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The Fast Imaging Solar Spectrograph (FISS) is a powerful instrument for studying photospheric and chromospheric events by simultaneously recording high-resolution spectrograms in two spectral bands. Recently, because of a change in the optical setup, unexpected fringes have been noticed on the Ca ii 8542Å band camera, which persist even after the standard flat-fielding. Here, we develop a new data calibration method for the FISS, including a wavelet-based fringe reduction technique. By applying the wavelet transform independently along each axis of the flat data, we successfully isolate the fringes using a low-pass filter and a Gaussian window in the power spectrum. We implement the phase correction to account for temporal phase shifts in the fringes, enabling their effective removal from object frames without altering spectral profiles. This preprocessing pipeline has been integrated into the Python-based FISSpy package for the FISS data analysis. We anticipate that the proposed calibration method will enhance data quality and be utilized for the next generation of the FISS. 

Abstract We report on a flare-driven coronal rain event observed along postflare loops during the decay phase of an X1.6-class solar flare. Although high-resolution studies of flare-driven coronal rain have been conducted, imaging spectroscopic studies are rare due to observational difficulties. Our observation taken with the Fast Imaging Solar Spectrograph, installed at the 1.6 m Goode Solar Telescope of the Big Bear Solar Observatory, provided unprecedented high-resolution spectroscopic imaging data of coronal rain in the Hαand Caii854.2 nm lines. We identify two locations along postflare loops with rain displaying distinctly different thermal properties, different Doppler velocities, and different patterns of acceleration and deceleration. We also observed intense brightening at one footpoint of coronal rain, where the spectroscopic analysis reveals an energy conversion process resulting in significant localized chromospheric heating. We thoroughly investigate the footpoint brightening Doppler velocities and compare their spectral line profiles to typical flare-ribbon line profiles. We estimate the spatial scale of the fine structure of the coronal rain and the footpoint brightening. Our results provide important insights into the dynamic and thermal properties of flare-driven coronal rain and the related chromospheric response, which will help validate the flare-driven modeling of coronal rain. 

This white paper is on the HMCS Firefly mission concept study. Firefly focuses on the global structure and dynamics of the Sun's interior, the generation of solar magnetic fields, the deciphering of the solar cycle, the conditions leading to the explosive activity, and the structure and dynamics of the corona as it drives the heliosphere.