NASA Technical Reports Server (NTRS) 19930005153: The solar wind interaction with Venus

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by  NASA Technical Reports Server (NTRS)

LPI ContributionNo. 789 65
rains, moderate-sized mountains, and sharp terrain boundaries. the constant fields of intHmic origin [5]. This "magnetotail" is
These features are chosen because the goodness of fit is likely to be further seen to be populated by heavy (O+)ions that are evidently
most affecte, cleither by departures from normal incidence angles or escaping from the planet at significant (~10 -_ s-t) rates [6]. These
by sharpchanges in terrain type within asingle footprint. Most large heavy ions arc also obscz_ed in the daysiclc magnetoshcath [7].The
features that are elevated with respect to their surroundings will intmpretation is that ions arc produced by both photoionization and
suffer from steep slope effects, and smaller coronae and impact solar wind electron impact ionization of the upper neutral atmo-
craters will probably suffer due torapid changes in their appearance sphere that extends into the magnetosheatlL The flowing solar wind
withinasinglefootprint (10-20 kin). plasma with its imbedded magnetic field "picks up" the ions and
Sincethe surfacepropertiesof Venus can be derivedonly carriesthem tailward. While many escape, some of the pickedup
throughmodels,it is crucialthat surface scatteringmodels beas ionsimpactthedaysideatmosphere and sputterneutrals[8].By
accurateaspossible.The characterizatioonfterraianndthephysical thesemeans,thesolarwind interactiopnlaysaroleintheevolution
quantities that are estimated from surface properties presume an of the Venus atraosphere, although its importance relative to other
acceptable level of precision in the data, and are misleading if truly loss mechanisms is still undetermined. In any event, because the
incorrect. Once the problem areas are correctly identified, better planetary heavy ion contribution tothe plasma inthemagnetosheath
estimates of surface properties may be obtained tlLmugh models var/es with the solar cycle, itmay bethe cause of the aforementioned
tailored toparticular fitting difficulties. These surface _es, in shift in the bow shock position. For all the above reasons, research-
turn, will provide ameans to estimate physical characteristics of the ers sometimes consider that the Venus-solar wind interaction is in
planet's surface, and address the underlying geological processes. many ways comedike. These features are all aconsequence of the
References: [1] Hagfors T. (1970) Radio Sci., 5, 189-227. weak intrinsic magnetism, and as such should be relevant to Mars
[2] Ford P. G. and Senske D. A. (1990) GRL, 17, 1361-1364. [9] whine future measurements are likely to further elucidate the
[3] Ford P.G. and Pettengili G. H. (1992)JGR, inpress. [4] Squyres scavenging processes.
S. W. et al.(1992) JGR, in _ss. References: [1]Lxthmann J.G. (1986) Space Sci. Rev., 44,241.
i=: N' 93-14341 [2] Russell C. T. andValsberg O. (1983) InVenus (13.M. Hunten et
al., eds.), Univ. of Arizona, Tucson. [3| Phillips I. L. and Russell
THE SOLAR WIND INTERACTION WITH VENUS. J.G. C.T. (1987) ]GR, 92, 2253. [4] Zhang T. L. et al. (1990) ]GR, 95,
Luhmann, IGPP-UCLA, Los Angeles CA 90024-1567, USA. 14961. [5] Saurtders M. A. and Russell C.T. (1986) ]GR, 91, 5589.
[6] Moore K. R. et al. (1990)JGR, 95, 12005. [7] Mihalov J.D. and
The Pioneer Venus Orbiter (PRO) mission has played akey role Barnes A. (1981) GRL, 8,1277. [8] Luhmarm J.G. andKozyra I. U.
in establishing the nature of the solar wind interaction with Venus (1991) JGR. 96, 5457. [9] JGR, special Venus andMars issue, July,
[1]. Although earlier probes had determined that Venus presented an 1991. ,-, --_
obstacle much smaller than the size of Earth's magnetosphere tothe N93-14342 ,t
solar wind, they did not carry out in situ measurements pertaining
to solar wind interaction studies at low enough altitudes to deter- EXTENSIVE LAVA FLOW FIELDS ON VENUS: PRELIMI-
mine why. They also did not provide datasets of sufficient duration NARY INVESTIGATION OF SOURCE ELEVATION AND
to study the variability of the interaction on both short (one day) and REGIONAL SLOPE VARIATIONS. K. Magee-Robertst0 J.W.
long (solar cycle) timescales [2]. Head t,J. E. Guest 2,andM. G. Lancaster 2, tDepmament of Geologi-
The f'trst 60(1 of the nearly 5000 orbits of PVO magnetometer cal Sciences, Brown University, Providence RI 02912, USA,
data have been used to determine avery low upper limit (-10 -5of 2Univcrsity of London Observatory, University College London,
the terrestrial value) on the intrinsic dipolar magnetic moment of London NW7 2QS, UK.
Venus [3]. The consequence of that low magnetic moment isthat the
solar wind interacts directly with the upper atmosphere and iono- Large-volume lava flow fields have been identified on Venus
sphcre. Relative to adipolar field obstacle, the ionospheric obstacle [1], the most areally extensive (>50,000 km 2)of which are known
israther incompressible. A"bow" shock isobserved tostand in front as "fluetus" and have been subdivided into six morphologic types
of the nearly Venus-sized ionospheric obstacle at a comparatively [2}. Sheetlike flow fields (Type 1) lack the numerous, closely
steady subsolar altitude of-1.5 Rv (Venus radii). This shock spaced, discrete lava flow lobes that characterize digitate flow
decelerates the supersonic solar wind plasma so that it can flow fields. Transitional flow fields (Type 2) aresimilar to sheedike flow
around the obstacle. It was found to change its average position in fields but contain one or more broad flow lobes. Digitate flow fields
the terminator plane frtma about 2.4 Rvto 2.1 Rv as the solar cycle are divided further into divergent (Types 3-5) and subparallel (Type
progressed from the 1978 orbit insertion near solar maximum 6) classes on the basis of variations in the amount of downstream
through the 1986--87 solar minimum, and back again during the flow divergence. Flows that are radially symmetric about acentral
latest solaractivity increase [4]. source (e.g., volcanic shield or corona) are typical of Type 3 flow
Between the bow shock and the ionosphere proper, the slowed fields, whereas a similar but slightly asymmetric apron of flows
solar wind plasma flow diverges near the subsolar poim and makes about acentral source is characteristic of Type 4 flow fields. A fan-
its way across the terminator where it reaccelerates and continues shaped flow field that widens substantially in its distal regions is
anti-Stmward. The solar wind magnetic field, which is in effect typical of Type 5 flow fields. Type 6 flow fields (e.g., Mylitta and
frozen into the flowing plasma, isdistorted in this"magnetosheath" Kaiwan Fluctus) are not radially symmetric about acentral source
region so that it appears to hang up or drape over the dayside and do not widen or diverge substantially downstream.
ionosphere before itslips around with the flow. These features of the As aresult of our previous analysis of the detailed morphology,
solar wind interaction are also seen when the obstacle is adipole stratigraphy, and tectonic associations of Mylitta Fluctus [3], we
magnetic field, but there are two important distinctions. have formulated anumber of questions to apply to all large flow
In the wake of the Venus obstacle one trmds an "induced" fields on Venus. In particular, wc would like to address the follow-
magnetic tall composed of varying interplanetary fields rather than ing: (1) eruption conditions and style of flow emplacement (effusion