Solar flares and coronal mass ejections (CMEs) are two of the most powerful eruptive phenomena on the Sun. These eruptions are driven by evolution of the solar magnetic field. The ASO-S mission is uniquely designed to reveal connections among the solar magnetic field, solar flares, and CMEs. Its major scientific objectives therefore can be summarized as '1M2B', standing for the Magnetic field and the two kinds of Bursts (flares and CMEs). Via simultaneous observations of the global vector magnetic field, high-energy emission, and evolution of different layers of the solar atmosphere, the mission aims to achieve the following goals:
1) Simultaneous observations of solar flares and CMEs, two dominating eruptive events that regulate the space weather, to understand their connections and formation mechanisms.
Solar flares and CMEs are two prominent solar activities. Their occurrence frequency varies with the 11 year cycle of solar activity. Simultaneously observations of them play an essential role in revealing their connections and uncovering the underlying physical processes. The triggering mechanisms and evolution of solar flares and CMEs have been the frontier of solar physics research for several decades. Although flares are usually confined in a local area, CMEs can originate from both local and large scale structures. There is a good correlation between large flares and CMEs. It is still a matter of debate how the two eruptive events are related. Imaging observations of the source region of these two types of eruptions in white light, UV, X-ray, and ϒ-ray will enable us to follow these eruptions from the photosphere to the corona to better appreciate the relevant physical processes.
2) Observation of the full-disc vector magnetic field to uncover the build up of magnetic energy and its eruptive release during flares and CMEs and to see how the evolution of flares and CMEs are affected by the magnetic field.
A consensus has been reached that solar flares and CMEs are driven by evolution of the magnetic field, and the energy involved in these two eruptions comes from a gradual build-up of magnetic energy stored in the non-potential coronal magnetic field. It remains to be seen whether the energy build-up is dominated by shearing motion of the photosphere or by emergence of magnetic flux, and whether the CMEs are triggered by reconnection on small scales or MHD instabilities on large scales. One of the key issues in solar physics research is the relation between magnetic field configuration and characteristics of these eruptive events. The full-disk vector magnetograph onboard ASO-S will provide detailed information on the magnetic field evolution. HXI and LST are dedicated for flare and CME observations, respectively. Simultaneous observations of solar magnetic field, solar flares, and CMEs will help us to disentangle the relationships among them, and most importantly to establish quantitative relationships between the magnetic field and these eruptions. In particular, the evolution of small scale magnetic fields in the early phase of CMEs has been well-covered by past solar missions.
3) Observation of different layers of the solar atmosphere in response to eruptions to uncover the conversion and transport of different forms of energies.
Flares and CMEs can not only produce huge numbers of energetic electrons and ions, they can also induce plasma waves on a variety of scales and drive bulk motions of the background plasmas. These accelerated particles will propagate along the magnetic field lines. Some of them can penetrate into the low atmosphere and heat the plasma there producing high-energy emission at the same time. Others may escape into the interplanetary space and be observed as solar energetic particles. The X-ray and γ-ray observations of the ASO-S can reveal properties of accelerated electrons and ions and constrain their propagation in the solar atmosphere.
Furthermore, although the ASO-S is primarily a science mission, it has important application in monitoring destructive space weather events:
4) Observation of solar eruptions and the magnetic field evolution to facilitate forecasting of the space weather and to safeguard valuable assets in space.
Flares and CMEs can have tremendous impact on the space weather and may lead to devastating space environment. Flare observations by the ASO-S can be used to predict the arrival of damaging energetic particles at the Earth a few tens of minutes in advance. From the CME observations by the ASO-S, we can determine their morphology and propagation direction, then predict the arrival of a CME at the Earth tens of hours or a few days in advance. A good understanding of the relationship between the magnetic field configuration and the eruptions can lead to much advanced space weather forecast based on magnetic field observations of the ASO-S.