In the music of the universe, one of the most important sounds to us here on Earth is the song the Sun sings: every system has a resonance, and especially large and energetic ones like the Sun produce sounds that rebound back and forth though its vast sphere. These sound waves can be studied to glean information about the interior workings of our spheres that we would otherwise not be able to access, and recent changes in the Sun’s music has researchers that pay attention to this tune concerned.
"The sun is very much like a musical instrument except that its typical notes are at a very low frequency – some 100,000 times lower than middle C," explains professor Yvonne Elsworth, of the School of Physics and Astronomy at the University of Birmingham. "Studying these sound waves, using a technique called helioseismology, enables us to find out what’s going on throughout the sun’s interior."
The University of Birmingham has been gathering helioseismological data on the Sun since 1985, a period covering three of our star’s 11-year activity cycles. These cycles refer to periods where the Sun’s generation of energetic particles wax and wane as the interaction between its magnetic field and outer physical layers fluctuate. Currently, we’re transitioning into a solar minimum, expected to be reached in either 2019 or 2020, a period of low magnetic activity for the Sun, when fewer sunspots and solar flares are generated.
The sound waves that reverberate back and forth throughout the Sun are generated by turbulence in the gases roiling in our star’s convection zone, an upper layer lying just under the surface’s photosphere shell. Much like how seismic waves rebound back and forth through the Earth’s interior, the Sun also traps these sound waves within itself, but research into this resonance is suggesting that its activity may continue a 23-year long trend of relative quiet.
The last solar minimum, having occurred in 2010, was unusually quiet, including a period where the Sun’s surface was nearly devoid of sunspots for almost two years — a phenomenon that hadn’t been seen since 1913. And, despite predictions that that period’s unusual quiet would result in a sudden explosion of activity when the minimum ended, the Sun still took several years to get back into its active cycle.
Even then, the Sun never really got its rhythm back, with its magnetic field remaining comparatively thin throughout the last solar maximum, with Elsworth explaining that "the acoustic properties have as such failed to re-set to their pre-1994 state."
"Recent activity maxima have actually been rather quiet and the last cycle had a long, extended minimum," continues Elsworth. "It will be interesting to see if the minimum of this current cycle is extended in the manner of the previous one or if it will soon be back to the conditions of the past. However, if it is a normal minimum it will also be interesting to ask why the previous one was unusual."
It was also discovered that, at certain latitudes, the Sun’s rotation is slowing: "This is not how it used to be and the rotation rate has slowed a bit at latitudes around about 60 degrees,” Elsworth says. "We are not quite sure what the consequences of this will be but it’s clear that we are in unusual times."
Elsworth and her team are unsure as to what these changes mean for our Sun, but when they apply their data to radical transitions recorded in other stars, this strange behavior "lends weight to the speculation that a fundamental change in the nature of the solar dynamo may be in progress."