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1.
Statistics on the temporal variability of uv cloud features on Venus during 66 days of nominal mission imaging by the Pioneer Venus Orbiter Cloud Photopolarimeter reveal at least five types of systematic variability on large scales: (1) a low-latitude global-scale wave of period 3.94 ± 0.1 days corresponding to longitudinal motion of the dark equatorial band and propagating westward relative to the mean flow; (2) a midlatitude wave of period 5.20 ± 0.2 days corresponding to wavenumber 1 oscillations of the latitude of the bright polar bands and propagating eastward relative to the mean flow; (3) ～2- to 3-week fluctuations in the slope of longitudinal cloud brightness power spectra at intermediate wavenumbers manifested by variations in the intensity of large bow-shaped features; (4) ～2-month variations in polar region brightness consistent with polar brightening episodes observed from Earth; and (5) a monotonic decrease in the disk-integrated brightness of Venus during the nominal mission which may be either a true time variation or a solar-locked longitudinal dependence of brightness. Small-scale features appear to correlate with large-scale albedo patterns. Specifically, cellular features exist primarily where large-scale dark material is present, while the orientation of streak features with respect to latitude circles oscillates with the same ～4-day period as the large-scale features at low latitudes. The wide range of time scales present in the data suggests the complexity of Venus stratospheric dynamics. Extended observations over many years may be becessary to define the general circulation.  相似文献   

2.
Mid-infrared images of almost the entire Venus nightside hemisphere obtained by the Longwave Infrared Camera (LIR) onboard Akatsuki on December 9 and 10, 2010 reveal that the brightness temperature of the cloud-top ranges from 237 K in the cold polar collars to 243 K in the equatorial region, significantly higher than the values obtained by Venera 15. Other characteristic features of the temperature distributions observed are zonal belt structures seen in the middle and low latitudes and patchy temperature structures or quasi-periodic streaks extending in a north–south direction in the northern middle latitudes and southern low latitudes.  相似文献   

3.
The problem of how to determine parameters of motion for an automated interplanetary probe (AIP) on a quasi-satellite orbit during flight to the small natural satellite of a planet is examined. The problem is solved by searching for an optimal state-space trajectory of automated interplanetary probe according to the least-squares criterion. The results of longitude and latitude measurements at several points are used as boundary conditions.  相似文献   

4.
Theoretical brightness distributions over the Venus disk have been computed for homogeneous and multilayered cloud models. With homogeneous models, the relative brightness does not depend very much on the optical properties of the cloud, except near the limb or for small phase angles. For multilayered cloud structures, the relative brightness is nearly fixed by the structures and the relative brightness is nearly fixed by the scattering function of the uppermost cloud; the vertical distribution of the absorption is unimportant if the spherical albedo of the planet is given. If the horizontal inhomogeneities currently seen on Venus are due to a layered structure, with an optical thickness of the upper cloud varying from point to point, large simplifications seem possible, and measured contrasts at various wavelengths should permit a test of such a model.  相似文献   

5.
An improved cloud tracking method for deriving wind velocities from successive planetary images was developed. The new method incorporates into the traditional cross-correlation method an algorithm that corrects for erroneous cloud motion vectors by re-determining the most plausible correlation peak among all of the local maxima on the correlation surface by comparing each vector with its neighboring vectors. The newly developed method was applied to the Venusian violet images obtained by the Solid State Imaging system (SSI) onboard the Galileo spacecraft during its Venus flyby. Although the results may be biased by the choice of spatial scale of atmospheric features, the cloud tracking is the most practical mean of estimating the wind velocities with extensive spatial and temporal coverage. The two-dimensional distribution of the horizontal wind vector field over 5 days was obtained. It was found from these wind maps that the solar-fixed component in 1990 was similar to that in 1982 obtained by the Pioneer Venus orbiter. The deviation of the instantaneous zonal wind field from the solar-fixed component shows a distinct wavenumber-1 structure in the equatorial region. On the assumption that this structure is a manifestation of an equatorial Kelvin wave, the phase relationship between the zonal wind and the cloud brightness suggests a short photochemical lifetime of the violet absorber. The momentum deposition by this Kelvin wave, which is subject to radiative damping, would induce a westward mean-wind acceleration of ～0.3 m s?1 per Earth day.  相似文献   

6.
Cloud motions were obtained from a number of images acquired in reflected solar ultraviolet light during spring and fall of 1979 from the Pioneer Venus Orbiter Cloud Photopolarimeter (OCPP) to determine the zonal mean circulation of the atmosphere of Venus at the cloud top level. The meridional profile of the zonal component of motion is somewhat different from that previously obtained from Mariner 10 and preliminary Pioneer Venus observations, although the equatorial magnitude is about the same (?94 m/sec). The mean meridional motion is toward the south pole south of about 5° south latitude, and toward the north pole north of this latitude, with peak mean magnitudes of about 7 m/sec polewards of 20° north and 40° south latitudes in the respective hemispheres. From the few measurements obtained at higher latitudes the magnitude of the mean meridional component appears to decrease although it is still directed toward the respective poles. Due to the evolution of the cloud patterns over the duration of the images from which the cloud velocities are obtained, the uncertainties in the mean zonal and meridional components may be as large as 5–10 and 2–4 m/sec, respectively. Preliminary estimates of meridional momentum transport show that the mean circulation dominates the eddy circulation transport completely, in agreement with the estimates obtained from Mariner 10 data, although the uncertainties in both the mean and eddy circulation transports are large. The momentum transports are polewards and their peak magnitudes occur at latitudes between 20° and 40° in both the hemispheres.  相似文献   

7.
We present an analysis of VIRTIS-M-IR observations of 1.74 μm emission from the nightside of Venus. The 1.74 μm window in the near infrared spectrum of Venus is an ideal proxy for investigating the evolution of middle and lower cloud deck opacity of Venus because it exhibits good signal to noise due to its brightness, good contrast between bright and dark regions, and few additional sources of extinction beside the clouds themselves. We have analyzed the data from the first 407 orbits (equivalent to 407 Earth days) of the Venus Express mission to determine the magnitude of variability in the 1.74 μm radiance. We have also performed an analysis of the evolution of individual features over a span of roughly 5–6 h on two successive orbits of Venus Express. We find that the overall 1.74 μm brightness of Venus has been increasing through the first 407 days of the mission, indicating a gradual diminishing of the cloud coverage and/or thickness, and that the lower latitudes exhibited more variability and more brightening than higher latitudes. We find that individual features evolve with a time scale of about 30 h, consistent with our previous analysis. Analysis of the evolution and motion of the clouds can be used to estimate the mesoscale dynamics within the clouds of Venus. We find that advection alone cannot explain the observed evolution of the features. The measured vorticity and divergence in the vicinity of the features are consistent with evolution under the influence of significant vertical motions likely driven by a radiative dynamical feedback. We measure a zonal wind speed of around 65 m/s, and a meridional wind speed around 2.5 m/s by tracking the motion of the central region of the features. But we also find that the measured wind speeds depend strongly on the points chosen for the wind speed analysis.  相似文献   

8.
Since the discovery of ultraviolet markings on Venus, their observations have been a powerful tool to study the morphology, motions and dynamical state at the cloud top level. Here we present the results of investigation of the cloud top morphology performed by the Venus Monitoring Camera (VMC) during more than 3 years of the Venus Express mission. The camera acquires images in four narrow-band filters centered at 365, 513, 965 and 1010 nm with spatial resolution from 50 km at apocentre to a few hundred of meters at pericentre. The VMC experiment provides a significant improvement in the Venus imaging as compared to the capabilities of the earlier missions. The camera discovered new cloud features like bright “lace clouds” and cloud columns at the low latitudes, dark polar oval and narrow circular and spiral “grooves” in the polar regions, different types of waves at the high latitudes. The VMC observations revealed detailed structure of the sub-solar region and the afternoon convective wake, the bow-shape features and convective cells, the mid-latitude transition region and the “polar cap”. The polar orbit of the satellite enables for the first time nadir viewing of the Southern polar regions and an opportunity to zoom in on the planet. The experiment returned numerous images of the Venus limb and documented global and local brightening events. VMC provided almost continuous monitoring of the planet with high temporal resolution that allowed one to follow changes in the cloud morphology at various scales.We present the in-flight performance of the instrument and focus in particular on the data from the ultraviolet channel, centered at the characteristic wavelength of the unknown UV absorber that yields the highest contrasts on the cloud top. Low latitudes are dominated by relatively dark clouds that have mottled and fragmented appearance clearly indicating convective activity in the sub-solar region. At ～50° latitude this pattern gives way to streaky clouds suggesting that horizontal, almost laminar, flow prevails here. Poleward from about 60°S the planet is covered by almost featureless bright polar hood sometimes crossed by dark narrow (～300 km) spiral or circular structures. This global cloud pattern can change on time scales of a few days resulting in global and local “brightening events” when the bright haze can extend far into low latitudes and/or increase its brightness by 30%. Close-up snapshots reveal plenty of morphological details like convective cells, cloud streaks, cumulus-like columns, wave trains. Different kinds of small scale waves are frequently observed at the cloud top. The wave activity is mainly observed in the 65–80° latitude band and is in particular concentrated in the region of Ishtar Terra that suggests their possible orographic origin. The VMC observations have important implications for the problems of the unknown UV absorber, microphysical processes, dynamics and radiative energy balance at the cloud tops. They are only briefly discussed in the paper, but each of them will be the subject of a dedicated study.  相似文献   

9.
The Community Atmosphere Model (CAM), a 3-dimensional Earth-based climate model, has been modified to simulate the dynamics of the Venus atmosphere. The most current finite volume version of CAM is used with Earth-related processes removed, parameters appropriate for Venus introduced, and some basic physics approximations adopted. A simplified Newtonian cooling approximation has been used for the radiation scheme. We use a high resolution (1° by 1° in latitude and longitude) to take account of small-scale dynamical processes that might be important on Venus. A Rayleigh friction approach is used at the lower boundary to represent surface drag, and a similar approach is implemented in the uppermost few model levels providing a ‘sponge layer’ to prevent wave reflection from the upper boundary. The simulations generate superrotation with wind velocities comparable to those measured in the Venus atmosphere by probes and around 50-60% of those measured by cloud tracking. At cloud heights and above the atmosphere is always superrotating with mid-latitude zonal jets that wax and wane on an approximate 10 year cycle. However, below the clouds, the zonal winds vary periodically on a decadal timescale between superrotation and subrotation. Both subrotating and superrotating mid-latitude jets are found in the approximate 40-60 km altitude range. The growth and decay of the sub-cloud level jets also occur on the decadal timescale. Though subrotating zonal winds are found below the clouds, the total angular momentum of the atmosphere is always in the sense of superrotation. The global relative angular momentum of the atmosphere oscillates with an amplitude of about 5% on the approximate 10 year timescale. Symmetric instability in the near surface equatorial atmosphere might be the source of the decadal oscillation in the atmospheric state. Analyses of angular momentum transport show that all the jets are built up by poleward transport by a meridional circulation while angular momentum is redistributed to lower latitudes primarily by transient eddies. Possible changes in the structure of Venus’ cloud level mid-latitude jets measured by Mariner 10, Pioneer Venus, and Venus Express suggest that a cyclic variation similar to that found in the model might occur in the real Venus atmosphere, although no subrotating winds below the cloud level have been observed to date. Venus’ atmosphere must be observed over multi-year timescales and below the clouds if we are to understand its dynamics.  相似文献   

10.
We present a new method of measuring the Venus winds by Doppler velocimetry on the full visible spectrum of solar light scattered by the clouds. In January 2003, we carried out observations to measure the winds of Venus, using the EMILIE high-resolution, cross-dispersed spectrograph and its associated calibrating instrument the Absolute Astronomical Accelerometer (AAA), at Observatoire de Haute-Provence, France. The motivation of this type of measurements is that it measures the actual velocity of cloud particles, while the other method (track of cloud features) may be sensitive to the deformation of the clouds. During observations, Venus was near maximum western elongation, at a phase angle near 90°. The EMILIE-AAA system allows us to measure accurately the Doppler shift induced in the reflected solar spectrum by the radial component of the motion of the clouds of Venus. We present the measurements and compare them with a forward simulation of a solid super-rotation of the atmosphere of Venus. Taking into account the Doppler shift relative to the Sun and that relative to the Earth, the theoretical total Doppler shift induced in the solar spectra is easily computed as a function of the velocity of the reflecting target. A first forward simulation is computed, with a wind model considering a purely horizontal and zonal wind. The magnitude of the wind is assumed to depend on cos(latitude), as for a solid-body rotation. The comparison with the measurements at various points on the illuminated semi-disc allowed us to determine an equatorial velocity of 66, 75, 91 and 85 m/s on 4 consecutive mornings, consistent with previous ultraviolet cloud tracking wind measurements, showing that wave propagation is not a major factor in the apparent motion of the cloud marks. Further, we discuss the effect of the finite angular size of the Sun and its rapid equatorial rotation (that we call the Young effect). It mainly affects measurements taken near the terminator, where the largest discrepancies are found. These discrepancies are alleviated when the Young effect is taken into account in the model but then the retrieved Venus equatorial velocity is reduced to only 48±3 m/s. This is well below classical ultraviolet markings velocities, but the altitude at which the visible photons are scattered (66 km) that we use is 5 km below the UV markings, confirming the vertical gradient of the horizontal winds shown by previous in-situ measurements.  相似文献   

11.
Jerome Apt  Johnny Leung 《Icarus》1982,49(3):427-437
A search was made for periodic fluctuations in the thermal brightness temperatures recorded by the Pioneer Venus orbiter's infrared radiometer. Data were averaged in 10 × 10° latitude-longitude bins for each of the 72 days the instrument was in operation. This time series of thermal brightness temperatures was then analyzed to determine the amplitude of fluctuations at periods from 2 to 64 days at four levels in the atmosphere (at the cloud tops and at approximately 70, 80, and 90 km). The amplitude of such fluctuations is small at equatorial latitudes and increases to a maximum at 60–70° latitude at most altitudes. The period of the highest amplitude fluctuation is 5.3±0.4 days (at all altitudes) except at 70–80°, where a 2.9-day period which appears to correspond to the polar dipole dominates the cloud-top channel. The amplitude of the periodic fluctuations is a maximum at the cloud tops, decreasing to a minimum at the 80-km channel, and increasing again at the 90-km channel.  相似文献   

12.
J.T. Schofield  F.W. Taylor 《Icarus》1982,52(2):245-262
Improved calculations of net emission from the northern hemisphere of Venus are presented. These are based on temperature profiles, water vapor mixing ratio profiles, and cloud models retrieved in 120 solar-fixed latitude-longitude bins from infrared measurements in six spectral channels made over a period of 72 days by the orbiter infrared radiometer (OIR) instrument of the Pioneer Venus mission. Only carbon dioxide, sulfuric acid cloud, and water vapor are considered as significant sources of atmospheric opacity, and the role of the latter component is found to be minor. The sensitivity of the calculations to extreme alternative cloud models, measurement errors, and calibration errors is also discussed. Net emission is found to be only weakly dependent on latitude and longitude during the period of observation with the exception of the high-latitude polar collar region, where emission is low. Mean net emission from the northern hemisphere is 157.0 ± 6.9 W.m?2, corresponding to an equivalent temperature of 229.4 ± 2.5°K. If this figure is characteristic of the whole planet and if thermal balance is assumed, the bolometric albedo of Venus is 0.762 ± 0.011. This value is consistent with the latest estimates within experimental error.  相似文献   

13.
Venus cloud covered atmosphere offers a well-suited framework to study the coupling between the atmospheric dynamics and the structure of the cloud field. Violet images obtained during the Galileo flyby from 12 to 17 February 1990 have been analyzed to retrieve the zonal power spectra of the cloud brightness distribution field between latitudes 70° N and 50° S. The brightness distribution spectra serve as a diagnostic of the eddy kinetic energy spectrum providing indirect information about the distribution of energy along different spatial scales. We composed images covering a full rotation of the atmosphere at the level of the UV contrasted clouds obtaining maps of almost 360° that allowed us to obtain the brightness power spectra from wavenumbers k=1 to 50. A full analysis of the spectrum slope for different latitude bands and ranges of wave numbers is presented. The power spectra follow a classical law k−n with exponent n ranging from −1.7 to −2.9 depending on latitude and the wavenumber range. For the whole planet, the average of this parameter is −2.1 intermediate between those predicted by the classical turbulence theories for three- and two-dimensional motions (n=−5/3 and n=−3). A comparison with previous analysis of Mariner 10 (in 1974) and Pioneer Venus (in 1979) shows significant temporal changes in the cloud global structure and in the turbulence characteristics of the atmosphere.  相似文献   

14.
Near-infrared observations of the nightside of Venus reveal regions of high brightness temperatures. These regions of high brightness temperatures are caused by the localized evaporation of the middle and lower cloud decks, which are about 50 to 60 km above the surface of the planet. We simulate the Venus condensational middle and lower cloud deck with the University of Colorado/NASA Ames Community Aerosol and Radiation Model for Atmospheres (CARMA). Our simulated clouds have similar characteristics to the observed Venus clouds. Our radiative transfer model reproduces the observed temperature structure and atmospheric stability structure within the middle cloud region. A radiative-dynamical feedback occurs which generates mixing due to increased absorption of upwelling infrared radiation within the lower cloud region, as previously suggested by others. We find that localized variations in temperature structure or in sub-grid scale mixing cannot directly explain the longevity and optical depth of the clouds. However, vertical motions are capable of altering the cloud optical depth by a sufficient magnitude in a short enough timescale to be responsible for the observed clearings.  相似文献   

15.
The interpretation of unexpected characteristics of Pioneer Venus temperature measurements, and of the large difference between these and the Venera results, is aided by new Venus temperature profiles derived from engineering measurements of the Pioneer Venus Small-Probe Net Flux Radiometer (SNFR) instruments. To facilitate correction of a temperature-dependent radiometric response, these instruments monitored the temperatures of their deployed radiation detectors. The accurate calibration of the temperature sensors, and their strong thermal coupling to the atmosphere, make it possible to deduce atmospheric temperatures within 2°K (at most altitudes) using a simple two-component thermal model to account for lag effects. These independent temperature profiles generally confirm to high accuracy, the small-probe results of A. Seiff, D. B. Kirk, R. E. Young, R. C. Blanchard, J. T. Findlay, G. M. Kelly, and S. C. Sommer (1980a, J. Geophys. Res.85, pp. 7903–7933) concerning vertical structure and horizontal contrast in the lower atmosphere, although the stable layer below 25 km is found to be slightly more stable (by about 0.4°K/km) and absolute temperatures are an average of 2°K higher. The measured Day-Night thermal contrast is compatible with predicted responses to the diurnal variation in solar heating, except near the cloud base, where 3–5°K differences may be due to thermal radiative heating differences associated with different cloud opacities. Temperature contrasts between latitudes 30 and 60° are roughly consistent with cyclostrophic balance. But pressure and temperature measurements by the Pioneer Venus Sounder probe at 4° latitude, when compared to Small-probe results, imply unreasonably large equatorward accelerations of 100 (m/sec)/day. Poleward accelerations compatible with cyclostrophic balance can be obtained if Sounder-probe temperatures are increased by a scale-factor correction reaching 6–7°K at 13 km.  相似文献   

16.
The results of the investigation of two regions of Venus by bistatic radiolocation are presented. The experiments were carried out at wavelength λ0 = 32 cm. Maps of the distribution of reflectivity were obtained and characteristics of the relief, dielectric permittivity, soil density, and refraction attenuation in the atmosphere were measured. The value of the dispersion of small-scale slopes in the observed regions, γ, varies between 0.4 and 2.2°. There are some features on the reflectivity maps. Some of these features may correspond to mountain slopes with values in the range 2 to 8°. Corresponding changes of relief heights are contained in the interval 0.8 to 2.6 km. The features are found within the region (in the venerocentric IAU system): ?26.5 to 25.0° latitude and 220.0?239.2° longitude. One area was revealed with large values of permittivity in the range 6.5–7.5, and soil density between 2.7 and 2.9 g/cm3. The center of this area is located at ?23.5° latitude and 230.4° longitude. The extent of this region is 80 km. The results of measurements of the refraction angle and the refraction attenuation of radio waves are in good agreement with the parameters of the atmosphere of Venus received from the Soviet landers.  相似文献   

17.
基于单站短弧段光学观测的低轨卫星轨道预报算法   总被引：1，自引：0，他引：1  
基于单站短弧段的目标跟踪预报技术是保证高精度光电经纬仪在非常规环境下正常跟踪捕获目标的重要途径.构建了基于非线性滤波器的跟踪预报算法,能够在正常情况下为闭环跟踪提供引导数据,同时构建了基于非线性变换的目标预报算法,可以在无有效观测数据情况下为经纬仪提供轨迹引导,保证目标不丢失.证明了非线性滤波在单站短弧段跟踪预报算法中比常规扩展卡尔曼滤波(Extended Kalman Filter,EKF)更有效.计算表明,本文设计的非线性滤波器可以作为光学跟踪设备的引导算法,引导精度同经纬仪的随机测量精度等量级,在设备系统误差达到50"时,预报60 s的精度可达到20",仍然满足跟踪设备的视场要求.  相似文献   

18.
The position and shape of the Gegenschein’s maximum brightness provide information on the structure of the interplanetary dust cloud. We show that the asteroidal dust bands, extended near the anti-solar point, play an important role in determining both the position of the maximum brightness and the shape of the Gegenschein. After removing the asteroidal dust bands from an observation of the Gegenschein on November 2, 1997, it was found that the maximum brightness point shifted −0.4° in ecliptic latitude, i.e., to the south of the ecliptic plane, at an ecliptic longitude of 180°, in contrast to a latitude value of +0.1° when the dust bands were included. Furthermore, the part of the Gegenschein to the south of the ecliptic plane was brighter than the northern part at the time of observation. Referring to the cloud model of T. Kelsall et al. (1998, Astrophy. J. 508, 44-73), it can be estimated that the ascending node of the symmetry plane of the dust cloud is 57°−3°+7° when its inclination is 2.03° ? 0.50°.  相似文献   

19.
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   

20.
Observations of the spotted rotating G8 V star HII 296, a member of the Pleiades cluster, are presented. The star has exhibited stable periodic brightness variations for several years. The apparent rotation period of the star in our observations was 2.53 and 2.58 days. The shape and amplitude of its light curve changed from year to year. During our observational monitoring from September to November 2012, we recorded an almost complete absence of brightness variations in September. A rapid (within a few weeks) and manyfold increase in the brightness variability amplitude was observed in October and November 2012. The changes in the rotation period, amplitude, and shape of the light curve suggest an intense relative motion of spots in latitude and longitude.  相似文献   

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