P33B-1290 

Global Hydrid Modelling of the Venus Express MAG Observations

* Jarvinen, R (riku.jarvinen@fmi.fi), Finnish Meteorological Institute, P.O. Box 503, Helsinki, 00101, Finland Kallio, E (esa.kallio@fmi.fi), Finnish Meteorological Institute, P.O. Box 503, Helsinki, 00101, Finland Barabash, S (stas.barabash@irf.se), Swedish Institute of Space Physics, P.O. Box 812, Kiruna, 98128, Sweden Zhang, T (tielong.zhang@oeaw.ac.at), Austrian Academy of Sciences, Space Research Institute, Schmiedlstr. 6, Graz, 8042, Austria Fedorov, A (andrei.fedorov@cesr.fr), Centre d'Etude Spatiale des Rayonnements, P.O. Box 4346, Toulouse, 31028, France Sillanpää, I (ilkka.sillanpaa@fmi.fi), Finnish Meteorological Institute, P.O. Box 503, Helsinki, 00101, Finland Janhunen, P (pekka.janhunen@fmi.fi), Finnish Meteorological Institute, P.O. Box 503, Helsinki, 00101, Finland

When the unmagnetized planet Venus with a dense atmosphere interacts with the solar wind, an induced magnetosphere is formed and the interplanetary magnetic field (IMF) is enhanced and draped near the planet. Hybrid modelling is a semi-kinetic method to study the plasma interactions of Venus-like objects in a global planetary scale. HYB simulation code solves numerically the hybrid model equations and provides, for example, a three dimensional structure of the magnetic field in the objects's near-space. The magnetic fields produced by the HYB-Venus runs are compared to the Venus Express MAG magnetometer observations. Also, the possiblity to derive more information from the MAG-HYB comparison by rotating a quasi-stationary simulation solution around the Venus-Sun axis along the spacecraft orbit is studied. The rotations correspond to a dynamically changing clock angle in the upstream IMF.

P33B-1291 

Venus Measurements by the MESSENGER Gamma-Ray and X-Ray Spectrometers

* Rhodes, E A (Ed.Rhodes@jhuapl.edu), Johns Hopkins Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, United States Starr, R D (Richard.D.Starr.1@gsfc.nasa.gov), The Catholic University of America, Department of Physics, Washington, DC 20064, United States Goldsten, J O (John.Goldsten@jhuapl.edu), Johns Hopkins Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, United States Schlemm, C E (Chuck.Schlemm@jhuapl.edu), Johns Hopkins Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, United States Boynton, W V (wboynton@lpl.arizona.edu), The University of Arizona, Department of Planetary Sciences, Tucson, Az 85721, United States

The Gamma-Ray Spectrometer (GRS), which is a part of the Gamma-Ray and Neutron Spectrometer Instrument, and the X-Ray Spectrometer (XRS) on the MESSENGER spacecraft made calibration measurements during the Venus flyby on June 5, 2007. The purpose of these instruments is to determine elemental abundances on the surface of Mercury. The GRS measures gamma-rays emitted from element interactions with cosmic rays impinging on the surface, while the XRS measures X-ray emissions induced on the surface by the incident solar flux. The GRS sensor is a high-resolution high-purity Ge detector cooled by a Stirling cryocooler, surrounded by a borated-plastic anticoincidence shield. The GRS is sensitive to gamma-rays up to ~10 MeV and can identify most major elements, sampling down to depths of about ten centimeters. Only the shield was powered on for this flyby in order to conserve cooler lifetime. Gamma-rays were observed coming from Venus as well as from the spacecraft. Although the Venus gamma-rays originate from its thick atmosphere rather than its surface, the GRS data from this encounter will provide useful calibration data from a source of known composition. In particular, the data will be useful for determining GRS sensitivity and pointing options for the Mercury flybys, the first of which will be in January 2008. The X-ray spectrum of a planetary surface is dominated by a combination of the fluorescence and scattered solar X-rays. The most prominent fluorescent lines are the Kα lines from the major elements Mg, Al, Si, S, Ca, Ti, and Fe (1-10 keV). The sampling depth is less than 100 u m. The XRS is similar in design to experiments flown on Apollo 15 and 16 and the NEAR-Shoemaker mission. Three large-area gas-proportional counters view the planet, and a small Si-PIN detector mounted on the spacecraft sunshade monitors the Sun. The energy resolution of the gas proportional counters (~850 eV at 5.9 keV) is sufficient to resolve the X-ray lines above 2 keV, but Al and Mg filters on two of the three gas counters are required to differentially separate the lower energy X-ray lines from Al, Mg, and Si. A Be-Cu honeycomb collimator provides a 12° field of view, which is smaller than the planet at apoapsis and reduces the X-ray sky background. The Venus atmosphere is almost entirely composed of carbon and oxygen that fluoresce below the energy range of the XRS, but the flyby still provided valuable experience in planning for the upcoming Mercury flybys.

P33B-1292 

The MESSENGER Venus Flyby: First results from the Mercury Atmospheric and Surface Composition Spectrometer

* Holsclaw, G (holsclaw@colorado.edu), University of Colorado, Laboratory for Atmospheric and Space Physics 1234 Innovation Dr, Boulder, CO 80303, United States McClintock, W (William.McClintock@lasp.colorado.edu), University of Colorado, Laboratory for Atmospheric and Space Physics 1234 Innovation Dr, Boulder, CO 80303, United States Robinson, M (mark.s.robinson@asu.edu), Arizona State University, School of Earth and Space Exploration Box 871404, Tempe, AZ 85287, United States Izenberg, N (noam.izenberg@jhuapl.edu), The Johns Hopkins University, Applied Physics Laboratory 11100 Johns Hopkins Road, Laurel, MD 20723, United States

The MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft, NASA's first mission to orbit the planet Mercury, was launched on August 3, 2004, from Cape Canaveral Air Force Station. A gravity-assist maneuver brought the spacecraft past the planet Venus with a closest approach on June 5, 2007. This event provided an important opportunity to observe the Venus clouds and atmosphere with several of MESSENGER's remote sensing experiments. The Mercury Atmospheric and Surface Composition Spectrometer (MASCS), one of seven instruments onboard the spacecraft, consists of a Cassegrain telescope that simultaneously feeds the Visible and Infrared Spectrograph (VIRS) and the Ultraviolet and Visible Spectrometer (UVVS). VIRS is a point spectrometer, covering the wavelength range 320-1450 nm with a field-of-view of 0.023°. The Sun-illuminated portion of Venus was visible to the body-fixed MESSENGER instruments for about 15 minutes, during which time VIRS recorded ~500 individual spectra and MESSENGER's Mercury Dual Imaging System (MDIS) acquired an 11-color sequence before and after the VIRS measurements. These dual observations provide a unique data set for inter-comparing the radiometric calibration of these complementary devices. Visible color ratios from VIRS show evidence for a heterogeneous atmosphere. Nightside near-infrared spectra from VIRS do not show evidence for thermal emission in the known atmospheric window bands. Preliminary analysis of visible and near-infrared spectra from VIRS and multispectral MDIS Wide Angle Camera (WAC) images are internally consistent with observations of the Moon acquired in August 2005.

P33B-1293 

Flux-rope Structure in the Ionospheres of Venus and Titan

* Wei, H Y (hwei@igpp.ucla.edu), Institute of Geophysics and Planetary Physics, 3846 Slichter Hall, University of California, Los Angeles, CA 90095-1567, United States Russell, C T (ctrussel@igpp.ucla.edu), Institute of Geophysics and Planetary Physics, 3846 Slichter Hall, University of California, Los Angeles, CA 90095-1567, United States Zhang, T L (Tielong.Zhang@oeaw.ac.at), Space Research Institute, Space Research Institute, Graz, A-8042, Austria Dougherty, M K (m.dougherty@ic.ac.uk), The Blackett Laboratory, Dept. of Physics, Imperial College, London, SW7 2BZ, United Kingdom Luhmann, J G (jgluhman@ssl.berkeley.edu), Space Sciences Laboratory, University of California, Berkeley, San Jose, CA 94720, United States Wahlund, J - (jwe@irfu.se), Swedish Institute of Space Physics, Uppsala Division, Box 537, Uppsala, SE-751 21, Sweden Delva, M (Magda.Delva@oeaw.ac.at), Space Research Institute, Space Research Institute, Graz, A-8042, Austria Ma, Y J (yingjuan@igpp.ucla.edu), Institute of Geophysics and Planetary Physics, 3846 Slichter Hall, University of California, Los Angeles, CA 90095-1567, United States

Magnetic flux ropes, which have the structure of twisted flux tubes, are created in the ionosphere of Venus by its interaction with the solar wind and in that of Titan during it interacting with the flowing magnetized plasma corotating with Saturn. At solar maximum, the Venus ionosphere was found to be generally field free by the PVO eccentric orbiter, and flux ropes are frequently observed in the field-free region below the ionopause. During solar minimum, Venus Express detects a largely magnetized ionosphere, and flux ropes are observed both in the lower ionosphere with no background field and near the ionopause with some background field. The latter appears to be in the early stage of flux-rope formation. Similarly, the repeated low-altitude passes of Cassini through the Titan atmosphere reveal a "strongly" magnetized ionosphere and twisted magnetic field lines are observed which resemble the flux rope in Venus ionosphere during formation. This paper studies the flux-rope structures in Venus ionosphere during solar maximum and solar minimum and in Titan's ionosphere to understand the formation of flux ropes and what controls the orientation and helicity of a flux rope.

P33B-1294 

Venus Wind Spin-up Experiments with Topography; Sensitivity to Model Resolution

* Sayanagi, K M (kunio.sayanagi@louisville.edu), Comparative Planetology Lab, University of Louisville, 200 Sackett Hall, University of Louisville, Louisville, KY 40292, United States Ghurtskaia, N (ngkhar01@louisville.edu), Comparative Planetology Lab, University of Louisville, 200 Sackett Hall, University of Louisville, Louisville, KY 40292, United States Dowling, T E (dowling@louisville.edu), Comparative Planetology Lab, University of Louisville, 200 Sackett Hall, University of Louisville, Louisville, KY 40292, United States

We present new increased-resolution Venus spin-up experiments with topography using the latest hybrid vertical coordinate EPIC atmosphere model (Dowling et. al., Icarus, 182, 2006). We previously reported lower-resolution simulations and showed that the presence of topography substantially accelerates the spin-up of Venusian superrotating winds (Herrnstein and Dowling, JGR, 112, 2007). Our experiments also showed that, with 5- degree resolution, a large stationary eddy develops over Ishtar Terra, north-polar highland region. Here, we test the sensitivity of our previous results to the model resolution, and investigate the effect of finer-scale topographic features on the wind spin-up. Topographic features including an equatorial mountain range were recently discovered on Titan, another slow-rotating planetary body with a thick atmosphere (Stofan et. al., Icarus 186, 2006), and we explore the effects of Titan mountain ranges on the superrotation spin-up also. We aim to compare the effects of prominent topographic features on the resulting equator-to-pole Hadley circulations when the mountains are in the polar region (e.g., Venus) and when they are around the equator (Titan). This research is funded by the NSF Planetary Astronomy Program and NASA Planetary Atmospheres Program.

P33B-1295 

Venus Airglow Measurements Obtained by the Mercury Atmospheric and Surface Composition Spectrometer During MESSENGER's Second Venus Flyby

* Vervack, R J (Ron.Vervack@jhuapl.edu), The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723-6099, McClintock, W E (william.mcclintock@colorado.edu), University of Colorado, Laboratory for Atmospheric and Space Physics, 1234 Innovation Drive, Boulder, CO 80303, Izenberg, N R (Noam.Izenberg@jhuapl.edu), The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723-6099, Bradley, E T (bradleet@colorado.edu), University of Colorado, Laboratory for Atmospheric and Space Physics, 1234 Innovation Drive, Boulder, CO 80303, Kochte, M C (Mark.Kochte@jhuapl.edu), The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723-6099, Stewart, A (Ian.Stewart@lasp.colorado.edu), University of Colorado, Laboratory for Atmospheric and Space Physics, 1234 Innovation Drive, Boulder, CO 80303, Sprague, A L (sprague@lpl.arizona.edu), The University of Arizona, Lunar and Planetary Laboratory, 1629 University Blvd., Tucson, AZ 85721-0092,

During MESSENGER's second flyby of Venus on 5 June 2007, the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) conducted several different observations of the Venus airglow using the Ultraviolet- Visible Spectrometer (UVVS) component of the instrument. On the inbound leg and crossing into the sunlit hemisphere, measurements of H Lyman α (121.6 nm) and O (130.4 nm) were obtained. Once the MASCS line of sight crossed the terminator into the night hemisphere, measurements of the NO δ and γ bands and of the O₂ Herzberg II band system were added. Because the MASCS instrument is a scanning spectrometer, these measurements were obtained in series, with the cycle repeating until the line of sight moved off of the surface by several hundred kilometers. The sequence then returned to H and O measurements only on the outbound leg. The rapid speed of the flyby and design of MASCS (optimized for Mercury orbit) limited the number of measurements that were possible. Nevertheless, a fair number of spectra were obtained and represent an opportunity not only to calibrate MASCS against other measurements but also to provide additional, perhaps unique, data on the Venus airglow. We present an analysis of these spectra and place them in the context of previous measurements by the Pioneer Venus Orbiter Ultraviolet Spectrometer (H, O, and NO), the Venera orbiters (O₂), and ground-based telescopes (O₂) as well as continuing measurements by the Venus Express spacecraft.

P33B-1296 

MDIS Observations from the Second MESSENGER Venus Flyby

* Robinson, M S (mrobinson@asu.edu), Arizona State University, School of Earth and Space Exploration Box 871404, Phoenix, AZ 85287-1404, United States Harch, A P (harch@astro.cornell.edu), Cornell University, 310 Space Sciences Building, Ithaca, NY 14853, United States Hawkins, S E (ed.hawkins@jhuapl.edu), Johns Hopkins University, Applied Physics Laboratory 11100 Johns Hopkins Road, Laurel, MD 20723, United States Head, J W (James.Head@brown.edu), Brown University, Department of Geological Sciences Box 1846, Providence, RI 02912, United States Kang, H (hong.kang@jhuapl.edu), Johns Hopkins University, Applied Physics Laboratory 11100 Johns Hopkins Road, Laurel, MD 20723, United States Laslo, N R (nori.laslo@jhuapl.edu), Johns Hopkins University, Applied Physics Laboratory 11100 Johns Hopkins Road, Laurel, MD 20723, United States Murchie, S L (Scott.Murchie@jhuapl.edu), Johns Hopkins University, Applied Physics Laboratory 11100 Johns Hopkins Road, Laurel, MD 20723, United States Prockter, L M (louise.prockter@jhuapl.edu), Johns Hopkins University, Applied Physics Laboratory 11100 Johns Hopkins Road, Laurel, MD 20723, United States Solomon, S C (scs@dtm.ciw.edu), Carnegie Institution, Department of Terrestrial Magnetism 5241 Broad Branch Road, Washington, DC 20015, United States Vaughan, R M (robin.vaughan@jhuapl.edu), Johns Hopkins University, Applied Physics Laboratory 11100 Johns Hopkins Road, Laurel, MD 20723, United States Watters, T R (watterst@si.edu), Smithsonian Institution, National Air and Space Museum MRC 315, Washington, DC 20560, United States

The MESSENGER spacecraft successfully executed a close-approach (338 km) flyby of Venus on 5 June 2007 with the goal of optimizing the spacecraft's trajectory for its first Mercury encounter on 14 January 2008. During the flyby, key Mercury Dual Imaging System (MDIS) sequences required for the upcoming Mercury flyby were exercised with the accompanying spacecraft slews. MDIS observations included scattered light tests, high- resolution mosaics, point-and-stare multispectral images, flat-field calibrations, optical navigation tests, nightside imaging of thermal emission from the Venus surface, and a departure sequence. A total of 638 images were acquired and successfully received on the ground. These data are being analyzed and mosaicked to support instrument characterization and science comparisons with data obtained by the Venus Express mission. We will present reduced data with implications for the upcoming Mercury flyby. http://messenger.jhuapl.edu

P33B-1297 

First Measurements of Neutrons at Venus Using the MESSENGER Neutron Spectrometer

* Lawrence, D J (djlawrence@lanl.gov), Los Alamos National Laboratory, Space Science and Applications, Mail Stop D466, Los Alamos, NM 87545, United States Feldman, W C (feldman@psi.edu), Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719, United States Boynton, W V (wboynton@lpl.arizona.edu), University of Arizona, Lunar and Planetary Lab, Tucson, AZ 85721, United States Goldsten, J O (john.goldsten@jhuapl.edu), Johns Hopkins Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, United States Rhodes, E A (ed.rhodes@jhuapl.edu), Johns Hopkins Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, United States Science Team, M

The MESSENGER Neutron Spectrometer (NS) is part of the Gamma-Ray and Neutron Spectrometer (GRNS) instrument on board the MESSENGER spacecraft. The measurement goals of the NS are to identify and measure the abundance of hydrogen at high northern latitudes on Mercury, measure composition-sensitive neutron-flux intensities related to iron and titanium abundances on Mercury's surface, and provide neutron flux measurements that will be used for analyzing data from the GRNS Gamma-Ray Spectrometer sensor. The NS uses lithium glass and borated plastic scintillators to measure thermal, epithermal, and fast neutrons. The MESSENGER spacecraft executed a gravity assist flyby of Venus on 5 June 2007 during which time the NS was turned on. Strong cosmic ray-induced neutron signals from the Venus atmosphere in all energy ranges covered by NS were clearly observed. This is the first time that neutrons have been observed from Venus as well as the first planetary neutrons seen with the NS. Since these neutrons originate from the Venus atmosphere, there is no new information about Venus surface composition in these data. However, because these neutrons come from a source with a known composition, they provide ideal calibration data that will be needed to understand future neutron observations at Mercury.

P33B-1298 

Improved Version of the Roldan et al's Non-Local Thermodynamic Equilibrium Model for the Infrared Emissions in the Atmosphere of Venus

* Martin-Torres, F J (Francisco.Martin-Torres-1@nasa.gov), SD-CLIMATE SCIENCE BRANCH AS&M, NASA/Langley Research Center Mail Stop 936, Hampton, VA 23681-2199, United States Roldan, C (cristina.roldan@vodafone.es

Crisp, D (David.Crisp@jpl.nasa.gov), Jet Propulsion Laboratory, Jet Propulsion Laboratory M/S 183-501 4800 Oak Grove Drive, Pasadena, CA 91109, United States

Roldan et al. (2000) described a state of the art non-local thermodynamic equilibrium model for the calculation of the vibrational populations of CO₂ and and the cooling and heating rates in the atmosphere of Venus. This is currently the most comprehensive and sophisticated model up to date for the study of the CO₂ infrared emissions and the radiative equilibrium temperature in atmosphere of Venus. It allows the calculation of the populations of 68 vibrational levels of the 4 major isotopes of CO2 and the cooling and heating rates of 100 transitions. In spite of its unique capabilities this model has been abandoned several years. Here we present an update of the code with an improved treatment of the physics of the problem (including more accurate radiative transfer and line mixing calculations and aerosol treatment) and state-of-the-art line spectroscopic parameters. This new model also includes the calculation of the near-infrared emissions from the O₂ electronic states. Two versions of the model (in Fortran 77 and Fortran 90) are now available. The modular structure of the Fortran 90 code allows for an easy adaptation to any General Circulation Model of the Venus atmosphere or planetary mission retrieval processor. In this paper we present the new capabilities and features of the model and their impacts in the calculations. References: Roldan, C., M. A. Lopez-Valverde, M. Lopez-Puertas, and D. P. Edwards, "Non-LTE Infrared Emissions of CO₂ in the Atmosphere of Venus", Icarus, vol. 147, Issue 1, pp. 11-25 (2000).

P33B-1299 

Venus Upper Atmosphere Winds Traced by Night Airglow Distributions: NCAR VTGCM Simulations

* Brecht, A (abrecht@umich.edu), Atmospheric, Oceanic, and Space Sciences - University of Michigan, 2455 Hayward St., Ann Arbor, MI 48109, Bougher, S (bougher@umich.edu), Atmospheric, Oceanic, and Space Sciences - University of Michigan, 2455 Hayward St., Ann Arbor, MI 48109, Rafkin, S (srafkin@boulder.swri.edu), Southwest Research Institute, 1050 Walnut Street Suite 300, Boulder, CO 80302, Foster, B (foster@ucar.edu), High Altitude Observatory - National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307,

The National Center for Atmospheric Research (NCAR) thermospheric general circulation model for Venus (VTGCM) has been upgraded to better simulate night airglow distributions. Night airglow, at this time, is an effective means to investigate and understand the variability behind the subsolar-to-antisolar and superrotating zonal wind structure on Venus. The model is a three dimensional model that can calculate temperatures, zonal winds, meridional winds, vertical winds, and concentration of specific species. The VTGCM can also compute the O2-IR and NO-UV night airglow intensity distributions. These results can be compared to Pioneer Venus and, when available, Venus Express data sets. The most recent change is the models bottom boundary, which has been lowered and now rests at about 70 km. This upgrade insures that all possible dynamical influences that contribute to maintaining these airglow layers, and driving their variations, can be captured within the VTGCM domain. Modeling these airglows will provide valuable insight into the changing circulation of Venus' upper atmosphere and lead to a better understanding of the atmospheric dynamics of the planet.

P33B-1300 

Venus Express Magnetometer Observations: Global Vortices at the Ionopause

* Balikhin, M A (m.balikhin@sheffield.ac.uk), Centre for Signal Processing and Complex Systems, The University of Sheffield, Mappin Street, Sheffield, S1 3JD, United Kingdom Pope, S A (S.a.pope@sheffield.ac.uk), Centre for Signal Processing and Complex Systems, The University of Sheffield, Mappin Street, Sheffield, S1 3JD, United Kingdom Zhang, T L (tielong.zhang@oeaw.ac.at), Space Res. Inst., Schmiedlstrasse, 6, Graz, 8042, Austria

The interaction of the solar wind with Venus differs from the terrestrial case because Venus does not posses intrinsic magnetic field. This leads to the direct interaction between the fast flowing solar wind and the Venusian ionosphere at the ionopause. Venus Express(VEX) magnetometer data show nonlinear steepened surface waves at the ionopause. In some crossings it shows that nonlinear evolution of these waves leads to the formation of vortices. We argue that these waves and vortex structures are the result of the Kelvin-Helmholtz instability excited due to the shear velocity profile at the ionopause. Analysis of VEX observations show that these vortices should contribute to the ionospheric and atmospheric escape and the formation of plasma clouds observed in previous experiments.

P33B-1301 

Plasma Wave Activity Near Venus as Seen by the Venus Express Spacecraft

* Guicking, L (l.guicking@tu-bs.de), Institute for Geophysics and extraterrestrial Physics, Mendelssohnstr. 3, Braunschweig, 38106, Germany Glassmeier, K (kh.glassmeier@tu-bs.de), Institute for Geophysics and extraterrestrial Physics, Mendelssohnstr. 3, Braunschweig, 38106, Germany Auster, H (uli.auster@tu-bs.de), Institute for Geophysics and extraterrestrial Physics, Mendelssohnstr. 3, Braunschweig, 38106, Germany Zhang, T (Tielong.Zhang@oeaw.ac.at), Austrian Academy of Sciences Space Research Institute, Schmiedlstr. 6, Graz, 8042, Austria Delva, M (magda.delva@oeaw.ac.at), Austrian Academy of Sciences Space Research Institute, Schmiedlstr. 6, Graz, 8042, Austria Fraenz, M (fraenz@mps.mpg.de), Max Planck Institute for Solar System Research, Max-Planck-Str. 2, Katlenburg-Lindau, 37191, Germany Martinecz, C (martinecz@mps.mpg.de), Max Planck Institute for Solar System Research, Max-Planck-Str. 2, Katlenburg-Lindau, 37191, Germany

We present a wave activity map of the Venusian space environment obtained from data of the Venus Express (VEX) magnetometer (MAG). Venus Express is the first European mission to the planet Venus and was inserted in a polar orbit with a duration of 24 hours in April 2006. Due to its different orbit in comparison to the long lasting mission of the Pioneer Venus Orbiter (PVO), it can complete the lack of measurments and leads to a better understanding of the processes occurring within the Venusian magnetosphere and the interaction between the solar wind and the planet's atmosphere. The magnetometer onboard VEX measures the magnetic field vector continously during one orbit. We used for our calculations the data from April to December 2006 with a resolution of four seconds. We made a Fourier Analysis of this data set and determined a mean value of the power spectral densities in different frequency bands. In doing so, we were mainly interested in ultra low frequency (ULF) waves, whose frequencies are less than the gyro-frequencies of the typical species in that region. The wave activity maps show a region of enhanced activity in the magnetosheath between the subsolar point and the terminator. Ion density measurements of the ASPERA-4 instrument onboard VEX shall help to explain these results by determination of the Alfvén velocity, in particular the Alfvén Mach number. We speculate that regions of high wave activity and regions of sub-Alfvén Mach numbers are in general overlapping, because the waves there can propagate in all directions. We discuss this phenomenon, including possible reasons for the occurrence.

P33B-1302 

Solar Wind Plasma Pile-up at the Magnetic Polar Regions of the Venus Ionosphere: Viscous Force Effects

* Perez-de-Tejada, H (perezdet@geofisica.unam.mx), Institute of Geophysics, UNAM, Ciudad Universitaria, Mexico City, DF 04510, Mexico Lundin, R (rickard.lundin@irf.se), Swedish Institute of Space Physics, Teknikhuset, Umea, SE-901 87, Sweden Reyes-Ruiz, M (maurey@astrosen.unam.mx), Institute of Astronomy, UNAM, Baja California, Ensenada, BC 2860, Mexico

Measurements conducted with the Mariner 5 and the Venus Express spacecraft near the Venus ionosphere reveal the presence of accumulated plasma fluxes located upstream from the polar regions. Along the Mariner 5 trajectory the plasma and magnetic field data show evidence of enhanced values of the plasma density, temperature, and magnetic field intensity, with a sharp change in the solar wind speed by the terminator at a position located nearly 4000 km above the planetary surface. This event was detected when the spacecraft was situated much farther away than the expected position of the ionopause at the terminator and suggests conditions unrelated to the pile-up of the solar wind over the latter boundary around the subsolar region. Similar conditions are inferred from measurements conducted with the ASPERA instrument in the Venus Express spacecraft which show data with enhanced plasma fluxes detected through regions that extend far upstream from the terminator. The presence of such enhanced plasma fluxes is interpreted as resulting from a drastic slow down that the solar wind experiences at the magnetic polar regions through momentum transferred to the upper ionosphere. The results of a numerical simulation of the solar wind interaction with the Venus ionosphere including viscous forces support the accumulation of the incident plasma fluxes upstream from the magnetic polar regions and lead to values of the Reynolds number and the Mach number that are consistent with those inferred from the geometry of the viscous boundary layer that extends downstream from them.

P33B-1303 

Numerical Simulation of a Viscous Flow Interaction of the Solar Wind with the Magnetic Polar Regions of the Venus Ionosphere

* Reyes-Ruiz, M (maurey@astrosen.unam.mx), Institute of Astronomy, UNAM, Baja California, Ensenada, BC 22860, Mexico Perez-de-Tejada, H (perezdet@geofisica.unam.mx), Institute of Geophysics, UNAM, Ciudad Universitaria, Mexico City, DF 04510, Mexico Lundin, R (rickard.lundin@irf.se), Swedish Institute of Space Physics, Teknikhuset, Umea, SE-901 87, Sweden

A numerical code has been developed to simulate the flow of the solar wind as it interacts with the magnetic polar regions of the Venus ionosphere in the presence of viscous-fluid forces. The geometry of the flow suggests that a flat plate immersed in a supersonic hydrodynamic flow is a suitable model to study the conditions encountered near the magnetic polar regions of the Venus ionosphere where the solar wind directly interacts with the upper ionospheric plasma. In its preliminary form the code does not take into account the presence of a magnetic field nor curvature effects associated with flow motion around the dayside ionosphere and it is basically directed to analyse the effects of viscous force terms in the flow. The velocity, density, pressure and temperature profiles derived from the numerical simulation have been compared with the position of the bow shock and the intermediate transition as measured by the Mariner 5 and the Venera 10 spacecraft along the flanks of the Venus ionosheath. Values in the 10 < R < 100 range for the Reynolds number and in the 2 < M < 3 range for the local Mach number are found as most suitable to account for the observed relative position of both transitions. These values are comparable to those inferred for the terminator region from the velocity and temperature diagrams derived from the analysis of the momentum equation of an MHD flow subject to viscous forces. The results of the numerical simulation also provide evidence that the incident plasma accumulates above the leading edge of the flat plate representing the response of the fluid to the drastic drop of the ~300 km/s solar wind velocity to conform with a very small velocity as boundary condition set at the plate and that represents the main effect of the viscous transport of solar wind momentum to the Venus ionosphere where the speed is only 2-3 km/s.

P33B-1304 

Coronae on Venus: Relationship of Geology to Gravity

* Stofan, E R (ellen@proxemy.com), Proxemy Research, PO Box 338, Rectortown, VA 20140, United States Smrekar, S E (ssmrekar@jpl.nasa.gov), Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA 91109, United States Martin, P (paula.martin@durham.ac.uk), Durham University, Dept. of Earth Sciences, Science Labs, Durham, DH1 3LE, United Kingdom

Analysis of the gravity signatures of coronae can provide insight into their formation, as well as information on lithospheric properties. Previous studies have shown a lack of correlation between elastic thickness and corona diameter, and that all topographic morphologies are represented in the group of isostatically compensated coronae, including morphologies believed to reflect active plumes (Smrekar et al., 2003; Smrekar and Stofan, 2003; Hoogenboom et al., 2004). In addition, Johnson and Richards (2003) found that uncompensated coronae are preferentially located in the Beta-Atla-Themis region, suggesting it is younger. However, only coronae that are well resolved in the gravity data and have well behaved admittance signatures can be analyzed (135 out of 513 coronae), which does not provide an adequate size population to ensure that results are statistically significant (i.e., Glaze et al., 2002). In order to determine how the 135 features examined in gravity data relate to the total corona population, we are examining the local/regional stratigraphic position, amount of associated volcanism, and geologic complexity (e.g., evolution of the annulus and interior) for the135 features analyzed in the gravity data (Smrekar et al., 2003; Smrekar and Stofan, 2003; Hoogenboom et al., 2004). Initial results suggest a general lack of correlation between the corona parameters studied in these populations: for example, compensated corona range from stratigraphically old features with relatively low topography and low amounts of associated volcanism to stratigraphically young features with high topography and high amounts of associated volcanism. However, coronae in specific regions, such as parts of Hecate Chasma, show a correlation between apparent stratigraphic position and elastic thickness (Smrekar et al., this meeting). We are assessing these results in comparison to the population as a whole in order to better understand the relationship of coronae to the geologic evolution of Venus.

P33B-1305 

Steep Sided Domes on Venus: A Re-evaluation of Morphologies and Emplacement Mechanisms

* Gleason, A L (agleason@gi.alaska.edu), University of Alaska, Fairbanks Department of Geology and Geophysics, Reichardt Building Room 308, 900 Yukon Drive P.O. Box 755780, Fairbanks, AK 99775, United States * Gleason, A L (agleason@gi.alaska.edu), Geophysical Institute, 903 Koyukuk Drive, Fairbanks, AK 99775, United States Herrick, R (rherrick@gi.alaska.edu), Geophysical Institute, 903 Koyukuk Drive, Fairbanks, AK 99775, United States

Venusian steep sided domes are a volcanic feature noted for their circularity, steep sides, large volumes and relatively flat tops. Several researchers have studied them in order to determine their origins, composition and relation to their surroundings. Previous studies, however, have focused on steep sided domes as a single group and do not often discuss the wide range of morphologies and differences within the dome population. The purpose of this study is to identify features of steep sided domes that can be used in a classification scheme to determine which formational, cooling processes and characteristics are universal to all domes and which are revealing of differing eruptive and cooling histories and mechanisms. A subset of the dome population will be examined for differences and similarities in morphology and for small-scale surface features. Digital elevation models (DEMs) are being generated using the Magellan stereo imagery at 1 km horizontal resolution for all the domes for which stereo coverage is possible. Previous studies have used altimetry data from the Magellan mission, which has ~10 km horizontal resolution. Use of the stereo technique allows for a more accurate estimation of heights and volumes of the domes as well as to better constrain the overall morphology. Initial results show that while many of the existing measurements are in agreement with ours, others are as much as an order of magnitude off. In addition, resolution of smaller scale topography, including depths and extents of surface features such as lineaments, is possible using stereo derived DEMs. Surface lineaments and fractures will be mapped and analyzed to try and determine which lineaments are due to formation or to subsequent cooling processes and how these relate to dome morphology. Preliminary results indicate that while some of the lineament patterns are related to the overall morphology or tectonic history, others seem to indicate multiple eruptive events, possibly indicating polygenetic growth.

P33B-1306 

Sensitivity of Simplified Venus General Circulation Models to Numerical Parameterizations

* Lee, C (lee@gps.caltech.edu), Division of Geological and Planetary Sciences. California Institute of Technology, 1200 E. California Blvd M/C 150-21, Pasadena, CA 91125, United States Richardson, M I (mir@gps.caltech.edu), Division of Geological and Planetary Sciences. California Institute of Technology, 1200 E. California Blvd M/C 150-21, Pasadena, CA 91125, United States

General circulation models (GCMs) of the Venus atmosphere are currently forced using only simplified parameterizations of the radiative forcing and boundary conditions, such as those discussed in Yamamoto and Takahashi (JAS, 2003) or Lee, Lewis and Read (JGR, 2007). Using these parameterizations, suitable dynamical cores produce a qualitatively realistic circulation with super-rotation and transient planetary waves. The quantitative accuracy of the GCMs varies with dynamical core and the particular numerical parameterizations employed. However, all current GCMs exhibit sensitivity to the numerical parameterizations used to describe the behavior of the boundary conditions and the sub grid-scale eddy activity. These parameterizations are often used to reduce the grid-scale noise and unwanted propagating waves caused by discretization in the models We discuss the response of the numerical cores used in a number of simplified Venus GCMs using their various boundary layer and sub grid-scale parameterizations, and forcing with the radiative parameterizations outlined in Lee, Lewis and Read (JGR, 2007). The numerical cores tested include the PlanetWRF model (Caltech/NCAR), FMS (GFDL/NOAA) and HadAM3 (Oxford/Hadley Centre). We also discuss a possible physical interpretation of the sensitivities we have investigated and suggest corrections to the parameterizations used in these simplified GCMs. These corrections should provide more physically reasonable parameterizations for the thick Venus atmosphere, and in turn reduce the sensitivity to the numerical parameterizations such as the horizontal hyper-diffusion used to damp grid-scale waves, or the Rayleigh friction used to damp vertically propagating waves.

Author(s) (2007), Title, Eos Trans. AGU, 88(52), Fall Meet. Suppl., Abstract #####-##.