LIR: Longwave Infrared Camera onboard the Venus orbiter Akatsuki
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The Longwave Infrared Camera (LIR) is one of a suite of cameras onboard the Venus orbiter Akatsuki. It will take images of thermal radiation in the wavelength range of 8–12 µm emitted by the Venus cloud tops. The use of an uncooled micro-bolometer array as an infrared image sensor makes LIR a lightweight, small and low-power consumption instrument with a required noise equivalent temperature difference of 0.3 K. Temperature and horizontal wind fields at the cloud-top will be retrieved for both dayside and nightside with equal quality. This will provide key observations to understand the mechanism of super rotation and the thermal budget of the planet. LIR will also monitor variations of the polar dipole and collar which are characteristic thermal features in the Venusian atmosphere. Mechanisms of the upper-cloud formation will be investigated using sequences of close-up images. The morphology of the nightside upper cloud will be studied in detail for the first time.
Key wordsVenus Planet-C atmosphere
The energy budget, dynamics and chemical cycle of the Venusian atmosphere is strongly influenced by the H2SO4-H2O clouds, which float at around 45–70 km altitudes (Esposito et al., 1983). H2SO4 is thought to be produced photochemically near the cloud top via the oxidation of SO2, which is abundant below the cloud top, and thus the clouds basically have the characteristics of photochemical aerosols. On the other hand, a strong coupling between cloud condensation and atmospheric motion is expected to occur in the lower part of the cloud layer, where the heating of clouds by upwelling infrared radiation drives vertical convection (Baker and Schubert, 1992; Imamura and Hashimoto, 2001).
The cloud layer absorbs solar radiation, thereby driving various atmospheric motions that might play a central role in the momentum balance of the super-rotation. The super-rotation is a planet-wide easterly wind on Venus; the wind speed increases with height at all latitudes and reaches about 100 m s−1 near the cloud top (Schubert et al., 1980). Various mechanisms explaining the super-rotation have been proposed to date (Gierasch et al., 1997). Among them is the combination of a thermally-driven Hadley circulation and large-scale eddies which transports angular momentum equatorward, causing a net upward transport of angular momentum (Gierasch, 1975; Rossow and Williams, 1979; Iga and Matsuda, 2005). Another candidate is acceleration by thermal tides, which are excited in the cloud layer by periodic solar heating and which propagate vertically to induce momentum exchange between atmospheric layers (Fels and Lindzen, 1974; Newman and Leovy, 1992; Takagi and Matsuda, 2005, 2007). Diurnal and semidiurnal tides are observed in the temperature structure above clouds (Taylor et al., 1980; Zasova et al., 2002; Tellmann et al., 2009) and in the cloud-top wind field (Rossow et al., 1990). The dissipation, at cloud heights, of Kelvin waves that originate in the lower atmosphere might also accelerate the atmosphere (Del Genio and Rossow, 1990; Yamamoto and Tanaka, 1997). Characterization of the dynamics at cloud heights is crucial for understanding the super-rotation.
The cloud-top wind field has been observed by tracking the movements of small-scale ultraviolet features seen in the dayside cloud images taken from Venus orbiters (e.g., Rossow et al., 1990; Moissl et al., 2009). These observations revealed a mean poleward flow with a velocity of up to 10 m s−1 at the cloud top. However, the limitation of the local time coverage may allow a significant contamination of the thermal tide component in the estimated meridional circulation, which can be as large as 10 m s−1 (Newman and Leovy, 1992). Characterization of the meridional circulation requires wind measurements at all local times.
It should be noted that these ultraviolet features do not necessarily represent the cloud structure, but reflect the horizontal inhomogeneities of ultraviolet absorbers. The solar ultraviolet radiation scattered by the cloud top shows absorption by SO2 at wavelengths shorter than 320 nm and by unknown materials at longer wavelengths (Esposito et al., 1997). The mixing ratios of both SO2 and the unknown absorber are considered to increase precipitously with decreasing altitude below the cloud top (Pollack et al., 1980; Bertaux et al., 1996), and thus the spatial distributions of these species should also be sensitive to vertical air motions. Such vertical winds may also influence the cloud-top height, but the relation of the cloud height to the vertical wind is uncertain. Recently, cloud altimetry using scattered near-infrared solar radiation showed that ultraviolet dark features tend to correspond to higher clouds in the southern high latitude (Ignatiev et al., 2009). Methods to map the cloud-top height over broad local times and latitudinal regions would further constrain the cloud dynamics.
Efforts to map the thermal emission from the cloud top of Venus have been made using ground-based telescopes (e.g., Apt et al., 1980) and infrared radiometers onboard the Pioneer Venus orbiter (Taylor et al., 1980) and Venera 15 and 16 (Zasova et al., 2007). They revealed the structures of thermal tides and other planetary-scale waves as well as the polar dipole structure. The visible and Infrared Thermal Imaging Spectrometer (VIRTIS) onboard Venus Express revealed that the temperature distribution in the south polar region is similar to that in the north polar region (Piccioni et al., 2007). However, the observations have been limited in spatial resolution, temporal resolution and the latitudinal coverage, preventing the studies of mesoscale processes and the derivation of wind field by cloud tracking.
Akatsuki, which is the first Japanese Venus orbiter, aims at understanding the atmospheric dynamics and cloud physics of Venus. Akatsuki maps clouds and minor constituents successively with four cameras covering wavelengths from infrared to ultraviolet, detects lightning flashes with a high-speed photometer, and retrieves the vertical structure of the atmosphere with a radio occultation technique (Nakamura et al., 2007, 2011). The planned orbit around Venus is a 30-hour-period elliptical orbit (370–78,500 km) near the ecliptic plane, and cloud images will be obtained every 2 hours for each observation wavelength. Science instruments altogether observe multiple height levels of the atmosphere to model the three-dimensional structure and dynamics. Among them is the Longwave Infrared Camera (LIR), which maps the thermal emission from the cloud top at 8–12 µm wavelengths (Taguchi et al., 2007). Unlike other cameras onboard Akatsuki, LIR is able to take images of dayside and nightside clouds with equal quality. This is advantageous not only to the studies of the diurnal cycles of cloud processes but also to the precise determination of the zonal-mean meridional circulation.
2. Instrument Design
Specification of LIR.
Field of view
16.4° × 12.4°
Number of pixels
0.3 K (at 230 K)
The control of LIR for sequential image acquisition and the subsequent data processing such as averaging and offset subtraction are performed by the Digital Electronics (DE), which is an onboard controller for four cameras including LIR (Nakamura et al., 2007). The amount of data transfer to the ground station is reduced by DE using a lossless data compression algorithm called HIREW (Takada et al., 2007). There still remains a slight inhomogeneity of the sensitivity, even in a subtracted image. This will be corrected after being transmitted to the ground. The resultant image, which gives the difference of the brightness temperature between the object and the shutter, is further converted to a brightness temperature map by adding the temperature of the shutter to all pixel values.
3. Evaluation Test of the Flight Model
3.1 Measurement of the NETD
3.2 Endurance test of the shutter
The stepping motor used for driving the shutter requires to be of high reliability. Since the number of operations in a mission period is expected to be a maximum of 120,000 times, a trial endurance test has been carried out in a vacuum environment using a shutter component. The motor has attained an operation of 240,000 times for about one month with round-the-clock operation.
3.3 Tolerance of the UMBA to vibration, radiation and sunlight
The tolerance of the UMBA to the launch environment was evaluated by using a vibration testing machine. Random vibration levels that were calculated using a mathematical structure model of LIR were imposed on the UMBA for 45 seconds for three axes; the spectrally-integrated levels were several tens of Grms. The function of the UMBA was normal after the vibration test without any pixel defects or malfunctions.
Since this is the first time for this type of UMBA to be used in the space environment, tolerance of the UMBA to high-energy protons was evaluated by exposing the UMBA to a 100 MeV proton beam at the rate of 4 Gy/min with a total dose of 300 Gy which exceeds what is expected during the mission life. The UMBA was set active during the test and no malfunction was observed during the exposure to the proton beam. After the exposure, the UMBA was inspected and shown to have the nominal sensitivity, NETD and no pixel defects.
3.4 Temperature control of camera
As stated in the previous section, the UMBA detects not only the thermal radiation from the Venusian atmosphere, but also that from the lens and optical mount. The temperatures of lens and optical mount must be controlled so that their influences on an image are minimized. It has been estimated by calculation, using the nominal transmittance of the lens, that a stability of ±2.3 K is required for the lens temperature, which is almost equivalent to the optical mount temperatures. In order to confirm the temperature range requirement, the flight model of LIR took images of a uniform temperature blackbody under different thermal conditions. Both of the signal outputs for the shutter and the object must be covered simultaneously by the 12 bits dynamic range of the A/D converter. It is concluded that if the optical mount temperature is within 293.5–304.6 K, LIR can take an image of an object whose temperature is as low as 203 K.
LIR developed for Akatsuki, the first Japanese Venus Climate Orbiter, the first lightweight infrared camera to use an uncooled micro-bolometer array as an image sensor. The infrared camera observes the thermal emission from the cloud top of Venus in the wavelength region 8 – 12 µm to obtain two-dimensional distributions of temperature and wind vector at the cloud top, where the temperature is typically as low as 230 K. The flight model of LIR has been manufactured and its performance was confirmed by several tests in a vacuum environment. It has been shown that the target performance of NETD∼0.3 K at an object temperature of ∼230 K is achieved by averaging several tens of images acquired within a few minutes. This NETD corresponds to a height difference at a cloud top of 100 m for an average temperature profile. The camera-case temperature should be stabilized within the temperature range of 293.5–304.6 K in order to take an image of an object having a temperature as low as 203 K. The tolerances of the bolometer array to sunlight, the mechanical environment, and high-energy protons, have been tested and satisfy the requirements. It is also confirmed that the shutter mechanism has enough endurance for the mission life of two years.
LIR has been mounted on Akatsuki which was launched on 21 May, 2010. LIR and the other cameras onboard Akatsuki have taken images of the Earth immediately after the launch. Furthermore, LIR has taken images of deep space for calibrations of the UMBA. However, the Venus orbit insertion maneuver for Akatsuki on December 7, 2010, failed. At present the spacecraft is orbiting the Sun, and it will have a chance to encounter Venus in 5 or 6 years time. JAXA is examining the possibility of conducting an orbit insertion maneuver again at this opportunity.
The authors wish to thank Mr. Higashino, Mr. Kashikawa and other engineers of NT Space Co. Inc. for their efforts in manufacturing LIR.
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