Characterization of Himawari-8 AHI 3.9 μm Channel Stray Light

摘要

The Advanced Himawari Imager (AHI) is the primary instrument aboard Himawari-8 and has 16 multispectral channels, including six visible and near infrared and 10 thermal emissive bands. The 3.9-μm channel imagery of AHI has spatial resolution of 2 km and performs routine full-disk imaging every 10 minutes. There have been stray light observed in the full disk imagery of the AHI 3.9-μm channel over a few weeks around February and October-November when the line of sight of the sun is at ～10 to ～20 degrees south of the nadir of the Himawari-8. In this paper, difference data between consecutive AHI 3.9-μm images have been processed to quantitatively characterize and monitor the AHI stray light. Stray light indices are also developed to trend the occurrence, position and magnitude of the stray light in the AHI 3.9-μm imageries. It is also found that the stray light is the greatest in the AHI 3.9-μm band but also is detectable in other Mid-Wavelength IR channels. Analysis of the ratio of stray light magnitude between AHI 3.9-μm and 6.2-μm band indicates that it is consistent with the ratio of solar radiance for these two bands. This suggests that the stray light is mainly due to direct illumination of the attenuated solar radiation on the AHI detector rather than from onboard thermal body emission due to heating. The upcoming Advanced Baseline Imager (ABI) onboard the GOES-R satellite has very similar spectral and spatial characteristics as AHI. Therefore, characterizing the stray light in the 3.9-μm channel of AHI helps support post-launch calibration activities of ABI.