Toru Kouyama

Stationary waves and slowly moving features in the night upper clouds of Venus

Venus Express wind measurements at Venus’s cloud top during the night show a different picture than dayside. Both fast and slow motions are detected (there are only fast ones during the day) as well as many stationary waves related to surface relief.

Equatorial jet in the lower to middle cloud layer of Venus revealed by Akatsuki

The Venusian atmosphere is in a state of superrotation where prevailing westward winds move much faster than the planet’s rotation. Venus is covered with thick clouds that extend from about 45 to 70 km altitude, but thermal radiation emitted from the lower atmosphere and the surface on the planet’s night-side escapes to space at narrow spectral windows of near-infrared. The radiation can be used to estimate winds by tracking the silhouettes of clouds in the lower and middle cloud regions below about 57 km in altitude. Estimates of wind speeds have ranged from 50 to 70 m/s at low- to mid-latitudes, either nearly constant across latitudes or with winds peaking at mid-latitudes. Here we report the detection of winds at low latitude exceeding 80 m/s using IR2 camera images from the Akatsuki orbiter taken during July and August 2016. The angular speed around the planetary rotation axis peaks near the equator, which we suggest is consistent with an equatorial jet, a feature that has not been observed previously in the Venusian atmosphere. The mechanism producing the jet remains unclear. Our observations reveal variability in the zonal flow in the lower and middle cloud region that may provide new challenges and clues to the dynamics of Venus’s atmospheric superrotation.

An overview of ISS HISUI hyperspectral imager radiometric calibration

The hyperspectral imager of the HISUI (Hyperspectral Imager Suite) project will be deployed and operated in the International Space Station (ISS) Japanese Experiment Module-Exposed Facility (JEM-EF) in FY2018. The pre-launch calibration is conducted using a large integrating sphere traceable to the SI, and after the launch, the onboard calibration will be carried out using onboard calibration unit including a lamp. The vicarious calibration will be conducted over the dry lake sites located both in northern hemisphere and in southern hemisphere, and we may also use automated calibration facilities due to the effects of orbital characteristics for the ISS. The HISUI hyperspectral imager will be cross-calibrated using forthcoming hyperspectral sensors flying on ISS. The data associated with these calibrations will be archived in the Calibration Data Archive System (CDAS), and the HISUI calibration working group plans to assess HISUI radiometric performance and produces radiometric (geometric) calibration databases on CDAS, which are required in Level 1 processing.

Assessment of HISUI radiometric performance using vicarious calibration and cross-calibration

The Hyperspectral Imager Suite (HISUI) is a future hyper-spectral and multispectral sensor developed by Japanese Ministry of Economy, Trade, and Industry (METI). Radiometric calibration of satellite sensor system is very important for the higher level products with high quality. This paper mainly shows the vicarious calibration and cross-calibration for assessment of HISUI radiometric performance in HISUI Calibration Working Group.

Development of an ensemble Kalman filter data assimilation system for the Venusian atmosphere

The size and mass of Venus is similar to those of the Earth; however, its atmospheric dynamics are considerably different and they are poorly understood due to limited observations and computational difficulties. Here, we developed a data assimilation system based on the local ensemble transform Kalman filter (LETKF) for a Venusian Atmospheric GCM for the Earth Simulator (VAFES), to make full use of the observational data. To examine the validity of the system, two datasets were assimilated separately into the VAFES forecasts forced with solar heating that excludes the diurnal component Qz; one was created from a VAFES run forced with solar heating that includes the diurnal component Qt, whereas the other was based on observations made by the Venus Monitoring Camera (VMC) onboard the Venus Express. The VAFES-LETKF system rapidly reduced the errors between the analysis and forecasts. In addition, the VAFES-LETKF system successfully reproduced the thermal tide excited by the diurnal component of solar heating, even though the second datasets only included horizontal winds at a single altitude on the dayside with a long interval of approximately one Earth day. This advanced system could be useful in the analysis of future datasets from the Venus Climate Orbiter ‘Akatsuki’.

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.

Moon observations for small satellite radiometric calibration

In recent years, more and more small satellites have been launched and operated for various purposes. Because of their sever restrictions of payload weight and cost, a radiometric calibration approach which does not need any special instrument has been desired. In this study, we demonstrate a radiometric calibration approach for Hodoyoshi-1, a Japanese small satellite, based on Moon observations utilizing SLENE/SP Lunar surface reflectance model. The model can simulate any satellite Moon observation considering observation geometry, and can be used for evaluating the consistency of the observed Moon brightness, which allows us to measure a time variation of a sensor sensitivity. Through Hodoyoshi-1s three Moon observations, gradual increasing in relative sensor sensitivities were confirmed.

HISUI vicarious calibration and CAL/VAL activities

The Hyperspectral Imager Suite (HISUI) is the Japanese next-generation Earth observation project, and is being developed by Japanese Ministry of Economy, Trade, and Industry (METI). We conduct the vicarious calibration for HISUI instrument as the pre-launch activities in summer in the southern hemisphere. Recent our activities focus on vicarious calibration experiment over Lake Lefroy, which is a large salt lake in southern Western Australia. Cross-calibration for HISUI is also discussed. This paper shows both of vicarious calibration and cross-calibration for HISUI instrument.

Usability of lunar reflectance model based on SELENE/SP for planned HISUI radiometric calibration

We have developed a method for radiometric calibration of HISUIs hyper and multi-spectral sensors using a lunar reflectance model developed from SELENE SP data, which involves lunar surface reflectance and photometric properties. For evaluating the utilization of the model, we simulated a lunar observation conducted by ASTER of its three visible and infrared bands and confirmed the model describes lunar surface photometric properties correctly because correlation coefficients of observed and modeled radiance exceed 0.99 for all bands. Although absolute radiance shows some discrepancy between the observed and the simulated Moon in visible band, the model is, at least, useful to evaluate relative degradation of sensors.

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