China's solar exploration: We have seen the sea of ​​stars, but our exploration of the sun is still far from unknown

The sun is the most dazzling celestial body in people's eyes and the source of growth for all things on earth. It brings us bright spring, blooming summer, abundant autumn fruits and flying winter snow. The sun makes our world full of vitality and colorful. However, what many people don't know is that the sun can sometimes bring disasters to mankind.

Around 2 a.m. on March 13, 1989, people in Quebec, Canada suddenly found that there was a power outage. Air conditioning, heating, elevators, lighting, subways, trains, and airports all stopped working, and even the operating tables in hospitals turned dark. The outage lasted for 12 hours, and nearly 6 million people suffered in the severe cold of minus 20 degrees Celsius, and many people lost their lives. The power outage affected Quebec and surrounding areas in North America. Afterwards, people found out that the disaster originated from two strong flares that occurred on the sun three or four days ago, on March 9 and 10 of that year. A large number of plasma groups were thrown out at high speed, merged into a stream and impacted the earth's magnetic field, and generated additional induced currents in the North American power grid, causing serious overload of the power grid and burning transformers and other equipment, resulting in large-scale power outages.

During Halloween in 2003, a series of solar flares caused damage to some scientific satellites in Europe, America and my country, disrupted global satellite communications, caused serious deviations in GPS positioning, and paralyzed some transportation systems that required instant communication and positioning to varying degrees. This was known as the "Halloween Storm". Solar storms have a very strong destructive effect on high-tech systems such as aviation, aerospace satellite communications and navigation, large power grid operations, and national security. How to reliably predict the occurrence of solar storms is particularly important for today's society, which is highly dependent on the above high-tech systems.

Figure 1: Schematic diagram of a solar storm

Whether it is a solar flare or a coronal mass ejection, it all comes from the solar magnetic field. The changing solar magnetic field can not only produce sunspots in the solar photosphere, but also trigger the occurrence of solar flares and coronal mass ejections. In order to study the origin and occurrence of solar storms, scientists have developed and built a variety of ground-based or space-based solar telescopes. Among them, ground-based solar telescopes are relatively low in cost and easy to maintain and operate. For example, the solar magnetic field telescope at the Huairou Solar Observatory of the National Astronomical Observatory of the Chinese Academy of Sciences, the solar radio spectrum heliograph MUSER at the Mingantu Observatory, and the one-meter vacuum solar telescope NVST at the Yunnan Observatory on the shore of Fuxian Lake. However, on the one hand, due to the influence of the earth's rotation, ground-based telescopes can only observe during the day. On the other hand, due to the absorption, shielding and interference of the earth's atmosphere, the sun cannot be observed in many bands, such as the ultraviolet band and the X-ray band. Therefore, the space solar telescope has become the most important tool for people to study and monitor the sun. Since the 1960s, countries around the world have launched more than 70 solar observation satellites. In 2018, the Parker solar probe was launched into space, observing and studying the sun at an unprecedented close distance and achieving a large number of scientific results. However, as a major space power, China has not yet made any progress in its journey to explore the sun and the sky!

Figure 2: my country’s main solar observation bases

As one of the strategic priority projects of the Chinese Academy of Sciences, the Advanced Space-based Solar Observatory (ASO-S) will be launched around the end of 2021, thus filling the gap in China's space exploration. The Advanced Space-based Solar Observatory will carry three instruments, namely the full-disk vector magnetograph to observe the solar magnetic field, the hard X-ray imager to observe the non-thermal phenomena of solar flare eruptions, and the Lyman-Alpha Solar Telescope to observe coronal mass ejections.

In addition to the combined features of the three instruments, each of them has its own characteristics. For example, the full-disk vector magnetoimager has a relatively high time resolution; the hard X-ray imager has 99 detectors, the largest number of similar telescopes in the world; and the Lyman-Alpha Solar Telescope itself works in an internationally latest observation band window.

In order to develop the above detection equipment, a large number of key technical problems need to be overcome. For example, the 99 probes of the hard X-ray imager are equivalent to 99 small eyes. The front of these small eyes is composed of gratings processed by hard metal. X-ray photons need to pass through the gaps in the grating. The narrowest gap is only 18 microns, which is thinner than a hair. The processing is very difficult, and it is also necessary to comprehensively consider factors such as thermal expansion and contraction, harsh space environment, and the launch process. The Lyman-Alpha Solar Telescope is a brand-new observation window both at home and abroad. There is no precedent to learn from, and everything needs to be explored from scratch.

The Advanced Space-Based Solar Observatory will operate in a sun-synchronous orbit 720 kilometers above the Earth's surface. The orbit passes through the Earth's North and South Poles and has an inclination of 98 degrees, ensuring continuous observation of the Sun 24 hours a day. It is expected to be in orbit for no less than 4 years. This period coincides with the peak year of the 25th solar activity cycle, and will conduct multi-band, high-quality observations and studies on the strong solar eruptions during this period, providing a reliable basis for the prediction and forecast of disastrous space weather events.

In fact, the sun is just an ordinary star among billions of stars in the vast starry sky. The reason why it is the most dazzling is simply because it is closest to us. Therefore, it is also the star sample that we have observed most carefully and with the highest spatial resolution. It is also the largest plasma fireball in space that we can observe in detail. It can serve as a natural laboratory for modern astrophysics and plasma physics. High-resolution, multi-band observations of the sun will greatly promote the development of disciplines such as astrophysics and basic plasma physics, and there are opportunities for major scientific discoveries here.

Figure 3: The main detectors on my country's Advanced Space-based Solar Observatory

For a long time, the total number of solar physics research papers in my country ranked second in the world, but most of them rely on observation data provided by European and American solar observation satellites, which to a certain extent constitutes a "bottleneck" problem. Because the research and observation level of solar physics is directly related to the ability to predict and forecast space weather, and the prediction and forecast of space weather is related to a series of major national needs such as aviation, aerospace satellite communications and navigation, and national security. If there is no independent observation data system, once the European and American powers close the relevant observation data to us, our various high-tech systems will face the risk of paralysis. After the "Advanced Space-based Solar Observatory" is launched into space, combined with a series of ground-based solar observation equipment, we will not only have first-hand, independent multi-band solar observation data, breaking through the risk of being stuck, but also contributing China's strength to international astronomical research.

After the "Advanced Space-based Solar Observatory", Chinese scientists will continue to promote a series of the latest ground-based and space solar exploration programs in their journey to explore the sun and the sky, so that my country can completely get rid of its dependence on major European and American powers and truly become a world aerospace power.

Author: Tan Baolin, National Astronomical Observatory, Chinese Academy of Sciences

Reviewer: Pang Zhihao, Chief Scientific Communication Expert of National Space Exploration Technology

This article was published by Tencent's "Everyone Loves Science" team

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