Takeshi Imamura

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. Temper

As the global distribution of Venusian H2SO4–H2O clouds is strongly related to the global circulation of H2SO4 governed by wind transport and sedimentation of droplets, the circulation of H2SO4 in the Tropics was studied by simultaneously solving advection and cloud microphysics equations using a one-dimensional model that includes a weak upwelling representing the rising branch of Hadley circulation near the equator. H2SO4 vapor in the upper cloud region is supplied by photochemical production

A two‐dimensional model study revealed that the meridional circulation can have a large influence on the distributions of the cloud and condensable gases on Venus. In our model, the generally observed depression of near‐infrared optical thickness at midlatitude is reproduced as a result of the enhancement of cloud thickness at low and high latitudes. At high latitudes, the modeled poleward transport of H 2 SO 4 ‐H 2 O droplets in the upper atmosphere makes a thick cloud, since most of the H 2 SO

Abstract 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 develop

Abstract 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 ti

Radio scintillation observations have been unable to probe flow speeds in the low corona where the scattering of radio waves is exceedingly strong. Here we estimate outflow speeds continuously from the vicinity of the Sun to the outer corona (heliocentric distances of 1.5–20.5 solar radii) by applying the strong scattering theory to radio scintillations for the first time, using the Akatsuki spacecraft as the radio source. Small, nonzero outflow speeds were observed over a wide latitudinal range

The Radio Science experiment (RS) in the Akatsuki mission of JAXA aims to determine the vertical structure of the Venus atmosphere, thereby complementing the imaging observations by onboard instruments. The physical quantities to be retrieved are the vertical distributions of the atmospheric temperature, the electron density, the H2SO4 vapor density, and small-scale density fluctuations. The uniqueness of Akatsuki RS as compared to the previous radio occultation experiments at Venus is that low

The electron density distribution in the vicinity of the lunar surface was explored with the radio occultation technique using a subsatellite on the SELENE mission. Although the measurements suffer from contamination by the terrestrial ionosphere and interplanetary plasma, an analysis of more than 300 measurements provides adequate statistics and reveals a general trend. The result suggests that a dense ionosphere covering the whole sunlit side, as suggested by the radio occultation measurements