Bowing
Something freaky happened to the Sun in the 17th century. Instead of the usual 11-year solar cycle of increasing and decreasing sunspot numbers, the Sun produced far fewer sunspots than normal from roughly 1645 to 1715.
Because the Maunder Minimum (as it was later named) occurred toward the very beginning of the era of solar observation with telescopes, scientists have long struggled to understand how frequently these events happen and what they mean for both the Sun and Earth.
In particular, solar astronomers have tried to pinpoint the transition from normal sunspot cycles to the Maunder Minimum (MM), with an eye toward possible understandings for precursors of grand solar minima. In a recent article, Hisashi Hayakawa and collaborators present evidence for normal cycles prior to the MM based on three drawings made in 1607 by the prominent astronomer Johannes Kepler.
“We found that the solar cycle duration was somewhat normal, which means the Sun was doing its everyday work.”
“The community has a general consensus that the transition was rather gradual, not a steep transition,” said Hayakawa, a space weather researcher at Nagoya University in Japan. “But we have a lot of controversy on what happened at the time, because it’s just around the beginning of [telescopic sunspot] observations.”
To record his observations, Kepler used a camera obscura: a dark chamber with a pinhole to allow light in. The light projects an image (in this case, of the Sun) onto the back of the chamber. Lacking the resolution provided by a telescope, Kepler saw only large sunspot groups, which he mistakenly identified as the planet Mercury.
Hayakawa and his collaborators re-created the exact placement of the sunspots recorded by Kepler, positioning them toward the end of the solar cycle; the earliest telescopic observations a few years later occurred at the beginning of a new cycle.
In other words, a regular solar minimum—the period of fewest sunspots—likely fell between 1607 and 1610. If Kepler had seen sunspots consistent with the earlier part of sunspot variations, the MM could have been preceded by an abnormally long solar cycle, which some have claimed. Instead, Hayakawa said, “we found that the solar cycle duration was somewhat normal, which means the Sun was doing its everyday work.”
Naturally, things are never that simple.
There’s a Little Black Spot on the Sun Today
Though sunspots have been observed since antiquity, Thomas Harriot, Galileo Galilei, Christoph Scheiner, and the father-son team David and Johannes Fabricius share credit for the first observations with telescopes in the period between 1610 and 1612. Ironically, just as solar observations were catching on, the Maunder Minimum arrived, though it wasn’t identified as such until the late 19th century. Today, credit for its discovery is shared by Gustav Spörer and the husband-and-wife partnership of Edward and Annie Russell Maunder.
Why grand minima like the MM happen at all is still a mystery, but they provide a look into the solar interior and the magnetic processes that affect Earth. High sunspot numbers bring increased solar storm activity, and low numbers might correlate with lower temperatures.
“We do not know when the next GM [grand minimum] will occur,” solar physicist José Vaquero Martínez of the Universidad de Extremadura in Spain wrote in an email; he was not involved with the Kepler study. “This could be the key to understanding the behavior of the Sun over time, because it is suspected that GMs show another operating regime of the solar [magnetic field].”
Hayakawa and his colleagues used Kepler’s original notes as well as modern geometrical methods to pinpoint where the sunspot groups were on the solar surface in 1607. They found that the spots were close to the equator on the basis of the relative orientation of Earth and the Sun in Prague, where Kepler worked at the time.
Low-latitude sunspots are more likely to occur late in the solar cycle, so the researchers concluded Kepler observed near the end of a cycle, whereas high-latitude sunspots observed by telescopes a few years later belonged to the next cycle.
Tree Rings and Solar Storms
Vaquero’s own work includes analyzing historical sunspot data, and “the real problem with studying past solar activity [is] we need more data,” he wrote. “My personal opinion is that it is almost impossible to continue the work of Hayakawa et al. because there are no more observations in our scientific archives.”
“It’s certainly just a single data point,” Hayakawa said, acknowledging Vaquero’s objections. “If we have more data, that would be excellent. But the question is whether these records exist.”
In the absence of reliable pre-1610 sunspot data, solar researchers rely on proxy methods, particularly carbon-14 concentration in tree rings. This isotope is produced by cosmic rays, but because the Sun’s magnetic fields help protect Earth from these particles, more carbon-14 is made during solar minima when magnetic activity is lowest.
“By knowing how much [carbon-14] was produced during a given year from the dated tree ring and by applying a chain of appropriate physical models, we can go back and reconstruct solar activity at that time,” said Natalie Krivova, a solar physicist at the Max Planck Institute for Solar System Research. This method is less precise than direct sunspot counts, but where those data aren’t available, carbon-14 can fill in the gaps. “While the exact value of the sunspot number at a given time has some uncertainty, the dates of solar maxima and minima are quite certain, within a year or two,” Krivova said.
Hayakawa and his colleagues combined their Kepler sunspot data with carbon-14 estimates of the boundary between solar cycles from Krivova’s group. The combined data support the hypothesis of a normal-length cycle preceding the MM.
Without a surprise set of historical data, this might be the best scientists can do.
—Matthew R. Francis (@DrMRFrancis), Science Writer
