Mission:
“ASO-S is a mission proposed for the 25th solar maximum by the Chinese solar community in 2011 and may become the first Chinese solar satellite. The scientific objectives may be summarized as 1M2B, standing for magnetic field, two kinds of burst events: coronal mass ejections and solar flares, respectively. The mission aims at exploringconnections among solar magnetic field, solar flares, and coronal mass ejections (CMEs). ASO-S consistsof three payloads: Full-disk Magnetograph (FMG), Lyman-alpha Solar Telescope (LST), and Hard X-ray Imager (HXI),to measure solar magnetic field, to observe CMEs and solar flares, respectively. The unique design of the payload allows simultaneous observations of vector magnetic field of the full Sun, Imaging spectroscopy at high energies and propagation of CMEs near the solar disk. It will not only advance our understanding of the underlying physics of solar eruptions, but also help to improve forecast of the space weather. ASO-S is now under the phase-C studies.”
About:
“It has been a long trek to develop space missions to study the Sun in China. The earliest effort could be dated back to1976, when the first solar mission proposal, named ASTRON-1, was accepted. In 1990s, some solar payloads onmanned spacecraft series "Shenzhou" were implemented. SST (Space Solar Observatory) was also proposed in 1990s .In 2000s, SMall Explorer for Solar Eruptions (SMESE, a joint Chinese-French mission), Kuafu, and others wereproposed and promoted. But none of them had gone into the engineering stage, except some solar payloads on"Shenzhou-2". In order to better organize the space science missions, the Chinese Academy of Sciences (CAS)opened a new domain named as "Strategic Priority Research Program of Space Science"in 2011. It in particular supports thedevelopment of scientific satellites at three different levels: conception study (Phase-0/A), background study (Phase-A/B), and mission engineering study (Phase-C/D).
The conception study of Advanced Space-based Solar Observatory (ASO-S) was carried out from September 2011 toMarch 2013. Its background studystarted in January 2014 and completed by the end of 2015.The mission engineering study will start in 2017 and the satellite design, a prototype, and the satellite will take respectively 20, 18, and 16 months to complete. The design lifetime of the satellite is 4 years.
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Spacecraft:
“ASO-S will have a solar synchronous orbit at an altitude of 720 km with an orbital period of ~99 minutes.The selection of the altitude takes into account both thelower particle background along the orbit for HXI and the lower scattered light level for LST. It has an inclination angleof around 98.2o. The satellite will only go through the shadow of the Earth between middle May and August and the maximum eclipse time is 18 minutes. The satellite will be launched with the CZ-2D rocket, which is capableof carrying 1000 kg mass into the orbit of 720 km. The altitude control uses the three-axes stability technology. ”
Instruments:
The proposed ASO-S mission has three payloads onboard, i.e., the Full-disk Vector Magnetograph, the Lyman-alpha Solar Telescope and the Hard X-ray Imager.
Full-disk vector Magnetograph (FMG)
TheFull-disk vector MagneoGraph (FMG) measures the magnetic fields of the photosphere over the entire solar disk. FMG consists of an imaging optical system, a polarization optical system, and a CCD image acquisition and processing system. The telescope is a telecentric optical design with 140 mm aperture, and the detector is a CMOS camera with 4k by 4k array and 16fps. The polarization optical system consists of a traditional Lyot-type birefringent and LCVR-type polarimeter. The birefringent filter works in the Fraunhofer line FeI 532.4nm with FWHM 0.01nm.
In order to get higher accuracy, FMG uses multi-frame add mode (deep-integration mode). In normal mode of observation, 512 frames (half for left and half for right) will be collected for one magnetogram. That means within 32s (for obtaining 512 frames) the pointing should be stabilized at least within half pixel, say 0.25". Thus, FMG has itself tip/tilt system. In normal mode, the sensitivities are 5G and 150G for longitudinal and transverse component, respectively.
Compared with the magnetograph onboard Hinode (SP), FMG has a much larger field of view and higher time cadence. Comparing to the magnetographs onboard SDO (HMI) and SOHO (MDI), FMG has a simpler observation mode and a higher measurement precision.
Lyman-alpha Solar Telescope (LST)
The Lyman-alpha Solar Telescope (LST) is composed of a solar disk imager (SDI) with an aperture of 60mm, a solar corona imager (SCI) also with an aperture of 60mm, and a white-light solar telescope (WST) with an aperture of 80mm.
The SDI is to image the Sun from the disk center to 1.2 solar radii in the Lyman–alpha waveband (121.67.5nm) with a cadence of 3 - 12s. The SDI uses a 4k by 4k CCD camera as the detector so that the pixel resolution is 0.56". The structure of SDI is similar to the LADI/LYOT for SMESE but with a larger aperture. Meanwhile, apiezoelectric image stabilization system is adopted for both theSDI and the SCI to achieve the high spatial resolution.
The SCI uses a 2k by 2k CCD camera to image the inner solar corona from 1.1 to 2.5 solar radii with a cadence of 3 - 120s in both the Lyman-alpha waveband (121.610nm) and white-light (70020nm). The SCI will use a filter wheel to realize the waveband and linear polarizer selection. The filter wheel has five holes: one is totally blocked (in case of strong sunlight comes into the instrument), one is installed with Lyman-alpha filter and the other three with white-light (70020nm) band filters and linear polarizers. Three polarization angles (0, 60) are used to conduct the polarization brightness measurement in the white-light waveband.
The WST is design to image the Sun in violet narrow-band continuum (360±(1.0-2.0) nm) from the disk center to 1.2 solar radii with a general cadence of 3 - 12s (it can be as high as 0.2s in the fast event mode). A 4k by 4k CMOS sensor is selected to be the detector, which can be easily windowed for output to get higher cadence in fast event mode. The WST also works as the guiding telescope, working in 675.8±5nm waveband. A beam-splitter in the optical path produces the two beams for the imaging section and the guiding section of the instrument. For the guiding, quadrant photodiode is used to monitor the solar limb, calculate the displacement and produce the guiding signal, which is converted to triggering signals to the piezoelectric actuator installed behind the main mirrors of both SCI and SDI.
Hard X-ray Imager（HXI）
The Hard X-ray Imager (HXI) aims to image the full solardisk in the high-energy rangeof 30 to 200 keVwithgood energy resolution (27%@30keV / 5.6%@1467keV), hightime cadence (0.5 s) and large effective area (200 cm2). It is designed forflare observations with a field of view (FOV) of 1 degree and an angular resolution of 6" at 30 keV. The HXI adopts the same principle as the Hard X-ray Telescope (HXI) onboard the Japanese YOHKOH satellite and the Spectrometer Telescope for Imaging X-rays (STIX) for the Solar Orbiter mission, i.e., using indirect imaging techniquevia spatial modulation.Thisis different from theReuvenRamaty High Energy Solar Spectroscopic Imager (RHESSI)thatimages the Sun with the indirect imaging technique via rotational modulation.
A guiding telescope is implemented with the HXI to monitor the Sun in white-light, which provides positioning information of the HXI and locates eruptions on the Sun.
Science Objectives:
Solar flares and coronal mass ejections (CMEs) are two of the most powerful eruptivephenomena 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 summarizedas ‘1M2B’, standingfor theMagnetic fieldand 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) Observationof the full-disc vector magnetic field to uncover the build up of magnetic energy and its eruptive release during flares and CMEs and to seehow 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 inthese two eruptions comes from a gradual build-up of magnetic energy stored in the non-potential coronal magneticfield. 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 ofsolar magnetic field, solar flares, and CMEs will help us to disentangle the relationships among them, and mostimportantly 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 ofdifferent 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 willpropagate along the magnetic field lines. Some of them can penetrate into the low atmosphere and heat the plasma thereproducing 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) Observationof 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 propagationdirection, then predict the arrival of a CME at the Earth tens of hours or a few days in advance. A good understanding of therelationship 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.”