ,,,, ,.,.,,,, tain scalingin the tableis that for electrontemperature which may notincrease as rapidly as has been assumed with decreas- • 102 •, b• ,,' ' ing radialdistance from the sun[Ogilvie and Scudder,1978], but as the use of thesenumbers here is only qualitative, sucha problemis not critical. •.• •o'/ • • • • • / Table 2 containsderived quantities that have beenuseful in • •N orderingthe wide variety of shockstructure observed at the earth [e.g., Dobrowolnyand Formisano,1973]. The slightly •c• 162 more radial IMF at 0.72 AU will result in some shift of the re- gionsof the shocksurface for whichthe anglebetween the up-

40 stream IMF and the shock normal 0s•vis greater or lessthan 50ø. In the latter case a thick transition region up to several • • 20 RE in width resultsat the earth, and the shockis termed quasi- • -

• 0 parallelin structure[Greenstadt et al., 1970].For 0s•v->50 ø the UT 1609 1611 1613 1615 Air{kin} 6766 6230 5691 5152 jump in the flow variablesis generaflywell defined,a transi- SZA{deg} 76 76 76 76 tion layer scalelength of the order of the speedof light di- Fig. 16. PioneerVenus ion massspectrometer, electron temper- vided by the ion plasma frequency•.opi leading to the ex- atureprobe, electfie field detector, andmagnetometer measurements pectation ofincreasing shock thickness with increasing radial acrossthe bow shock on orbit 18. distancefrom the sun, an expectationwhich has been con- 7636 SLAVINET AL.: VENUSBOW SHOCK

nary survey.Table 4 liststhree basicparameters describing the interplanetaryconditions for eachof the shockcrossings. The normalsto the shocksurfaces, n, neededin computingall saveone of thosequantities, were obtaineddirectly from the 5.4 KHz bestfit conicsurface found earlieras shownby Holzer et al. [1972] a = tan-' [e sin (SVS)/(l + • cos(SVS))] _ _ '0 • 10_4 730 Hz tan-' (Z'/Y') _ _ nx,= cos (SVS - a) (7) 0 113-6 n,, = cos(8) sin (SVS - a) 10-4 nz,= sin (8) sin (SVS - a) Coplanaritydetermined normals obtained where the magnetic I I I I field was steady gave generally similar resultsas expected from past experience at the earth [Russelland Greenstadt, 20 x 1979].As displayedin the table, SeNis the angle betweenthe o r i.l,ln.,-r--r,-,---- upstream IMF and the normal to the shock surface at the locationof the crossing,fii is the ratio of solarwind ion pressure E o E nkTi to the magneticpressure B2/8•r, and Mms*is the magne-

• ø20 tosonicMach number, the asteriskindicating that it is based upon the solar wind velocity componentperpendicular to the ._• ii 20 .- _ shocksurface, as opposedto the free stream value, calculated .• An ' with an assumedelectron temperature of 2.5 x 105øKfrom • o e- Table 1. Thus the shocksshown here span a range in SeNof • -20_ -- 35ø to 89ø, 0.3 to 1.2 for fl•, and 1.9 to _>4.4in Mms*. 40 _ --

o •- 20 -

0 LIT 1527 1529 1531 1533 1535 10'7 AIt(Km) 12783 13257 13725 14189 14648 $ZA(deg) 106 106 106 107 107

Fig. 18. Pioneer Venus electric and magnetic field observations 5.4KH• across the bow shock on orbit 3 outbound. ',• •o_ ? O. _ firmed at Jupiter[Scarfet at., 1979].At any giventime, different portionsof the shockwave are either quasi-perpendicular or quasi-parallel,the large physicaldimensions of the terres- '0• 730• trial shock and the rarity of simultaneousmultiple-satellite 100 Hz boundary crossingsgenerally precludingconcurrent observations of both regions.As was noted by Greenstadt[1970], shock measurementsat the spatially smaller bow shocksof Mercury, Venus, and Mars by relatively fast moving space- I I I I I I I craft at shock altitudes, such as PVO, will be able to examine the global condition of the shock at two widely separated points over intervalsof time sufficientlyshort that the upstream variables will have remained constant in some cases. The other quantitiesin Table 2 suchas fii and sonic,Alfv6nic, and magnetosonicMach numbers all show some changes from the earth to Venus, but accordingto presenttheory should not result in greatly different shockstructure than has already been observedat 1 AU [Greenstadtand Fredricks, 1979]unless caused by the nature of the solarwind interaction with Venus. In particular, the ionosheathof Venus has a non- :• -20 negligibleneutral particle population which may participate 40- in chargeexchange and photo-ion pickup as mentionedear- lief [see Gombosiet at., this issue],thereby modifying the sheathflow field and henceperhaps the shock[Wallis, 1973]. 0 1419 1421 1423 1425 Figures 15-20 displayhigh-resolution Pioneer Venus obser- A•(Km) 88•3 8298 7773 7244 6712 vationsacross six bow shockcrossings from the first 20 orbits SZA(deg) 87 87 86 85 84 chosenboth on the basisof data availabilityand to showsome Fig. 19. Pioneer Venus electric and magnetic field observations of the variationsin structureencountered in this very prelimi- across the bow shock on orbit 6 inbound. SLAVIN ET AL.: VENUS BOW SHOCK 7637

For the two shocksshown in Figures 15 and 16 it was pos- TABLE 4. UpstreamParameters for the ShockCrossings in sible to compare the shock signaturesof four PVO experi- Figures 15-20 ments: the UCLA (University of California at Los Angeles) Shock Os•r fii MMs* magnetometer[see Russell et al., this issue]the TRW electric I out 51 o 1.0 2.2 field detector[see Scarf et al., this issue]the GSFC (Goddard 3 out 65ø 1.2 2.2 SpaceFlight Center) electrontemperature probe [seeBrace et 6 in 82 ø 0.3 2.7 al., this issue],and the GSFC ion massspectrometer [see H. A. 6 out 53 ø 0.4 1.9 Taylor et al., this issue].Such a comparisonof shock data is 17 in 350-65 ø 1.0 3.9 highly desirable not only for the purpose of studying struc- 18 in 89 ø 0.5 __.4.4 ture, but also in determining the position of this boundary as evidencedby the discrepanciesamong the Mars 2 and 3 satel- detector) is also accompaniedby a reduction in the 30-kHz lite particles and fields instrumentsas to the location of the channelwhich detectselectron plasma oscillations (see Table Martian bow shock[e.g., Vaisberg,1976]. While only the total 2) ahead of the shock associatedwith suprathermalelectrons magnitudeof the magneticfield is shown for orbit 17 inbound reflected back from the shock. The other two OEFD channels in Figure 15, the IMF orientation was highly variable, 8s•v at 730 Hz and 5.4 kHz in the ion acousticfrequency ranges ranging from---35 ø to ---65ø . Consistentwith the variable and showedsmaller changesat the shockfor this particular shock at least occasionallyquasi-parallel 8s•r, the magnetic field is crossing.Owing to the limited number of channelsand their highly irregular after first encounteringthe shockat -1602:48 finite bandwidthsthe frequencyat which the electric field de- probablyowing to both temporaland spatialchanges such as tector producesthe strongestshock signature is very much a shock motion. Coincident with the first increase in B, the 100- function of the plasmaparameters at the time of a given shock Hz channel of the electric field detector shows an enhanced crossing which determine the natural frequencies of the value for the spectraldensity which being betweenthe ion and plasma waves associatedwith the foreshock and transition electrongyrofrequencies corresponds to whistler mode waves layer. The Goddard electron temperature probe experiment asare typically observed in planetarybow shock waves [Scarf usesaxially (vertical dash marks in the figure)and radially et aL, this issue].In addition, while not displayedthe increase (horizontal dash marks in the figure) mounted langrauir in B and the 100-Hz channel of OEFD (orbiter electric field probes to measure electron temperature and density as discussedin Brace et al. [this issue].While the absolute calibration usedhere to convertprobe current to density.doesnot take into account the higher temperature of the ionosheath plasma relative to that of the ionosphere and thus over- estimatesNe by a factor of 10 or more, OETP detectsan electron density increase very near the first increasesin B and wave activity. This is in good agreement with the terrestrial observations,which indicate electronheating beginningat the foot of the gradient in the magnetic field whereasion therma- lization continuesat a slowerrate through the entire transition •'•.10.4 layer and into the sheath [e.g., Greenstadtand Fredricks, 1979]. Further, the electron density envelope appearsto fol- low that of the magneticfield into the ionosheathas expected where the field is frozen into the plasma. Finally, the up- LLI permost panel shows the Goddard ion mass spectrometer shocksignature, which is in the form of ion currentswhich re- suit from the influx of particles with sufficient energy that thermal ion analysistechniques [H. A. Taylor et al., this issue] are not applicable,and at this point, no unique determination of particlemass or energyhas been made. Examinationof the detailed behavior of the currents for several orbits indicates that the sensorresponse is rather impulsive,which would be consistentwith the entry of particleswith energiesof the order of 100 eV or higher. The plot symbolsare coded for the mea- surementsequence and are not indicative of any variation in mass. As is displayed, the main peak in OIMS (orbiter ion massspectrometer) current comes ---4 rain (i.e., • 103km in altitude) later than the first clearshock signature in the other in- -20 struments.However, while not shown,owing to plotting limi- 40 tations, the earliest significant ion currents which are much lower in magnitudethan the ---1607level did occur near 1603. 20 A more comprehensivecomparison of shock cotssingtimes 0 with all four experimentsfor the early orbitsshows agreement UT 1544 1546 1548 1550 1552 A•(Km) 1O977 11470 11958 12441 12919 to betterthan a minutein generalfor OMAG (orbitermagne- SZA(deg) 104 104 105 105 105 tometer) OEFD, and OETP (orbiter electron temperature Fig.20. PioneerVenus electric and magnetic field observations probe), those of OIMS tending to lie closer to the planet by up acrossthe bowshock on orbit6 outbound. to severalminutes, suggesting that the ion massspectrometer 7638 SLAVIN ET AL.: VENUS BOW SHOCK

may have to penetrate into the denser and hotter ionosheath and 3, respectively.For orbit 6 outbound with the lower ratio plasmasbelow the shockbefore registeringa peak in current. of ion pressureto magneticpressure the ionosheathfluctua- However, this difficulty might be overcome by refining the tion amplitudeis small in comparisonwith either of the other definition of the OIMS shock signaturethrough further com- two crossingswith larger/•i. Again, this responseis similar to parison with other instruments. that found at the terrestrialshock by more comprehensive The orbit 18 inbound shock crossingin Figure 16 differs studies[Formisano and Hedgecock,1972]. from orbit 17, consideredabove, principally in its cleaner With only one spacecraftat Venusit is not in generalpos- ramp in B up to ionosheathvalues and the more regular and sible to separatespatial and temporal variationsin the data smaller amplitude fluctuations in the sheath. These differ- and measurethe thicknessof the shockdirectly, as is done,for encesare mainly due to the nearly perpendicular0•N for orbit example,by the ISEE missionwith its twin satellites[Russell 18 and the lower/•i which createa smoothertransition layer and Greenstadt,1979]. Observationsmade at the earth have and more regular ionosheathfield as has been found to be the shownthe shocksurface to be constantlyin motion[e.g., Hol- caseat the earth [e.g., Dobrowolnyand Formisano,1973]. This zer et al., 1966] with velocities of the order of 10ø-10: km/s shock encounter is also well defined in the OEFD and OETP relativeto the planet[Greenstadt et al., 1972].As the velocity measurementsat nearly the same time as observedwith the of PVO at shockaltitudes, 3-4 km/s, is not large in com- magnetometer,but the main rise in OIMS ion currents is parisonwith the expectedspeed of shockmotions, it is neces- about a minute later than for the other experiments,as was sary to rely on the terrestrial estimatesof mean radial shock the casefor the previousorbit. The periodic variation in'Ne is speedwhich are of the order of 10 km/s. Examination of the a spin modulation of the particle flux enteringthe detectoras- highest-resolutionmagnetic field data shows that the mean sociatedwith the langmuir probespassing through the space- timespent by PVO in the shocktransition layer for the quasi- craft wake. In addition, the strengthof this high Mach shock perpendicularshocks examined here is of the order of-•5 s, may be examined by comparing the change in the magnetic which for a mean relativespeed of 10 km/s impliesa shock field componenttangential to the boundary surfaceand the thicknessof about -•50 km, which is similarto the accepted electron density in passingthrough the shocktransition. The scalingof c/%i shownin Table 2 and in agreementwith the jump in the transverseB is a factor of •4, which is near the similarconclusion reached by Keriginet al. [1978]using Ven- gas dynamic upper limit of (¾ + 1)/(¾ - 1) for ¾ = 5/3, era 9 and 10measurements. By comparison,at JupiterScarf et whereasthe ratio of downstreamto upstreamelectron density al. [1979] usingVoyager 1 observationshave estimatedshock is larger near a factor of •7, which is also bigger than the thicknessto also be of the order of the ion inertial length, jump in plasma ion number density found by Mihalov et al. which for thoseobservations was nearly 400 kin. [this issue]. However, the greatnessof the change in Ne as Hence in this very preliminary survey of Pioneer Venus measured by OETP may be due to the constant calibration shock structure,no overt deviationsfrom expectationsbased factor used in Figure 16 which should vary with the increase on terrestrial experiencehave been uncovered.In other stud- in plasma temperature acrossthe shock.Thus the PVO obser- ies, Scarf et al. [this issue]have comparedthe OEFD observa- vationsin Figures 15 and 16 of two very differentshock cross- tions with thosetaken in the vicinity of the earth's bow shock ings have shown general agreementas to the location of the and found evidenceof higher relative spectralamplitudes at boundary.In addition, further work quantifyingthe high-alti- 30 kHz and 100 Hz, more diffusecrossings in general with tude measurementsof OETP and OIMS may enable them to electron oscillationsbeing generatedwithin the shock transi- support the plasma analyzer ion observationswith thermal tion layer, and extensive upstream wave activity out to electron and perhapsalso someadditional ion measurements. apoapsis possibly associated with solar wind interactions Figures 17-20 display magnetometerand electric field ob- with exospheric neutrals as discussed earlier. In servationsacross four additionalshocks for the purposeof addition,Russell et al. [1979d]have found that the average briefly consideringthe effects associatedwith varying 0•N, magnetic field jump across the Venus shock is smaller Mms*,and/•i. As is shownin Table 4, two of the crossings,6 than has beenobserved at the earth.The studyof the varia- inbound and 18, have 0s• values in excessof 80ø. In both tion in plasma ion density through the Venus shock wave casesthere is an absenceof upstreamactivity and the presence by Mihalov et al. [this issue] produces similar results. If of a well-defined magneticramp up to the ionosheathB level. confirmed,these findings indicate the need to carefully By comparison,orbits 1, 3, and 6 outbound with 51o <_0• <_ evaluate conservationof mass, momentum, and energy 65o do showevidence of upstreamactivity in both the OMAG with Pioneer Venus observationsalong streamlinesbegin- andOEFD data,indicative of a well-developedforeshock as ningwell upstream of Venusat apoapsison thedayside and mentionedearlier. Finally, for orbit 17, where t?• wassmaller extendingdownstream into the wake region for the purpose of and more variable, a much thicker and irregular transition re- identifyingthe effectsof neutralson the flow field in general gion from solar wind to ionosheathconditions was found. Or- and on the bow shock in particular. bits 6 inbound and 18 have similar t?• and/•i but different Mach numbers, with that of the latter at least 60% greater 6. SUMMARY than the former. At the earth, increasingMach number has In this paper an attempt has been made using Pioneer been found to result in a more turbulent shock with larger- Venus observationsto conduct a comprehensive,albeit in amplitude magnetic fluctuationsin the magnetosheath[e.g., many ways cursory, examination of the Venus bow shock Formisanoand Hedgecock, 1972]. Similar effectsare seenhere problem in its entirety drawing heavily on past work at both with the double amplitude of the ionosheathfluctuations in Venus and earth. The shapeand locationof the forward por- orbit 18 about 25% the mean field magnitudecompared with tion of the shockis determinedin section1. Its mean altitude onlyabout 5% on oribt6 inbound.Finally, orbits 1, 3, and 6 in the aberratedterminator plane is foundto be highlysym- outboundexhibit comparable values of t?•Nand Mms*,but metricabout the X' axisand independentof upstreamIMF with/•,equal to 0.3for 6 outboundand 1.0and 1.2for orbits1 orientation.The extrapolatedsubsolar shock height is shown SLAVIN ET AL.: VENUS BOW SHOCK 7639 through gas dynamic relations and scaling of the terrestrial REFERENCES analogue to imply an effective obstacle altitude below the Alfv6n, H., On the theory of comet tails, Tellus, 9, 92, 1957. height of the daysideionopause. Short-term variability in po- Bauer, S. J., Solar wind control of the extent of planetary ionospheres, sition about the mean shocksurface is similar in magnitudeto NASA Spec. 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