The Sun’s 11-year magnetic cycle appears to be ending, but that won’t happen anytime soon. In a paper submitted on May 26 to the journal Solar Physics, two solar scientists are reinterpreting earlier evidence to hypothesize that the Sun’s rotation rate and magnetic field are in a transitional phase that could lead to lengthening solar cycles, with the cycle ultimately disappearing altogether between 800 million and 2.4 billion years from now. Travis S. Metcalfe and Jennifer van Saders propose the scenario in their paper “Magnetic Evolution and the Disappearance of Sun-like Activity Cycles.”
“After decades of effort, the solar activity cycle is exceptionally well characterized, but it remains poorly understood,” the authors say in the paper’s abstract. “Pioneering work at the Mount Wilson Observatory demonstrated that other Sun-like stars also show regular activity cycles and suggested two possible relationships between the rotation rate and the length of the cycle. Neither of these relationships correctly describe the properties of the Sun, a peculiarity that demands explanation.”
The authors cite stellar evidence for the shutdown of “magnetic braking” in stars similar to our Sun. “The new picture of rotational and magnetic evolution provides a framework for understanding some observational features of stellar activity cycles that have until now been mysterious,” they said.
Metcalfe explained their observations through a recent Forbes magazine article. “Our previous discoveries identified an unexpected transition in the rotation and magnetism of middle-aged stars,” Metcalfe is quoted in the article, “The Sun’s Magnetic Dynamo Is Weakening” by Bruce Dorminey. “We now have direct evidence that the stellar dynamo — the mechanism inside stars that sustains their magnetic fields — actually shuts down during this transition.”
In their paper, the authors said that future observations with the Las Cumbres Observatory global telescope network “promise to probe the onset and duration of the magnetic transition that drives the evolution and eventual disappearance of Sun-like activity cycles.”
A 2016 paper Travis co-authored — “Stellar Evidence that the Solar Dynamo May Be in Transition,” published in The Astrophysical Journal Letters, concluded, “The Sun still exhibits a dipole component to its global field, particularly near magnetic minimum, but the solar analogs also suggest a gradual concentration of the field into smaller spatial scales, leading to weakened magnetic braking,”
Metcalfe is listed on the paper as being associated with the Space Science Institute and the White Dwarf Research Corp, both in Boulder, Colorado. Van Saders is listed as being associated with the Observatories of the Carnegie Institution for Science in Pasadena, California, and the Department of Astrophysical Sciences at Princeton University in New Jersey.