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Venus was the first of the planets of the Solar System in the atmosphere of which electrical phenomena similar to lightning in the Earth's atmosphere were discovered. Electrical discharges (lightning in the atmosphere of Venus) were discovered in 1978 in the Venera-12, -11, and Pioneer Venus missions, based on their electromagnetic radiation. The paradox, however, was that the search for optical flares remained unsuccessful for the past 40 years. In 2015, the Akatsuki spacecraft of the Japan Aerospace Exploration Agency was launched into orbit Venus satellite. It was designed to search for lightning and other studies of the meteorology of Venus by recording radiation in selected spectral ranges. In 2016, the Akatsuki orbiter successfully performed detailed observations of Venus in the near-infrared range in the "transparency windows" of the planet's atmosphere, as well as in the ultraviolet and other ranges. The article presents the results of an alternative search for and successful detection of lightning flashes according to the Akatsuki project, but not in the ultraviolet or visible ranges, but in the near-IR range. A comparison of the results of calculations based on models of terrestrial lightning with the results of measurements made by the IR2 camera of the Akatsuki mission on Venus at a wavelength of 2.26 μm shows a close agreement between the experimental and calculated data. [ABSTRACT FROM AUTHOR]

Images of Venusian clouds taken at ultraviolet (UV) wavelengths frequently exhibit mesoscale cellular structures at low latitudes. To obtain clues regarding the origin of these structures, we studied the temporal variation of the mesoscale UV contrast using 283‐ and 365‐nm images taken by the ultraviolet imager (UVI) onboard the Akatsuki spacecraft. The 283‐nm channel is mostly affected by SO2 at the cloud top and the 365‐nm channel is mostly affected by an unknown absorber, though both absorbers are thought to influence both wavelengths. We defined the mesoscale as a spatial scale roughly below 1,000 km, where the effect of large‐scale streaky patterns is not significant according to the spatial spectra of 283‐nm images taken by UVI and 2.02‐μm images taken by the near‐infrared camera (IR2) onboard Akatsuki. We found that the temporal variation of the standard deviation of the mesoscale UV contrast is dominated by quasi‐periodic oscillations with amplitudes of 30%–50% relative to the mean value and periods of around 4–5 Earth days. Oscillations of the planetary‐scale UV contrast with similar periods were also observed, suggesting that the mesoscale structures are influenced by planetary‐scale waves. The mesoscale contrast tends to be enhanced when the background albedo decreases in the course of the propagation of planetary‐scale waves. This correlation suggests that the solar heating near the cloud top level feeds the mesoscale dynamics. An enhancement of the mesoscale contrast in the afternoon was seen at a wavelength of 365 nm. Plain Language Summary: On Venus, the brightness pattern of the cloud top shows signs of convection, where heat is transported either upward or horizontally through the fluid motion. These processes could have a large impact on material and energy transportation on the planet. We examined ultraviolet (UV) cloud images captured by the Akatsuki Venus orbiter to track changes in contrast over time at both a planetary and mesoscale (below 1,000 km). Unlike visible wavelengths, Venus' clouds display changes in brightness over time and space when viewed at UV wavelengths. The UV contrast at the mesoscale was found to oscillate with periods of around 4–5 Earth days. Similar periods of oscillation are observed in the planetary‐scale brightness variation, suggesting that planetary‐scale waves influence the motion at a scale below 1,000 km. The contrast at this scale tends to be enhanced when the background becomes darker. This relation implies that the heating of clouds by the Sun enhances vertical or horizontal inhomogeneity of the temperature and then powers the observed convection. Key Points: The temporal variation of the mesoscale ultraviolet (UV) contrast of Venusian clouds was studied using images taken by the UV camera on AkatsukiThe mesoscale contrast was found to oscillate with periods of 4–5 Earth days, which are also the periods of planetary‐scale wavesThe mesoscale contrast tends to be enhanced when the background becomes darker, suggesting that solar heating feeds the mesoscale dynamics [ABSTRACT FROM AUTHOR]

Abstract Impacts of temperature assimilation on Venusian thermal tides are investigated by the observing system simulation experiments assuming Akatsuki Longwave Infrared Camera (LIR) observations. Synthetic temperature data are prepared by a Venusian general circulation model (VGCM) to test if LIR temperature data resolves a discrepancy in the structure of thermal tides between observations and the VGCM. They are assimilated at 70 km altitude with several combinations of frequency and horizontal region. The result shows that the three-dimensional structure of thermal tides is significantly improved not only in temperature but also in horizontal wind, even if observations are available only at a limited frequency of 6-hourly or on the dayside. The zonal–mean zonal wind and temperature fields are also modified at 60–80 km altitudes globally through the vertical momentum transport of thermal tides. It would be promising to assimilate Akatsuki LIR observations to produce realistic objective analysis of the Venus atmosphere.

The characteristic features of different layers in the Venus ionosphere during the deep solar minimum of the solar cycle 24 have been studied using a radio science experiment onboard the Akatsuki spacecraft. Radio signals from the spacecraft were tracked at the Indian deep space network, Bangalore; Usuda Deep Space Center, Japan; and DLR Ground station, Weilheim, Germany. The orbital geometry of the spacecraft provides rare opportunities to probe the Venusian ionosphere and atmosphere in the equatorial region at low solar zenith angles (SZAs). The height of the peak plasma density of the V2 layer (∼141 km) was seen to remain almost constant for the SZA ≤ 90° while the peak density was the least among the published results. Features of the V1 layer (∼125 km) agree well with the Venus Express radio occultation measurements. All three types of V1 layers were observed in Akatsuki observations. The V0 layer (∼110 km) was observed in about 15% of cases and was seen to occur irrespective of SZA and geographical constraints. Nighttime occurrence of the ionosphere was very limited despite regular observations. Plain Language Summary: The planetary ionosphere forms by the ionization of available neutrals via various ionization sources, like solar radiations, and solar energetic particles. Depending upon the neutral composition and solar flux, different layers of the ionosphere form at different heights. Based on the ion distribution, the Venusian ionosphere is divided into four layers, namely V3, V2, V1, and V0. The impact of low solar activity conditions on the characteristics of these layers has been studied using the radio science payload onboard the Akatsuki orbiter, which is truly the first near‐equatorial orbiting satellite. Observations show a clear impact of the low solar activity period on its primary (V2) and secondary (V1) peak electron densities which are the minimum reported so far. The heights corresponding to these peaks do not vary with respect to the solar zenith angle, unlike on Earth. A major highlight of observation is the presence of the V0 layer at 110 ± 4 km altitude. Earlier it was observed to be sporadic in nature occurring mostly during morning and evening hours. The nightside ionosphere has been rarely observed despite good coverage, which helps to surmise that no discernible electron density is transported from dayside to nightside during the profound solar minima. Key Points: First regular observations of Venus ionosphere in the equatorial regionMarginal variations in the peak V2 and V1 heights with respect to solar zenith angle, with the peak plasma density being the minimum reported so farLimited nighttime occurrence of ionosphere despite regular observations [ABSTRACT FROM AUTHOR]

Impacts of temperature assimilation on Venusian thermal tides are investigated by the observing system simulation experiments assuming Akatsuki Longwave Infrared Camera (LIR) observations. Synthetic temperature data are prepared by a Venusian general circulation model (VGCM) to test if LIR temperature data resolves a discrepancy in the structure of thermal tides between observations and the VGCM. They are assimilated at 70 km altitude with several combinations of frequency and horizontal region. The result shows that the three-dimensional structure of thermal tides is significantly improved not only in temperature but also in horizontal wind, even if observations are available only at a limited frequency of 6-hourly or on the dayside. The zonal–mean zonal wind and temperature fields are also modified at 60–80 km altitudes globally through the vertical momentum transport of thermal tides. It would be promising to assimilate Akatsuki LIR observations to produce realistic objective analysis of the Venus atmosphere. [ABSTRACT FROM AUTHOR]

The differences and similarities in morphologies acquired across various wavelengths in simultaneously obtained Venus' dayside images may provide clues on cloud/atmospheric physics and chemistry. Here, we focus on spatial scales smaller than ∼600 km, where cell‐like or streaky features seem to dominate in ultraviolet images. Using images acquired at ultraviolet wavelengths of 283 nm, 365 nm, and infrared wavelengths of 0.90, 2.02, and 10 μm by the Venus orbiter Akatsuki, correlation coefficient maps between pairs of wavelengths, such as 2.02 μm/10 μm, 2.02 μm/283 nm, 2.02 μm/365 nm, 283 nm/365 nm, and 0.90 μm/2.02 μm were created. Our results show a clear negative correlation between images obtained at 2.02 μm (CO2 absorption) and 10 μm (cloud top temperature), meaning that elevated clouds are cooled by adiabatic expansion or the ambient air. A clear negative correlation was found between 2.02 μm and 283 nm (SO2 absorption), suggesting that SO2 is transported to the cloud top region during cloud ascent. We observed a clear positive correlation between images obtained at 283 and 365 nm (unknown absorber), implying a close relationship between the unknown absorber and SO2 or a non‐negligible contribution of the unknown absorber at 283 nm. We found a low correlation between images obtained at 0.90 μm (middle/lower cloud) and 2.02 μm, suggesting a weak vertical coupling in the clouds. Plain Language Summary: Sulfuric acid clouds cover the whole globe of Venus at altitudes of 50–70 km. These clouds show various patterns at ultraviolet and infrared wavelengths. The physical/chemical processes that create the cloud patterns are not fully known. Atmospheric variables such as cloud height and temperature can be mapped at various wavelengths. The relationship between the spatial patterns at these wavelengths provides crucial information on the processes that create such cloud features. We used image data obtained at multiple wavelengths by the Venus orbiter Akatsuki to investigate the correlation of small‐scale (<600 km) cloud patterns acquired at various wavelengths, namely 283 nm (SO2 abundance), 365 nm (the unknown absorber), 0.9 μm (middle/lower cloud), 2.02 μm (cloud top altitude), and 10 μm (cloud top temperature). The correlations among these wavelengths place new constraints on the processes responsible for the cloud production. For example, a negative correlation was found between 2.02 μm and 283 nm, suggesting that SO2 is transported to the cloud top region during cloud ascent. Key Points: We performed a statistical analysis on the correlation of small‐scale patterns on the Venus dayside acquired at various wavelengthsSignificant correlation was found between the ultraviolet absorption, the cloud top altitude and the cloud top temperatureWe propose that vertical motions in the upper cloud region play an important role in creating small‐scale patterns [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]

Abstract The existence of lightning discharges in the Venus atmosphere has been controversial for more than 30 years, with many positive and negative reports published. The lightning and airglow camera (LAC) onboard the Venus orbiter, Akatsuki, was designed to observe the light curve of possible flashes at a sufficiently high sampling rate to discriminate lightning from other sources and can thereby perform a more definitive search for optical emissions. Akatsuki arrived at Venus during December 2016, 5 years following its launch. The initial operations of LAC through November 2016 have included a progressive increase in the high voltage applied to the avalanche photodiode detector. LAC began lightning survey observations in December 2016. It was confirmed that the operational high voltage was achieved and that the triggering system functions correctly. LAC lightning search observations are planned to continue for several years.

Abstract 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 The Venus Climate Orbiter Akatsuki arrived at Venus in December 2015, and the Longwave Infrared Camera (LIR) onboard the spacecraft started making observations. LIR has acquired more than 8000 images during the first two Venusian years since orbit insertion without any serious faults. However, brightness temperature derived from LIR images contained an unexpected bias that related not to natural phenomena but to a thermal condition of the instrument. The bias could be partially eliminated by keeping the power supply unit for LIR always active, while the residual bias was simply correlated with the baffle temperature. Therefore, deep-space images were acquired at different baffle temperatures on orbit, and a reference table for eliminating the bias from images was prepared. In the corrected images, the brightness temperature was ~ 230 K at the center of the Venus disk, where the effect of limb darkening is negligible. The result is independent of the baffle temperature and consistent with the results of previous studies. Later, a laboratory experiment with the proto model of LIR showed that when the germanium (Ge) lens was heated, its actual temperature was slightly higher than the temperature measured by a thermal sensor attached to the lens holder. The experiment confirmed that transitory baffle heating accounted for the background bias found in the brightness temperature observed by LIR.

Abstract After the arrival of Akatsuki spacecraft of Japan Aerospace Exploration Agency at Venus in December 2015, the radio occultation experiment, termed RS (Radio Science), obtained 19 vertical profiles of the Venusian atmosphere by April 2017. An onboard ultra-stable oscillator is used to generate stable X-band downlink signals needed for the experiment. The quantities to be retrieved are the atmospheric pressure, the temperature, the sulfuric acid vapor mixing ratio, and the electron density. Temperature profiles were successfully obtained down to ~ 38 km altitude and show distinct atmospheric structures depending on the altitude. The overall structure is close to the previous observations, suggesting a remarkable stability of the thermal structure. Local time-dependent features are seen within and above the clouds, which is located around 48–70 km altitude. The H2SO4 vapor density roughly follows the saturation curve at cloud heights, suggesting equilibrium with cloud particles. The ionospheric electron density profiles are also successfully retrieved, showing distinct local time dependence. Akatsuki RS mainly probes the low and middle latitude regions thanks to the near-equatorial orbit in contrast to the previous radio occultation experiments using polar orbiters. Studies based on combined analyses of RS and optical imaging data are ongoing. Graphical abstract .

Planetary‐scale waves at the Venusian cloud‐top cause periodic variations in both winds and ultraviolet (UV) brightness. While the wave candidates are the 4‐day Kelvin wave and 5‐day Rossby wave with zonal wavenumber 1, their temporal evolutions are poorly understood. Here we conducted a time series analysis of the 365‐nm brightness and cloud‐tracking wind variations, obtained by the UV Imager onboard the Japanese Venus Climate Orbiter Akatsuki from June to October 2017, revealing a dramatic evolution of planetary‐scale waves and corresponding changes in planetary‐scale UV features. We identified a prominent 5‐day periodicity in both the winds and brightness variations, whose phase velocities were slower than the dayside mean zonal winds (or the super‐rotation) by >35 m/s. The reconstructed planetary‐scale vortices were nearly equatorially symmetric and centered at ~35° latitude in both hemispheres, which indicated that they were part of a Rossby wave. The amplitude of wind variation associated with the observed Rossby wave packet was amplified gradually over ~20 days and attenuated over ~50 days. Following the formation of the Rossby wave vortices, brightness variation emerges to form rippling white cloud belts in the 45–60° latitudes of both hemispheres. An ~3.8‐day periodic signals were observed in the zonal wind and brightness variations in the equatorial region before the Rossby wave amplification. Although the amplitude and significance of the 3.8‐day mode were relatively low in the observation season, this feature is consistent with a Kelvin wave, which may be the cause of the dark clusters in the equatorial region.Plain Language Summary: The Earth's twin planet Venus is mysterious for the fast atmosphere circulation called as the super‐rotation. The cloud top atmosphere rotates around the planet with 100 m/s, which corresponds to ~4‐day circulation. There are many types of atmospheric waves, which are crucial for understanding how the wind blows on the planet. In this study, we analyzed one thousand Venus ultraviolet (UV) images taken by Japanese Venus Climate Orbiter Akatsuki and first captured the continuous temporal evolution of a planetary‐scale wave with a period of 5 days. The 5‐day wave had large equatorially symmetric vortices at the cloud top. According to the wave evolution, the shape of the dark UV cloud (absorption) features dramatically changed over the planet. Since the UV absorption is important for the radiative energy balance, we open the door for the future investigations of the long‐term impacts of planetary‐scale waves to the atmospheric dynamics and chemistry.Key Points: Periodic variations in winds and UV brightness observed by Akatsuki were analyzed to study planetary‐scale waves at the Venusian cloud topThe temporal evolution of Rossby and Kelvin waves was captured for the first timeThe relation between the waves in the winds and UV brightness is described [ABSTRACT FROM AUTHOR]

Longwave Infrared Camera (LIR) on board Akatsuki first revealed the global structure of the thermal tides in the upper cloud layer of Venus. The data were acquired over three Venusian years, and the analysis was done over the areas from the equator to the midlatitudes in both hemispheres and over the whole local time. Thermal tides at two vertical levels were analyzed by comparing data at two different emission angles. Dynamical wave modes consisting of tides were identified; the diurnal tide consisted mainly of Rossby‐wave and gravity‐wave modes, while the semidiurnal tide predominantly consisted of a gravity‐wave mode. The revealed vertical structures were roughly consistent with the above wave modes, but some discrepancy remained if the waves were supposed to be monochromatic. In turn, the heating profile that excites the tidal waves can be constrained to match this discrepancy, which would greatly advance the understanding of the Venusian atmosphere. Plain Language Summary: On Venus, the atmosphere circulates 60 times faster than the solid body of Venus; this phenomenon is called "superrotation," and it is one of the mysteries of the Venusian atmosphere. To maintain the fast circulation, thermal tides, which are global‐scale atmospheric waves excited by solar heating, have been considered a very important candidate because they have the ability of accelerating the atmosphere through propagating. A midinfrared camera onboard the Japanese Venus orbiter, Akatsuki, can capture temperature perturbations due to the thermal tides in the upper cloud level (60‐ to 70‐km altitude), and it revealed their global and vertical structures with a long‐term observation (more than three Venusian years) for the first time. Interestingly, we found that the location of the maximum temperature at the cloud top level was different from noon where solar energy input is at a maximum. In addition, the location was shifted toward the morning side as the sensing altitude increased. This finding is an evidence of the vertical traveling of the thermal tides, indicating the wave's atmospheric acceleration. Key Points: Akatsuki/LIR revealed the global structures of thermal tides across the equator in the upper cloud layer of Venus for the first timeUsing the emission angle dependence of LIR's sensing altitude, upward propagation of the semidiurnal tide was confirmedWave types consisting of the thermal tides were identified [ABSTRACT FROM AUTHOR]

Observations by the Lightning and Airglow Camera on Japan's Venus Climate Orbiter "Akatsuki" over its first 3 years in orbit are reported. Forty‐two opportunities during low‐altitude nightside passes have accumulated 16.8 hr of observation, yielding an area‐time product of 81.6 ×106 km2‐hr, by far the largest at Venus itself to date. No flashes attributable to lightning have been detected, whereas similar observations at Earth would yield thousands of detections. A low flash rate of ~0.005 per million km2‐hr indicated in ground‐based observations is not excluded (but would require that there are not many more smaller flashes). The allowable flash rate is incompatible with the much higher rates of bursts recorded by magnetic and electric field sensors at Venus, indicating that electrical discharges at Venus lack optical emission or that the electromagnetic detections have a nonlightning explanation or both. Key Points: Nondetection in 42 observations by a dedicated flash detector on Akatsuki suggests a Venus lightning rate >~1,000 lower than EarthResult is compatible with zero flashes, with claimed ground‐based detections or with lightning that is highly clustered in space or timeNondetection is incompatible with the rate of whistler mode radio waves, suggesting that their origin is not associated with optical flashes [ABSTRACT FROM AUTHOR]

Stationary features indicative of topographic gravity waves were identified at the cloud top of Venus with the 283‐nm channel of the Ultraviolet Imager (UVI) onboard Akatsuki, and their geographical and local time dependences were studied. At this wavelength the absorption by SO2 dominates. To extract stationary structures with respect to the surface, we averaged multiple images to smooth out moving features and applied high‐pass filtering to emphasize small structures. We found that stationary features appear exclusively above highlands and that they tend to appear between noon and evening. The stationary features seem to be synchronized with those observed in the cloud top temperature maps taken by the Longwave Infrared Camera (LIR). It was shown using a gravity wave model that the scale height of SO2 should be smaller than that of the cloud around the cloud top to reproduce the observed phase relationship between the stationary features seen in the Ultraviolet Imager and Longwave Infrared Camera images. Plain Language Summary: Topographic gravity waves, which are atmospheric waves created by winds flowing over mountains, are found to be ubiquitous in the Venus atmosphere. These waves are expected to play crucial roles in the momentum balance of the global‐scale atmospheric circulation. The spatial and temporal variabilities of such waves were studied using 283‐nm images taken by an ultraviolet camera onboard the Venus orbiter Akatsuki. To extract cloud structures related to topographic gravity waves that are stationary with respect to the surface, multiple images were averaged to smooth out moving features. Topographic waves were found to appear above highlands from noon to evening. The vertical distribution of SO2, which is the main absorber at this wavelength, around the cloud top was constrained from the observed wave features based on the modeling of wave‐induced transport. Key Points: Stationary features were observed at the cloud top of Venus in the ultraviolet and attributed to topographic gravity wavesThe features appear above highlands and between noon and the eveningThe scale height of SO2 was constrained based on modeling of the observed features [ABSTRACT FROM AUTHOR]

Temperature profiles of the Venus atmosphere obtained by the Akatsuki radio occultation measurements showed a prominent local time dependence above 65‐km altitude at low latitudes equatorward of 35°. A zonal wavenumber 2 component is predominant in the temperature field, and its phase (i.e., isothermal) surfaces descend with local time, suggesting its downward phase propagation. A general circulation model (GCM) for the Venus atmosphere, AFES‐Venus, reproduced the local time‐dependent thermal structure qualitatively consistent with the radio occultation measurements. Based on a comparison between the radio occultation measurements and the GCM results, the observed zonal wavenumber 2 structure is attributed to the semidiurnal tide. Applying the dispersion relationship for internal gravity waves to the observed wave structure, the zonally averaged zonal wind speed at 75‐ to 85‐km altitudes was found to be significantly smaller than that at the cloud top. The decrease of the zonal wind speed with altitude is attributed to the momentum deposition by the upwardly propagating semidiurnal tide excited in the cloud layer. Plain Language Summary: Akatsuki radio occultation measurements showed the local time dependence of the Venus atmosphere in the equatorial region. By comparing the measurements with a general circulation model, it is attributed to the upward propagating semidiurnal tide generated in the cloud layer. And then, we proposed a new method to estimate the zonal wind speed above the cloud layer, where any optical instruments cannot be measured, for the first time. Key Points: Akatsuki RO measurements show the local time dependence of the thermal structure of the Venus upper atmosphere in the equatorial regionComparison of RO measurements and the GCM suggests the importance of semidiurnal tide to the local time dependencyA new method to estimate the zonal wind speed in the upper atmosphere in the equatorial region is proposed [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]

Abstract: The existence of large stationary gravity waves was discovered during Akatsuki's first observation sequence in 2015. In this study, the further detection of large stationary gravity waves in brightness temperature images over a 1.5 year period is reported. The waves periodically appeared mostly above four specific highland regions in the low latitudes when these regions were in the local afternoon. The wave amplitudes attenuated after the wave locations passed beyond the evening terminator, and the locations of the waves tended to slowly drift eastward over their lifetimes. The appearances of stationary waves depend not only on surface topography but also on latitude and local time, suggesting that solar heating during the daytime and atmospheric structure affected by solar heating may control the excitation and propagation of stationary waves. [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]

The Venus Climate Orbiter Akatsuki arrived at Venus in December 2015, and the Longwave Infrared Camera (LIR) onboard the spacecraft started making observations. LIR has acquired more than 8000 images during the first two Venusian years since orbit insertion without any serious faults. However, brightness temperature derived from LIR images contained an unexpected bias that related not to natural phenomena but to a thermal condition of the instrument. The bias could be partially eliminated by keeping the power supply unit for LIR always active, while the residual bias was simply correlated with the baffle temperature. Therefore, deep-space images were acquired at different baffle temperatures on orbit, and a reference table for eliminating the bias from images was prepared. In the corrected images, the brightness temperature was ~ 230 K at the center of the Venus disk, where the effect of limb darkening is negligible. The result is independent of the baffle temperature and consistent with the results of previous studies. Later, a laboratory experiment with the proto model of LIR showed that when the germanium (Ge) lens was heated, its actual temperature was slightly higher than the temperature measured by a thermal sensor attached to the lens holder. The experiment confirmed that transitory baffle heating accounted for the background bias found in the brightness temperature observed by LIR. [ABSTRACT FROM AUTHOR]

After the arrival of Akatsuki spacecraft of Japan Aerospace Exploration Agency at Venus in December 2015, the radio occultation experiment, termed RS (Radio Science), obtained 19 vertical profiles of the Venusian atmosphere by April 2017. An onboard ultra-stable oscillator is used to generate stable X-band downlink signals needed for the experiment. The quantities to be retrieved are the atmospheric pressure, the temperature, the sulfuric acid vapor mixing ratio, and the electron density. Temperature profiles were successfully obtained down to ~ 38 km altitude and show distinct atmospheric structures depending on the altitude. The overall structure is close to the previous observations, suggesting a remarkable stability of the thermal structure. Local time-dependent features are seen within and above the clouds, which is located around 48-70 km altitude. The HSO vapor density roughly follows the saturation curve at cloud heights, suggesting equilibrium with cloud particles. The ionospheric electron density profiles are also successfully retrieved, showing distinct local time dependence. Akatsuki RS mainly probes the low and middle latitude regions thanks to the near-equatorial orbit in contrast to the previous radio occultation experiments using polar orbiters. Studies based on combined analyses of RS and optical imaging data are ongoing. [ABSTRACT FROM AUTHOR]

AKATSUKI is the Japanese Venus Climate Orbiter that was designed to investigate the climate system of Venus. The orbiter was launched on May 21, 2010, and it reached Venus on December 7, 2010. Thrust was applied by the orbital maneuver engine in an attempt to put AKATSUKI into a westward equatorial orbit around Venus with a 30-h orbital period. However, this operation failed because of a malfunction in the propulsion system. After this failure, the spacecraft orbited the Sun for 5 years. On December 7, 2015, AKATSUKI once again approached Venus and the Venus orbit insertion was successful, whereby a westward equatorial orbit with apoapsis of ~440,000 km and orbital period of 14 days was initiated. Now that AKATSUKI's long journey to Venus has ended, it will provide scientific data on the Venusian climate system for two or more years. For the purpose of both decreasing the apoapsis altitude and avoiding a long eclipse during the orbit, a trim maneuver was performed at the first periapsis. The apoapsis altitude is now ~360,000 km with a periapsis altitude of 1000-8000 km, and the period is 10 days and 12 h. In this paper, we describe the details of the Venus orbit insertion-revenge 1 (VOI-R1) and the new orbit, the expected scientific information to be obtained at this orbit, and the Venus images captured by the onboard 1-µm infrared camera, ultraviolet imager, and long-wave infrared camera 2 h after the successful initiation of the VOI-R1. [ABSTRACT FROM AUTHOR]

Bow-shaped stationary gravity waves (mountain waves) have been observed in Venusian cloud images taken by the cameras onboard the Akatsuki spacecraft. The waves are most pronounced in the thermal infrared images taken by the Longwave Infrared Camera (LIR), and have been shown to appear at longitudes where high mountains exist. To better characterize mountain waves in the Venusian atmosphere, we applied a new analysis method to the LIR data. By averaging multiple sequentially obtained images in the geographical coordinates, noise and drifting features were suppressed and stationary features were emphasized, thereby identifying even weak wave features whose amplitudes were smaller than those identified in previous studies. The waves were observed in six highland regions, and two of these wave features have not been previously reported. Each wave packet extends both downstream and upstream by thousands of kilometers from the expected source region. Such characteristics might be attributed to the trapping and horizontal propagation of mountain waves in the low-static stability layers below the clouds. • Mountain waves at Venusian cloud tops observed by a thermal infrared camera onboard the Venus orbiter Akatsuki. • Detection of weak wave features whose amplitudes were smaller than those identified in previous studies • Extension of each wave packet both downstream and upstream by thousands of kilometers from the expected source region • Trapping and horizontal propagation of mountain waves in the low-static stability layers below the clouds [ABSTRACT FROM AUTHOR]

Abstract: Recent years have seen a renewed interest in exploration of the interior of the solar system. A number of missions are currently under way, in planning as well as in space, with the primary goal to expand our knowledge on the planets Mercury and Venus. Chemical propulsion missions to Mercury in particular require an extended cruise phase prior to arrival at their destination, usually involving multiple planetary fly-by manoeuvres and many revolutions in heliocentric orbit. The difficulties in discovering and tracking small objects interior to Earth's orbit, mainly due to unfavourable viewing geometry as well as atmospheric interference, have long been noted by the solar system science and planetary defence communities. Space probes in the interior of the solar system are in a position to observe objects near or interior to Earth's orbit in favourable opposition geometry. They are also usually free from planet-related interference, at least while in cruise, and often can be while in planetary eclipse. Dedicated search and survey missions to look for Near and Inner Earth Objects (NEO, IEO) from the vicinity of Earth or low Earth orbit are being planned. In this article, the ad-hoc available as well as near-term planned in-situ capabilities of the optical instrument payloads of space probes to Venus and Mercury are compiled from publications by the respective instrument teams. The small-object detection capabilities of cameras and spectrographs in opposition geometry are estimated by a common method, using data from comparable instruments to supplement missing information where necessary. The on-board cameras are classified according to their small-object detection potential in a technology demonstration of asteroid detection from a heliocentric orbit substantially interior to Earth's. [Copyright &y& Elsevier]

We present new wind velocities and its variability at Venus cloud-tops (≈70 km) based on coordinated observations with two complementary techniques: (1) space-based cloud-tracking using JAXA's Akatsuki/UVI imaging and (2) ground-based Doppler velocimetry using the TNG/HARPS-N (Telescopio Nazionale Galileo). This was the first use of HARPS-N to observe a Solar system planetary atmosphere. The measurement of both zonal and meridional wind velocities and the constraints of temporal and spatial wind variability at cloud top is essential to our understanding of the mechanisms which yield and maintain the dynamics of Venus' atmospheric superrotational circulation. From space, clouds features were tracked on images obtained by the Akatsuki instrument (UVI) operating in the ultraviolet range (365 nm filter), acquired in orbit #39, between 26 and 31 of January 2017. From the ground, data acquired on 28–29 January 2017 using TNG/HARPS-N, Doppler velocimetry measurements are based on high-resolution spectra (resolution of ∼115,000) of Fraunhofer lines in the visible range (383–690 nm) using incoming solar radiation scattered by cloud top particles in the observer's direction (Widemann et al., 2008; Machado et al., 2014; Machado et al., 2017). The analysis and results of this new coordinated dataset indicate (1) cross-validation and complementarity of cloud-tracking and Doppler velocimetry measurements; (2) a significant North-South asymmetry of zonal wind circulation, of the order of 10 ms −1 higher in the South hemisphere, in Akatsuki/UVI observations (in accordance with previous detection by Horinouchi et al. (2018)); (3) we present a high precision cloud-top meridional wind flow, obtained with HAPRS-N; a poleward meridional flow, symmetrical with respect to equator, clearly measured in both hemispheres; it peaks around 40° N and S with an amplitude of v ¯ m = 30±5 m s −1; (4) spatial and temporal variability of Akatsuki's zonal and meridional wind flow is also observed, with a significant increase of wind flow near evening terminator. • Cross-validation of cloud-tracking and Doppler velocimetry measurements • North-South asymmetry of zonal wind circulation, in Akatsuki/UVI observations • HARPS-N used for the first time to observe a solar system planetary atmosphere • HARPS-N meridional wind profile is the most complete and precise ever retrieved [ABSTRACT FROM AUTHOR]