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This study investigated gravity waves in the Martian polar lower atmosphere using Mars Global Surveyor (MGS) radio occultation data from February 1998 to March 1999. In addition to the conventional geometrical optics method, Full Spectrum Inversion (FSI) was used to achieve a vertical resolution of ∼300 m in terms of the wavelength. In the polar night in the southern high latitude, wavelike structures with vertical wavelengths ranging from ∼1 km to several km extend from the surface to high altitudes. The positive static stability near the surface and the high terrain in the southern high latitude suggest the waves were topographically generated. The near-surface wind speed was estimated to be a few m s−1. In the midnight sun in the northern high latitude, waves show smaller amplitudes up to 10–15 km altitudes and attenuation above, which is attributable to reduced background stability. Near-neutral thin layers were ubiquitously observed in both groups, indicating the generation of turbulent layers by saturated gravity waves. It was shown that high-resolution radio occultations with radio holographic analysis offer a valuable tool for understanding atmosphere-solid planet interactions. • Gravity waves in Mars' polar atmosphere were studied using MGS radio occultation data • A radio holographic method, FSI, was used to achieve a high vertical resolution • Short-vertical wavelength waves and near-neutral thin layers were frequently observed • Orographic generation and saturation of gravity waves were suggested • Surface wind speed was estimated to be a few m s−1 from the observed wave structure [ABSTRACT FROM AUTHOR]

Planetary‐scale waves are thought to play a crucial role in the maintenance of the atmospheric circulation of Venus. They have been mainly studied using ultraviolet images. In this study, cloud top temperature oscillations associated with planetary‐scale waves were extracted from thermal images taken over 142 Earth days by the Longwave Infrared Camera onboard JAXA's Akatsuki spacecraft. A spectral analysis showed that multiple planetary‐scale waves overlap. The latitudinal structures of four distinct waves with periods of 3.6, 5.0, 5.4, and 6.1 days were studied. The 3.6‐day wave is considered to be a Kelvin wave and the other waves are considered to be hemispherically symmetric Rossby waves. Although the Rossby waves have similar periods, their latitudinal structures are different. The Rossby waves are considered to originate from different sources based on a comparison with analytical solutions. Plain Language Summary: The motion of the Venus's atmosphere is dominated by a global‐scale westward wind in a superrotation state. Momentum transport by planetary‐scale waves may help maintain the superrotation. The characteristics of such waves are not well‐known. Here, we use the cloud top temperature data taken by a thermal imager onboard JAXA's Akatsuki spacecraft to detect planetary‐scale waves at the cloud top. A spectral analysis identified multiple waves that exist simultaneously. The structures of four waves with periods of 3.6, 5.0, 5.4, and 6.1 Earth days were studied. The 3.6‐day wave will be a Kelvin wave, and the other waves will be Rossby wave. The structures of the Rossby waves were investigated based on a comparison with theoretical wave solutions. It is supposed that the observed Rossby waves originate from different wave sources. Key Points: Planetary‐scale waves were observed at the Venusian cloud top in thermal infrared images taken by the Venus orbiter AkatsukiFour distinct modes were identified in the spectra; one is attributed to a Kelvin wave and the others are attributed to Rossby wavesThe Rossby waves are thought to originate in different regions based on a comparison with analytical solutions [ABSTRACT FROM AUTHOR]

The characteristics of gravity wave packets in the Venusian atmosphere were studied using high‐vertical‐resolution temperature profiles obtained by ESA's Venus Express and JAXA's Akatsuki radio occultation experiments with radio holographic methods. Localized disturbances were detected by applying a wavelet transform to the temperature profiles. The packet lengths were found to be distributed over 0.6–10 km, in which typically 1.5–4.0 oscillations are included. The number of oscillations per wave packet was found to have only a slight dependence on the wavelength, which is consistent with the −3 power law dependence of the spectral density on the wavenumber in the saturation model. The spectral densities of the wave packets are roughly aligned with the power law of the saturation model, while the saturation ratio for each quasi‐monochromatic wave is low. This suggests that the saturated spectrum is produced by the superposition of individually unsaturated quasi‐monochromatic waves. Waves with short vertical wavelengths (<1.5 km) were found to be more prevalent at lower altitudes than at higher altitudes, implying an effect of radiative damping during upward propagation. The amplitude was found to be larger at higher latitudes, which might be attributed to an increase in background static stability at high latitudes, which allows larger saturation amplitudes. Plain Language Summary: Although gravity waves are thought to play crucial roles in transporting momentum and energy in the Venusian atmosphere, their characteristics are not well constrained from observations. Here the characteristics of Venusian gravity waves were studied by extracting wave packets from temperature profiles obtained by radio occultation experiments using Venus orbiters. The number of oscillations per wave packet was found to have only a slight dependence on the wavelength. The observed amplitudes and the vertical wavenumber spectra suggest that the superposition of quasi‐monochromatic waves causes convective instability to limit their amplitudes. The decrease of short vertical‐scale waves with altitude is thought to be caused by radiative dampling, and the increase of the amplitude at high latitudes is attributed to an effect of the background static stability. The finding that the wave amplitude is mostly limited by convective instability serves as the basis for gravity wave parameterization in numerical models. Key Points: Gravity wave packets in the Venusian atmosphere were detected in radio occultation temperature profiles using wavelet transformThe characteristics of quasi‐monochromatic wave packets were studied in terms of the consistency with the saturation modelAltitude and latitude dependences might be caused by radiative damping and the latitudinal variation of static stability, respectively [ABSTRACT FROM AUTHOR]

Superrotation is a dynamical regime where the atmosphere circulates around the planet in the direction of planetary rotation with excess angular momentum in the equatorial region. Superrotation is known to exist in the atmospheres of Venus, Titan, Jupiter, and Saturn in the solar system. Some of the exoplanets also exhibit superrotation. Our understanding of superrotation in a framework of circulation regimes of the atmospheres of terrestrial planets is in progress thanks to the development of numerical models; a global instability involving planetary-scale waves seems to play a key role, and the dynamical state depends on the Rossby number, a measure of the relative importance of the inertial and Coriolis forces, and the thermal inertia of the atmosphere. Recent general circulation models of Venus's and Titan's atmospheres demonstrated the importance of horizontal waves in the angular momentum transport in these atmospheres and also an additional contribution of thermal tides in Venus's atmosphere. The atmospheres of Jupiter and Saturn also exhibit strong superrotation. Recent gravity data suggests that these superrotational flows extend deep into the planet, yet currently no single mechanism has been identified as driving this superrotation. Moreover, atmospheric circulation models of tidally locked, strongly irradiated exoplanets have long predicted the existence of equatorial superrotation in their atmospheres, which has been attributed to the result of the strong day-night thermal forcing. As predicted, recent Doppler observations and infrared phase curves of hot Jupiters appear to confirm the presence of superrotation on these objects. [ABSTRACT FROM AUTHOR]

Abstract: Radio occultation (RO) is one of the most efficient techniques for studying fine vertical structures in planetary atmospheres. However, the geometrical optics (GO) method, which has been used for the analysis of RO data, suffers blurring by the finite width (Fresnel scale) of the radio ray and cannot decipher multipath propagation, which also prevents retrieval of fine structures. Here we apply Full Spectrum Inversion (FSI), which is one of the radio holographic methods, to RO data taken in Venus Express and Akatsuki missions to retrieve fine structures in Venus' cloud‐level atmosphere. The temperature profiles obtained by FSI achieve vertical resolutions of ~150 m, which is much higher than the typical resolution of 400–700 m in GO, and resolve structures in multipath regions. Thin, near‐neutral layers are found to be ubiquitous at cloud heights; we suggest here that those layers are caused by the mixing associated with the breaking of short‐wavelength gravity waves. The wavenumber spectra of small‐scale structures are consistent with the semiempirical spectrum of saturated gravity waves and show larger amplitudes at higher latitudes. Temperature profiles in the high latitudes frequently show a sharp temperature minimum near the cloud top, below which the vertical temperature gradient is near adiabat, implying that the sharp temperature minimum is created by adiabatic cooling associated with convective plumes that impinge on the overlying stable layer. [ABSTRACT FROM AUTHOR]

The first year (December 2015 to November 2016) of IR2 after Akatsuki's successful insertion to an elongated elliptical orbit around Venus is reported with performance evaluation and results of data acquisition. The single-stage Stirling-cycle cryo-cooler of IR2 has been operated with various driving voltages to achieve the best possible cooling under the given thermal environment. A total of 3091 images of Venus (1420 dayside images at 2.02 μm and 1671 night-side images at 1.735, 2.26, and 2.32 μm) were acquired in this period. Additionally, 159 images, including images of stars for calibration and dark images for the evaluation of noise levels, were captured. Low-frequency flat images (not available in pre-launch calibration data) have been constructed using the images of Venus acquired from near the pericenter to establish the procedure to correct for the IR2 flat-field response. It was noticed that multiple reflections of infrared light in the PtSi detector caused a weak but extended tail of the point-spread function (PSF), contaminating the night-side disk of Venus with light from the much brighter dayside crescent. This necessitated the construction of an empirical PSF to remove this contamination and also to improve the dayside data by deconvolution, and this work is also discussed. Detailed astrometry is performed on star-field images in the H-band (1.65 μm), hereby confirming that the geometrical distortion of IR2 images is negligible. [ABSTRACT FROM AUTHOR]

Vertical wavenumber spectra of Martian gravity waves were obtained for the altitude range 3-32 km from temperature profiles acquired by the Mars Global Surveyor (MGS) radio occultation experiments. The spectra, which cover vertical wavelengths from 2.5 to 15 km, generally show a decline of the spectral density with wavenumber similar to those obtained in the terrestrial stratosphere and mesosphere. The power-law spectral index is typically around -3 except near the low-wavenumber end, and the spectra frequently lie along the theoretical spectrum for saturated gravity waves developed for Earth's atmosphere. The results suggest that gravity wave saturation occurs in the atmosphere of Mars as well as that of Earth. [ABSTRACT FROM AUTHOR]

Abstract: 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 ∼0.3ms−1 per Earth day. [Copyright &y& Elsevier]

Abstract: The Venus Climate Orbiter mission (PLANET-C), one of the future planetary missions of Japan, aims at understanding the atmospheric circulation of Venus. Meteorological information will be obtained by globally mapping clouds and minor constituents successively with four cameras at ultraviolet and infrared wavelengths, detecting lightning with a high-speed imager, and observing the vertical structure of the atmosphere with radio science technique. The equatorial elongated orbit with westward revolution fits the observation of the movement and temporal variation of the atmosphere which as a whole rotates westward. The systematic, continuous imaging observations will provide us with an unprecedented large data set of the Venusian atmospheric dynamics. Additional targets of the mission are the exploration of the ground surface and the observation of zodiacal light. The mission will complement the ESA''s Venus Express, which also explores the Venusian environment with different approaches. [Copyright &y& Elsevier]

Wavenumber spectra of the atmospheric potential energy of Mars at mesoscales (wavelengths of 64–957 km) were obtained as a function of latitude, season, and Martian year using infrared radiance data obtained by the Thermal Emission Spectrometer (TES) onboard the Mars Global Surveyor (MGS) spacecraft. Spectral slopes tend to be flatter at smaller scales, and the slopes are usually flatter than -1 near small-scale ends. Near large-scale ends, the spectra sometimes show prominent steepening with slopes from -2 to -3. The power peaks in the high latitudes in winter and equinoxes, suggesting that eddies are generated preferentially in baroclinic zones. The seasonal variation at each latitude band, on the other hand, tends to be obscured by large interannual variability. An enhancement in the power was observed around the storm tracks in the Southern Hemisphere. Spectra of the terrestrial stratosphere were also obtained with a similar method from data taken by the Aura satellite and compared to the results for Mars. [ABSTRACT FROM AUTHOR]

It is shown that planetary-scale waves are inherently accompanied by latitudinal momentum transport when they propagate vertically in vertically sheared zonal flows. Because of the dependence of the wave's latitudinal scale on the intrinsic phase speed, positive (negative) vertical shear should force prograde (retrograde) waves to focus equatorward and retrograde (prograde) waves to expand poleward in the course of upward propagation. Consequently, Eliassen–Palm (EP) flux vectors are tilted from the vertical and nonzero latitudinal momentum fluxes occur. The direction of momentum transport should always be equatorward (poleward) in positive (negative) vertical shear irrespective of the zonal propagation direction. The idea was applied to upwardly propagating waves in the Venusian middle atmosphere, where vertical shear of strong midlatitude jets and equatorial superrotation exist. Numerical solutions showed that Kelvin and prograde inertio-gravity waves focus equatorward and mixed Rossby–gravity and Rossby waves expand poleward below the cloud top. The former is attributed primarily to the vertical shear of the superrotation, while the latter to the vertical shear beneath the midlatitude jets. Such characteristics of planetary-scale waves will cause angular momentum separation between high and low latitudes and, at least partly, contribute to the maintenance of the superrotation. [ABSTRACT FROM AUTHOR]