Never Before Seen Anomalies in Solar Cycles in the 1600s

Records of aurora observations found in royal chronicles from Korea show that during the 'Maunder Minimum' between 1645 and 1715, the sun's solar cycles were much shorter than we see today.  During that period, recognized as among the coldest periods in the global temperature records and frequently called "the Little Ice Age", the solar cycles were eight years long, three years shorter than they are today (both numbers are averages - it's the difference that matters).   

An annotated section of the historical Korean texts that mentions auroras occurring during the Maunder Minimum. (Image credit: Yan et al. 2023)  

While difficult to read, by enlarging and rotating the original graphic from Space.com in my photo editor (Corel PSP, 2022), we can read (first rotated left 90 degrees) "Emperor Shunzi, third year, second lunar month, 23rd day [Apr 1646]".  The left side is longer and has to be read by rotating the above image right 90 degrees, "There were vapors like fire light in the S direction at night in the first watch and the second watch."

The reference to "vapors like fire light" is thought to refer to the appearance of auroras in the Western Pacific Anomaly, an area above Korea that produces regular red auroras despite being far from the magnetic poles. Unlike other auroras at the time, these light shows persisted despite a decrease in solar activity because the Earth's magnetic field is thinner in this region, which makes them a great proxy for solar cycle progression, the researchers wrote.

The Maunder Minimum, sometimes referred to as the Grand Solar Minimum, was a period of greatly reduced solar activity between 1645 and 1715 when sunspots "effectively disappeared," Scott McIntosh, a solar physicist at the National Center for Atmospheric Research in Colorado who was not involved in the recent research, told Live Science in an email.

In the new study, published Oct. 3 in the journal AGU Advances, researchers analyzed historic auroral records from Korea and found that solar cycles during the Maunder Minimum were only eight years long on average — three years shorter than modern cycles.

The aurora records were part of three separate books, or chronicles, written on behalf of Korean kings that contained detailed daily reports of royal business, state affairs, weather and astronomical phenomena that occurred within the Korean peninsula between 918 and 1910, according to the 2021 study that first described them.

The circled area includes the Maunder minimum, and shows solar activity as small variations in the solar irradiance.  (Image credit: NASA/University of Colorado'/LASP Interactive Solar Irradiance Datacenter)  Note that total solar irradiance includes broad-spectrum daylight, and beyond into both infrared and ultraviolet.  It won't be exactly like plots of smoothed sunspot numbers or the 10.7 cm solar flux, but it will resemble them.

There is no current theoretical explanation for how things like the Maunder minimum happen, and this observation of the solar cycles being three years shorter is new information.  Perhaps it can help Dr. McIntosh and other solar physicists to develop models.  We've talked lots about solar cycles and particularly the predictions for this cycle (25) on this blog, and even the cycle before that, which was starting in the early days of this blog.  There have been many people predicting we might be heading toward another grand solar minimum, but cycle 25 has been stronger than the consensus predictions and Dr. McIntosh was one of the first to predict that.  It also appears to be peaking earlier than the predictions.  Could that be an early indication of the cycles getting shorter? 

All humanity can do is observe.