Shin-ya MURAKAMI

© 2017 The Author(s). 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 H2SO4vapor 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.[Figure not available: see fulltext.]

Copyright © 2016 by the International Astronautical Federation (IAF). All rights reserved. Japan's Venus Climate Orbiter Akatsuki was proposed to ISAS (Institute of Space and Astronautical Science) in 2001 as an interplanetary mission. We made 5 cameras with narrow-band filters to image Venus at different wavelengths to track the cloud and minor components distribution at different heights to study the Venusian atmospheric dynamics in 3 dimension. It was launched on May 21st, 2010 and reached Venus on December 7th, 2010. With the thrust by the orbital maneuver engine, Akatsuki tried to go into the westward equatorial orbit around Venus with the 30 hours' orbital period, however it failed by the malfunction of the propulsion system. Later the spacecraft has been orbiting the sun for 5 years. On December 7th, 2015 Akatsuki met Venus again after the orbit control and Akatsuki was put into the westward equatorial orbit whose apoapsis is about 0.44 million km and orbital period of 14 days. Its main target is to shed light on the mechanism of the fast atmospheric circulation of Venus. The systematic imaging sequence by Akatsuki is advantageous for detecting meteorological phenomena with various temporal and spatial scales. We have five photometric sensors as mission instruments for imaging, which are 1 m-infrared camera (IR1), 2 m-infrared camera (IR2), ultra-violet imager (UVI), long-wave infrared camera (LIR), and lightning and airglow camera (LAC). These photometers except LIR have changeable filters in the optics to image in certain wavelengths. Akatsuki's long elliptical orbit around Venus is suitable for obtaining cloud-tracked wind vectors over a wide area continuously from high altitudes. With the observation, the characterizations of the meridional circulation, mid-latitude jets, and various wave activities are anticipated. The technical issues of Venus orbit insertion in 2015 and the scientific new results will be given in this paper.

tudy on dynamics of single vortex would be helpful for farther understanding of two-dimensional turbulence, since two-dimensional turbulence can be considered as collective motion of vortices.Melander, et al.(1987) studied an axisymmetrization process of an elliptic vortex with non-uniform vorticity distribution.They discussed qualitatively the effect of the filamentson the axisymmetrization process of the elliptic vortex.However, quantitative studies on such effect have not been performed yet.Thus, we investigate the effect of the filaments on the axisymmetrization of the elliptic vortex through numerical simulations.We find that the filaments play a significant role on the axisymmetrizationof the elliptic vortex only at the early stage of evolution.In contrast, the velocity induced by the vortex core contributes to both the axisymmetrization and the anti-axisymmetrization.

We examine the role of the filaments on the axisymmetrization of an isolated elliptic vortex with non-uniform vorticity distribution for a two-dimensional incompressible barotropic fluid, numerically. In order to perform quantitative examination of the role of filaments on the axisymmetrization, firstly, we divide the vorticity field into the elliptic region and the hyperbolic region. The former corresponds to the core of the vortex and the latter the filaments and a weak vorticity region just outside the vortex core. Secondly, we analyse the radial displacement of the maximum and minimum curvature points on a vorticity contour advected by velocities induced by the vorticity of those regions. These investigations result that the vorticity of the hyperbolic region largely contributes to the axisymmetrization at the both points, especially at the time when the filaments are forming.

Evolution of an isolated 2-D elliptic vortex with non-uniform vorticity distribution is investigated from the viewpoint of the palinstrophy generation. As pointed out in a previous study, the palinstrophy generation rate has a quadrapole structure over the core of the vortex. The palinstrophy generation rate depends on the positive eigenvalue of the rate of strain tensor, the palinstrophy and the alignment angle between the vorticity gradient vector and the eigenvector of the rate of strain tensor associated with the negative eigenvalue. We show that the quadrapole structure over the vortex core, which is pointed out in a previous study, is caused by the alignment angle. We also show that the magnitude of the palinstrophy generation rate is mainly determined by the palinstrophy.