Synthetic Lyman-α emissions for the coronagraph aboard the ASO-S mission

Context. Strong ultraviolet (UV) emission from the sun will be observed by the Lyman-alpha Solar Telescope (LST) on board the Advanced Space-based Solar Observatory (ASO-S), scheduled for launch in 2022. It will provide continuous observations from the solar disk to the corona below a 2.5 solar radius with high resolution. To configure the appropriate observing modes and also to better understand its upcoming observations, a series of simulations and syntheses of different structures and processes need to be done in advance. Aims. As prominence eruptions are the main drivers of space weather, the need to monitor such phenomena has been set as a priority among the objectives of ASO-S mission. In this work, we synthesize the evolution of a modeled prominence-cavity system before and during its eruption in the field of view (FOV) of LST. Methods. We adopted the input magnetohydrodynamic (MHD) model of a prominence-cavity system, which is readily comparable to the Atmospheric Imaging Assembly (AIA) observations. The Lyman-alpha emission of the prominence and its eruptive counterparts are synthesized through the PRODOP code, which considers non-local thermodynamic equilibrium (NLTE) radiative transfer processes, while the other coronal part such as the cavity and surrounding streamer, are synthesized with the FORWARD package, which deals with optically thin structures. Results. We present a discussion of the evolution of the eruptive prominence-cavity system, analyzing the synthetic emissions both on the disk near the limb and above the limb as viewed by the coronagraph, as well as the three-dimensional (3D) data of the MHD simulation. Conclusions. The evolution of the prominence-cavity system exhibits the condensation of cavity mass onto the prominence and the evaporation of prominence plasma into the central cavity. The synthetic emission in Lyman-alpha shows a similar pattern as in the AIA extreme ultraviolet (EUV) wavelengths before eruption, namely, the appearance of a "horn" substructure as a precursor to the eruption. The emission of prominence with an optically thick assumption is one to two orders of magnitude lower than the optically thin one. Here, the dimming effect in Lyman-alpha is analyzed, for the first time, for the eruptive prominence-cavity system. Accompanying the prominence plasma motion during the eruption, the apparent dimming shows a preferred location evolving from the top and bottom of the bright core to the whole body above the bottom part, while the collisional component progressively dominates the total emission of the flux rope bright core at these locations. By analyzing the signal-to-noise ratio (S/N) with a consideration of LST's optical design, we conclude that the substructures in the cavity and the bright core of the CME can be observed with sufficient S/N at different stages in the FOV of LST.