Scientists have delved into the Korean historical chronicles to study the minimum of solar activity that occurred at the turn of the XVII–XVIII centuries. It is evidenced by the number of auroras observed at this time. The data showed that the cycles of activity of our luminary then were not only weaker, but also shorter.
Research on solar cycles in the past
Scientists continue to uncover the secrets of how solar activity has changed over time. Data from annual tree rings and Antarctic ice helped them find extremely powerful flares in the distant past. In the new study, scientists could study the time when our sun, on the contrary, was very quiet.
Observations of the number of spots on the Sun have been going on since the end of the XVIII century. No one had conducted such studies before. However, they can be studied according to other data. Flares on our luminary cause the appearance of auroras, and people have seen them all the time and recorded them in historical documents.
Especially in this regard, the chronicles of the Korean Joseon dynasty, which cover the XIV–XIX centuries, are interesting. They are quite detailed, and the magnetic anomaly over the northwestern Pacific Ocean contributes to the fact that during powerful flares, auroras are visible at low latitudes.
Mystery revealed by the chronicles
Most of all, scientists were interested in the Maunder minimum previously discovered from other sources. It falls in the years 1645-1715. The activity of the Sun was greatly reduced during it. Such events have occurred repeatedly over the past few millennia.
The researchers studied the chronicle records between 1620 and 1810. They found 1012 mentions of the aurora observed at that time in Korea. At the same time, the distribution of these phenomena fully confirms the existence of the Maunder minimum.
Moreover, scientists came to the unexpected conclusion that solar cycles at this time were not only quieter, but also shorter. Now they last 11 years. However, then it lasted only 8 years. Scientists conclude from this that with a decrease in the activity of the Sun, its dynamo changes its rhythm, but they cannot name the reasons for this phenomenon.
According to phys.org.
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