Initial performance of the radio occultation experiment in the Venus orbiter mission Akatsuki

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 H 2 SO 4 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.

Stationary Features at the Cloud Top of Venus Observed by Ultraviolet Imager Onboard Akatsuki

“…The parameters of the background atmosphere are taken from the values at 70‐km altitude at latitudes <30° of Venus International Reference Atmosphere (Seiff et al, ): The temperature is 229.8 K, the gas constant used for calculating H is 191.3 J·K ‐1 ·kg ‐1 , γ = 1.32, g = 8.67 m/s 2 , T 0 = 229.8 K, and the Brunt‐Väisälä frequency is N B = 0.018 s ‐1 . Though newer data are available (Limaye et al, ), the difference from Venus International Reference Atmosphere is small around the cloud top (65–70 km) at low latitudes according to radio occultations (Imamura et al, ; Tellmann et al, ); we confirmed that the difference hardly changes the results below. The scale height is calculated to be H ~ 5.1 km.…”

Three‐Dimensional Turbulence‐Resolving Modeling of the Venusian Cloud Layer and Induced Gravity Waves: Inclusion of Complete Radiative Transfer and Wind Shear

“…A strong latitudinal variability of the vertical extent of the convective layer was observed (Tellmann et al, ), with the thickness of the convective layer reaching 10 km in polar regions, almost twice thicker than in the equatorial regions. No variability of the thickness of the convective layer with local time was measured in this data set, though the radio occultations measured with the ongoing spacecraft Akatsuki measured a convective layer that appears to be thicker in the morning (Imamura et al, ). The amplitude of the vertical convective plumes, as well as the width of the convective cells, was measured in situ by the VEGA balloons flying in the Venusian convective layer: vertical winds range between −3.5 and 2 m/s (Linkin et al, ) and convective cells extend horizontally from several hundred meters to tens of kilometers (Kerzhanovich et al, ).…”

“…This is in line with the convective features observed in the UV by Pioneer Venus and Venus Express. However, no mixed layer has been detected in radio occultations at this altitude, neither by Magellan (Hinson & Jenkins, ) nor Venus Express (Tellmann et al, ) nor Akatsuki (Imamura et al, ). At this altitude, the atmosphere profiled by radio occultations is very stable (the static stability is several kelvins per kilometer).…”

“…In the radio occultation measurements of VeRa the convective layer at the equator was located between approximately 49 and 59 km (Tellmann et al, ). The first radio occultations on board Akatsuki (Imamura et al, ) measured a vertical extension of the convective layer from 50 to 58 km. Thus, the bottom of the convective layer resolved in our Venus LES is slightly lower than the observations, but the thickness is consistent.…”

“…Different regimes have been determined, from mottled dark clouds at low latitude to streaky clouds around 50° and bright and almost featureless clouds at high latitude. Interestingly, while the low‐latitude cloud top images are reminiscent of convective activity, the Venus Express and Akatsuki radio occultation measurements at those altitudes in tropical regions do not show any clear neutral‐stability layers (Ando et al, ; Imamura et al, ; Tellmann et al, ).…”