Why does the sun have a cycle when it loses its temper?

It is reported that China's first comprehensive solar exploration satellite, the Advanced Space-based Solar Observatory (ASO-S), will be launched in October this year. Taking advantage of the 25th solar activity week, ASO-S takes "one magnetic field and two storms" as its scientific goal, and conducts simultaneous observations of solar flares, coronal mass ejections and full-disk vector magnetic fields, providing support for the forecast of space catastrophic weather that seriously affects normal human life.

"We are currently in the rising phase of the 25th solar activity cycle, and solar activity is becoming increasingly frequent. The second half of this year is one of the best windows for conducting systematic and in-depth observations of the sun," Tan Baolin, a researcher at the National Astronomical Observatory of the Chinese Academy of Sciences, told Science and Technology Daily.

Scientists generally believe that the solar activity cycle is about 11 years. So how did scientists discover this law? Why is solar activity cyclical? What impact does the solar activity cycle have on solar science observations and daily life?

Summary of patterns from 250 years of sunspot records

The sun is not usually "quiet". Yan Yihua, a researcher at the National Space Science Center of the Chinese Academy of Sciences, said that transient phenomena caused by solar eruptions such as sunspots, flares and coronal mass ejections will appear on the sun. In addition, the sun will continue to "blow" out a stream of high-temperature charged particles, namely the solar wind.

"Sunspots are a basic sign of solar activity," Tan Baolin said. Sunspots are like "moles on the face" of the sun. But in fact, a medium-sized sunspot is about the same size as the earth, and the sunspot itself is not black, but appears black because its temperature is lower than that of the solar photosphere. Existing research shows that the low temperature of sunspots is caused by a strong magnetic field, which is about 10,000 times stronger than the earth's magnetic field.

In 1610, Italian astronomer Galileo observed sunspots with an astronomical telescope. In 1851, German astronomy enthusiast Schwabe discovered that the number of sunspots has a cycle of about 10 years through 17 years of continuous observation and mapping.

Inspired by Schwabe, Wolf, director of the Zurich Observatory in Switzerland, obtained more than 250 years of sunspot records after analysis and compilation. Wolf found that when the number of sunspots or sunspot groups increased, solar activity phenomena such as prominences and flares also increased accordingly. Therefore, Wolf proposed that the number of sunspots can represent the average level of solar activity.

Wolf also observed that the number of sunspots changes periodically over time, ranging from 9 years to nearly 14 years, with an average of about 11.1 years. He named the period from 1755 to 1766 the first solar cycle. Based on this, we are currently in the rising period of the 25th solar cycle.

Later, German astronomer Spörer discovered that the latitude of sunspots also changes periodically with the solar activity cycle, which is Spörer's law. At the beginning of each solar activity cycle, sunspots appear near 30 to 45 degrees latitude in the northern and southern hemispheres of the sun. As the solar activity cycle progresses, sunspots gradually approach the equator. In the maximum year of the solar activity cycle (the year with the largest number of sunspots), sunspot groups mainly appear near 15 degrees north and south latitude of the sun; at the end of the activity cycle, sunspot groups mainly appear near 8 degrees north and south latitude of the sun.

In 1904, British astronomers Simon and Mary Maunder drew a graph showing the changing distribution of sunspots at different latitudes on the solar disk. The sunspots in the graph were distributed in a butterfly shape.

Yan Yihua introduced that in 1908, American astronomer Hale discovered that sunspots are areas of strong magnetic fields on the sun, which confirmed for the first time the existence of magnetic fields in celestial bodies in the universe and revealed that solar activity is caused by the sun's magnetic field.

The solar cycle cannot be accurately predicted

What drives the sun's magnetic cycle? This question was listed as one of the 125 most cutting-edge scientific questions by Science magazine.

"If the magnetic field on a stellar scale decays naturally, it will take more than 10 billion years. However, the sunspot magnetic poles in the northern and southern hemispheres of the sun are opposite, and there is a 22-year magnetic pole reversal cycle, which means that the magnetic field polarity reverses once every 11 years. This shows that the sun is not an ordinary star, and its magnetic field does not decay naturally," said Yan Yihua.

In order to reveal the origin of the sun's magnetic field, American astrophysicist Parker proposed the solar dynamo theory in 1955. Tan Baolin explained that the sun is a rapidly rotating plasma "fireball". Plasma is a good conductor. The movement of its internal matter may form electric currents in local areas, and electric currents can form induced magnetic fields. This process is vividly called the solar dynamo process. The solar dynamo theory is the theoretical basis for explaining the origin of the magnetic fields of the earth, the sun and other celestial bodies.

"The solar dynamo theory can not only explain the origin of the solar magnetic field, but also explain the 22-year magnetic cycle. It is also consistent with the latitudinal distribution of the sunspot 'butterfly diagram' and the observation results. However, the theory cannot currently accurately predict the cycle of solar activity, and cannot explain the various cycles and irregularities of the cycles presented by sunspots." Yan Yihua pointed out.

In the 1960s, American scientists Babcock and his son and Leighton et al. proposed an early solar generator model: the interaction between the solar differential rotation, the solar dipole magnetic field, and the meridional circulation caused the periodicity of solar activity. This model was called the Babcock-Leighton model. Later, some astronomers used numerical simulations based on this model to make semi-quantitative predictions of some solar activity cycles.

Tan Baolin introduced that the Babcock-Leighton model is based on three basic facts about the sun. The first is that there is differential rotation on the sun, that is, the rotation periods at different latitudes of the sun are different. The rotation near the equator of the sun is faster, with a period of about 27 days. As the latitude increases, the rotation gradually slows down. Near 45 degrees north and south latitude, the rotation period is about 31 days. In the polar regions, the rotation period is close to 40 days.

Second, there are weaker magnetic fields in the Sun's quiet zones (areas without sunspots) and polar regions, and the magnetic fields in the Sun's northern and southern hemispheres are in opposite directions.

Third, there is a meridional circulation in the solar troposphere, that is, the material on the surface of the sun flows from the equator to the poles, while the material at the bottom of the solar troposphere flows from the poles to the equator, forming a closed circulation. This phenomenon is called the solar meridional circulation.

But in Tan Baolin's view, the generator model is still a semi-quantitative empirical model. People have not yet been able to accurately explain theoretically which physical factors determine the length and occurrence time of the solar activity cycle, and why the solar activity cycle is about 11 years.

It will have a serious impact on satellites, power grid systems, etc.

Tan Baolin pointed out that the solar activity cycle is not only a periodic reflection of the number of sunspots on the sun, but also a periodic manifestation of the sun's release of energy into the surrounding space, solar eruptions, and solar storms.

"Studying the solar activity cycle will have an important impact on the development of many disciplines." Tan Baolin said that studying the origin of the solar activity cycle will help us better understand the formation and evolution process of the sun and many stars similar to the sun in the universe.

In addition, research on the solar activity cycle will help to better predict the development trend of future space weather activities and learn in advance about future solar eruptions and possible solar storms.

"The study of the solar activity cycle may also directly promote the development of disciplines such as plasma physics, fluid physics and astrophysics, and even have an important impact on engineering technology," said Tan Baolin.

Does the solar activity week have a big impact on the lives of ordinary people? Yan Yihua said that in general, it is not a big impact, and there is no direct impact. However, solar activity will have a very significant impact on satellite systems, power grid systems, etc., and may even cause serious damage to them, thereby indirectly affecting people's lives. For example, it will cause mobile phone signals to become weak or lost, and navigation cannot be used.

For example, the Carrington solar flare event in 1859 severely damaged the global telegraph network at the time, making mankind realize for the first time the impact of solar activity on the Earth's space environment; in March 1989, the violent coronal mass ejection triggered an extremely strong geomagnetic explosion, causing the power grid in Quebec, Canada to be completely paralyzed within 90 seconds, causing direct economic losses of approximately US$500 million; the major solar activity event in October 2003 caused the interruption of shortwave communications on a global scale, the interruption of over-the-horizon radar and civil aviation communications, the interruption of the Swedish power grid for 1 hour, the failure of the Global Positioning System navigation, and the loss of data from multiple scientific satellites.

In addition, Yuan Lanfeng, deputy director of the Science Communication Research Center of the Chinese Academy of Sciences, said that the long-term changes in sunspots are also closely related to the earth's climate. For example, there was a small ice age from 1300 to 1850, when the activity of the sun was significantly weaker than the average level in subsequent years.