Photometric Calibration of Spectral Imaging CoronaGraph with Dual-wavelength Observation

Abstract The Spectral Imaging CoronaGraph (SICG) serves as the optical observation equipment of E-corona in the Chinese Meridian Project Phase II, which aims at monitoring the initial source of solar activities. For the purpose of in-depth exploration and space weather forecast in the full chain of Sun–Earth space, SICG is designed to work at two wavelengths of 637.4 and 530.3 nm in the quasi-simultaneous observation mode. Thus, the photometric calibration is more challenging to guarantee accurate scientific data of SICG. Two solar photometers are specially developed to match the observing wavelengths and make the photoelectronic conversion traceable. Correspondingly, the calibration process selects the solar disk center as the brightness reference, which compensates for the photometric losses along the atmospheric transmission path. This study derives the calibration coefficients from the two photometers for the E-coronal brightness processing in real time. By modeling aerosol absorption and scattering and comparing with continuous flat-field observation, the photometric calibration of SICG is evaluated with deviations of 2.1% and 2.3% at 637.4 nm and 530.3 nm, respectively. Based on this, the evolution speed of a multi-temperature coronal loop was analyzed, facilitating further research into the physical mechanisms of coronal mass ejections.