Earth, Planets and Space Ultraviolet Imager on Venus Orbiter Akatsuki and Its Initial Results

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1 Earth, Planets and Space Ultraviolet Imager on Venus Orbiter Akatsuki and Its Initial Results --Manuscript Draft-- Manuscript Number: Full Title: Article Type: Section/Category: EPSP-D--000 Ultraviolet Imager on Venus Orbiter Akatsuki and Its Initial Results Full paper Aeronomy Funding Information: Abstract: Corresponding Author: The ultraviolet imager (UVI) has been developed for the Akatsuki spacecraft (Venus Climate Orbiter mission). The UVI takes ultraviolet (UV) images of the solar radiation reflected by the Venusian clouds with narrow bandpass filters centered at the nm and nm wavelengths. There are absorption bands of SO and an unknown absorber in these wavelength regions. The UV images provide the spatial distribution of SO and the unknown absorber around cloud top altitudes. Also the images allow us to understand the cloud top morphologies and haze properties. Nominal sequential images with hours interval are used to understand the dynamics of the Venusian atmosphere by estimating wind vectors at the cloud top altitude, as well as mass transportation of the UV absorbers. UVI is equipped with an off-axial catadioptric optics, two bandpass filters and a diffuser installed in a filter wheel moving with a stepping motor, and a high-sensitive CCD detector with a UV coating. The UVI images have spatial resolutions from 0 m to km at sub spacecraft points. The UVI has been kept in a good condition during the extended interplanetary cruise thanks to the carefully designed operations such as temperature maintenance and avoiding solar radiance damage. The images have signal-to-noise ratios of over 0 after onboard desmear processing. Atsushi Yamazaki Japan Aerospace Exploration Agency JAPAN Corresponding Author Secondary Information: Corresponding Author's Institution: Japan Aerospace Exploration Agency Corresponding Author's Secondary Institution: First Author: Atsushi Yamazaki First Author Secondary Information: Order of Authors: Atsushi Yamazaki Manabu Yamada Yeon Joo Lee Shigeto Watanabe Takeshi Horinouchi Shin-ya Murakami Toru Kouyama Kazunori Ogohara Takeshi Imamura Takao M. Sato Yukio Yamamoto Tetsuya Fukuhara Powered by Editorial Manager and ProduXion Manager from Aries Systems Corporation

2 Hiroki Ando Ko-ichiro Sugiyama Seiko Takagi Hiroki Kashimura Shoko Ohtsuki Naru Hirata George L. Hashimoto Makoto Suzuki Chikako Hirose Munetaka Ueno Takehiko Satoh Takumi Abe Nobuaki Ishii Masato Nakamura Order of Authors Secondary Information: Suggested Reviewers: Sanjay Limaye Masahiro Takagi Woiciech Markiewicz Dmitri V Titov dmitri.titov@esa.int Opposed Reviewers: Manuscript Classifications: 0.0: Middle atmosphere Powered by Editorial Manager and ProduXion Manager from Aries Systems Corporation

3 Cover Letter Click here to download Cover Letter CoverLetter.pdf Dear Editor-in-Chief of Earth, Planets and Space We are sending under separate cover a manuscript entitled Ultraviolet Imager on Venus Orbiter Akatsuki and Its Initial Results by A. Yamazaki et al. together with the graphical abstract for the special issue of Akatsuki at Venus: The First Year of Scientific Operation. The contents include the calibration results before the launch and in orbit and the initial scientific results of UV image on Akatsuki. Please find the manuscript and the abstract. The authors would like to thank you for your kindness and attention. Sincerely yours, Atsushi Yamazaki

4 Manuscript Click here to download Manuscript VCO_UVI_EPS_.pdf Click here to view linked References Title page: Title: Ultraviolet Imager on Venus Orbiter Akatsuki and Its Initial Results Author #1: Atsushi Yamazaki (Corresponding author), Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), -1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa -0, Japan. ( nd affiliation) Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, --1 Hongo, Bunkyo-ku, Tokyo -00, Japan. Author #: Manabu Yamada, Planetary Exploration Research Center (PERC), Chiba Institute of Technology, --1 Tsudanuma, Narashino, Chiba -001, Japan. Author #: Yeon Joo Lee, Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), -1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa -0, Japan. Author #: Shigeto Watanabe, Hokkaido Information University, - Nishinopporo, Ebetsu, Hokkaido 0-, Japan. Author #: Takeshi Horinouchi, Faculty of Environmental Earth Science, Hokkaido University, NW Sapporo, Hokkaido 00-0, Japan.

5 Author #: Shin-ya Murakami, Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), -1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa -0, Japan. Author #: Toru Kouyama, Artificial Intelligence Research Center, National Institute of Advanced Industrial Science and Technology, -- Aomi, Koto-ku, Tokyo 1-00, Japan. Author #: Kazunori Ogohara, School of Engineering, University of Shiga Prefecture, 00 Hassaka-cho, Hikone, Shiga -, Japan. Author #: Takeshi Imamura, Graduate School of Frontier Sciences, The University of Tokyo, -1- Kashiwanoha, Kashiwa, Chiba -1, Japan. Author #: Takao M. Sato, Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), -1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa -0, Japan. Author #: Yukio Yamamoto, Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), -1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa -0, Japan.

6 Author #: Tetsuya Fukuhara, Department of Physics, Rikkyo University, --1 Nishi-Ikebukuro, Toshima-ku, Tokyo 1-01, Japan. Author #1: Hiroki Ando, Faculty of Science Kyoto Sangyo University, Motoyama, Kamigamo, Kita-ku, Kyoto, Kyoto 0-, Japan. Author #1: Ko-ichiro Sugiyama, Department of Information Engineering, National Institute of Technology, Matsue College, 1- Nishi-Ikuma, Matsue, Shimane 0-, Japan. Author #1: Seiko Takagi, Research and Information Center, Tokai University, -1-1 Kitakaname, Hiratsuka, Kanagawa -, Japan. Author #1: Hiroki Kashimura, Department of Planetology/Center for Planetary Science, Kobe University, -1-, Minamimachi, Minatojima, Chuo-ku, Kobe 0-00, Japan. Author #: Shoko Ohtsuki, School of Commerce, Senshu University, -1-1 Higashimita, Tama-ku, Kawasaki, Kabagawa -0, Japan. Author #: Naru Hirata, School of Computer Science and Engineering, The University of Aizu, 0 Kami-Iawase, Tsuruga, Ikki-machi, Aizu-Wakamatsu, Fukushima -0,

7 Japan. Author #: George L. Hashimoto, Department of Earth Science, Okayama University, -1-1 Tsushimanaka, Kita, Okayama 00-, Japan. Author #: Makoto Suzuki, Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), -1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa -0, Japan. Author #: Chikako Hirose, Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), -1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa -0, Japan. Author #: Munetaka Ueno, Center for Planetary Science (CPS), Graduate School of Science, Kobe University, -1- Minatojima-minamimachi, Chuo-ku, Kobe, Hyogo 0-00, Japan. Author #: Takehiko Satoh, Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), -1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa -0, Japan. ( nd affiliation) Department of Space and Astronautical Science, School of Physical Sciences, SOKENDAI, -1-1 Yoshinodai, Chuo-ku,

8 Sagamihara, Kanagawa -0, Japan. Author #: Takumi Abe, Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), -1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa -0, Japan. (nd affiliation) Department of Space and Astronautical Science, School of Physical Sciences, SOKENDAI, -1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa -0, Japan. Author #: Nobuaki Ishii, Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), -1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa -0, Japan. Author #: Masato Nakamura, Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), -1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa -0, Japan. Corresponding author: Author #1, Atsushi Yamazaki (yamazaki@stp.isas.jaxa.jp)

9 Abstract The ultraviolet imager (UVI) has been developed for the Akatsuki spacecraft (Venus Climate Orbiter mission). The UVI takes ultraviolet (UV) images of the solar radiation reflected by the Venusian clouds with narrow bandpass filters centered at the nm and nm wavelengths. There are absorption bands of SO and an unknown absorber in these wavelength regions. The UV images provide the spatial distribution of SO and the unknown absorber around cloud top altitudes. Also the images allow us to understand the cloud top morphologies and haze properties. Nominal sequential images with hours interval are used to understand the dynamics of the Venusian atmosphere by estimating wind vectors at the cloud top altitude, as well as mass transportation of the UV absorbers. UVI is equipped with an off-axial catadioptric optics, two bandpass filters and a diffuser installed in a filter wheel moving with a step motor, and a high-sensitive charge-coupled device (CCD) with a UV coating. The UVI images have spatial resolutions from 0 m to km at sub spacecraft points. The UVI has been kept in a good condition during the extended interplanetary cruise thanks to the carefully designed operations such as

10 temperature maintenance and avoiding solar radiance damage. The images have signal-to-noise ratios of over 0 after onboard desmear processing. Keywords Venus orbiter Akatsuki, Ultraviolet imager (UVI), Performance of UVI, UV images of Venus at the cloud top altitude, Initial results of cloud tracking

11 Main Text 1. Introduction Venus has a higher albedo than the earth, and its cloud scatters well the solar radiation. The Venus image is featureless in the range of the visible wavelength; on the other hand, the observed ultraviolet (UV) images have high contrast of bright and dark features, which reflected the distribution of UV absorbers. One of the UV absorbers is SO that has an absorption band in the wavelengths from 0 to nm. Another is an unknown absorption matter that shows absorption around 00 nm and shorter wavelengths according to previous observations (Esposito et al., ; Pollack et al., 0). Therefore differently from featureless visible Venus images, the UV images present unique cloud morphologies, including well known Y-feature even observable from ground-based observations (Dollfus, ). The Venus UV image observations have about 0 years of history starting from ground-based observations (Wright, ; Ross, ) to Hubble space telescope (Na and Esposito, ), and several spacecrafts observations were performed by the use of flybys opportunities (Mariner (Bruce et

12 al., ), Galileo (Belton et al., 1) and Messenger), and by using Venus orbiters (Venera (Ksanfomaliti et al., ), Pioneer Venus (Travis et al., ; Pollack et al., ; Stewart et al., ), and Venus Express (Markiewicz et al., 0a, b; Titov et al., 0)). Venus orbiters could provide astonishing details of cloud features that cannot be resolved by ground-based telescopes, such as polar vortex, equatorial convective cells, bright polar hoods, and so on (Rossow et al., 0;Titov et al., 0). As a new generation of Venus orbiter, Akatsuki (Nakamura et al., 0, ) has successfully inserted into the Venus orbit in December 1 (Nakamura et al., 1), and the onboard ultraviolet imager (UVI) continues Venus UV observations at and nm. These images are used to investigate the distributions of the absorbers, such as SO and the unknown matter in the Venus mesosphere. Other scientific targets are the derivation of wind vectors with cloud tracking techniques (e.g., Limaye and Suomi, 1; Moissl et al., 0; Ogohara et al., ; Kouyama et al., ; Ikegawa and Horinouchi, 1; Horinouchi et al., a), and the retrieval of the vertical distribution of haze from limb observations.

13 The H SO clouds (Esposito et al.,, ) are formed by photochemical reactions from SO and H O near the cloud top altitude. However, the process of SO transportation in the Venus atmosphere is not yet well understood. SO is abundant below the clouds, but it is unclear how SO could be transported to the cloud top region (Ignatiev et al., 0), where the stratification is statically stable. On the other hand, the unknown UV absorber is responsible for a considerable solar heating at the cloud top level (Crisp and Titov ; Lee et al. 1). The heating might results in the generation of thermal tides that would contribute to the maintenance of the strong zonal winds near the cloud top level (e.g., Takagi and Matsuda 0). Winds at the Venusian cloud top have been obtained from tracking of cloud morphologies observed by Orbiter Cloud Photo-polarimeter (OCPP) on Pioneer Venus Orbiter (Rossow et al., 0), Venus Monitoring Camera (VMC) on Venus Express (Markiewicz et al., 0b) and other spacecraft missions. The flow is almost zonal and westward, i.e., in the same direction as Venus rotation. The wind is so-called super-rotation due to about sixty times faster speed than the Venus ground. The speed of the zonal westward wind increases with altitude and reaches ~0 m/s near the cloud

14 top altitude. The generation mechanisms of super-rotation have been proposed by many authors (e.g., Gierasch, ; Gierasch et al., ), but the mechanism is still unclear because we do not have enough data of the three-dimensional wind distribution. The coupling among waves, cloud processes and global-scale winds is also important to understand the dynamics of the Venus atmosphere. We have the following research targets to be pursued by UVI: (1) Large-scale (1,000-0,000 km) to mesoscale (1-00 km) cloud morphologies, () Three-dimensional haze distribution, () Interaction between the lower and the middle atmosphere, () Generation, propagation and dissipation of planetary waves and gravity waves, and their interaction with general circulation, () Generation of super-rotation, () Distribution of unidentified ultraviolet absorber, () Distribution of SO, and the photochemical processes related to H SO formation, and () Cloud aerosol microphysical properties.. Instrumentation and Observation.1 Instrumental design

15 The UVI instrument consists of two parts: a sensor (UVI-S) and an analog electronics (UVI-AE) shown in Figure 1. The UVI-S consists of band pass filters installed in a filter wheel turret, off-axial catadioptric optics, and the detector of a charge-coupled device (CCD) with a preamplifier circuit. The optical layout is shown in Figure. The UVI-AE includes a power supply unit and a control and readout circuits of the CCD detector. The electric power is W in the observation mode, and W at the filter wheel rotation. The UVI specification is shown in Table 1. The UVI-S has an off-axial catadioptric optics that consists of two lenses and two reflecting surfaces. The optics has a composite focal length of. mm and an F-number of 1. The x field-of-view (FOV) can capture the whole Venus disk during % of one orbit, except ~ hours near a periapsis. The filter wheel has four positions. Two interference band pass filters and one diffuser are installed in the filter wheel. The interference filters select the observational wavelengths of nm and nm both with a bandpass width of 1 nm. The wavelength of nm is located in the midst of the strong absorption band of SO (Stewart et al., ). The wavelength of nm is in the broad absorption of the unknown UV absorbers, whose strong contrasts

16 enable us to track the cloud morphology easily, as previous Venus orbiters observations had reported. The diffuser is used for the onboard flat field calibration, such as measurement of correction of relative sensitivity between pixels. The shutter position is used to obtain the noise counts. The wheel positions are controlled by a step motor and two hall sensors to know the wheel rotation angle, and they are also used to automatically return the wheel to the shutter position. UVI-S has a full-frame transfer CCD of a back illuminated type with a UV sensitive coating, the pixel size of 1 μm, and the imaging area of x pixels. The angular resolution is 0.0, which corresponds to the spatial resolutions of ~0 m and ~ km on the cloud top level in the observations from the altitudes of ~00 km at the periapsis and ~0 R V (the radius of Venus) at apoapsis, respectively. The readout signal of the CCD has the data depth of bits. The data processing at the Digital Electronics (DE) equipment for the imaging sensors on Akatsuki, such as median filtering, subtraction of dark current, and desmearing, is performed after converting the original bits-depth data to 1 bits-depth data. The image is compressed to reduce the size from Mbytes to several hundred KBytes using an onboard application program.

17 Observations and onboard data processing The exposure time of UVI can be selected from 0.00 to sec with steps by a command. The chosen exposure time is 0. s before June 1 and or 0.0 s after June 1 for nm, while the exposure time is 0.0 sec for nm. The CCD detector with no electrical cooler system is thermally in contact with a cooling radiator on the outside panel of the spacecraft body to reduce the dark current. The radiator has an area to make the CCD temperature be less than C in the observation mode. The signal-to-noise ratio of raw images exceeds with this temperature before image data processing. The CCD has no mechanical shutter, so a smear noise in transferring from the image area to the storage area on the chip degrades the signal-to-noise ratio of the obtained image, especially in the case of the short exposure. In the nominal operation of UVI, images ( Venus and shutter images) are used for smear correction for each wavelength in the onboard data processing at the DE equipment. A set of three images taken in the same condition yields one median image. Then, letting T be the chosen exposure time, the breakdown of the images is: a set of 0-s exposure shutter images

18 and a set of T-s exposure shutter images before Venus shots; a set of 0-s Venus images and a set of T-s Venus images; and a set of 0-s shutter images and a set of T-s shutter images after Venus shots. The Venus and shutter images with the same exposure time are used to remove the dark and noise count. The image with 0-s exposure is subtracted from the image with the chosen exposure time; this procedure is called desmearing. As a result of the data processing, the signal-to-noise ratio of the UVI image improves to over 0. The above onboard processing is done in the portion of the orbit where Venus moves slowly as seen from Akatsuki. When Venus is moving fast as is observed around the periapsis, the above method of the smear correction is not performed effectively. In this case the count in an optical black area of CCD, which is permanently masked outside the imaging area, is used to make the smear correction at the ground. Calibration Results.1 Performances measured before the launch The transmittances of two interference filters and one diffuser installed in the filter

19 wheel were measured before the launch to be shown in Figures a and b. The measurement errors are within the size of marks in the figures. Both filters have the effective bandwidth of 1 nm, and the transmittance is ~0 % for nm and ~ % for nm. The diffuser has a broad bandpass of 0 nm spanning these two observational wavelengths with the transmittance of a few %. The CCD detector adopted for UVI has the UV coating to enhance its quantum efficiency up to 0 % in the observational wavelengths range, and has a saturation level of 1,000 electrons per pixel. It is one of the full-frame transfer types, and has three areas: the image area, storage area, and optical black area. The counts of the first two areas are used for the data processing at DE, and the counts of the last area are used for the smear correction at the ground data processing. The noise count rate of the CCD detector through the readout electronics tends to increase rapidly with temperature at the CCD temperature over - C, and is less than counts/sec at the temperature below a few degrees (Figure ), which is achieved by cooling with the radiator under the normal observation mode. The nominal exposure time for the Venus observation is determined as the time where the signal level becomes

20 half the saturation level of the CCD device. The signal-to-noise ratio achieves over for the raw image and over 0 after the onboard smear correction at the normal observation, which satisfies the scientific requirement. The flat field image, the distortion image, and the spatial frequency response (SFR) were obtained as the total optical performance check of UVI-S. The image of a surface light source at the wavelength of nm was obtained using the integrating sphere at the optical facility in the Earth Observation Research Center (EORC) at JAXA. The system is absolutely calibrated with the nonuniformity of the area light source of below 1%. The obtained raw image without any optical correction, such as the cosine fourth law correction due to vignetting, is shown in Figure. The count rate derives the sensitivity of UVI throughout the filter, optical lens, and CCD detector before the launch. It is noted from Figure that the angular diameter of the integrating sphere aperture viewed from UVI was circle, and that the FOV of UVI is square. The count rate directed to the center area of the integrating sphere aperture is correct in order to derive UVI's total sensitivity. Therefore the direction of UVI field-of-view was changed into x directions to obtain images of the integrating sphere and the

21 sensitivity of areas of UVI field-of-view was independently calibrated. The averaged image created from the images derived the sensitivity of the whole field-of-view. The flat field pattern was also obtained as Venus images taken by using the diffuser in the orbit around Venus. The diffuser images of Venus are used to create the calibration flat pattern for the data processing on the ground. The UVI image data released from DARTS (Data ARchives and Transmission System) of ISAS are corrected by the flat pattern and did not have the clear nonuniformity caused from the sensitivity difference of CCD detector pixel by pixel. However, the flat pattern is not a perfect correction for photometry analysis which requires careful treatment of the brightness. It is because the brightness gradient of one image due to vignetting slightly remains. Therefore the factor (a flat conversion factor), which is based on the pixel sensitivity estimated from the calibration results using the integrating sphere before the launch, is also prepared at DARTS. It is recommended that the released image data set is used basically for morphology analysis and that the flat conversion factors are used for photometry studies. The product of the conversion factors and the released data for morphology analysis serves as the absolutely-calibrated brightness for photometry

22 analysis. The distortion was measured by using black-light lamps with two different sizes of rectangle masks. One pattern has three.-cm and one -cm wide masks. Four patterns e are set up at one time at.-m distance from UVI-S with -cm interval. UVI-S is mounted on a tilt-and-swivel base to change its line-of-sight direction to positions ( elevation angles x 1 azimuth angles). images are superimposed to create one image shown in Figure. The image reveals that the distortion is 0. % at the direction of. angle away from the optical axis. Several images of a test chart were taken to estimate an index of the optical performance, SFR. The measured SFR shown in Figure as a function of the chart pattern frequency of the line width per picture height (LW/PH) indicates that the limiting resolution of UVI is 0 LW/PH with SFR of %.. Variation of total sensitivity estimated from the onboard calibration The total sensitivity of UVI is calibrated from the results of the ground experiments before the launch. The values of the - and -nm channels, which convert pixel count rate to radiance, are and 0

23 [W/m /sr/m/count rate], respectively. The onboard calibration is performed based on the star observations after the launch. Star fields of Sagittarius and Scorpius were measured during cruising (Oct. ; before Venus orbit insertion) and in orbit (Feb. and Sep. 1). The observed star flux is compared to a known value, so a calibration factor (β) can be derived as where Fobs is the observed star flux by UVI, and Fexp is the known star flux. Fexp is calculated as where λ is wavelength, T is the transmittance profile of the nm filter, and Fstar is a star flux spectrum, which is taken from Pulkovo Spectrophotometric Catalog (Alekseeva et al., the data is downloaded from The observed radiance (W/m /sr/μm) of stars is converted to flux (W/m /μm), multiplying a solid angle of one pixel, pix = (0.000) square radian. Then, the star flux Fobs has been calculated using the aperture

24 photometry technique which is widely used in star flux calculations for ground-based observations (Mighell ; Laher et al. ). The -nm channel is also measured in the same method using data obtained by International Ultraviolet Explorer (IUE). Table shows the summary of measured calibration factor β for both of - and -nm channels. Initial Results.1 Operation in the orbit of Venus UVI is activated by executing the observation programs on the DE equipment, and is nominally performed every hours; higher observation frequencies are inhibited in order to keep the thermal condition of the instrument. UVI images taken so far cover scattering phase angles from 0 to 1 degrees. The scattering property as a function of the phase angle is reported by Lee et al. (). The observation programs enable us to perform collaborative observations with the Longwave Infrared Camera (LIR) (Taguchi et al., 0; Fukuhara et al.,, ), the 1μm camera (IR1)

25 (Iwagami et al.,, ) and the μm camera (IR) (Satoh et al., 1, ). Some special observations of two UVI shots with short time interval (minimum of minutes) can be performed around the periapsis. Quasi-simultaneous observations with the radio occultation experiment (Radio Science) (Imamura et al.,, ) and the Lightning and Airglow Camera (LAC) (Takahashi et al., 0; Imai et al., ) are also performed around the opportunities of radio occultation and eclipse.. Initial Venus images UVI sample images of - and -nm wavelength are displayed in Figure. The images are taken at :1, and : UTC on April in 1, respectively, with the exposure times of 0.0 sec and 0. sec, and with the distance of Venus from Akatsuki of,1 km and,km. The local time and the latitude at the sub-spacecraft point are. LT and. degrees, respectively. The upside of images corresponds to northward. The large-scale structures seen in the images, such as the bright structure extended in the east-west direction in the mid-latitudes, are similar with each other. But in

26 detail there are features quite different from each other. The radiance of -nm image is about ten times larger than that of -nm image. The -nm image shows more contrast and the bright area in the equatorial region near the center of the image. These differences shown in the UVI images suggest that the spatial distributions of SO and the unknown UV absorber are governed by, at least partly, different chemical and/or dynamical processes.. Cloud tracking UVI images are used to estimate horizontal winds by tracking cloud features. An example derived from the three -nm images taken at,, h UTC on December, 1, the day of Akatsuki s orbital insertion, is shown in Figure. The tracking is based on the automated method described in Ikegawa and Horinouchi (1) and Horinouchi et al. (a). The method is based on the template matching, but unlike in the earlier studies (e.g., Kouyama et al ), it utilizes image combinations from more than two images. We utilized a measure of precision based on the sharpness

27 of the cross-correlation surfaces (Ikegawa and Horinouchi, 1), so that the results are shown in the figures only when the estimated precision is better (smaller) than m/s; a complete description of their derivation is found in the online supplement of Horinouchi et al. (b). The zonal wind exhibits the superrotation at around 0 m/s. The spatial distribution of horizontal winds is consistent with the divergent tidal flow as shown in earlier studies (e.g., Del Genio and Rossow, 0); the sub-solar longitude at observation time was 1, so the region where winds are obtained is in the local afternoon. Summary The ultraviolet imager (UVI) on the Akatsuki spacecraft has started the Venus observations successfully. The imager nominally takes a pair of UV images at -nm, which is sensitive to SO, and -nm, which is in the absorption band of an unknown material, every two hours by executing the observation programs of the DE application. The spatial resolution is 0 m

28 in the observation from the periapsis of ~00 km altitude, while it is ~ km from the apoapsis of ~0 Rv altitude. After the onboard processing in including dark count subtraction, median filtering and desmearing at DE, the signal-to-noise ratio of UVI images achieves over 0, which satisfies the scientific requirement. Correction for the pixel-to-pixel inhomogeneity of the detector sensitivity is made at the ground. The imager has been kept under good temperature condition during the years of cruise, and does not show a sign of significant degradation yet. The images obtained so far show both similarities and differences between the two wavelengths. Comparison of the images would provide clues to the three-dimensional distributions of the UV absorbers and clouds as well as cloud morphologies. Cloud tracking using sequential images reveals the circulation structure in the cloud top region. The scattering properties at the cloud top altitude in UV range, such as the phase angle dependence, can be analyzed to reveal microphysical parameters of cloud particles and constrain the vertical distributions of SO and unknown absorbers (Lee et al.

29 ). Combining the information, UVI has a potential to reveal photochemical and dynamical processes that play crucial roles in the formation of HSO clouds. UVI data, combined with data from other onboard instruments, are also used for the investigation of the generation mechanism of super-rotation.

30 Declarations List of abbreviations UVI : Ultraviolet Imager UV : ultraviolet SO : Sulfur Dioxide CCD : Charge Coupled Device HSO : Sulfuric Acid HO : water OCPP : Orbiter Cloud Photo-polarimeter VMC : Venus Monitoring Camera UVI-S : Sensor of UVI UVI-AE : Analog Electronics of UVI FOV : Field-Of-View R V : the radius of Venus DE : Digital Electronics

31 SFR : Spatial Frequency Response EORC : Earth Observation Research Center JAXA : Japan Aerospace Exploration Agency DARTS : Data Archives and Transmission System ISAS : Institute of Space and Astronautical Science LW/PH : Line Width per Picture Height IUE : International Ultraviolet Explorer LIR : Longwave Infrared Camera IR1 : 1μm camera IR : μm camera LAC : Lightning and Airglow Camera UTC : Coordinated Universal Time LT : Local Time Ethics approval and consent to participate and consent to publish Not applicable.

32 Availability of data and materials Akatsuki data will be available from the DARTS of ISAS as well as NASA s Planetary Data System in summer. Competing interests There are no financial and non-financial competing interests. Authors' contributions AY is responsible to all elements of this manuscript. MY, YJL, SW, TI, as the UVI team members, enabled the UVI observations and contributed to analysis of the onboard calibration. GLH, MU, MS, MN, as the UVI team members, contributed to design of UVI. TH, SM, TK, KO, enabled the derivation of the wind vector from the cloud tracking methods. TMS, TF, HA, KS, ST, HK, SO, TA, TS, enabled scientific operations including the Venus orbit insertion of spacecraft. NH, YY contributed to data-processing pipeline.

33 CH, NI enabled special operations such as the Venus orbit insertion of spacecraft. Acknowledgements The authors are grateful to all the members of the UVI instrument team for their excellent support and effort in completing UVI, especially NEC corporation, NEC Space Technologies, Ltd., Nikon Corporation, FUJITOK Corporation, Tamagawa Seiki Co., Ltd., and Cornes Technologies. The authors are also grateful to all the members of the Akatsuki/VCO project team for their remarkable support in developing the Akatsuki spacecraft. The authors appreciate to Drs. Markiewicz, W. J., Keller, H. U., and Titov, D. V. for their kind and helpful implications and suggestions about the design of UVI. The authors also thank to the members of the optical facility in Earth Observation Research Center (EORC) of JAXA and the National Astronomical Observatory of Japan (NAOJ) for the ground-based calibration experiments before the launch. IUE UV star spectra were obtained from the Mikulski Archive for Space Telescopes (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under

34 NASA contract NAS-. Support for MAST for non-hst data is provided by the NASA Office of Space Science via grant NNX0AF0G and by other grants and contracts. Authors' information SW is the Principal Investigator of UVI. MN is the Project Manager, NI is Project Engineer, TS is the Project Scientist, and TI is the former Project Scientist of Akatsuki.. Endnotes None.

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48 Violet Light. Publications of the Astronomical Society of the Pacific :0. doi:./ Figure legends Figure 1: Photographs of UVI-S (left) and UVI-AE (right). The radiator for the CCD cooling is hiding under the baseplate of the instrument in the left panel. Figure : Schematic layout of UVI. Figure : (a) Transmittance of the interference filters for the and nm channels (circles and squares) and (b) transmittance of diffuser (open circles). Figure : The temperature dependence of the CCD dark counts through the electronics.

49 Figure : Raw flat image of the integrating sphere using the diffuser with no correction. Figure : Measured distortion pattern of UVI. Figure : SFR of the UVI imager. Figure : Time variation of calibration factor for the nm channel. Figure : Examples of UVI -nm image (left) and -nm image (right). Figure Horizontal velocities derived from three -nm images on the day of Akatsuki s Venus orbit insertion ( December ). Arrows show their deviation from the uniform westward wind of m/s The length scale of the arrows is shown in m/s near the lower-right corner. Gray-scale shading

50 shows the highcloud tracking. Table 1: Specification of UVI. Table : Calibration factor β at and nm.

51 Figures Figure 1: Photographs of UVI-S (left) and UVI-AE (right). The radiator for the CCD cooling is hiding under the baseplate of the instrument in the left panel.

52 Figure : Schematic layout of UVI

53 a) b) Figure : (a) Transmittance of the interference filters for the and nm channels (circles and squares) and (b) transmittance of diffuser (open circles).

54 Figure : The temperature dependence of the CCD dark counts through the electronics.

55 Figure : Raw flat image of the integrating sphere using the diffuser with no correction.

56 Figure : Measured distortion pattern of UVI

57 Figure : SFR of the UVI imager

58 Figure : Time variation of calibration factor for the nm channel.

59 Radiance [W/m /sr/m] Figure : Examples of UVI -nm image (left) and -nm image (right).

60 Figure : Horizontal velocities derived from three -nm images on the day of Akatsuki s Venus orbit insertion ( December ). Arrows show their deviation from the uniform westward wind of m/s The length scale of the arrows is shown in m/s near the lower-right corner. Gray-scale shading shows the high-passed radiance (W m sr 1 m 1 ) at the beginning of cloud tracking.