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

Abstract

The Venus Climate Orbiter Akatsuki arrived at Venus in December 2015, and the Longwave Infrared Camera (LIR) onboard the spacecraft started making observations. LIR has acquired more than 8000 images during the first two Venusian years since orbit insertion without any serious faults. However, brightness temperature derived from LIR images contained an unexpected bias that related not to natural phenomena but to a thermal condition of the instrument. The bias could be partially eliminated by keeping the power supply unit for LIR always active, while the residual bias was simply correlated with the baffle temperature. Therefore, deep-space images were acquired at different baffle temperatures on orbit, and a reference table for eliminating the bias from images was prepared. In the corrected images, the brightness temperature was ~ 230 K at the center of the Venus disk, where the effect of limb darkening is negligible. The result is independent of the baffle temperature and consistent with the results of previous studies. Later, a laboratory experiment with the proto model of LIR showed that when the germanium (Ge) lens was heated, its actual temperature was slightly higher than the temperature measured by a thermal sensor attached to the lens holder. The experiment confirmed that transitory baffle heating accounted for the background bias found in the brightness temperature observed by LIR.

Global Structure of Thermal Tides in the Upper Cloud Layer of Venus Revealed by LIR on Board Akatsuki

“…The LIR images have a background bias that strongly depends on the baffle temperature of LIR. This bias was determined from deep space observations and successfully removed in all LIR image (Fukuhara et al, ). An additional temperature calibration was performed to reduce the unexpected gradual temperature increase seen in deep space images (supporting information Text S1).…”

Stationary Features at the Cloud Top of Venus Observed by Ultraviolet Imager Onboard Akatsuki

“…The pixel resolution is~1 km at periapsis and~300 km at apoapsis. Details on the LIR are given in Fukuhara et al (2011) andFukuhara, Taguchi, et al (2017).…”

Venus looks different from day to night across wavelengths: morphology from Akatsuki multispectral images

“…Instrumental constraints prevent concurrent images from different filters from the same camera, but the time interval is relatively short (about 214 s for UVI) which provides near simultaneous imaging at different wavelengths except at close approach to Venus. The performance of the four cameras during the first year of operations in orbit is presented in this special issue by Iwagami et al (IR1), Satoh et al (2017) for the IR2, Yamazaki et al (UVI) and Fukuhara et al (2017b) for the LIR camera. An electronics problem with a unit in late 2016 has affected the operation of the IR1 and IR2 cameras, and while efforts continue to address this problem, the cameras have not sent back any data since the anomaly.…”

“…There are five cameras on Akatsuki to image Venus at the ultraviolet to thermal infrared wavelengths-UVI (Iwagami et al 2018, submitted to EPS Akatsuki special issue), IR2 (Satoh et al 2016), LIR (Fukuhara et al 2011(Fukuhara et al , 2017b) and the Lightning and Airglow Camera (LAC) which does not produce an image (Takahashi et al 2008), and is used when the nightside of Venus is visible to Akatsuki to detect lightening and airglow. Table 1 provides the characteristics of the four imaging cameras.…”