Tetsuya Fukuhara

MSI: a visible multispectral imager for 1.6-m telescope of Hokkaido University

We have built a visible multi-spectral imager (MSI) for the 1.6-m Pirka telescope of the Hokkaido University in Hokkaido, Japan. The instrument is equipped with two liquid crystal tunable filters and a 512 × 512 pixel EMCCD camera. One of the major purposes of this instrument is to obtain multi-spectral images (series of narrow-band images at many different wavelengths) of the solar planets rapidly. These tunable filters are a Lyot filter with liquid crystal variable retarders and thus can tune the transmitting wavelength rapidly without moving parts. Their spectral ranges are 400–720 nm and 650–1100 nm and the bandwidth is typically 10 nm on both filters. The EMCCD camera can obtain images at a frame rate of about 32 Hz, which also enables us to improve the spatial resolution with the shift-and-add or the Lucky imaging techniques. The field of view is 3.3 × 3.3 arcmin with a pixel scale of 0.39 arcsec pixel−1. The instrument also has UBV RI-band broad-band filters and several narrow-band filters. MSI is mounted at the f/12 Cassegrain focus of the telescope. It had the first light on February 2011, and then have been used for several astronomical and planetary science programs as a major facility instrument at this telescope. We describe the design, construction, integration, and performance of this multi-spectral imager.

Mesoscale Spectra of Mars’s Atmosphere Derived from MGS TES Infrared Radiances

Wavenumber spectra of the atmospheric potential energy of Mars at mesoscales (wavelengths of 64–957 km) were obtained as a function of latitude, season, and Martian year using infrared radiance data obtained by the Thermal Emission Spectrometer (TES) onboard the Mars Global Surveyor (MGS) spacecraft. Spectral slopes tend to be flatter at smaller scales, and the slopes are usually flatter than 1 near small-scale ends. Near large-scale ends, the spectra sometimes show prominent steepening with slopes from 2t o3. The power peaks in the high latitudes in winter and equinoxes, suggesting that eddies are generated preferentially in baroclinic zones. The seasonal variation at each latitude band, on the other hand, tends to be obscured by large interannual variability. An enhancement in the power was observed around the storm tracks in the Southern Hemisphere. Spectra of the terrestrial stratosphere were also obtained with a similar method from data taken by the Aura satellite and compared to the results for Mars.

Overview of Akatsuki data products: definition of data levels, method and accuracy of geometric correction

We provide an overview of data products from observations by the Japanese Venus Climate Orbiter, Akatsuki, and describe the definition and content of each data-processing level. Levels 1 and 2 consist of non-calibrated and calibrated radiance (or brightness temperature), respectively, as well as geometry information (e.g., illumination angles). Level 3 data are global-grid data in the regular longitude–latitude coordinate system, produced from the contents of Level 2. Non-negligible errors in navigational data and instrumental alignment can result in serious errors in the geometry calculations. Such errors cause mismapping of the data and lead to inconsistencies between radiances and illumination angles, along with errors in cloud-motion vectors. Thus, we carefully correct the boresight pointing of each camera by fitting an ellipse to the observed Venusian limb to provide improved longitude–latitude maps for Level 3 products, if possible. The accuracy of the pointing correction is also estimated statistically by simulating observed limb distributions. The results show that our algorithm successfully corrects instrumental pointing and will enable a variety of studies on the Venusian atmosphere using Akatsuki data.

Development of the longwave infrared imager (LIR) onboard PLANET-C

The Longwave Infrared Camera (LIR), which mounts an uncooled micro-bolometer array (UMBA), is under development for the Japanese Venus orbiter mission, PLANET-C. LIR detects thermal emission from the top of the sulfur dioxide cloud in a wavelength region 8--12 μm to map the cloud-top temperature which is typically as low as 230 K. The requirement for the noise equivalent temperature difference (NETD) is 0.3 K. Images of blackbody targets in room temperature (~300 K) and low temperature (~230 K) have been acquired in a vacuum environment using a prototype model of LIR, showing that the NETD of 0.2 K and 0.8 K are achieved in ~300 K and ~230 K, respectively. We expect that the requirement of NETD<0.3 K for ~230 K targets will be achieved by averaging several tens of images which are acquired within a few minutes. The vibration test for the UMBA was also carried out and the result showed the UMBA survived without any pixel defects or malfunctions. The tolerance to high-energy protons was tested and verified using a commercial camera in which a same type of UMBA is mounted. Based on these results, a flight model is now being manufactured with minor modifications from the prototype.

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

Plain Language Summary Temperature profiles of the Venus atmosphere obtained by the Akatsuki radio occultation measurements showed a prominent local time dependence above 65-km altitude at low latitudes equatorward of 35 degrees. A zonal wavenumber 2 component is predominant in the temperature field, and its phase (i.e., isothermal) surfaces descend with local time, suggesting its downward phase propagation. A general circulation model (GCM) for the Venus atmosphere, AFES-Venus, reproduced the local time-dependent thermal structure qualitatively consistent with the radio occultation measurements. Based on a comparison between the radio occultation measurements and the GCM results, the observed zonal wavenumber 2 structure is attributed to the semidiurnal tide. Applying the dispersion relationship for internal gravity waves to the observed wave structure, the zonally averaged zonal wind speed at 75- to 85-km altitudes was found to be significantly smaller than that at the cloud top. The decrease of the zonal wind speed with altitude is attributed to the momentum deposition by the upwardly propagating semidiurnal tide excited in the cloud layer. Akatsuki radio occultation measurements showed the local time dependence of the Venus atmosphere in the equatorial region. By comparing the measurements with a general circulation model, it is attributed to the upward propagating semidiurnal tide generated in the cloud layer. And then, we proposed a new method to estimate the zonal wind speed above the cloud layer, where any optical instruments cannot be measured, for the first time.

HEAT: Image and database browser for the thermal imager on Hayabusa2

Hayabusa2 is a sample return mission to asteroid 162173 Ryugu, which will arrive in 2018. The Thermal Infrared Imager (TIR) is onboard the spacecraft and will be used to make thermal observations to ensure a safe touch down and sample return, and to study the nature of the asteroid. This study developed a TIR database and data browser. Our developments are called Hayabusa2 Exploration Assistant for TIR (HEAT). HEAT has three uses: visualization, calibration, and analysis. It covers all of the uses based on functions, browsers, the database, and a data processor. This study considers two ways to perform calibrations: regression and direct conversion, which is interpolation with all of the ground test data. The TIR data analysis involves the following. First, TIR images are collected. Then, the local time is obtained for a specified longitudinal zone of the shape model. A TIR image includes surface temperature information. Third, the temporal profile of surface temperature is combined with individual TIR images and the local time. Fourth, the thermal inertia of the local site is estimated from the profile. Fifth, the thermal inertia is mapped. Finally, the thermal model is established by integrating the thermal inertia.

Detection of Small Wildfire by Thermal Infrared Camera With the Uncooled Microbolometer Array for 50-kg Class Satellite

The thermal infrared camera with the uncooled microbolometer array based on commercial products has been developed in a laboratory of a Japanese university and mounted to a 50-kg class small satellite specialized for discovering wildfire. It has been launched in 2014 and successfully detected considerable hotspots not only wildfire but also volcanoes. Brightness temperature derived from observation has been verified, and the scale of observed wildfire has been provisionally presumed; the smallest wildfire ever detected has a flame zone less than

Planet-C: Venus Climate Orbiter mission of Japan

\sim 300~\text{m}^{\mathrm {\mathbf {2}}}

Overview of Venus orbiter, Akatsuki

and the fire radiative power = ~35.4 mW. It is 1/30th the size of the initial requirement estimated in the design process. Our thermal infrared camera developed in a short time with low cost has attained enough ability to discover small wildfire which is suppressive at initial attack.