The Maunder Minimum, also called the "prolonged sunspot minimum," was a time between about 1645 and 1715 when sunspots became very rare. During the 28 years from 1672 to 1699, scientists saw fewer than 50 sunspots. This is much lower than the typical 40,000 to 50,000 sunspots observed in modern times over a similar period.
The Maunder Minimum was first noticed by Gustav Spörer in papers published in 1887 and 1889. These works were shared with the Royal Astronomical Society in London and later expanded by Edward Walter Maunder and his wife, Annie Russell Maunder. They studied how the positions of sunspots changed over time. Two papers were published under Edward Maunder's name in 1890 and 1894, and he referenced Gustav Spörer's earlier work. Because Annie Maunder did not earn a university degree, her contributions were not publicly acknowledged at the time. The term "Maunder Minimum" became widely used after John A. Eddy published an important paper in Science in 1976.
The Maunder Minimum happened during the Little Ice Age, a long period from about 1300 to 1850 when European temperatures were lower than average. Scientists believe reduced solar activity may have contributed to the cooling, but the cooling began before the solar minimum. The main cause of the cooling is thought to be volcanic activity.
Sunspot observations
The Maunder Minimum happened between 1645 and 1715, a time when very few sunspots were seen. This was not because people were not watching the Sun. During the 17th century, Giovanni Domenico Cassini made regular solar observations at the Paris Observatory, with help from astronomers Jean Picard and Philippe de La Hire. Johannes Hevelius also made his own observations. Here are the total sunspots recorded in some years (without Wolf numbers):
Even during the Maunder Minimum, enough sunspots were seen to identify 11-year cycles. The highest activity happened in 1676–1677, 1684, 1695, 1705, and 1718. Most sunspots were in the Sun’s southern hemisphere, except during the last cycle, when they appeared in the northern hemisphere. According to Spörer’s law, sunspots begin at high latitudes at the start of a cycle and then move to lower latitudes until they reach about 15° latitude at the cycle’s peak. After the peak, they continue to move to lower latitudes, around 7°, while new sunspots start forming at high latitudes again. The visibility of sunspots is also influenced by how fast the Sun’s surface rotates at different latitudes:
Observations from Earth are affected by the ecliptic, which is tilted 7° from the Sun’s equator (latitude 0°).
Eclipses during the Maunder Minimum
In his important study, John A. Eddy examined solar eclipses that occurred during the Maunder Minimum. He used written accounts from people who saw the events in 1652, 1706, and 1715. From these, he concluded that the solar corona was not very bright and did not have a clear shape during the Maunder Minimum. However, there were no pictures or drawings of these events available to him. Some images were found on political cartoons and coins, but these were likely not made by people who saw the events. Two prints were made by people who saw the 1706 eclipse, but they were created for commercial purposes, not by scientists. In 2012, Markus Heinz of the Berlin State Library discovered two paintings of the 1706 eclipse that had been thought lost. These paintings were created by Maria Clara Eimmart, a skilled astronomer and observer, and the daughter of the director of an observatory in Nuremberg Castle. The paintings matched detailed written descriptions of the event by Johann Philipp Wurzelbau and by French mathematician and cartographer Jean de Clapiès and astronomer François de Plantade, who observed the same event from Montpellier. This supported Eddy's findings about a weak and structureless corona during the Maunder Minimum and matched computer models showing a structureless F-corona without the K-corona, which is shaped by magnetic fields. More details about these observations and how the K-corona returned by the time of the 1715 event are discussed in a 2020 study by Hayakawa and others.
Little Ice Age
The Maunder Minimum happened around the same time as the middle part of the Little Ice Age, a period when Europe and North America had colder than usual temperatures. However, scientists are still studying whether these events are connected. The best idea currently is that the Little Ice Age was caused by volcanic activity. The Little Ice Age began before the Maunder Minimum started, and temperatures in the Northern Hemisphere during the Maunder Minimum were not much different from the 80 years before it. This suggests that lower solar activity was not the main reason for the Little Ice Age.
Scientists have studied the connection between low sunspot activity and cold winters in England using the longest temperature record available, called the Central England Temperature record. NASA's Solar Radiation and Climate Experiment found that the sun's ultraviolet light output changes more during the solar cycle than scientists previously believed. A 2011 study showed that low solar activity affected the jet stream, causing some areas to have mild winters (such as southern Europe and Canada/Greenland) while others had colder winters (like northern Europe and the United States). In Europe, very cold winters occurred during 1683–84, 1694–95, and the winter of 1708–09.
Other observations
Solar activity from the past can be studied using different signs, such as carbon-14 and beryllium-10. These signs show that solar activity was lower during the Maunder Minimum. The changes in carbon-14 levels during one cycle are small (about one percent of average levels) and are considered when scientists use radiocarbon dating to find the age of old objects. Understanding the amounts of beryllium-10 and carbon-14 found in places like ice sheets and tree rings has been helped by studies of the Sun's magnetic field and historical records of geomagnetic storms. These studies connect the time between the end of old isotope data and the start of modern spacecraft data.
Other times when sunspots were less common, called sunspot minima, have been found through direct observations or by studying cosmogenic isotopes. These include the Spörer Minimum (1450–1540) and the Dalton Minimum (1790–1820). In a 2012 study, sunspot minima were found by studying carbon-14 in lake sediments. In total, scientists have found about 18 sunspot minima in the last 8,000 years. Studies suggest that the Sun spends up to a quarter of its time in these minima.
A paper based on a drawing by John Flamsteed suggests that the Sun's surface rotation slowed during the deep Maunder Minimum (1684).
During the Maunder Minimum, auroras were observed regularly, following a cycle that repeats every 10 years. This is surprising because a later, less deep sunspot minimum called the Dalton Minimum clearly shows changes in aurora frequency, especially at lower magnetic latitudes. Since magnetic latitude affects how often auroras are seen, it is important to consider factors like population movement that might have changed the number of people observing auroras in the past. Decadal cycles during the Maunder Minimum are also visible in beryllium-10 levels (which can be studied with yearly precision), but these patterns appear opposite to any remaining sunspot activity. A theory about solar cycles and the loss of solar magnetic flux was proposed in 2012.
Important research on the Maunder Minimum has been published in studies comparing the Spörer, Maunder, and Dalton Minima.