Radio occultation experiment of the Venus atmosphere and ionosphere with the Venus orbiter Akatsuki

Takeshi Imamura,Tomoko Hayashiyama,Daichi Hirahara,Martin Pätzold,Takahiro Iwata,Yoshifumi Futaana,Nanako Mochizuki,Alexander Nabatov,Tomoaki Toda,Bernd Häusler,Hiroki Ando,Atsushi Tomiki,Zen-Ichi Yamamoto,Hirotomo Noda,Takumi Abe

doi:10.5047/eps.2011.03.009

Abstract

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 latitudes can be probed many times thanks to the near-equatorial orbit. Systematic sampling in the equatorial region provides an opportunity to observe the propagation of planetary-scale waves that might contribute to the maintenance of the super-rotation via eddy momentum transport. Covering the subsolar region is essential to the understanding of cloud dynamics. Frequent sampling in the subsolar electron density also helps the understanding of ionosphere dynamics. Another unique feature of Akatsuki RS is quasi-simultaneous observations with multi-band cameras dedicated to meteorological study; the locations probed by RS are observed by the cameras a short time before or after the occultations. An ultra-stable oscillator provides a stable reference frequency which is needed to generate the X-band downlink signal used for RS.

Highlights

Radio occultation is one of the major applications of radio science in space missions, and has played a central role in determining the vertical structures of planetary atmospheres from the early stage of the planetary exploration in the 1960’s (e.g., Eshleman et al, 1987; Tyler, 1987)
In a radio occultation experiment conducted with an orbiter, the spacecraft transmits radio waves toward a tracking station on the earth and sequentially goes behind the planet’s ionosphere, neutral atmosphere, and solid planet as seen from the tracking station, and reemerges in the reverse sequence
To achieve a high frequency stability in the X-band downlink signal, the spacecraft is equipped with an ultra-stable oscillator (USO) as a reference frequency source

Summary

Introduction

Radio occultation is one of the major applications of radio science in space missions, and has played a central role in determining the vertical structures of planetary atmospheres from the early stage of the planetary exploration in the 1960’s (e.g., Eshleman et al, 1987; Tyler, 1987). In a radio occultation experiment conducted with an orbiter, the spacecraft transmits radio waves toward a tracking station on the earth and sequentially goes behind the planet’s ionosphere, neutral atmosphere, and solid planet as seen from the tracking station, and reemerges in the reverse sequence. Radio occultation experiments have been conducted many times in the previous Venus missions, dense sampling at low latitudes and coordination with other measurements will enable a unique, efficient observation of the atmospheric structure and its temporal variation. Within the cloud layer (60–70 km) the temperature drops by ∼20 K around 60–75◦ latitude, suggesting via thermal wind balance a prominent easterly vertical shear associated with the mid-latitude jet around 60◦ latitude (Newman et al, 1984; Tellmann et al, 2009). The structure of the ionosphere is affected by the input of energy and momentum from the solar wind to the upper atmosphere, and important for understanding the escape of the atmosphere to space, which should have driven the climate evolution of Venus in a geological time scale

Objectives

Derivation of atmospheric density and temperature

Vertical and horizontal resolution