Day-to-day

  How do stars evolve?

How do galaxies form?

Is there a boundary in the universe... With the development of science and technology, people's understanding of the universe is gradually expanding, and these terms are becoming familiar to the public day by day.

The successful launch of the "Xihe" solar exploration science and technology experimental satellite has made people realize that, in fact, our understanding of the solar system is far from enough.

What are the unsolved mysteries of the sun, a star closely related to the survival of mankind?

How do scientists plan to explore the sun?

Three unsolved mysteries of the sun

  The sun is the closest star to the earth, providing light and heat for the survival and development of life on the earth.

At the same time, the sun has a huge impact on the earth. Its total radiation changes (slow and large time scale) regulate the earth’s long-term climate changes. Technical systems and long-distance oil, power, gas and other systems have caused catastrophic consequences.

  There are still many major unsolved mysteries about the sun, the most important of which are three scientific questions.

  The first question: Why is there periodicity in solar activity?

  The driving force of the solar activity phenomenon comes from the magnetic field on the sun, and its strong magnetic field structure is characterized by sunspots, which are also relatively low temperature regions on the solar photosphere.

Sunspots have an 11-year cycle, and their long-term changes are closely related to the Earth’s climate. For example, during the Little Ice Age (about 1300-1850) in climatology, solar activity was significantly weaker than the average level of subsequent years.

Not only that, the activities of sunspots also have a great impact on the earth's magnetic field and wireless communications.

The "Carrington" solar flare event that occurred in 1859 caused violent geomagnetic disturbances and severely damaged the high-tech system of human society at that time-the global telegraph network, which made it the first time that humans realized the impact of solar activity on the earth's space environment. Influence.

  In 1908, the American astronomer Haier discovered that sunspots are a strong magnetic field on the sun, confirming the existence of magnetic fields in cosmic bodies for the first time and also revealing that solar activity originates from the solar magnetic field.

However, how is the solar magnetic field produced?

Why does the solar activity dominated by the solar magnetic field have periodic changes?

These problems are called the mystery of the origin of the solar activity cycle. This is the essential problem of the sun, a highly nonlinear and complex system. At present, the scientific community still cannot give a clear explanation. It is listed as 125 problems faced by contemporary human society by the "Science" magazine. One of the most cutting-edge scientific issues.

  Second question: Why is the corona so hot?

  Like the earth, the sun can also be divided into many levels from the inside to the outside, including the solar nucleus (its temperature is about 15 million ℃), the radiation area (about 7 million ℃), the convection area (about 2 million ℃), and the photosphere (number of Thousands of degrees Celsius), chromosphere (thousands to tens of thousands of degrees Celsius), corona (millions of degrees Celsius).

It can be seen that the temperature structure of the sun is obviously different from that of the earth, the outermost corona of the sun’s atmosphere presents an abnormally high temperature state, which violates the second law of thermodynamics. How to explain the heating mechanism of the corona is an important part of astrophysics. The scientific problem is called one of the "eight unsolved astronomical mysteries" in astrophysics by "Science" magazine.

  The third question: How to observe and forecast the solar-geophysical propagation process and interplanetary effects of solar activity in real time?

  This is a vital application requirement to ensure the smooth progress of high-tech equipment and deep space exploration in neighboring space.

In March 1989, a violent coronal mass ejection triggered a very strong geomagnetic explosion, leading to a large-scale power outage in Quebec, Canada.

In this magnetic storm, the entire power grid of Quebec was completely paralyzed within 90 seconds, causing a direct economic loss of approximately US$500 million.

The major solar event in October 2003 caused the interruption of global shortwave communications, the interruption of over-the-horizon radar and civil aviation communications, the interruption of the Swedish power grid for one hour, the failure of GPS navigation, and the loss of data from multiple scientific satellites.

The National Aeronautics and Space Administration (NASA) "Heliophysics and Space Physics Development Report" also clearly pointed out that the main goal of solar exploration is to better understand the solar-earth system, predict changes in the space environment and their social impacts.

Therefore, a complete physical image depicting the spread and influence of solar activities in the solar-terrestrial space is the most cutting-edge scientific problem of contemporary solar-terrestrial physics, and it is also an urgent need of various scientific and technological powers.

  In the above three scientific issues, the magnetic field plays a vital role.

In fact, the understanding of the nature of the magnetic fields of the sun and stars is also an important scientific issue.

However, due to the limitations of current observation capabilities and technical means, humans have not yet fully understood the solar magnetic field.

This leads to another basic problem of astrophysics: the process of magnetic convection of fibrillated radiation in the stellar atmosphere.

  In addition to the above scientific issues, there are many important uses for understanding the sun.

For example, searching for another habitat for humans in the universe-habitable planets is not as simple as "with water" and "atmosphere." One of the important aspects is the need to determine the space weather effects of host stars on habitable planets.

To carry out these studies, it is particularly important to study "the overall space weather behavior of the sun as a star", which is the basis for the study of the habitability of exoplanets in space weather.

"I don't know the true face of Mount Lu, but I am only in this mountain." At present, research in this area needs to be strengthened urgently.

International competition for solar exploration

  Regardless of the development of science and technology or national security, solar exploration is so important, and it has become the focus of competition in the fields of astronomy and space physics in various countries.

  Compared with other fields of astronomy, the characteristics of solar observation can be briefly described as "three highs and one precise imaging", that is, high time resolution, high spatial resolution, high spectral resolution, accurate polarization (magnetic field) measurement, Imaging observations, of course, observations in other fields of astronomy also emphasize these indicators, but those indicators are often orders of magnitude or even many orders of magnitude different than those pursued by solar observations.

Solar physics research is also undergoing extensive and in-depth intersection with other astronomical disciplines.

For example, the concept of solar generator model is introduced into the process of stars and galaxy generators to explain the origin and evolution of the magnetic fields of stars and galaxies; the standard model of solar flares and the concepts of coronal mass ejection are used to explain stellar flares and coronal mass ejections; Apply the solar-terrestrial interaction model to the exploration of understanding the life signals in the atmosphere of exoplanets.

Solar physics research is a research discipline that can start from the basic principles and equations of physics and construct a three-dimensional radiant magnetic fluid model describing plasma from the ground up.

  Solar observations are always pursuing high time resolution to see the detailed process of evolution, pursuing large apertures to improve resolution to see the spatial details of evolution, pursuing high spectral resolution to explore more detailed solar atmospheric radiation processes, and pursuing high resolution. Polarization accuracy to obtain more accurate solar magnetic field measurement results.

  In order to achieve this goal, scientific researchers from all over the world have done their best.

  On the earth, ground-based solar observation has the characteristics of flexible upgrade, low cost, and strong sustainability.

In the 21st century, several one-meter solar optical telescopes have been put into operation in the world, including Sweden's 1-meter SST, China's 1-meter NVST, and Germany's 1.5-meter GREGOR.

At present, the most advanced ground-based solar telescope in the world is the 4-meter DKIST of the United States.

It has begun trial operation and is expected to be the most important solar optical observation equipment in the world in the next 10 years.

The 1-meter mid-infrared telescope AIMS, which is about to be put into trial observation in China, will be at the international leading level in terms of magnetic field measurement accuracy, and related scientific results are also worth looking forward to.

In terms of future planning, projects that can match or surpass DKIST are represented by China's 8m CGST and European 4m EST, but they are all in the stage of advancing the project.

  However, the earth’s atmosphere absorbs radiation from celestial bodies, and even makes observations in many bands impossible on the ground; atmospheric turbulence will limit observation resolution and reduce measurement accuracy; day and night alternation leads to discontinuity of observations... In order to make up for these shortcomings, space astronomical observations Has become the main stage of future competition.

  Space observation has unparalleled advantages for astronomy-continuous, stable, full-wavelength, resolution and observation accuracy are not affected by the atmosphere, etc.

Space solar exploration has risen along with artificial satellites going into the sky. By the 1990s, it entered the golden age. Several heavyweight solar satellites were launched and many important scientific results were obtained.

In recent years, as the Parker Solar Probe PSP achieves deep solar atmosphere exploration for the first time, and the Solar Orbiter achieves imaging observations that deviate from the ecliptic plane, solar observation research will enter a new period of development.

  Here, we want to focus on China's solar physics research.

The "Xihe", a solar exploration science and technology experimental satellite successfully launched a few days ago, is one of the important achievements of China's solar exploration.

Over the past 40 years of reform and opening up, the level of solar physics research in China has developed from tracking to parallel and partially advanced.

Beginning in the 1980s, guided by the internationally advanced solar magnetic field telescope independently developed by Chinese scientists, and with other distinctive observation equipment, China’s solar physics has gradually entered the international advanced ranks, and its comprehensive research capabilities are among the best. Mao.

Compared with other countries, China's solar physics research, especially the ground-based actual solar observation research, has formed an independent observation ability, and has less "stuck neck" technology, and can even export advanced solar observation equipment and instruments in the same field abroad.

  But it must be acknowledged that compared with the highest level of international space solar exploration, China’s space solar exploration is far from the international level-although Chinese scientists have successively proposed the "Astronomical Satellite No. 1" plan and the "Space Solar Telescope" to observe the sun. The "Kwafu" plan for comprehensive exploration of the sun at the L1 point of Lagrange on the Sun and Earth was planned, but these plans were not implemented due to various reasons.

  But we have begun to catch up-China’s Fengyun Meteorological Satellite has released China’s first solar extreme ultraviolet observation image, and the "Xihe" equipped with the Solar Hα telescope on the "dual super" high-tech satellite platform has been successfully upgraded. These events marked the beginning of our country’s process of catching up with the international advanced ranks.

In 2017, the Space Science Pilot Project of the Chinese Academy of Sciences officially launched the "Advanced Space-Based Solar Observatory" satellite engineering ASO-S project.

At present, the ASO-S project has been transferred to the prototype development stage and will be launched in 2022.

The future competition focus of solar exploration

  After the successful implementation of the landmark PSP and Solar Orbiter in the United States and Europe, the detection hotspots in related international fields have focused on the last gap in solar exploration-the exploration of the solar polar region.

  Regardless of space-based or ground-based observation, when we are in the ecliptic plane, it is very difficult for us to observe the solar polar region due to the projection effect and the edge dimming effect.

This difficulty is equivalent to the fact that two people of similar size cannot see the top of each other's heads.

Humans have not yet achieved frontal imaging observations of the solar polar region. Although Solar Orbiter deviates from the ecliptic plane, its deviation angle is only more than 20 degrees. Even if the mission's final plan is increased to 34 degrees, the height difference between the two is not enough to see. Clear the top of the other's head.

  Studies have shown that the magnetic field and flow field of the solar polar region play a vital role in the evolution of the solar cycle.

Improving the observational data in this area, so as to complete the last observation puzzle of the "generator model" about the origin of the solar activity week, is expected to bring a major breakthrough in the study of this scientific problem.

At the same time, the high-speed solar wind originating in the polar regions is the core element of the solar-terrestrial space-environmental connection. At present, human beings know little about its origin mechanism and process.

Polar exploration can also bring great progress to the study of the solar-geophysical propagation process and interplanetary effects of solar activity.

  Because of these major scientific opportunities, the next hot spot for international space exploration must be solar polar orbit exploration.

At present, space science development plans in Europe and the United States have all put forward the idea of ​​solar polar orbit detection.

As early as 10 years ago, China put forward the project proposal for the development of solar polar orbit exploration in the "Moon and Deep Space Exploration" plan.

At present, China's pre-research in this area is in a position equal to or even slightly ahead of the international level. Considering that the PSP and Solar Orbiter have just been implemented in the United States and Europe, China has an excellent opportunity to take the lead in implementation. Once the project is successful, it will surely enable China's space solar exploration has achieved rapid overtaking.

At the same time, seize the opportunity in the research of major scientific issues such as the origin of solar cycles and the origin of high-speed solar wind.

  Another hot spot of competition in international space solar exploration is the monitoring of the solar-geophysical causality chain for solar activity.

Place multiple satellites on the ecliptic plane (e.g. it would be better to combine solar polar orbit observations) to realize all-round three-dimensional detection of the sun-earth space, and strive to understand the behavior and mechanism of the super-complex system of solar-terrestrial space. On the one hand, it can satisfy the country. The strategic needs for monitoring and forecasting of the solar-terrestrial space environment, on the other hand, provide observation and theoretical foundations for the study of the habitability of exoplanets in space weather.

China has also launched a number of research programs related to this aspect. The first group in the international competition can also bring breakthroughs in the research of major scientific issues.

  As for the solar physics related to the nature of the solar magnetic field, the coronal heating and other scientific issues, compared to the aforementioned aspects, they are more basic scientific issues.

The study of the intrinsic properties of the solar magnetic field is actually a century-old problem in the history of solar magnetic field measurement. To overcome this problem requires the implementation of the space large-aperture solar optical telescope project.

For the problem of coronal heating, new observation theories and methods need to be developed on this basis, such as breaking through the technical bottleneck of coronal magnetic field measurement.

Our country has launched some new attempts in this regard, which may bring some surprises.

  Thanks to the substantial increase in national power, China has already possessed the economic strength to invest in the exploration of major scientific issues; and relying on the hard power enhancements brought about by the national lunar exploration, deep space exploration, and manned spaceflight, China has also possessed The technical strength to implement major solar exploration missions.

Time does not wait for me. I look forward to China's solar observation and research reaching the top of the world as soon as possible.

(Author: Deng Yuanyong and Yang Shangbin, researcher and associate researcher of the National Astronomical Observatory of the Chinese Academy of Sciences)

Keywords: temperature regions, exploration, technology, understanding, important, magnetic field, observation, satellite, china, sun, development, launch, systems, goal, driving force