Dec 30, 1980 - shock crossings with those observed by the Pioneer Venus orbiter indicate that both ... the Mariner 5 flyby [Bridge et al., 1967] and the Venera 4.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 85, NO. AI3, PAGES 7651-7654. DECEMBER 30, 1980
An Empirical Model of the VenusianOuter Environment2. The Shapeand Location V. N.
of the Bow Shock
SMIRNOV
AND
O. L. VAISBERG
SpaceResearchInstitute, USSR Academyof Sciences, Moscow,USSR D.
S. INTRILIGATOR
Departmentof Physics,Universityof SouthernCalifornia,LosAngeles,California90007 Yenera 9 and Yenera 10 bow shockcrossingsare analyzedfor solar zenith anglesin the range from --,25ø to -•153ø. The bestfit to 62 observedcrossings givesthe locationof the subsolarpoint of the bow shockas •7600 km from the centerof Venus or 1.27 Rv. Comparisonof Venera 9 and Venera 10 bow shockcrossingswith thoseobservedby the Pioneer Venus orbiter indicate that both the secularvariation and the latitudinal asymmetrymay be responsiblefor the closershockcrossingsof the Venera 9 and Venera 10 spacecraft.
The data presentedcover the time interval from October 1975 to April 1976.Figure I showsall 62 positivelyidentified shockcrossings in the Venera 9 and Venera 10 data. Crossings are shown in a cylindrical coordinatesystemwith the X axis along the Venus-sunline and with distancefrom this line
INTRODUCTION
Since the first observations of the Venusian
bow shock with
the Mariner 5 flyby [Bridge et al., 1967] and the Venera 4 probe [Dolginovet al., 1969],the locationof the bow shockhas been usedto estimatethe propertiesof an obstaclein the solar wind flow [Spreiteret al., 1970;Gringauzet al., 1976; Vaisberg (x/Y• + Za) asthe secondcoordinate.The aberrationangleof the solar wind was taken into account with the use of correet al., 1976; Russellet al., 1977]. spondingsolarwind velocitymeasurements. The estimates of the mean subsolar distance of the bow The crossingclosestto the subsolarpoint was at SZA ~25 ø. shockvary from 1.23 Rv [Russellet al., 1979]to 1.5 R, [Verigin Shock crossingswere also identified as far as ~93,000 kilomeet al., 1978]. There are two main reasonsfor this variation. First, mostof the shockcrossings were observedat large solar ters (---15.3R0 downstream. An approximationof the mean shockwasmade by employzenith angles (SZA), which makes it difficult to extrapolate ing a second-ordercurve symmetric relative to the X axis, meaningfullyto the subsolarpoint. Second,differentfit procewithout employing the often used suppositionthat the focus dureswere applied to obtain the mean shockcurve from the of the curve coincides with the center of the planet. Bow various observedcrossings. shock crossings were modeled by the curve It is important to determine the location of the shock for comparisonwith modelsof the interactionand for estimation of the possibleabsorptionof the solarwind by Venus [Russell, 1977].The purposeof this paper is to evaluate the mean location of the Venusian shock for the period of Venera 9 and Venera 10 observationsby using the shockcrossingsat large SZA and to comparethis location of the shockwith the shock location obtained by usingthe more recent Pioneer Venus orbiter (PVO) observations.
33 + Bx2 + Dx + E--0 The coefficients of the best fit curve were used then to cal-
culate the parameters of the bow shock: the planetocentric subsolar distance Ro, the terminator distance R r• the focus location on the X axis, the eccentricitye, and the formal Math number correspondingto the asymptoticangle of the curve. A least squaresbest fit program was used to minimize the mean squareddistancesof individual points from the secondSHOCK LOCATIONS order curve. Figure 1 showsthe result of this best fit to all the Data from the RIEP plasma spectrometer[ Vaisberget al., data points.The bestfit curve parametersare shownin Table 1976]and the Venera magneticfield measurements[Dolginov 1. The subsolarpoint of this best fit shock is located at 1.295 et al., 1978]were usedto identify shockcrossings. Data were R,. The formal Mach number calculatedfrom the asymptotic obtained in two measurementmodes.In the periapsismode angle of the shockis ~3.8. To decreasethe influenceof remote flank crossingson the the counting rate of the RIEP plasma spectrometersand the vector measurementsof the magnetic field were recorded determination of the shape and location of the shock on the every secondin the spacecraftcore memory. In this mode the dayside, only 48 crossingsfor the SZA range from ~25 ø to accuracyof the shocklocationcan dependon the shockthick- •-165 ø were used. These crossingsare shown in Figure 2a ness. In the orbital mode the measurements of the RIEP ion along with two best fit curves.Curve 1 was obtained with the distributionfunction and of the eight vector magneticmea- use of all 48 crossingsand curve 2 was obtained when the surementswere made during a 1-min interval followedby a 9- three most remote crossingsfrom the mean curve were omitmin interruption of the measurements. Therefore, in this ted. The difference in the parameters associatedwith these mode the shock location identification
has a ñ5-min
uncer-
tainty. Copyright¸ 1980 by the American GeophysicalUnion. Paper number 80A1142. 0148-0227/80/080A-
1142501.00
two curves allows one to estimate, as discussedbelow, the ac-
curacy of the mean curves(seeTable 1). Ten runs of the best fit program were made for varying initial conditionsand different weightingsof the individual shockcrossings depending 7651
7652
SMIRNOV ET AL.:VENUSBOWSHOCK
o o
+
++
I
I
i
I
SMIRNOV ET AL.: VENUS BOW SHOCK
TABLE
1.
7653
Venusinn Bow Shock Parameters RMS
Distance Subsolar at
Deviation Number Normal to of
Focus Mach Distance Terminator Location Eccentri- Number, the Curve, Crossings Case
Ro,km
Rr, km
X•, km
city,E
M
km
Used,N
V9/10 all points
7,840
12,330
3,220
1.037
3.78
1.82
62
7,670
12,720
2,660
1.040
3.62
0.82
48
V9/10 (curve2, Figure
7,340
12,940
1,870
1.044
3.49
0.80
45
•) Mean 10 runsV9/10
7,540
12,660
2,500
3.52
...
+245
+190
+460
8,155
14,770
0
8,230
14,820
1,730
1.022
4.82
1.1
7,590
14,760
530
1.015
5.85
1.05
(Figure 1)
V9/10 (curve1,Figure •)
PVO (curve3, Figure 2). [Slavineta!., 1979b] PVO (curve5, Figure 2) PVO + V9/10 (curve4, Figure 2)
1.044 +0.006
0.80
45-62
+0.20 86
......
86
107
25
t +4
20F
2O
5
0 '
0
-
' '
25
20
•: 15
•: 15
I0
5
0
I0
5
0 Xs•,I0• KM
-5
5
0
5
0
-5
,,
Xs•,I0• KM
Fig. 2. Near planetarybow shockcrossings. (a) Venera9 and Venera 10 data. (b) PioneerVenusOrbiter data (crosses) combinedwith Venera 9 and 10 data for lower SZA's (dots). (c) PioneerVenus orbiter data. (d) Combined Venera 9/10 and PVO data. Curves I and 2 are the best fit to Venera 9/10 data; curves3 and 5 are the best fit to the PVO data; curve 4 is the bestfit to both the PVO and the low SZA Venera 9/10 data (seetext and Table 1).
7654
SMIRNOV ET AL.: VENUS BOW SHOCK
on their respective SZA's. The means were calculated from theseten setsof best fit curve parameters.These mean values of the shockparametersare alsogivenin Table 1 and showthe accuracyof the approximation.
Recently,the data for 86 Venusianshockcrossings by the PioneerVenus orbiter (PVO) were published[Slavinet al., 1979b].These86 PVO crossings are shownin Figure2b along with the approximation of the mean shock curve (curve
made at large SZA. All the measurementsused in this study are made at large or intermediate SZA. The differencebetween the Venera and the Pioneer data could be due either to
a secularchange,presumablydue to changesin the solar activity cycle, or to a latitudinal asymmetry,due to the interplanetary magneticfield. Evidencefor a latitudinal asymmetry has been presented.A solar cycle changeis plausiblebut has not yet been confirmedby other observations.
3) from Slavinet al. [1979b],whichemploysa Venus-cenAcknowledgments.The authorsare indebtedto W. David Miller teredellipse.The parameters of thisellipsearegivenin Table 1. Slavinet al. [1979a]showedthat thereis a systematic dif- for his participationin the analyses.This work was performedat the University of Southern California. The participation of D. S. In-
ference between the PVO mean shock location and Venera 9/
triligator was supportedby NASA contractNAS2-9478.
10 mean shock location. REFERENCES
It shouldbe noted, however,that Slavin et al. [1979a] com-
paredthe PVO shockcrossings withVenera9/10 datataken Bridge,H. S., A. J. Lazarus,C. W. Snyder,E. J. Smith, L. Davis, P. J. from Veriginet al. [1978].Veriginet al., however,apparently Coleman,and D. E. James,Plasmaand magneticfield observations made no allowancefor the solarwind aberrationangle.As the
Venera 9/10 shock crossingsoccurredcloseto the ecliptic
planeat the Ys•raxisthisomission of anaberration termleads to the exaggerationof the differencein the observedbow shock location between PVO and Venera 9/10.
Slavin et al. [1979a] suggestedthat this differencemay ariseeither from solarcyclevariationsor from differencesbetween the Venera 9/10 orbits and the PVO orbits. That is, for
near Venus, Science,158, 1669, 1967.
Dolginov,Sh.Sh.,Ye. G. Yeroshenko, andL. Lewis,On thenatureof the magneticfield near Venus, CosmicRes., 7, 675, 1969. Gringauz,K. I., V. V. Bezrukilda, T. K. Breus,T. Gombosi,A. P. Remizov,M. L. Verigin, and G. T. Volkov, Plasmaobservations near Venus onboard the Venera 9 and 10 satellitesby means of
wideangleplasmadetectors, in Physics of SolarPlanetaryEnvironments,editedby D. J. Williams,p. 918,AGU, Washington, D.C., 1976.
Romanov,S. A., Asymmetryof the regionof the solarwind interactionwith Venus accordingto data of Venera 9 and Venera 10, example,owingto the asymmetryof the Venusianbowshock
relativeto the orientationof the interplanetarymagneticfield
[Romanov et al., 1978;Romanov, 1978],thelowlatitudeVen-
CosmicRes., 16, 256, 1978.
Romanov,S. A., V. N. Smirnov,and O. L. Vaisberg,On the natureof solar wind-Venus interaction, CosmicRes., 16, 603, 1978.
era 9/10 crossings may have occurredcloserto the planet, on the average,than the high latitudePVO crossings.
Russell C. T., The Venus bow shock: Detached or attached?,J. Geophys. Res.,82, 625, 1977. Assuming that the difference betweenthe PVO shockcross- RussellC. T., R. C. Elphic, and J. A. Slavin, Initial PioneerVenus
magnetic fieldresults: Daysideoperations, Science, 203,745, 1979. ings and the Venera9/10 shockcrossings are mainly con- Slavin, J. A., R. C. Elphic,and C. T. Russell,A comparison of Pio-
nected to the latitudinal asymmetry,we may estimatethe
neer Venus and Venera bow shock observations: Evidence for a so-
meanshapeof the high-latitudeshockby combining the PVO
lar cyclevariation,Geophys. Res.Lett., 6, 1089,1979a. Slavin,J. A., R. C. Elphic, C. T. Russell,J. H. Wolfe, and D. S. Intriligator,Positionand shapeof the Venus bow shock:Pioneer Venus orbiter observations,Geophys.Res. Lett., 6, 901, 1979b.
data with the Venera 9/10 data for lower SZA. The resulting
bestfit curve(curve4) is shownin Figure 2b, and the corre-
sponding parameters aregivenin Table 1.Whilethe obtained Spreiter,J. R., A. L. Summers, andA. W. Rizzi,Solarwindflowpast valuesdo not contradictthe degreeof shockasymmetryfound
by Romanovet al. [1978]andRomanov[1978],the resultdoes not excludethe possibilitythat the differencebetweenthe Venera 9/10 and the PVO shocklocationsare connectedwith secular variations. CONCLUSIONS
The bestfits to the Venera 9/10 bow shockcrossingsappear significantly different from those for the PVO bow shock crossings. Almost all the PVO best-fitparametersare outside the range of uncertainty estimatedfrom the mean of ten runs of Venera 9/10 data. The exceptionis the subsolardistance, which is difficult to determineaccuratelyfrom measurements
nonmagnetic planetsVenusand Mars, PlanetSpaceSci.,18, 1281, 1970.
Vaisberg,O. L., S. A. Romanov,V. N. Smirnov,I. P. Karpinsky,B. I. Khazanov, B. V. Polenov, A. V. Bogdanov, and N.M.
Antonov,
Ion flux parameters in the solarwind-Venusinteractionregionaccordingto the Venera 9 and Venera 10 data, in Physicsof Solar PlanetaryEnvironments, edited by D. J. Williams, p. 904, AGU, Washington,D.C., 1976.
Verigin,M. I., K. I. Gringauz,T. Gombosi,T. K. Breus,V. V. Bezrukikh, A. P. Remizov,and G. I. Volkov, Plasmanear Venus from the Venera9 and 10 wide-angleanalyzerdata, J. Geophys. Res.,83, 3721, 1978.
(ReceivedMarch 6, 1980; revisedJuly 3, 1980; acceptedJuly 3, 1980.)
