The Advanced Space-based Solar Observatory (ASO-S) is a mission proposed for the 25th solar maximum by the Chinese solar community. The scientific objectives are to study the relationships between the solar magnetic field, solar flares and coronal mass ejections (CMEs). Three payloads are deployed: the Full-disk vector MagnetoGraph (FMG), the Lyman- α Solar Telescope (LST) and the Hard X-ray Imager (HXI). ASO-S will perform the first simultaneous observations of the photospheric vector magnetic field, non-thermal imaging of solar flares, and the initiation and early propagation of CMEs on a single platform. ASO-S is scheduled to be launched into a 720 km Sun-synchronous orbit in 2022. This paper presents an overview of the mission till the end of Phase-B and the beginning of Phase-C.

As one of the payloads for the Advanced Space-based Solar Observatory (ASO-S) mission, the Lyman-alpha (Ly α ) Solar Telescope (LST) is aimed at imaging the Sun and the inner corona up to 2.5R _⊙ (mean solar radius) in both the Ly α (121.6 nm) and visible wavebands with high temporo-spatial resolution, mainly targeting solar flares, coronal mass ejections (CMEs) and filaments/prominences. LST observations allow us to trace solar eruptive phenomena from the disk center to the inner corona, to study the relationships between eruptive prominences/filaments, solar flares and CMEs, to explore the dynamical processes and evolution of solar eruptions, to diagnose solar winds, and to derive physical parameters of the solar atmosphere. LST is actually an instrument suite, which consists of a Solar Disk Imager (SDI), a Solar Corona Imager (SCI), aWhite-light Solar Telescope (WST) and two Guide Telescopes (GTs). This is the first paper in a series of LST-related papers. In this paper, we introduce the scientific objectives, present an overview of the LST payload and describe the planned observations. The detailed design and data along with potential diagnostics are described in the second (Paper II) and third (Paper III) papers, respectively, appearing in this issue.

We studied the magnetic topology of active region 12158 on 2014 September 10 and compared it with the observations before and early in the flare that begins at 17:21 UT (SOL2014-09-10T17:45:00). Our results show that the sigmoidal structure and flare ribbons of this active region observed by the Solar Dynamics Observatory/Atmospheric Imaging Assembly can be well reproduced from a Grad–Rubin nonlinear force-free field extrapolation method. Various inverse-S- and inverse-J-shaped magnetic field lines, which surround a coronal flux rope, coincide with the sigmoid as observed in different extreme-ultraviolet wavelengths, including its multithreaded curved ends. Also, the observed distribution of surface currents in the magnetic polarity where it was not prescribed is well reproduced. This validates our numerical implementation and setup of the Grad–Rubin method. The modeled double inverse-J-shaped quasi-separatrix layer (QSL) footprints match the observed flare ribbons during the rising phase of the flare, including their hooked parts. The spiral-like shape of the latter may be related to a complex pre-eruptive flux rope with more than one turn of twist, as obtained in the model. These ribbon-associated flux-rope QSL footprints are consistent with the new standard flare model in 3D, with the presence of a hyperbolic flux tube located below an inverse-teardrop-shaped coronal QSL. This is a new step forward forecasting the locations of reconnection and ribbons in solar flares and the geometrical properties of eruptive flux ropes.

The Lyman-alpha Solar Telescope (LST) is one of the three payloads onboard the Advanced Space-based Solar Observatory (ASO-S) mission. It aims at imaging the Sun from the disk center up to 2.5 R _⊙ targeting solar eruptions, particularly coronal mass ejections (CMEs), solar flares, prominences/filaments and related phenomena, as well as the fast and slow solar wind. The most prominent speciality of LST is the simultaneous observation of the solar atmosphere in both Ly α and white light (WL) with high temporospatial resolution both on the solar disk and the inner corona. New observations in the Ly α line together with traditionalWL observations will provide us with many new insights into solar eruptions and solar wind. LST consists of a Solar Corona Imager (SCI) with a field of view (FOV) of 1.1 – 2.5R _⊙ , a Solar Disk Imager (SDI) and a full-disk White-light Solar Telescope (WST) with an identical FOV up to 1.2R _⊙ . SCI has a dual waveband in Ly α (121.6 ± 10 nm) and in WL (700 ± 40 nm), while SDI works in the Ly α waveband of 121.6 ± 7.5 nm and WST works in the violet narrow-band continuum of 360 ± 2.0 nm. To produce high quality science data, careful ground and in-flight calibrations are required. We present our methods for different calibrations including dark field correction, flat field correction, radiometry, instrumental polarization and optical geometry. Based on the data calibration, definitions of the data levels and processing procedures for the defined levels from raw data are described. Plasma physical diagnostics offer key ingredients to understand ejecta and plasma flows in the inner corona, as well as different features on the solar disk including flares, filaments, etc. Therefore, we are making efforts to develop various tools to detect the different features observed by LST, and then to derive their physical parameters, for example, the electron density and temperature of CMEs, the outflow velocity of the solar wind, and the hydrogen density and mass flows of prominences. Coordinated observations and data analyses with the coronagraphs onboard Solar Orbiter, PROBA-3, and Aditya are also briefly discussed.

As one of the three payloads for the Advanced Space-based Solar Observatory (ASO-S) mission, the Lyman-alpha (Ly α ) Solar Telescope (LST) is composed of three instruments: a Solar Corona Imager (SCI), a Ly α Solar Disk Imager (SDI) and a full-disk White-light Solar Telescope (WST). When working in-orbit, LST will simultaneously perform high-resolution imaging observations of all regions from the solar disk to the inner corona up to 2.5 R _⊙ (R _⊙ stands for the mean solar radius) with a spatial resolution of 4.8″ and 1.2″ for coronal and disk observations, respectively, and a temporal resolution of 30 – 120 s and 1 – 120 s for coronal and disk observations, respectively. The maximum exposure time can be up to 20 s due to precise pointing and image stabilization function. Among the three telescopes of LST, SCI is a dual-waveband coronagraph simultaneously and independently observing the inner corona in the HI Ly α (121.6±10 nm) line and white light (WL) (700±40 nm) wavebands by using a narrowband Ly α beam splitter and has a field of view (FOV) from 1.1 to 2.5 R _⊙ . The stray-light suppression level can attain <10 ⁻⁶ B _⊙ (B _⊙ is the mean brightness of the solar disk) at 1.1 R _⊙ and ≤5×10 ⁻⁸ B _⊙ at 2.5 R _⊙ . SDI and WST are solar disk imagers working in the Ly α line and 360.0 nm wavebands, respectively, which adopt an off-axis two-mirror reflective structure with an FOV up to 1.2 R _⊙ , covering the inner coronal edge area and relating to coronal imaging. We present the up-to-date design for the LST payload.

By using a new method of forced-field extrapolation, we study the emerging flux region AR11850 observed by the Interface Region Imaging Spectrograph and Solar Dynamical Observatory. Our results suggest that the bright points (BPs) in this emerging region exhibit responses in lines formed from the upper photosphere to the transition region, which have relatively similar morphologies. They have an oscillation of several minutes according to the Atmospheric Imaging Assembly data at 1600 and 1700 Å. The ratio between the BP intensities measured in 1600 and 1700 Å filtergrams reveals that these BPs are heated differently. Our analysis of the Helioseismic and Magnetic Imager vector magnetic field and the corresponding topology in AR11850 indicates that the BPs are located at the polarity inversion line and most of them are related to magnetic reconnection or cancelation. The heating of the BPs might be different due to different magnetic topology. We find that the heating due to the magnetic cancelation would be stronger than the case of bald patch reconnection. The plasma density rather than the magnetic field strength could play a dominant role in this process. Based on physical conditions in the lower atmosphere, our forced-field extrapolation shows consistent results between the bright arcades visible in slit-jaw image 1400 Å and the extrapolated field lines that pass through the bald patches. It provides reliable observational evidence for testing the mechanism of magnetic reconnection for the BPs and arcades in the emerging flux region, as proposed in simulation studies.