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1.
The Venus Express Radio Science Experiment VeRa retrieves atmospheric profiles in the mesosphere and troposphere of Venus in the approximate altitude range of 40–90 km. A data set of more than 500 profiles was retrieved between the orbit insertion of Venus Express in 2006 and the end of occultation season No. 11 in July 2011. The atmospheric profiles cover a wide range of latitudes and local times, enabling us to study the dependence of vertical small-scale temperature perturbations on local time and latitude.Temperature fluctuations with vertical wavelengths of 4 km or less are extracted from the measured temperature profiles in order to study small-scale gravity waves. Significant wave amplitudes are found in the stable atmosphere above the tropopause at roughly 60 km as compared with the only shallow temperature perturbations in the nearly adiabatic region of the adjacent middle cloud layer, below.Gravity wave activity shows a strong latitudinal dependence with the smallest wave amplitudes located in the low-latitude range, and an increase of wave activity with increasing latitude in both hemispheres; the greatest wave activity is found in the high-northern latitude range in the vicinity of Ishtar Terra, the highest topographical feature on Venus.We find evidence for a local time dependence of gravity wave activity in the low latitude range within ±30° of the equator. Gravity wave amplitudes are at their maximum beginning at noon and continuing into the early afternoon, indicating that convection in the lower atmosphere is a possible wave source.The comparison of the measured vertical wave structures with standard linear-wave theory allows us to derive rough estimates of the wave intrinsic frequency and horizontal wavelengths, assuming that the observed wave structures are the result of pure internal gravity waves. Horizontal wavelengths of the waves at 65 km altitude are on the order of ≈300–450 km with horizontal phase speeds of roughly 5–10 m/s.  相似文献   

2.
We investigate the Venus cloud top structure by joint analysis of the data from Visual and Thermal Infrared Imaging Spectrometer (VIRTIS) and the atmospheric temperature sounding by the Radio Science experiment (VeRa) onboard Venus Express. The cloud top altitude and aerosol scale height are derived by fitting VIRTIS spectra at 4–5 μm with temperature profiles taken from the VeRa radio occultation. Our study shows gradual descent of the cloud top from 67.2 ± 1.9 km in low latitudes to 62.8 ± 4.1 km at the pole and decrease of the aerosol scale height from 3.8 ± 1.6 km to 1.7 ± 2.4 km. These changes correlate with the mesospheric temperature field. In the cold collar and high latitudes the cloud top position remarkably coincides with the sharp minima in temperature inversions suggesting importance of radiative cooling in their maintenance. This behaviour is consistent with the earlier observations. Spectral trend of the cloud top altitude derived from a comparison with the earlier observations in 1.6–27 μm wavelength range is qualitatively consistent with sulphuric acid composition of the upper cloud and suggests that particle size increases from equator to pole.  相似文献   

3.
《Planetary and Space Science》2006,54(13-14):1298-1314
The planetary fourier spectrometer (PFS) for the Venus Express mission is an infrared spectrometer optimized for atmospheric studies. This instrument has a short wavelength (SW) channel that covers the spectral range from 1700 to 11400 cm−1 (0.9–5.5 μm) and a long wavelength (LW) channel that covers 250–1700 cm−1 (5.5–45 μm). Both channels have a uniform spectral resolution of 1.3 cm−1. The instrument field of view FOV is about 1.6 ° (FWHM) for the short wavelength channel and 2.8 ° for the LW channel which corresponds to a spatial resolution of 7 and 12 km when Venus is observed from an altitude of 250 km. PFS can provide unique data necessary to improve our knowledge not only of the atmospheric properties but also surface properties (temperature) and the surface-atmosphere interaction (volcanic activity).PFS works primarily around the pericentre of the orbit, only occasionally observing Venus from larger distances. Each measurements takes 4.5 s, with a repetition time of 11.5 s. By working roughly 1.5 h around pericentre, a total of 460 measurements per orbit will be acquired plus 60 for calibrations. PFS is able to take measurements at all local times, enabling the retrieval of atmospheric vertical temperature profiles on both the day and the night side.The PFS measures a host of atmospheric and surface phenomena on Venus. These include the:(1) thermal surface flux at several wavelengths near 1 μm, with concurrent constraints on surface temperature and emissivity (indicative of composition); (2) the abundances of several highly-diagnostic trace molecular species; (3) atmospheric temperatures from 55 to 100 km altitude; (4) cloud opacities and cloud-tracked winds in the lower-level cloud layers near 50-km altitudes; (5) cloud top pressures of the uppermost haze/cloud region near 70–80 km altitude; and (6) oxygen airglow near the 100 km level. All of these will be observed repeatedly during the 500-day nominal mission of Venus Express to yield an increased understanding of meteorological, dynamical, photochemical, and thermo-chemical processes in the Venus atmosphere. Additionally, PFS will search for and characterize current volcanic activity through spatial and temporal anomalies in both the surface thermal flux and the abundances of volcanic trace species in the lower atmosphere.Measurement of the 15 μm CO2 band is very important. Its profile gives, by means of a complex temperature profile retrieval technique, the vertical pressure-temperature relation, basis of the global atmospheric study.PFS is made of four modules called O, E, P and S being, respectively, the interferometer and proximity electronics, the digital control unit, the power supply and the pointing device.  相似文献   

4.
Observations of Venus using the ultraviolet filter of the Venus Monitoring Camera (VMC) on ESA’s Venus Express Spacecraft (VEX) provide the best opportunity for study of the spatial and temporal distribution of the venusian unknown ultraviolet absorber since the Pioneer Venus (PV) mission. We compare the results of two sets of 125 radiative transfer models of the upper atmosphere of Venus to each pixel in a subset of VMC UV channel images. We use a quantitative best fit criterion based upon the notion that the distribution of the unknown absorber should be independent of the illumination and observing geometry. We use the product of the cosines of the incidence and emission angles and search for absorber distributions that are uncorrelated with this geometric parameter, finding that two models can describe the vertical distribution of the unknown absorber. One model is a well-mixed vertical profile above a pressure level of roughly 120 mb (～63 km). This is consistent with the altitude of photochemical formation of sulfuric acid. The second model describes it as a thin layer of pure UV absorber at a pressure level roughly around 24 mb (～71 km) and this altitude is consistent with the top of upper cloud deck. We find that the average abundance of unknown absorber in the equatorial region is 0.21 ± 0.04 optical depth and it decreases in the polar region to 0.08 ± 0.05 optical depth at 365 nm.  相似文献   

5.
Nightglow emissions provide insight into the global thermospheric circulation, specifically in the transition region (～70–120 km). The O2 IR nightglow statistical map created from Venus Express (VEx) Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS) observations has been used to deduce a three-dimensional atomic oxygen density map. In this study, the National Center of Atmospheric Research (NCAR) Venus Thermospheric General Circulation Model (VTGCM) is utilized to provide a self-consistent global view of the atomic oxygen density distribution. More specifically, the VTGCM reproduces a 2D nightside atomic oxygen density map and vertical profiles across the nightside, which are compared to the VEx atomic oxygen density map. Both the simulated map and vertical profiles are in close agreement with VEx observations within a ～30° contour of the anti-solar point. The quality of agreement decreases past ～30°. This discrepancy implies the employment of Rayleigh friction within the VTGCM may be an over-simplification for representing wave drag effects on the local time variation of global winds. Nevertheless, the simulated atomic oxygen vertical profiles are comparable with the VEx profiles above 90 km, which is consistent with similar O2 (1Δ) IR nightglow intensities. The VTGCM simulations demonstrate the importance of low altitude trace species as a loss for atomic oxygen below 95 km. The agreement between simulations and observations provides confidence in the validity of the simulated mean global thermospheric circulation pattern in the lower thermosphere.  相似文献   

6.
Sub-millimeter 12CO (346 GHz) and 13CO (330 GHz) line absorptions, formed within the mesospheric to lower thermospheric altitude (70–120 km) region of the Venus atmosphere, have been mapped across the nightside disk of Venus during 2001–2009 inferior conjunctions, employing the James Clerk Maxwell Telescope (JCMT). Radiative transfer analysis of these thermal line absorptions supports temperature and CO mixing profile retrievals, as described in a companion paper (Clancy et al., 2012). Here, we consider the analysis of the sharp line absorption cores of these CO spectra in terms of accurate Doppler wind profile measurements at 95–115 km altitudes versus local time (～8 pm–4 am) and latitude (～60N–60S). These Doppler wind measurements support determinations of the nightside zonal and subsolar-to-antisolar (SSAS) circulation components over a variety of timescales. The average behavior fitted from 21 retrieved maps of 12CO Doppler winds (obtained over hourly, daily, weekly, and interannual intervals) indicates stronger average zonal (85 m/s retrograde) versus SSAS (65 m/s) circulation at the 1 μbar pressure (108–110 km altitude) level. However, the absolute and relative magnitudes of these circulation components exhibit extreme variability over daily to weekly timescales. Furthermore, the individual Doppler wind measurements within each nightside mapping observation generally show significant deviations (20–50 m/s, averaged over 5000 km horizontal scales) from the simple zonal/SSAS solution, with distinct local time and latitudinal characters that are also time variable. These large scale residual circulations contribute 30–70% of the observed nightside Doppler winds at any given time, and may be most responsible for global variations in nightside lower thermospheric trace composition and temperatures, as coincidentally retrieved CO abundance and temperature distributions do not correlate with solution retrograde zonal and SSAS winds (see companion paper, Clancy et al., 2012). Limited comparisons of these nightside submillimeter results with dayside infrared Doppler wind measurements suggest distinct dayside versus nightside circulations, in terms of zonal winds in particular. Combined 12CO and 13CO Doppler wind mapping observations obtained since 2004 indicate that the average zonal and SSAS wind components increase by 50–100% between altitudes of 100 and 115 km. If gravity waves originating from the cloud levels are responsible for the extension of zonal winds into the thermosphere (Alexander, M.J. [1992]. Geophys. Res. Lett. 19, 2207–2210), such waves deposit substantial momentum (i.e., break) in the lower nightside thermosphere.  相似文献   

7.
The Venus Express (VEX) mission has been in orbit to Venus for more than 4 years now. The Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument onboard VEX observes Venus in two channels (visible and infrared) obtaining spectra and multi-wavelength images of the planet that can be used to sample the atmosphere at different altitudes. Day-side images in the ultraviolet range (380 nm) are used to study the dynamics of the upper cloud at 66–72 km while night-side images in the near infrared (1.74 μm) map the opacity of the lower cloud deck at 44–48 km. Here we present a long-term analysis of the global atmospheric dynamics at these levels using a large selection of orbits from the VIRTIS-M dataset covering 860 Earth days that extends our previous work (Sánchez-Lavega, A. et al. [2008]. Geophys. Res. Lett. 35, L13204) and allows studying the variability of the global circulation at the two altitude levels. The atmospheric superrotation is evident with equatorial to mid-latitudes westward velocities of 100 and 60 m s?1 in the upper and lower cloud layers. These zonal velocities are almost constant in latitude from the equator to 50°S. From 50°S to 90°S the zonal winds at both cloud layers decrease steadily to zero at the pole. Individual cloud tracked winds have errors of 3–10 m s?1 with a mean of 5 m s?1 and the standard deviations for a given latitude of our zonal and meridional winds are 9 m s?1. The zonal winds in the upper cloud change with the local time in a way that can be interpreted in terms of a solar tide. The zonal winds in the lower cloud are stable at mid-latitudes to the tropics and present variability at subpolar latitudes apparently linked to the activity of the South polar vortex. While the upper cloud presents a net meridional motion consistent with the upper branch of a Hadley cell with peak velocity v = 10 m s?1 at 50°S, the lower cloud meridional motions are less organized with some cloud features moving with intense northwards and southwards motions up to v = ±15 m s?1 but, on average, with almost null global meridional motions at all latitudes. We also examine the long-term behavior of the winds at these two vertical layers by comparing our extended wind tracked data with results from previous missions.  相似文献   

8.
On its highly elliptical 24 h orbit around Venus, the Venus Express (VEX) spacecraft briefly reaches a periapsis altitude of nominally 250 km. Recently, however, dedicated and intense radio tracking campaigns have taken place in August 2008, October 2009, February and April 2010, for which the periapsis altitude was lowered to the 186–176 km altitude range in order to be able to probe the upper atmosphere of Venus above the North Pole for the first time ever in situ. As the spacecraft experiences atmospheric drag, its trajectory is measurably perturbed during the periapsis pass, allowing us to infer total atmospheric mass density at the periapsis altitude. A Precise Orbit Determination (POD) of the VEX motion is performed through an iterative least-squares fitting process to the Doppler tracking data, acquired by the VEX radioscience experiment (VeRa). The drag acceleration is modelled using an initial atmospheric density model (VTS3 model, Hedin, A.E., Niemann, H.B., Kasprzak, W.T., Seiff, A. [1983]. J. Geophys. Res. 88, 73–83). A scale factor of the drag acceleration is estimated for each periapsis pass, which scales Hedin’s density model in order to best fit the radio tracking data. Reliable density scale factors have been obtained for 10 passes mainly from the second (October 2009) and third (April 2010) VExADE campaigns, which indicate a lower density by a factor of about 1.8 than Hedin’s model predicts. These first ever in situ polar density measurements at solar minimum have allowed us to construct a diffusive equilibrium density model for Venus’ thermosphere, constrained in the lower thermosphere primarily by SPICAV-SOIR measurements and above 175 km by the VExADE drag measurements (Müller-Wodarg et al., in preparation). The preliminary results of the VExADE campaigns show that it is possible to obtain with the POD technique reliable estimates of Venus’ upper atmosphere densities at an altitude of around 175 km. Future VExADE campaigns will benefit from the planned further lowering of VEX pericenter altitude to below 170 km.  相似文献   

9.
《Planetary and Space Science》2006,54(13-14):1315-1335
The Venus Express Radio Science Experiment (VeRa) uses radio signals at wavelengths of 3.6 and 13 cm (“X”- and “S”-band, respectively) to investigate the Venus surface, neutral atmosphere, ionosphere, and gravity field, as well as the interplanetary medium. An ultrastable oscillator (USO) provides a high quality onboard reference frequency source; instrumentation on Earth is used to record amplitude, phase, propagation time, and polarization of the received signals. Simultaneous, coherent measurements at the two wavelengths allow separation of dispersive media effects from classical Doppler shift.VeRa science objectives include the following:
  • (1)Determination of neutral atmospheric structure from the cloud deck (approximately 40 km altitude) to 100 km altitude from vertical profiles of neutral mass density, temperature, and pressure as a function of local time and season. Within the atmospheric structure, search for, and if detected, study of the vertical structure of localized buoyancy waves, and the presence and properties of planetary waves.
  • (2)Study of the H2SO4 vapor absorbing layer in the atmosphere by variations in signal intensity and application of this information to tracing atmospheric motions. Scintillation effects caused by radio wave diffraction within the atmosphere can also provide information on small-scale atmospheric turbulence.
  • (3)Investigation of ionospheric structure from approximately 80 km to the ionopause (<600 km), allowing study of the interaction between solar wind plasma and the Venus atmosphere.
  • (4)Observation of forward-scattered surface echoes obliquely reflected from selected high-elevation targets with anomalous radar properties (such as Maxwell Montes). More generally, such bistatic radar measurements provide information on the roughness and density of the surface material on scales of centimeters to meters.
  • (5)Detection of gravity anomalies, thereby providing insight into the properties of the Venus crust and lithosphere.
  • (6)Measurement of the Doppler shift, propagation time, and frequency fluctuations along the interplanetary ray path, especially during periods of superior conjunction, thus enabling investigation of dynamical processes in the solar corona.
  相似文献   

10.
《Planetary and Space Science》2007,55(12):1741-1756
The dynamics of Venus’ mesosphere (70–110 km) is characterized by the superposition of two different wind regimes: (1) Venus’ retrograde superrotation; (2) a sub-solar to anti-solar (SS–AS) flow pattern, driven by solar EUV heating on the sunlit hemisphere. Here, we report on new ground-based velocity measurements in the lower part of the mesosphere. We took advantage of two essentially symmetric Venus elongations in 2001 and 2002 to perform high-resolution Doppler spectroscopy (R=120,000) in 12C16O2 visible lines of the 5ν3 band and in a few solar Fraunhofer lines near 8700 Å. These measurements, mapped over several points on Venus’ illuminated hemisphere, probe the region of cloud tops. More precisely, the solar Fraunhofer lines sample levels a few kilometers below the UV features (i.e. near ∼67 km), while the CO2 lines probe an altitude higher by about 7 km. The wind field over Venus’ disk is retrieved with an rms uncertainty of 15–25 m s−1 on individual measurements. Kinematical fit to a one- or two-component circulation model indicates the dominance of the zonal retrograde flow with a mean equatorial velocity of ∼75 m s−1, exhibiting very strong day-to-day variations (±65 m s−1). Results are very consistent for the two kinds of lines, suggesting a negligible vertical wind shear over 67–74 km. The SS–AS flow is not detected in single-day observations, but combining the results from all data suggests that this component may invade the lower mesosphere with a ∼40 m s−1 velocity.  相似文献   

11.
Observations of the dayside of Venus performed by the high spectral resolution channel (–H) of the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on board the ESA Venus Express mission have been used to measure the altitude of the cloud tops and the water vapor abundance around this level with a spatial resolution ranging from 100 to 10 km. CO2 and H2O bands between 2.48 and 2.60 μm are analyzed to determine the cloud top altitude and water vapor abundance near this level. At low latitudes (±40°) mean water vapor abundance is equal to 3 ± 1 ppm and the corresponding cloud top altitude at 2.5 μm is equal to 69.5 ± 2 km. Poleward from middle latitudes the cloud top altitude gradually decreases down to 64 km, while the average H2O abundance reaches its maximum of 5 ppm at 80° of latitude with a large scatter from 1 to 15 ppm. The calculated mass percentage of the sulfuric acid solution in cloud droplets of mode 2 (～1 μm) particles is in the range 75–83%, being in even more narrow interval of 80–83% in low latitudes. No systematic correlation of the dark UV markings with the cloud top altitude or water vapor has been observed.  相似文献   

12.
Between November 23 and 28, 2007, the Cologne Tuneable Heterodyne Infrared Spectrometer THIS was installed at the McMath-Pierce Solar Telescope (Kitt Peak, Arizona, USA) to determine zonal wind velocities and to estimate the subsolar-to-antisolar flow. We investigate dynamics in the upper atmosphere of Venus by measuring the Doppler shift of fully-resolved non-LTE CO2 emission lines at 959.3917 cm?1 (10.423 μm), which probe a narrow altitude region in Venus’ atmosphere around 110 ± 10 km (～1 μbar). The results show no significant zonal wind velocity at the equator. An increase with latitude up to 43 ± 13 m/s at a latitude of 33°N was observed. This confirms the deduction of a minor influence of Venus superrotation at an altitude of 110 km from previous measurements in May 2007 (Sornig et al., 2008). The specific observing geometry enables estimating the maximum cross terminator velocity of the subsolar-to-antisolar flow at 72 ± 47 m/s.  相似文献   

13.
《Planetary and Space Science》2007,55(12):1701-1711
The Venus Express mission will focus on a global investigation of the Venus atmosphere and plasma environment, while additionally measuring some surface properties from orbit. The instruments PFS and SPICAV inherited from the Mars Express mission and VIRTIS from Rosetta form a powerful spectrometric and spectro-imaging payload suite. Venus Monitoring Camera (VMC)—a miniature wide-angle camera with 17.5° field of view—was specifically designed and built to complement these experiments and provide imaging context for the whole mission. VMC will take images of Venus in four narrow band filters (365, 513, 965, and 1000 nm) all sharing one CCD. Spatial resolution on the cloud tops will range from 0.2 km/px at pericentre to 45 km/px at apocentre when the full Venus disc will be in the field of view. VMC will fulfill the following science goals: (1) study of the distribution and nature of the unknown UV absorber; (2) determination of the wind field at the cloud tops (70 km) by tracking the UV features; (3) thermal mapping of the surface in the 1 μm transparency “window” on the night side; (4) determination of the global wind field in the main cloud deck (50 km) by tracking near-IR features; (5) study of the lapse rate and H2O content in the lower 6–10 km; (6) mapping O2 night-glow and its variability.  相似文献   

14.
Venus Express measurements of the vertical profiles of SO and SO2 in the middle atmosphere of Venus provide an opportunity to revisit the sulfur chemistry above the middle cloud tops (～58 km). A one dimensional photochemistry-diffusion model is used to simulate the behavior of the whole chemical system including oxygen-, hydrogen-, chlorine-, sulfur-, and nitrogen-bearing species. A sulfur source is required to explain the SO2 inversion layer above 80 km. The evaporation of the aerosols composed of sulfuric acid (model A) or polysulfur (model B) above 90 km could provide the sulfur source. Measurements of SO3 and SO (a1Δ → X3Σ-) emission at 1.7 μm may be the key to distinguish between the two models.  相似文献   

15.
An automated cloud tracking algorithm is applied to Cassini Imaging Science Subsystem high-resolution apoapsis images of Saturn from 2005 and 2007 and moderate resolution images from 2011 and 2012 to define the near-global distribution of zonal winds and eddy momentum fluxes at the middle troposphere cloud level and in the upper troposphere haze. Improvements in the tracking algorithm combined with the greater feature contrast in the northern hemisphere during the approach to spring equinox allow for better rejection of erroneous wind vectors, a more objective assessment at any latitude of the quality of the mean zonal wind, and a population of winds comparable in size to that available for the much higher contrast atmosphere of Jupiter. Zonal winds at cloud level changed little between 2005 and 2007 at all latitudes sampled. Upper troposphere zonal winds derived from methane band images are ～10 m s?1 weaker than cloud level winds in the cores of eastward jets and ～5 m s?1 stronger on either side of the jet core, i.e., eastward jets appear to broaden with increasing altitude. In westward jet regions winds are approximately the same at both altitudes. Lateral eddy momentum fluxes are directed into eastward jet cores, including the strong equatorial jet, and away from westward jet cores and weaken with increasing altitude on the flanks of the eastward jets, consistent with the upward broadening of these jets. The conversion rate of eddy to mean zonal kinetic energy at the visible cloud level is larger in eastward jet regions (5.2 × 10?5 m2 s?3) and smaller in westward jet regions (1.6 × 10?5 m2 s?3) than the global mean value (4.1 × 10?5 m2 s?3). Overall the results are consistent with theories that suggest that the jets and the overturning meridional circulation at cloud level on Saturn are maintained at least in part by eddies due to instabilities of the large-scale flow near and/or below the cloud level.  相似文献   

16.
The most significant aspect of the general circulation of the atmosphere of Venus is its retrograde super-rotation. A complete characterization of this dynamical phenomenon is crucial for understanding its driving mechanisms. Here we report on ground-based Doppler velocimetry measurements of the zonal winds, based on high resolution spectra from the UV–Visual Echelle Spectrograph (UVES) instrument at ESO’s Very Large Telescope. Under the assumption of predominantly zonal flow, this method allows the simultaneous direct measurement of the zonal velocity across a range of latitudes and local times in the day side. The technique, based on long slit spectroscopy combined with the high spatial resolution provided by the VLT, has provided the first ground-based characterization of the latitudinal profile of zonal wind in the atmosphere of Venus, the first zonal wind field map in the visible, as well as new constraints on wind variations with local time. We measured mean zonal wind amplitudes between 106 ± 21 and 127 ± 14 m/s at latitudes between 18°N and 34°S, with the zonal wind being approximately uniform in 2.6°-wide latitude bands (0.3 arcsec at disk center). The zonal wind profile retrieved is consistent with previous spacecraft measurements based on cloud tracking, but with non-negligible variability in local time (longitude) and in latitude. Near 50° the presence of moderate jets is apparent in both hemispheres, with the southern jet being stronger by ～10 m/s. Small scale wind variations with local time are also present at low and mid-latitudes.  相似文献   

17.
We have obtained spatially resolved near-infrared spectroscopy of the Venus nightside on 15 nights over three observing seasons. We use the depth of the CO absorption band at 2.3 μm to map the two-dimensional distribution of CO across both hemispheres. Radiative transfer models are used to relate the measured CO band depth to the volume mixing ratio of CO. The results confirm previous investigations in showing a general trend of increased CO abundances at around 60° latitude north and south as compared with the equatorial regions. Observations taken over a few nights generally show very similar CO distributions, but significant changes are apparent over longer periods. In past studies it has been assumed that the CO latitudinal variation occurs near 35 km altitude, at which K-band sensitivity to CO is greatest. By modeling the detailed spectrum of the excess CO at high latitudes we show that it occurs at altitudes around 45 km, much higher than has previously been assumed, and that there cannot be significant contribution from levels of 36 km or lower. We suggest that this is most likely due to downwelling of CO-rich gas from the upper atmosphere at these latitudes, with the CO being removed by around 40 km through chemical processes such as the reaction with SO3.  相似文献   

18.
《Planetary and Space Science》2007,55(12):1673-1700
Spectroscopy for the investigation of the characteristics of the atmosphere of Venus (SPICAV) is a suite of three spectrometers in the UV and IR range with a total mass of 13.9 kg flying on the Venus Express (VEX) orbiter, dedicated to the study of the atmosphere of Venus from ground level to the outermost hydrogen corona at more than 40,000 km. It is derived from the SPICAM instrument already flying on board Mars Express (MEX) with great success, with the addition of a new IR high-resolution spectrometer, solar occultation IR (SOIR), working in the solar occultation mode. The instrument consists of three spectrometers and a simple data processing unit providing the interface of these channels with the spacecraft.A UV spectrometer (118–320 nm, resolution 1.5 nm) is identical to the MEX version. It is dedicated to nadir viewing, limb viewing and vertical profiling by stellar and solar occultation. In nadir orientation, SPICAV UV will analyse the albedo spectrum (solar light scattered back from the clouds) to retrieve SO2, and the distribution of the UV-blue absorber (of still unknown origin) on the dayside with implications for cloud structure and atmospheric dynamics. On the nightside, γ and δ bands of NO will be studied, as well as emissions produced by electron precipitations. In the stellar occultation mode the UV sensor will measure the vertical profiles of CO2, temperature, SO2, SO, clouds and aerosols. The density/temperature profiles obtained with SPICAV will constrain and aid in the development of dynamical atmospheric models, from cloud top (∼60 km) to 160 km in the atmosphere. This is essential for future missions that would rely on aerocapture and aerobraking. UV observations of the upper atmosphere will allow studies of the ionosphere through the emissions of CO, CO+, and CO2+, and its direct interaction with the solar wind. It will study the H corona, with its two different scale heights, and it will allow a better understanding of escape mechanisms and estimates of their magnitude, crucial for insight into the long-term evolution of the atmosphere.The SPICAV VIS-IR sensor (0.7–1.7 μm, resolution 0.5–1.2 nm) employs a pioneering technology: an acousto-optical tunable filter (AOTF). On the nightside, it will study the thermal emission peeping through the clouds, complementing the observations of both VIRTIS and Planetary Fourier Spectrometer (PFS) on VEX. In solar occultation mode this channel will study the vertical structure of H2O, CO2, and aerosols.The SOIR spectrometer is a new solar occultation IR spectrometer in the range λ=2.2–4.3 μm, with a spectral resolution λ/Δλ>15,000, the highest on board VEX. This new concept includes a combination of an echelle grating and an AOTF crystal to sort out one order at a time. The main objective is to measure HDO and H2O in solar occultation, in order to characterize the escape of D atoms from the upper atmosphere and give more insight about the evolution of water on Venus. It will also study isotopes of CO2 and minor species, and provides a sensitive search for new species in the upper atmosphere of Venus. It will attempt to measure also the nightside emission, which would allow a sensitive measurement of HDO in the lower atmosphere, to be compared to the ratio in the upper atmosphere, and possibly discover new minor atmospheric constituents.  相似文献   

19.
《Planetary and Space Science》2006,54(13-14):1482-1495
Venus has no internal magnetic dynamo and thus its ionosphere and hot oxygen exosphere dominate the interaction with the solar wind. The solar wind at 0.72 AU has a dynamic pressure that ranges from 4.5 nPa (at solar max) to 6.6 nPa (at solar min), and its flow past the planet produces a shock of typical magnetosonic Mach number 5 at the subsolar point. At solar maximum the pressure in the ionospheric plasma is sufficient to hold off the solar wind at an altitude of 400 km above the surface at the subsolar point, and 1000 km above the terminators. The deflection of the solar wind occurs through the formation of a magnetic barrier on the inner edge of the magnetosheath, or shocked solar wind. Under typical solar wind conditions the time scale for diffusion of the magnetic field into the ionosphere is so long that the ionosphere remains field free and the barrier deflects almost all the incoming solar wind. Any neutral atoms of the hot oxygen exosphere that reach the altitude of the magnetosheath are accelerated by the electric field of the flowing magnetized plasma and swept along cycloidal paths in the antisolar direction. This pickup process, while important for the loss of the Venus atmosphere, plays a minor role in the deceleration and deflection of the solar wind. Like at magnetized planets, the Venus shock and magnetosheath generate hot electrons and ions that flow back along magnetic field lines into the solar wind to form a foreshock. A magnetic tail is created by the magnetic flux that is slowed in the interaction and becomes mass-loaded with thermal ions.The structure of the ionosphere is very much dependent on solar activity and the dynamic pressure of the solar wind. At solar maximum under typical solar wind conditions, the ionosphere is unmagnetized except for the presence of thin magnetic flux ropes. The ionospheric plasma flows freely to the nightside forming a well-developed night ionosphere. When the solar wind pressure dominates over the ionospheric pressure the ionosphere becomes completely magnetized, the flow to the nightside diminishes, and the night ionosphere weakens. Even at solar maximum the night ionosphere has a very irregular density structure. The electromagnetic environment of Venus has not been well surveyed. At ELF and VLF frequencies there is noise generated in the foreshock and shock. At low altitude in the night ionosphere noise, presumably generated by lightning, can be detected. This paper reviews the plasma environment at Venus and the physics of the solar wind interaction on the threshold of a new series of Venus exploration missions.  相似文献   

20.
The Visible and Infra-Red Thermal Imaging Spectrometer (VIRTIS) instrument on board the Venus Express spacecraft has measured the O2(a1Δ) nightglow distribution at 1.27 μm in the Venus mesosphere for more than two years. Nadir observations have been used to create a statistical map of the emission on Venus nightside. It appears that the statistical 1.6 MR maximum of the emission is located around the antisolar point. Limb observations provide information on the altitude and on the shape of the emission layer. We combine nadir observations essentially covering the southern hemisphere, corrected for the thermal emission of the lower atmosphere, with limb profiles of the northern hemisphere to generate a global map of the Venus nightside emission at 1.27 μm. Given all the O2(a1Δ) intensity profiles, O2(a1Δ) and O density profiles have been calculated and three-dimensional maps of metastable molecular and atomic oxygen densities have been generated. This global O density nightside distribution improves that available from the VTS3 model, which was based on measurements made above 145 km. The O2(a1Δ) hemispheric average density is 2.1 × 109 cm?3, with a maximum value of 6.5 × 109 cm?3 at 99.2 km. The O density profiles have been derived from the nightglow data using CO2 profiles from the empirical VTS3 model or from SPICAV stellar occultations. The O hemispheric average density is 1.9 × 1011 cm?3 in both cases, with a mean altitude of the peak located at 106.1 km and 103.4 km, respectively. These results tend to confirm the modeled values of 2.8 × 1011 cm?3 at 104 km and 2.0 × 1011 cm?3 at 110 km obtained by Brecht et al. [Brecht, A., Bougher, S.W., Gérard, J.-C., Parkinson, C.D., Rafkin, S., Foster, B., 2011a. J. Geophys. Res., in press] and Krasnopolsky [Krasnopolsky, V.A., 2010. Icarus 207, 17–27], respectively. Comparing the oxygen density map derived from the O2(a1Δ) nightglow observations, it appears that the morphology is very different and that the densities obtained in this study are about three times higher than those predicted by the VTS3 model.  相似文献   

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