Initial performance of the radio occultation experiment in the Venus orbiter mission Akatsuki
- Takeshi Imamura1Email authorView ORCID ID profile,
- Hiroki Ando2,
- Silvia Tellmann3,
- Martin Pätzold3,
- Bernd Häusler4,
- Atsushi Yamazaki5,
- Takao M. Sato5,
- Katsuyuki Noguchi6,
- Yoshifumi Futaana7,
- Janusz Oschlisniok3,
- Sanjay Limaye8,
- R. K. Choudhary9,
- Yasuhiro Murata5,
- Hiroshi Takeuchi5,
- Chikako Hirose5,
- Tsutomu Ichikawa5,
- Tomoaki Toda5,
- Atsushi Tomiki5,
- Takumi Abe5,
- Zen-ichi Yamamoto5,
- Hirotomo Noda10,
- Takahiro Iwata5,
- Shin-ya Murakami5,
- Takehiko Satoh5,
- Tetsuya Fukuhara11,
- Kazunori Ogohara12,
- Ko-ichiro Sugiyama13,
- Hiroki Kashimura14,
- Shoko Ohtsuki15,
- Seiko Takagi16,
- Yukio Yamamoto5,
- Naru Hirata17,
- George L. Hashimoto18,
- Manabu Yamada19,
- Makoto Suzuki5,
- Nobuaki Ishii5,
- Tomoko Hayashiyama20,
- Yeon Joo Lee5 and
- Masato Nakamura5
© The Author(s) 2017
Received: 4 July 2017
Accepted: 20 September 2017
Published: 3 October 2017
The main goal of the Venus orbiter mission Akatsuki is to understand the mechanisms driving the atmospheric circulation and maintaining the cloud layer (Nakamura et al. 2011). For this purpose, five cameras onboard, having bandpass filters, take images of Venus at different wavelengths to observe the horizontal distributions of clouds and minor constituents at different heights. Radio occultation measurement in Akatsuki, termed RS (Radio Science), aims at exploration of the vertical structure of the atmosphere (Imamura et al. 2011), being complementary to the imaging observations by onboard cameras.
The orbiter was launched in May 2010 and arrived at Venus in December 2010. The first attempt on Venus orbit insertion has failed, and the second attempt conducted in December 2015, after 5-years of interplanetary cruise, was successful (Nakamura et al. 2016). Regular observations of the Venusian atmosphere with radio occultation technique have started in March 2016. In addition to the observation of Venus, radio occultation observations of the solar corona were conducted during the solar conjunction periods in 2011 and 2016 using the same equipment (Imamura et al. 2014b).
Radio occultation has played a crucial role in determining the structures of the planetary atmospheres (e.g., Eshleman 1973; Tyler 1987; Pätzold et al. 2007; Imamura et al. 2012). In a radio occultation experiment conducted with a spacecraft equipped with a stable frequency source, the spacecraft transmits radio waves toward the Earth, while it goes behind the planet and reemerges as seen from the Earth. During such occultation events, the planetary atmosphere causes bending and attenuation of the radio waves. Assuming a local spherical symmetry, the analysis of the frequency and the signal intensity time series obtained at the tracking station yields vertical profiles of the refractive index and the absorption coefficient. Temperature profiles are obtained from the refractive index profiles assuming hydrostatic balance. The advantages of this technique over other remote sensing techniques are its high vertical resolution (typically < 1 km) and high temperature resolution (typically < 1 K) (Hinson and Jenkins 1995; Tellmann et al. 2009). Radio occultation can cover altitudes from the sub-cloud region (< 50 km) to the upper atmosphere (~ 90 km). The quality of the measurement declines below 40 km due to defocusing loss and absorption, and no information is obtained below ~ 32 km since the curvature of the ray path exceeds that of the planetary surface (Fjeldbo et al. 1971; Häusler et al. 2006).
The scientific background of the measurement has been described in Imamura et al. (2011); here, we briefly describe the background with recent updates. Previous radio occultation experiments of the Venusian atmosphere revealed the meridional structure of the atmospheric temperature, including the horizontally near-uniform temperature below the cloud layer, the cold collar and the mid-latitude jet at cloud heights, and the warm high latitudes above clouds (Kliore and Patel 1980; Newman et al. 1984; Piccialli et al. 2012). The dynamical stability of the atmosphere was studied using radio occultation temperatures (Piccialli et al. 2012). The static stability is near neutral in the middle and lower cloud regions (50–58 km), suggesting occurrence of vertical convection (Pollack et al. 1980; Tellmann et al. 2009). At altitudes above ~ 58 km, propagation of gravity waves is suggested from wavelike features in the temperature profiles (Hinson and Jenkins 1995; Tellmann et al. 2012; Ando et al. 2015a). In spite of these findings, the relationship among the variations of the temperature field, the wind field and the cloud distribution has not been studied. Local time dependence of the thermal structure in the cloud is also unclear, although numerical models predict significant diurnal variations (Imamura et al. 2014a).
Mixing ratios of H2SO4 vapor, which is concentrated in the sub-cloud region, have been retrieved from the attenuation of radio occultation signals (Jenkins et al. 1994; Kolodner and Steffes 1998; Oschlisniok et al. 2012). The observed H2SO4 vapor distribution suggests control of the H2SO4 distribution by condensation and evaporation in clouds, thermal decomposition in the lower atmosphere, and global-scale circulation (Krasnopolsky and Pollack 1994; Imamura and Hashimoto 1998, 2001). Using Venus Express radio occultation data, Oschlisniok et al. (2012) revealed complicated meridional distribution and local time dependence of the mixing ratio, which are not reproduced in numerical models. The relationship among the variations of the thermal structure, the H2SO4 vapor distribution, and the cloud distribution is a key to understanding the cloud system; regional cloud models predict significant variability of the clouds (McGouldrick et al. 2008a, b), and Venus Express VIRTIS showed mesoscale variability of the cloud optical thickness (McGouldrick et al. 2012).
The occurrence of small-scale atmospheric density fluctuations, having scales smaller than the Fresnel diameter given later, has been inferred from the scintillation of radio occultation signals. The scintillation, which is enhanced around 60 km altitude and at high latitudes, might be caused by turbulence (Woo et al. 1980) or gravity waves (Leroy and Ingersoll 1996). Ando et al. (2015a) argued, based on a spectral analysis of radio occultation temperatures, that saturation of gravity waves through convective instability occurs above clouds, although there is no direct evidence for wave breaking. A detailed comparison of the distribution of scintillation with wavy temperature structures might provide clues to wave breaking and resultant turbulence.
Ionospheric electron densities have primarily been measured by radio occultation technique (Kliore and Luhmann 1991). The peak electron density is an order of magnitude higher on the dayside than nightside, and multiple layers are observed on the dayside (Pätzold et al. 2007). The structure of the ionosphere is affected by the input of energy and momentum from the solar wind to the upper atmosphere, and also by the dynamical coupling with the lower neutral atmosphere. The latter is particularly unexplored and should be studied with simultaneous observations of the variability of the lower atmosphere and the upper atmosphere.
This paper presents the initial performance, the observation plan, and the expected science themes of Akatsuki RS. Scientific results will be reported elsewhere. The observation plan was largely changed from the original given in Imamura et al. (2011) because of the change of the orbit around Venus. The originally planned orbit is a 30 h-period elliptical orbit with the apoapsis altitude of ~ 13 Venus radii, while the current orbit is a 10.5-day-period elliptical orbit with the apoapsis altitude of 59 Venus radii.
Radio occultation method
One-way downlink at X-band (8410.932 MHz) is used in the experiment. The experiment relies on the frequency stability of both the onboard radio wave source and the recording system at the ground station. Akatsuki RS employs an ultra-stable oscillator (USO) as the onboard frequency source; the performance of the USO is given in “Performance of the ultra-stable oscillator”. The USO is almost identical to those onboard ESA’s Rosetta and Venus Express spacecraft (Häusler et al. 2007). The absence of the second frequency (such as S-band) prevents dual-frequency occultation method, which is sensitive only to plasma along the radio propagation path (e.g., Imamura et al. 2010; Pätzold et al. 2007). This might have some influence on the detection of ionospheric fine structures at low ionospheric altitudes but will not alter the coarse ionospheric electron density profiles.
The 3 dB half beam width of the high-gain antenna is ~ 1°; since the ray bending exceeds the beam width, the spacecraft performs attitude maneuvers to compensate for the changing direction of the signal path. The direction of the antenna beam is changed along a polygonal (zigzag) curve in such a way that the difference between the controlled beam direction and the ideal beam direction based on the Venus International Reference Atmosphere (VIRA) (Seiff et al. 1985) is less than 0.05°. The difference sometimes exceeds this limit because of an inaccurate trajectory prediction at the time of command generation, leading to non-negligible decline of the signal intensity. The effect of such antenna mispointing on the measured signal intensity is corrected before analysis using the measured antenna pattern (Toda et al. 2010) and the trajectory data reconstructed after the experiment. The accuracy of this intensity correction is estimated to be better than 1 dB in most of the cases.
The accuracy of the altitudes in the derived atmospheric profiles relies on the accuracy of trajectory determination. According to the analysis of flight dynamics, the position of Akatsuki relative to Venus typically has an error of 100–500 m during radio occultation experiments. This error causes uncertainties of the altitude offset of similar magnitude in the atmospheric profiles.
The primary ground station used for the experiment is the 64-m antenna of Usuda Deep Space Center (UDSC) of JAXA, which is located at 138°21′54″ East longitude, 36°07′44″ North latitude. In addition to UDSC, for increasing the number of observation opportunities, we started to use the 32-m antenna of Indian Deep Space Network (IDSN) of Indian Space Research Organization (ISRO), which is located at 77°22′08″ East longitude, 12°54′11″ North latitude, from March 2017.
The received signals are down-converted to < ~ 1 MHz by an open-loop heterodyne system stabilized by a hydrogen maser (Allan deviation < 3 × 10−13 for 1 s, < 3 × 10−15 for 1000 s) and 8-bits digitized. The sampling rate is 0.5–4 MHz so that the received signal can be confined in the recording bandwidth, while the frequency changes with time due to Doppler shift. The detail of the observation system is given in Imamura et al. (2011).
Locations of observations
The uniqueness of the Akatsuki RS is that the observation points cluster in the low and middle latitude because of the near-equatorial orbit. Since the onboard cameras are designed to observe wide areas in the low latitude, the locations probed by RS can be observed by the cameras a short time (h) before or after each occultation experiment. Mostly dawn and dusk regions have been probed by April 2017.
Performance of the ultra-stable oscillator
In the processing of recorded data, we first subtract the Doppler shift calculated from the orbital information and a model atmosphere from the original signal by heterodyning, thereby suppressing the frequency variation and enabling narrow-band filtering. Then, approximate carrier frequencies are determined for successive time blocks by fitting a theoretical signal spectrum (sinc function) to the discrete Fourier transforms (Lipa and Tyler 1979). The resultant frequency variation is subtracted from the signal by heterodyning in order to apply another narrow-band filtering. The signal frequency and intensity time series to be used for the analysis of the atmospheric structure are obtained by successively fitting a sinc function to the narrow-band filtered data.
The sum of the frequency variation in the final product like Fig. 4 and the frequency variation that has been subtracted via heterodyning in the course of the processing gives the total Doppler shift. Subtraction of the “straightline Doppler shift”, which is the frequency variation to be observed if Venus does not exit, from the total Doppler shift gives the contribution of the Venusian atmosphere termed “atmospheric Doppler shift”. Although the atmospheric Doppler shift should be zero in the portion of the data above the height of the ionosphere, we usually observe a smoothly varying frequency component that is attributed to the error in the trajectory data. To remove this frequency offset, we fit a linear function to this portion of the frequency time series and subtract it from the whole time series; this procedure is termed “baseline fit”. The atmospheric Doppler shift corrected by baseline fit is combined with the trajectory data to calculate the bending angle and the impact parameter.
The temperature profiles clearly show differences in the stratification characteristics among the altitude regions. Almost constant lapse rate of ~ 10 K/km, which is close to the adiabatic lapse rate (Seiff et al. 1985), is observed in the middle and lower cloud region (50–58 km); this region is considered as a convective layer driven by the heating of the cloud base by the thermal infrared flux from below (Pollack et al. 1980). The region below the cloud (< 50 km) is weakly stable. The region above 58 km is also stable and is dominated by short-vertical scale (< 5 km) fluctuations, being consistent with Venus Express radio occultation experiments (Tellmann et al. 2012). The range of the temperature variation near the top is ~ 50 K, which is smaller than the day-to-day variability of ~ 80 K in the same height region observed by the solar occultation experiment SOIR onboard Venus Express (Mahieux et al. 2012; Limaye et al. 2017). More measurements are needed to better understand the variability.
Comparing the morning and the afternoon profiles, local time-dependent features are seen at various vertical scales. The amplitudes of the fine structures with vertical scales of < 5 km below 70 km altitude seem to be larger on the morning side, and the phase of the background wavy structure with a vertical scale of > 10 km above 70 km seems to be shifted in altitude between these local time regions. The detail of the local time dependence will be reported elsewhere.
Radio occultation observation of the Venusian atmosphere in the Akatsuki mission has started in March 2016. Ten occultation experiments, including 10 ingress and 9 egress measurements, have been conducted by February 2017. From the open-loop recorded data, the altitude distributions of the temperature, the H2SO4 vapor mixing ratio, and the electron density are being successfully retrieved.
The dataset will be used for a variety of scientific researches. The temperature and H2SO4 vapor profiles are analyzed in combination with cloud images obtained successively from Akatsuki and also with cloud-tracked vectors obtained from those images. Complementary uses of those data enables studies on the interaction between cloud physics and thermal structure, the three-dimensional structures of atmospheric circulation and waves, and the vertical coupling between the cloud-level atmosphere and the mesosphere. The combination of the electron density profiles with the whole Akatsuki dataset will provide clues to the role of the variation of the neutral atmosphere in the variability of the ionosphere. Comparison of Akatsuki RS results with the previous radio occultation results from Pioneer Venus (Kliore and Patel 1980) and Venus Express (Pätzold et al. 2007; Tellmann et al. 2009) would reveal long-term variation of the Venusian atmosphere. The radio occultation temperatures are used in the radiative transfer calculation for the analyses of data taken by ground-based telescopes. The results from those studies will be presented in separate papers.
Apart from Venus researches, radio occultation observations of the solar corona were conducted during the solar conjunction periods in June–July 2011 and May–June 2016. The outward flow velocity can be retrieved from the signal intensity fluctuation, and the plasma density fluctuation can be retrieved from the frequency fluctuation. The 2011 observation has revealed the radial profile of the flow velocity (Imamura et al. 2014b), the radial distribution of compressible waves (Miyamoto et al. 2014), and the internal structure of a coronal mass ejection (Ando et al. 2015a, b).
TI is the principal investigator of the experiment. HA, ST and JO contributed to data analysis. MP, BH, YM, HT, TT, AT, ZY, IN, TH and MN contributed to the development of the observation system. KN, YF, SL, TA, HN and TI contributed to science planning. AY, TS, TS, CH, TF, KO, KS, HK, SO, ST, MY, MS and LY contributed to command planning and spacecraft operation. TI represents the orbital dynamics team. SM, YY, NH and GLH contributed to data archiving. RKC represents the ISRO RS Team. All authors read and approved the final manuscript.
We would like to acknowledge all members of the Akatsuki project team and the staff of UDSC for supporting the preparation of the experiment. The high performance of the USO was achieved by the tremendous efforts made by Theo Schwall, Wolfgang Schäfer and other engineers of TimeTech GmbH. We thank Richard Simpson at Stanford University and Kevin McGouldrick at University of Colorado, Boulder for supporting data archiving. We also thank Koh-ichiro Oyama for continuous encouragement.
The authors declare that they have no competing interests.
Availability of data and materials
The radio occultation data used in this study will become open to the public at DARTS (Data ARchives and Transmission System) of JAXA and the PDS (Planetary Data System) of NASA.
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Ethics approval and consent to participate
This work was supported by JSPS KAKENHI Grant Nos. JP22244060, JP24540482, JP16H04060, JP00706335 and JP16H02231.
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