Absolute calibration of brightness temperature of the Venus disk observed by the Longwave Infrared Camera onboard Akatsuki

The Venus Climate Orbiter Akatsuki was successfully inserted into a Venus orbit in December 2015 after spending 5 years orbiting the sun under unintended environmental conditions. Subsequently, five cameras and an ultra-stable oscillator for a radio occultation experiment onboard Akatsuki started observation of the Venus atmosphere (Nakamura et al. 2016). The Longwave Infrared Camera (LIR), which is one of the five cameras onboard Akatsuki, maps of brightness temperature of the Venus disk on both day and night hemispheres by detecting emissions at wavelengths from 8 to 12 μm (Fukuhara et al. 2011). Radiative transfer calculations using a typical cloud height distribution of Venus atmosphere indicate that the thermal infrared radiation emitted from an altitude of ~ 65 km contributes most to the thermal contrast seen in the LIR images (Taguchi et al. 2007). The brightness temperature at that altitude is generally ~ 230 K according to the vertical temperature distribution of the Venus atmosphere derived from previous observations (e.g., Seiff et al. 1985; Pollack et al. 1993; Zasova et al. 2007; Tellman 2009). Thus, the wavelength band of LIR was designed to observe the temperatures expected at ~ 65 km. The noise equivalent temperature difference (NETD), which describes relative temperature resolution, is ~ 0.3 K when the temperature of a target is ~ 230 K. The NETD corresponds to an altitude difference of a few hundred meters at the cloud-top layer on Venus (Fukuhara et al. 2011).

Immediately after the orbit insertion of the spacecraft, LIR discovered a large stationary gravity wave that appeared above Aphrodite terra close to 18:00 local solar time (Fukuhara et al. 2017). Although LIR cannot directly observe this wave at lower altitudes, a simulation showed that a wave generated by an atmospheric perturbation caused by the surface topography could propagate to the cloud-top layer. Since 1- and 2-μm cameras (IR1 and IR2) observe an aspect of the lower atmosphere in nightside by detecting the wavelengths in the atmospheric windows of Venus (Iwagami et al. 2011; Satoh et al. 2015), comparison of images acquired by LIR with those acquired by IR1 and IR2 should reveal the propagation process of the stationary gravity wave in some detail.

The radio occultation experiment termed Radio Science (RS) retrieves vertical profiles of the atmospheric temperature, the pressure, mixing ratio of the sulfuric acid vapor, and the electron density (Imamura et al. 2011). Synchronous observation by LIR and RS identifies the altitude of the cloud-top layer observed by LIR. On the other hand, Ultraviolet Imager (UVI), which is sensitive to wavelengths of 283 and 365 nm, observes SO₂ and an unknown UV absorber distributed above ~ 65 km (Nakamura et al. 2011). Observations by UVI and LIR are typically synchronized. When horizontal distributions of the UV absorbers derived from UVI observation are correlated with the brightness temperature distributions derived from LIR observation, the altitude of the UV absorbers can be identified by comparison of the UV images with LIR images. Furthermore, vertical variation of the UV absorbers may be revealed by the continuously synchronized observation.

Brightness temperatures derived from a sequence of LIR images are expected to show signatures of thermal tides, convection and perhaps other atmospheric processes. Further, since LIR measures a major portion of the thermal emission from Venus, the accumulated data will provide useful information about the radiative balance of its atmosphere. The LIR images acquired immediately after the failure of orbit insertion in 2010 showed that the mean brightness temperature at the low latitudes in the nightside of Venus was ~ 243 K. This temperature is higher than those previously inferred and not consistent with the expected variation. The reason for the discrepancy could not be determined at this time, despite some speculation about local time dependence, as discussed in Taguchi et al. (2012).

As discussed above, not only thermal contrast but also brightness temperature is a major scientific product for LIR. Moreover, the brightness temperature is often compared with the results acquired from other instruments mounted on the spacecraft in order to retrieve physical quantities. Accuracy of the physical quantities derived from such comparisons partly depends on the accuracy of the absolute brightness temperature, whose error was estimated in the design process at ~ 3 K as a standard deviation of brightness temperature at 230 K. The standard deviation may have increased due to degradation of the sensitivity of the detector during 5 years of extended orbiting around the sun, or some other unexpected condition affecting LIR at the time of observation. The unexpected standard deviation would introduce some ambiguity in the interpretation of the atmospheric processes. For instance, the temperature discrepancy seen in Taguchi et al. (2012) is a suspicious case in which a systematic bias far larger than 3 K standard deviation may be involved. Therefore, we undertook an effort to verify the performance of LIR observation on orbit at an early stage so that any error could be precisely estimated.

LIR has continuously acquired more than 8000 images without serious faults for the first two Venus years since the orbit insertion. The mechanical shutter with a stepping motor, whose prototype has been driven 650,000 times in the laboratory experiment (Fukuhara et al. 2011), has been operated 86,000 times on orbit without any mechanical trouble. However, the observational lifetime of Akatsuki was originally designed for 4 years after a half-year cruising (Nakamura et al. 2011). Therefore, major elements that may affect contrast in LIR images were initially checked on orbit.

Background variation in the observed image

Average temperatures of the Venus disk and deep space were retrieved from the brightness temperature of each LIR image derived from formula (3). Figure 2 shows the temperature variations in the first Venusian year after the orbit insertion. Both the Venus disk and deep space have a temperature variation with amplitude of more than 20 K and a similar trace. Since formula (3) does not follow Plank’s law except at temperatures from 210 to 250 K, the deep-space temperature should converge at ~ 180 K. However, the large variation shown in Fig. 2 is contrary to this expectation. The short-term variation seen in the figure is coincident with the orbital period of approximately 10 earth days. Meanwhile, the long-term variation depends on the orbital period of Venus, which gradually changes the geometry of the spacecraft and Venus at the time of observation. The baffle and the power supply unit were major suspects for the temperature variations because they are not thermally stabilized, and hence, they could be the source of background bias.

Brightness temperature of the Venus disk.

Figure 7 shows the same as Fig. 6 except that the brightness temperature distribution is for the Venus disk. A thermal infrared image of Venus taken by Pioneer Venus orbiter (PVO) showed the limb darkening effect in which temperature distribution systematically decreases as a function of the zenith angle of the emission (Taylor 1980). The effect depended on the vertical structure of the Venus atmosphere, the local time, and the emission angle at the time of observation. Images of the Venus disk obtained by LIR also exhibit the limb darkening effect even after correction of the background bias. The effect should be carefully removed with consideration of the atmospheric conditions. The effect is negligible at the disk center, where the emission angle is low. Therefore, the mean brightness temperature on the Venus disk after the correction in Fig. 7 was simply obtained within the area of an emission angle of less than 10°; they mostly converged to ~ 230 K as expected without dependence on the baffle temperature. The result is consistent with the temperature deduced from previous studies, as described in Introduction. On the other hand, the brightness temperature derived in Taguchi et al. (2012) is obviously inconsistent with this result. However, we were not aware of the background bias in 2010; the baffle temperature ranged up to 327.4 K at the time of observation. When we applied the correction of the background bias to the Venus disk image observed in 2010, the brightness temperature predictably decreased to ~ 231 K.

Origin of the background bias

Since infrared radiation from the baffle does not directly enter the UMBA, we assumed at the time of the design process that the image brightness was completely independent of the baffle temperature. However, the observed bias suggests that the baffle has radiative coupling with the Ge lens, and the radiative coupling may indirectly affect image brightness when the baffle is heated. The baffle was cooled to 230 K in the laboratory experiment before the launch; however, the baffle heating by solar radiation could not be reproduced in this experiment. The temperature of the Ge lens is monitored via telemetry and kept thermally stable by a heater to prevent radiative coupling. However, the thermal sensors are not put on the lens itself but on the lens holder, as shown in Fig. 8. Thus, the thermal sensor may not measure the actual temperature of the Ge lens when the baffle is temporarily heated.

The temperature difference between the lens and the lens holder was verified in the heating scenario by using the proto model of LIR in an air environment in the laboratory. In order to measure the actual lens temperature, thermocouples were put directly on the center of the Ge lens and the lens holder where temperature for the telemetry is measured. A sheet heater was placed in front of the lens in Fig. 8 to simulate infrared radiation from the baffle, and it was heated from 300 to 320 K. When the temperature of the heater attained a maximum of 320 K, the temperature of the Ge lens, which was initially equivalent to that of the lens holder, became ~ 0.6 K higher than that of the lens holder. Temperature of the Ge lens on orbit also must become higher than that of the lens holder monitored by the thermal sensor when the baffle is temporarily heated by solar radiation. Thus, we confirmed that the temperature difference between the Ge lens and the lens holder probably caused the background bias seen in the LIR images.

The result of initial checkouts on orbit indicated that LIR worked without serious faults despite 5 years of extra cruise under harsh solar exposure. However, early LIR images contained a noticeable background bias dependent on the thermal condition of the instrument. When the power supply unit for LIR was kept active since October 2016, the bias could be partially removed from the images. The residual component was simply correlated with the baffle temperature and was successfully corrected by using a reference table based on the deep-space images acquired at different baffle temperatures on orbit. The cloud-top temperature at the center of the Venus disk retrieved from LIR observation was consistent with that deduced from previous studies. The reference table for the correction is provisional; an authentic reference table will be shortly provided based on the deep-space images acquired with the thermal behavior of the baffle strictly controlled.

An observation program with m = 32 and n = 1 was additionally prepared for the acquisition of the close-up images with a high spatial resolution. This observation program enables us to investigate not only the fine structure of the cloud-top temperature but also the horizontal temperature distribution along an entire longitude during orbit. When the whole longitude is continuously observed, the local time dependence of the cloud-top temperature will be revealed.

We must note that the behavior of the baffle temperature during periapsis passage is not completely understood because the solar incident angle varies rapidly there. We have attempted to eliminate the background bias from the images acquired near periapsis using formula (4); however, the deep-space temperature still contained a residual component of the background bias. Thus, the background bias in the images acquired near periapsis currently cannot be corrected by the procedure described in this paper. When more Venus images are acquired near periapsis, fluctuation of the baffle temperature will be systematically revealed enough to eliminate the residual background bias.