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The Average Magnetic Field Draping and Consistent Plasma Properties of the Venus Magnetotail

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The Average Magnetic Field Draping and Consistent Plasma Properties of the Venus Magnetotail University of New Hampshire University of New Hampshire Scholars' Repository Physics Scholarship Physics 7-1986 The average magnetic field draping and consistent plasma properties of the Venus magnetotail David J. McComas Los Alamos National Laboratory Harlan E. Spence University of New Hampshire, [email protected] C. T. Russell University of California - Los Angeles M. A. Saunders Imperial College of Science and Technology Follow this and additional works at: https://scholars.unh.edu/physics_facpub Part of the Physics Commons Recommended Citation McComas, D. J., H. E. Spence, C. T. Russell, and M. A. Saunders (1986), The average magnetic field draping and consistent plasma properties of the Venus magnetotail, J. Geophys. Res., 91(A7), 7939–7953, doi:10.1029/JA091iA07p07939. This Article is brought to you for free and open access by the Physics at University of New Hampshire Scholars' Repository. It has been accepted for inclusion in Physics Scholarship by an authorized administrator of University of New Hampshire Scholars' Repository. For more information, please contact [email protected]. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 91, NO. A7, PAGES 7939-7953,JULY 1, 1986 The Average Magnetic Field Draping and ConsistentPlasma Properties of the Venus Magnetotail D. J. MCCOMAS Los AlamosNational Laboratory, Los Alamos,New Mexico H. E. SPENCE AND C. T. RUSSELL Instituteof Geophysicsand PlanetaryPhysics, University of California,Los Angeles M. A. SAUNDERS The BtackettLaboratory, Imperial Collegeof Scienceand Technology,London A new techniquehas beendeveloped to determinethe averagestructure of the Venusmagnetotail (in the range from -8 R V to -!2 R V) from the PioneerVenus magnetometerobservations. The spacecraft positionwith respectto the cross-tailcurrent sheetis determinedfrom an observedrelationship between the field-draping angle and the magnitude of the field referencedto its value in the nearby mag- netosheath.This allowsus statisticallyto removethe effectsof tail flappingand variabilityof drapingfor the first time and thus to map the averagefield configurationin the Venus tail. From this average configurationwe calculatethe cross-tailcurrent density distribution and J x B forces.Continuity of the tangentialelectric field is utilized to determinethe averagevariations of the X-directed velocitywhich is shownto vary from -250 km/s at -8 Rv to -470 km/s at -12 Rv. From the calculatedJ x B forces, plasmavelocity, and MHD momentumequation the approximateplasma acceleration, density, and temperaturein the Venustail are determined.The derivedion densityis approximately•0.07 p+/cm3 (0.005O+/cm 3) in the lobesand •0.9 p+/cm3 (0.06 O+/cm3) in the currentsheet, while the derived approximateaverage plasma temperature for the tail is •6 x '10• K for a hydrogenplasma or •9 x 10? K for an oxygenplasma. 1. INTRODUCTION and stretchedout generally sunward and antisunward to the Extensive in situ observations of the magnetic field and sidesof the Venus magnetotail. plasmapopulations in the Venus environshave shownthat (1) In addition to deflectionof the flow by the conductiveiono- Venusdoes not have a significantintrinsic magnetic field [Rus- sphere, viscousslowing of the flow by the ionosphere and sell et al., 1980a] and (2) a magnetotail is a regular feature of massloading of the magnetosheathplasma with material from the region antisunwardof the planet [Russell,1976; Dolginov the extendedVenus atmosphereand ionospherealso contrib- et al., 1978; Russellet al., 1981, 1985; lntriligator and Scarf, ute to Venus magnetotailformation. The mass-loadedplasma 1984; Slavin et al., 1984; Saunders and Russell, 1986]. The flow slows in order to conserve momentum, which signifi- Venusmagnetotail is generallybelieved to form by magnetic cantly enhancesthe draping of field lines around the conduc- field draping about the Venus ionospherein a manner first ting ionosphereand substantiallyhelps produce the Venus suggestedto explaincomet tails by Alfv•n [1957]. magnetotail. Eventually, somewherevery far downstream,we A steady state configuration of this interaction is shown expect that the field lines should once again assumetheir in- schematicallyin Figure 1. The solar wind, with the imbedded terplanetary configuration owing to the Maxwell stressesin interplanetary magnetic field (IMF), flows radially outward the draped magneticfield configuration.These stresses should from,the rotating sun, causingthe well-known Parker spiral causethe kinked portions of the field lines to accelerateback pattern of the IMF. The solar wind carries the IMF flux up to, and beyond, the solar wind speed,while mass-loaded through Venus'sbow shock and massloading, extended,neu- material tends to diffuse along the field lines owing to its tral exosphere,and then past the generallyunmagnetized but parallel plasmatemperature. conductingionosphere. Little, if any, of the upstreamplasma There is now much direct and indirect evidence for mass flow enters Venus's ionosphere.Rather, the flow is slowed, loading of the Venus magnetosheathby the pickup of newly compressed,and deflectedabove the daysideionopause and created ions and other processes,such as charge exchange. eventuallyslips aroun.dthe planet. The plasma in the mag- Oxygen ions have been seenin the near-terminatorregion of netosheathto the sidesof the Venus obstacleand magnetotail the Venus magnetosheath[Mihalov and Barnes, 1981] and regionstravels at the largervelocity of the magnetosheathoccasionally in the distant magnetosheathand magnetotail flow, which is a function of position behind the shock. Since region [Mihalov and Barnes, 1982]. An enhancementof the the magnetic field lines link the magnetosheath and near- magnetosheathmagnetic field strengthhas been found in the Venus regions,the lines becomebent. This has the effect of hemispherewhere most ion masspickup is expected,presum- draping the field lines so that they are "hung up" on Venus ably due to a decreaseof the magnetosheathflow speedand the consequentpileup of the plasma density and magnetic Copyright1986 by the AmericanGeophysical Union. field [Luhmannet al., 1985]. Other indirect evidenceis that the Paper number 5A8292. position of the bow shock at the Venus terminator has been 0148-0227/86/005A-8292505.00 found to dependon solaractivity. When the solar EUV flux is 7939 7940 MCCOMAS ET AL.' VENUS MAGNETOTAIL X SOLAR WIND the variability of the data ordered by spatial location and lays FLOW the groundwork for developing a coordinate system which measureslocations with respect to the tail structures them- selves. Section 3 shows how we reconstruct the structure of the Y• tail in the presenceof flappingand examinesthe averagevari- ations in the field components,culminating in the average field vectors,cross-tail current density distribution, and J x B forces as functions of location across the tail. Section 4 derives AWAY the average downtail velocity as a function of distanceand SECTOR defines a simple model based on the field variations from which the average plasma acceleration as a function of dis- ,ow ;HOCK tance, density, and temperature are obtained. Finally, the ; "Summary"reviews the most salientsteps and resultsof our ß analysis. 2. THE AVERAGE VENUS TAIL IN MAGNETIC / COORDINATES / The data set usedin this studyis a large subsetof the data // set chosenby Saundersand Russell[1986]. It consistsof mag- / ,•MAGNETOPAUSEnetic field data from the Pioneer Venus Orbiter (PVO) magne- ,/ / tometer [Russell et al., 1980b] and contains the magnetotail portions of 38 orbits which were selectedfrom the first 10 /' / / / / • seasonsof data (June 1979 to May 1984). In all, 9423 1-min averagedmagnetic field measurementsare used.The criteria Fig. 1. Schematicdiagram of the postulated solar wind interac- tion with Venus in the plane containingthe upstreamIMF and solar used to identify magnetotailportions of the data are vari- wind flow vector. The upstreamsolar wind carriesthe IMF through ations in field strength and orientation, and changesin the the bow shock and magnetosheath.At the obstaclethe magneticfield spectrum of field variations' they are describedin greater must flow perpendicular to this plane and around the conductive detail, with examples,by Saundersand Russell[1986]. While ionosphere.This diversion of the flow, enhancedby mass loading of the flow near to the obstacle,causes the field linesto drapeand form the identificationof theseregions is not perfect,it is correctfor a magnetotail. Note also that the spiral orientation of the upstream thevast majority of thedata, and the mixing in of Small magnetic field causesthe magnetic flux in the left lobe to be greater portions of magnetosheathdata will not substantiallyaffect than in the right at every distancedowntail. This has important dy- the statistical results described here. namical implicationsfor the actual tail configuration(see text). The coordinatesystem used in this sectionof our studywill be called the B-r coordinatesystem. The term B-p is used to high,it appearsthat more massis beingadded to the shocked indicate that the orientation of the coordinatesystem is deter- solar wind, forcing the bow shock to recedefrom the planet mined by the averageupstream magnetic field and solar wind [Alexander and Russell,1985]. flow directions.In the B-r coordinatesystem the sølar wind An interestingasymmetry between the two draped lobes is flows parallel to the -X direction,and the magneticfield observedin Figure 1. If this schematicis correct, the X com- componentperpendicular to the solarwind
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