Editorial

Abstract

The Chinese Hα Solar Explorer (CHASE) was successfully launched on October 14, 2021 as the first solar space mission of China National Space Administration (CNSA). The CHASE mission, dubbed "Xihe" in Chinese (Goddess of the Sun), is designed to test a newly developed satellite platform and to spectroscopically observe the Sun at the Hα line, which is one of the most important optical lines reflecting the dynamics of solar activities in the lower atmosphere, namely the photosphere and chromosphere. Since the first application of spectrographs and filters in the earlier 20th century, the solar Hα images have usually been obtained with ground-based telescopes, thus suffering from seeing and weather effects of the Earth's atmosphere. The CHASE mission provides, for the first time, the seeing-free Hα spectroscopic observations of the full solar disk or region of interest with high spectral and temporal resolutions. The unique characteristics of the CHASE data are critical for the research of solar and stellar physics. Since its launch, the on-orbit performance of the CHASE satellite has been excellent. This special topic aims to help users of CHASE data better understand its instrumentation, technical parameters, observational modes, and data processing.This special topic on CHASE mission is composed of five articles. In the first article, Li et al. [1] give a general introduction of the scientific objectives of the CHASE mission, an overview of the scientific payload-Hα Imaging Spectrograph (HIS), a brief description of the data processing, and the first results of on-orbit observations. The second article [2] provides detailed calibration procedures of the CHASE/HIS science data, including the correction of the slit image curvature, dark-and flat-field corrections, intensity calibration, and the generation of higher-level products. In the third article, Zhang et al. [3] describe the design of the ultra-high precision and stability platform of the CHASE satellite, and its on-orbit performance. The fourth article [4] gives a detailed description of the HIS instrument, including its optical systems, mechanics, and electronics. The last article [5] introduces another payload onboard the CHASE satellite-the solar atomic frequency discriminator for autonomous navigation.