Jupiter's cyclones and anticyclones vorticity from Voyager and Galileo images

We have measured the internal velocity field in jovian synoptic-scale cyclones and anticyclones by tracking cloud elements in very high spatial resolution images obtained by the Voyager 1 and 2 (in 1979) and Galileo (1996-2000) spacecrafts. In total we have studied 24 different closed vortices (6 cyclones, 18 anticyclones) spanning a latitude range from ∼60° N to 60° S and with East-West sizes larger than ∼2000 km. The tangential component of the velocity as a function of the distance to the vortex center and position angle is used to retrieve the vorticity field. We find that the velocity increases in all the vortices from a nearly quiescent center to a maximum at the vortex periphery, with a record of about 180 m s⁻¹ for the GRS. The vorticity of cyclones and anticyclones increases in general toward their periphery with absolute values in the range from ∼2-14×¹⁰⁻⁵ s⁻¹. There is a marked tendency to increase the vortices vorticity with their latitude location. However the vorticity does not depend on the vortex size, circulation sense, or ambient background meridional wind shear. The vortex Rossby number ranges from ∼0.2 to 0.5. A study of the interaction between the Great Red Spot with other vortices show that the GRS does not change its vorticity upon their absorption. The two White Ovals mergers showed contradictory results, with greater vorticity in the case of BE, but lower vorticity in the case of BA, although data are poorer for this last case. We present the case of a short lived but large coherent cyclone at −59° that was embedded in a weakly anticyclone wind shear domain. We show that jovian vortices do not follow the simple Kida vortex relationship between vorticity and aspect ratio as it has been previously suggested.     

Coupling of acoustic waves to clouds in the jovian troposphere

Seismology is the best tool for investigating the interior structure of stars and giant planets. This paper deals with a photometric study of jovian global oscillations. The propagation of acoustic waves in the jovian troposphere is revisited in order to estimate their effects on the planetary albedo. According to the standard model of the jovian cloud structure there are three major ice cloud layers (e.g., [Atreya et al., 1999. A comparison of the atmospheres of Jupiter and Saturn: Deep atmospheric composition, cloud structure, vertical mixing, and origin. Planet Space Sci. 47, 1243-1262]). We consider only the highest layers, composed of ammonia ice, in the region where acoustic waves are trapped in Jupiter's atmosphere. For a vertical wave propagating in a plane parallel atmosphere with an ammonia ice cloud layer, we calculate first the relative variations of the reflected solar flux due to the smooth oscillations at about the ppm level. We then determine the phase transitions induced by the seismic waves in the clouds. These phase changes, linked to ice particle growth, are limited by kinetics. A Mie model [Mishchenko et al., 2002. Scattering, Absorption, and Emission of Light by Small Particles. Cambridge Univ. Press, Cambridge, pp. 158-190] coupled with a simple radiation transfer model allows us to estimate that the albedo fluctuations of the cloud perturbed by a seismic wave reach relative variations of 70 ppm for a 3-mHz wave. This albedo fluctuation is amplified by a factor of ∼70 relative to the previously published estimates that exclude the effect of the wave on cloud properties. Our computed amplifications imply that jovian oscillations can be detected with very precise photometry, as proposed by the microsatellite JOVIS project, which is dedicated to photometric seismology [Mosser et al., 2004. JOVIS: A microsatellite dedicated to the seismic analysis of Jupiter. In: Combes, F., Barret, D., Contini, T., Meynadier, F., Pagani, L. (Eds.), SF2A-2004, Semaine de l'Astrophysique Francaise, Les Ulis. In: EdP-Sciences Conference Series, pp. 257-258].     

Analysis of Thermal Emission Spectrometer data using spectral EOF and tri-spectral methods

We introduce two new techniques in analyzing martian spectrally resolved radiance data obtained by the Thermal Emission Spectrometer (TES): spectral empirical orthogonal function (EOF) analysis and the tri-spectral algorithm. Spectral EOF analysis allows us to obtain the variability of spectra and associated temporal and spatial patterns. The case study with TES 20° S-20° N data shows that the first principal component (PC1) dominates the total variance and is associated with surface or near-surface brightness temperature variations. The PC2 is associated with atmospheric variability, and a negative correlation between dust and ice absorptions can be clearly seen over many regions. The annual cycle is a major component of the PC1 temporal patterns. The fingerprint of the dust storm can be clearly seen in the PC2 temporal patterns in most areas except the highlands. Spectral EOF can be used for validation of the variability of martian GCMs. The tri-spectral algorithm is based on the differences between three bands (dust, ice and a weak CO₂ absorption band) to distinguish spectra sampled in different situations: water ice cloud, dust, and surface anisothermality. We use a line-by-line radiative transfer model coupled with multiple scattering to investigate the sensitivity of this algorithm to dust and ice optical depth as well as surface emissivity. The comparisons between results of this algorithm and the TES team's retrieved dust and ice opacity are consistent over all studied periods except during the peak of the dust storm. Our algorithm is complementary to the more sophisticated TES retrieval and can be used to screen large amounts of data to get an overview.     

标准大气模型建立映射函数的可靠性讨论

随着新空间技术观测精度的不断提高，大气传播误差的研究已经成为改进观测精度的主要途径之一．为了提高大气延迟改正映射函数的计算精度，在大气剖面的选取上，近年来已经开始从过去的模型大气，逐渐地向实测大气转变．结合个别具有代表性的探空气球站观测资料，比较用标准大气模型建立的映射函数与探空气球资料路径积分的结果，研究用标准大气模型建立映射函数的可靠性，并简单讨论在映射函数中地球物理参数选择的若干问题．     

Dust deposition at the Mars Pathfinder landing site: observations and modeling of visible/near-infrared spectra

Temporal variations in the visible/near-infrared reflectance spectra of the radiometric calibration targets on the Mars Pathfinder (MPF) lander obtained by the Imager for Mars Pathfinder (IMP) camera reveal the effects of aeolian dust deposition at the MPF site throughout the mission. Sky brightness models in combination with two-layer radiative transfer models were used with these data to track changes in dust opacity on the radiometric calibration targets (RCTs) to constrain the dust deposition rate and the spectral properties of the deposited dust. Two-layer models were run assuming both linear and nonlinear dust accumulation rates, and suggest that RCT dust optical depth at the end of the 83-sol mission was 0.08 to 0.16, or on the order of 5- to 10-μm thickness for plausible values for dust porosity and grain size. These values correspond to dust fall rates of about 20-45 μm per Earth year, consistent with previous studies of dust deposition on Mars. The single scattering albedos of the dust derived from the models fall between those previously determined for atmospheric dust and bright soils. Comparisons of relative reflectance spectra calibrated using observed RCT radiances from late in the mission versus using radiances from modeled (dust-free) RCTs also reveal distinct spectral differences consistent with dust on the RCTs. Temporal variations in RCT dust opacity are not clearly linked to known passages of vortices at the MPF site, but overall suggest that deposition of dust onto the targets by local dust devils may be favored over erosion. Analyses of temporal changes in visible/near-infrared spectra will provide valuable information for future missions by constraining how dust deposition affects landed spacecraft operability (e.g., solar power availability), instrument calibration, and interpretations of surface mineralogy and composition.     

GCM simulation of balloon trajectories on Titan

Possible trajectories of passive balloons in Titan's troposphere are simulated with the instantaneous wind field predicted by a GCM (general circulation model). In most areas the basic motion of a balloon is a predominantly eastward or westward drift, depending on altitude, latitude and season of the balloon release point. Some meridional oscillation is always superposed on this basic motion, resulting in a wavy trajectory, with a maximum extent (of 40^°) at high latitudes of the winter hemisphere. As a general rule, the meridional oscillation can be maximised if a balloon is deployed in altitudes and latitudes where the mean zonal wind is eastward and smaller than the phase speed of Saturn's gravitational tide on Titan. A balloon's groundtrack does not repeat as it makes successive circuits around Titan, but rather makes a spiral or braided pattern. The summer pole is rather difficult to access for a balloon not directly introduced there because of small meridional oscillation, while the winter pole can be readily accessed and left several times. A preferred zonal drift direction can be achieved by choosing a proper altitude and hemisphere, but choosing a preferred meridional direction is not possible.     

天文高分辨像复原技术检测地基天文光学望远镜成像质量

在天文高分辨像复原技术的基础上，根据谱比法较准确地测量视宁度参数r0后，计算得到大气系统的平均短曝光传递函数，从而把大气湍流对检测结果的影响从综合成像系统中分离了出来。利用望远镜摄取大量目标源的短曝光像(斑点图)作为原始数据，通过分析这些含有望远镜衍射极限分辨率信息的斑点图，实现天文光学望远镜系统点扩展函数(PSF)中低频信息的重建，得到半峰全宽(FWHM)和80％的能量集中度(EE)。     

Sea‐level response at Nain,Labrador, to atmospheric pressure and wind

Abstract

Two years of subtidal sea‐level data from Nain, Labrador, are analysed in terms of local atmospheric pressure and the two components of geostrophic wind or stress. Frequency‐dependent response coefficients are determined by multiple regression analysis involving inversion of the cross‐spectral matrix of the inputs. At very low frequencies the response to pressure is isostatic and the wind stress coefficients are consistent with those determined by Thompson et al. (1985) from analysis of a longer series of monthly means. There is very little change in the response between icy and ice‐free seasons. The wind, or stress, coefficients correspond to geostrophic set‐up by a narrow longshore current but do not show as much of an increase of phase lag with increasing frequency as expected. The pressure response is less than isostatic and lags as the frequency increases from zero to about 0.02 cph. Possible reasons for this are discussed. Removal of wind as well as pressure effects ffom the sea‐level data makes only minor changes to the monthly mean residual sea‐level.     

Photon-stimulated desorption of Na from a lunar sample: temperature-dependent effects

We report recent results in an investigation of source mechanisms for the origin of Na atoms in tenuous planetary atmospheres, focusing on non-thermal processes. Experiments include photon stimulated desorption (PSD), electron stimulated desorption (ESD), and ion sputtering of Na atoms from the surface of a lunar basalt sample. Bombardment of the sodium covered surface by 3 keV Ar⁺ ions removes Na from the surface by sputtering into vacuum and by implantation into the sample bulk. Bombardment of the Na covered surface by ultraviolet photons or by low energy electrons (E>3 to 4 eV) causes desorption of “hot” Na atoms. These results are consistent with our previous measurements of sodium and potassium desorption from a silica surface: electron- or photon-induced charge transfer from the substrate to the ionic adsorbate causes formation of a neutral alkali atom in a repulsive configuration, from which desorption occurs. There is a strong temperature-dependence of Na ESD and PSD signals, under conditions where the Na surface coverage is constant and thermal desorption is negligible. The yield of Na (atoms/photon) increases by 10× from 100 to 470 K; an activation energy of ∼20 meV is measured. This phenomenon may be attributed to thermally-induced changes in surface bonding sites, and will affect recent modeling of the sodium atmospheres of Mercury and the Moon.     

基于迭代分形法的大气波前仿真

大口径望远镜受大气湍流的影响,光学分辨率远远小于其自身光路所决定的衍射极限。为了相应的自适应光学系统设计,首先有必要对大气波动进行仿真以提供环境数据。通常的大气波前仿真方法需要通过计算结构函数,得到功率谱函数,进而得到仿真波前,但该方法存在计算速度慢,中间变量存储空间大的问题,给大口径望远镜或者长时间仿真带来很大不便。介绍了一种可行的基于迭代分形法的波前仿真方法,复杂度达到O（N）,可以大大提高波前仿真的速度。