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

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 H 2 SO 4 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 nearequatorial 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.

Local Time Dependence of the Thermal Structure in the Venusian Equatorial Upper Atmosphere: Comparison of Akatsuki Radio Occultation Measurements and GCM Results

“…The radio occultation temperatures have accuracies of ∼0.1 K and vertical resolutions of ∼1 km. Details of the Akatsuki radio occultation measurements are described in Imamura et al, (2011Imamura et al, ( , 2017.…”

Fine Vertical Structures at the Cloud Heights of Venus Revealed by Radio Holographic Analysis of Venus Express and Akatsuki Radio Occultation Data

“…In the GO method, the geometry of the raypath (the ray impact parameter a and the asymptotic bending angle α ) is retrieved at each time step from the instantaneous frequency on the assumption that a single ray reaches the receiver. The frequency time series is obtained by applying fast Fourier transform (FFT) to successive short time blocks of the recorded signal and finding the frequency in each time block (Imamura et al, , ). In regions where multipath propagation occurs, this procedure selects the strongest subsignal or obtains an approximate average frequency of the subsignals.…”

“…Though FSI has been applied to GPS radio occultation data (e.g., Tsuda et al, ), it has not been used for planets other than the Earth. The data used are those obtained in the European Space Agency (ESA)'s Venus Express mission (Häusler et al, ) and JAXA's Akatsuki mission (Imamura et al, , ). In the traditional analysis method, the raypath at each time step is retrieved from the instantaneous frequency measured at the ground station based on geometrical optics (GO).…”

How to process radio occultation data: 1. From time series of frequency residuals to vertical profiles of atmospheric and ionospheric properties

“…This legacy at Cornell continues into the present day, with independent analysis of Cassini radio occultation measurements at Titan and Saturn (Schinder et al, 2011a(Schinder et al, ,b, 2012. Expertise also exists in Europe, where groups from Cologne and Munich lead radio science investigations on Rosetta, Mars Express, and Venus Express (Pätzold et al, 2004;Häusler et al, 2006;Pätzold et al, 2007); Japan, where the Sakigake, Nozomi, Selene, and Akatsuki spacecraft carried radio occultation investigations (Oyama et al, 2001;Noguchi et al, 2002;Imamura et al, 2011Imamura et al, , 2012; and China (Hu et al, 2010). Several groups also possess related expertise concerning radio occultations of Earth's atmosphere and ionosphere (e.g.…”