Aurora observation map of the Korean Peninsula. The ancient observatory in Seoul, South Korea is represented by a white dot. The pink and cyan circles indicate the visible range of Seoul at altitudes of 400 kilometers and 200 kilometers, respectively. Photo provided by interviewee

As the only star in the solar system, the sun has a broad and profound impact on the earth and planets. For 200 years, the cyclical changes in solar activity have been the focus of attention in various fields of natural science, and the 11-year cycle has long been known to everyone.

However, during the 70 years from 1645 to 1715, the regular 11-year cycle of sunspot activity was broken. During this period, the number of sunspots dropped to extremely low levels, making it difficult to show obvious cyclical changes. Solar activity entered a special period of unusually weak activity, called the Maunder Minimum. The periodic changes in solar activity during this period are the key to understanding the solar/stellar dynamo, and are also a focus of ongoing debate in the solar physics community.

Recently, the team of Wei Yong, a researcher at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, and his collaborators analyzed and studied the ancient equatorial auroral database, confirming that ancient equatorial auroral records can characterize changes in solar activity, and discovered changes in solar activity during the Maunder Minimum. It has a periodicity of about 8 years. This discovery provides new important constraints for the dynamo theory of the Sun and other stars, and helps to better understand the generation mechanism of the superminimum of the Sun and other stars and to more accurately understand the Sun-Earth relationship during the Maunder Minimum. . Relevant research results were recently published in "Progress of the American Geophysical Union".

The sun's "black box"

In 1957, the artificial satellite opened the era of space science and gave us an in-depth understanding of the current relationship between the sun and the earth. However, the solar system has a history of 4.6 billion years. How can we understand the relationship between the sun and the earth on a longer time scale?

"Compared with the Earth's 4.6 billion years of evolution, the existing observational data can be said to be insignificant. Only by understanding the past can we predict the future. Only by understanding the Earth's past can we know how long the Earth's habitability will last." Wei Yong told China Science News that he started thinking about this issue in 2010 when he was a postdoctoral fellow at the Max Planck Institute for Solar System Research in Germany.

Different types of activity phenomena occur on the sun all the time, including sunspots, jets, flares, prominences and coronal mass ejections. Changes in solar activity depend on solar dynamo processes. The overall level of solar activity changes with an approximately 11-year cycle, which is called the solar activity cycle. Sometimes the regular 11-year cyclical changes in sunspot activity will be broken, and the sun will enter a period of abnormally weak activity that lasts for decades, called a solar superminimum. The most famous one is from 1645 to 1715. Maunder minimum. During this period, the number of sunspots, commonly used to characterize solar activity, dropped to extremely low levels. Other alternative indicators, such as the auroras in the Earth's polar regions and the solar activity index derived from cosmic isotopes, also plummeted to extremely low levels. Due to the limitations of these indicators, there has been controversy in the academic community about the cyclic changes in solar activity during the Maunder Minimum.

Superminimum is not unique to solar activity. Abnormal periods similar to superminimum have also been found in other sun-like stars. The existence of superminimum brings challenges to the solar/stellar dynamo theory. The changing pattern of solar activity during Maunder Minimum is the key to understanding the solar/stellar dynamo. Some current dynamo models can reproduce Maunder Minimum-like behavior in one or both solar hemispheres, but the simulated solar activity periodic changes are not the same. This is due to the lack of Maunder Minimum solar activity cycles. Important observational constraints on changes.

"The Maunder Minimum is like a 'black box'. We don't know what exactly happened to the sun." Wei Yong said that the academic community has been looking for new independent indicators to reveal the changes in solar activity during the Maunder Minimum.

Wei Yong's team found the answer in ancient books and proposed that equatorial auroras can be used as a new indicator to reflect the level of solar activity.

Looking for Aurora

"Auroras are the product of the interaction between the solar wind and the Earth's magnetic field, which can reflect the level of solar activity." Wei Yong said that there are two types of aurora on the earth. In addition to the well-known polar aurora, there is also one that occurs in the negative geomagnetic anomaly area at low latitudes. of equatorial aurora. Equatorial auroras are related to solar activity levels and occur more frequently during periods of high solar activity.

In the past, the research team analyzed and found that between the 16th and 19th centuries, a negative magnetic anomaly in the geomagnetic field appeared in the western Pacific region, creating conditions for the generation of equatorial auroras and providing an opportunity to study the changes in solar activity during the Maunder Minimum. .

In ancient times, China, Korea and Japan were all within the visible range of the equatorial aurora, leaving a large number of records of the equatorial aurora in ancient books. Wei Yong proposed that the equatorial aurora recorded in these ancient books can be used as a new indicator reflecting the level of solar activity.

Wei Yong’s idea at the time was approved and supported by his mentor, space physicist, and academician of the Chinese Academy of Sciences Wan Weixing (1958-2020). They systematically organized the official history of the Korean Peninsula and unearthed a total of 2013 auroral records from 1012 to 1811 AD. The relevant ancient book records have been compiled and published in the monograph "Aurora Chronology of Ancient Korea" (2020). This list of ancient equatorial aurora provides a unique database for studying the changing patterns of solar activity during the Maunder Minimum.

In the spring of 2019, the Aurora database was completed. Wan Weixing personally analyzed the data on his sickbed and discovered that changes in solar activity during the Maunder Minimum have a periodicity of about 8 years, not 11 years. This result shocked and confused everyone. After several debates, the research team decided to study it from the perspective of solar physics, mainly led by Yan Limei, an associate researcher at the Institute of Geology and Geophysics, Chinese Academy of Sciences.

Yan Limei introduced that the changes in the ancient equatorial aurora can represent the observational evidence of changes in solar activity, that is, in the two solar activity cycles before entering the Maunder Minimum, the changes in the ancient equatorial aurora are consistent with the changes in the sunspot number. The two cycles of the group sunspot number The peak value is basically consistent with the two periodic peak values recorded in ancient equatorial auroras. This shows that the variability of ancient equatorial aurora can represent changes in solar activity, that is, ancient equatorial auroral records can be used to study the changes in solar activity during the Maunder Minimum.

"The ancient equatorial aurora shows that after entering the Maunder Minimum in 1645 (1645-1678), solar activity went through four consecutive and complete cycles of about 8 years that can be distinguished by the naked eye. This gives the Maunder Minimum period Direct observational evidence that the solar activity dominates the cycle of about eight years during this period." Yan Limei said.

Find the "coordinates" in history

This study provides a new data source to characterize solar activity during Maunder Minimum and provides evidence of the eight-year cycle of solar activity during Maunder Minimum.

In addition, the Maunder Minimum (1645-1715) coincides with the coldest period of the Earth's Little Ice Age (1300-1850). The Maunder Minimum is often considered to be potentially significant to the Earth's Little Ice Age. Contribution. However, the contribution of the Maunder Minimum of solar activity to the Earth's Little Ice Age has been controversial. This study provides an eight-year cycle change in solar activity during the Maunder Minimum, which helps to better understand the potential impact of solar activity on the Earth's climate during the Maunder Minimum.

"In the future, we hope to extend the results of this study to understand the relationship between solar activity and Earth's climate." Wei Yong said.

In the past 10 years, Wei Yong has been trying to combine subject history research with scientific research and promote subject construction. This year, the Institute of Geology and Geophysics of the Chinese Academy of Sciences has officially established the history of science and technology discipline and begun recruiting graduate students.

“I offer the history of science and technology course in the hope that students can establish their own historical coordinates of the discipline and understand the background, history and development process of the birth of the discipline, so that they can grasp the scientific frontiers and national needs, and truly understand the connotation and spiritual essence of the development of the discipline.” In Wei Yong's view, people engaged in scientific research should do some research on the history of the discipline, "and always have a sense of coordinates in the development of the discipline."

Related paper information:

https://doi.org/10.1029/2023AV000964

(The original title is "Chinese scientists focused on research on the "Maunder Minimum" and found that the 70-year solar activity cycle is 8 years")