The sun is about to enter a period of quiet, known as a solar minimum. This cycle happens every 11 years and is characterized of decreased activity—when sunspots fade away and produce fewer solar flares. With this latest period of inactivity approaching, scientists have been monitoring the sun to better understand some of the unusual activity observed over recent years.

In a study published in May the Monthly Notices of the Royal Astronomical Society, a team of scientists from the U.S., U.K. and Denmark analyzed 31 years' worth of data from the Birmingham Solar Oscillations Network (BiSON)—a group of six ground-based telescopes that provide constant monitoring of the sun's oscillations.

In the study, Yvonne Elsworth and colleagues studied the sound waves from the sun over the last three solar minimums to see how they have changed during different periods of activity. Elsworth will present the findings at the National Astronomy Meeting at the University of Hull, U.K., on Tuesday.
"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," she said in a statement. "Studying these sound waves, using a technique called helioseismology, enables us to find out what's going on throughout the Sun's interior."

The solar minimum is expected at some point between 2019 and 2020 and it provides astronomers with the opportunity to study certain activities on the sun not normally possible, such as the development of coronal holes. These are regions where the sun's magnetic field opens up, allowing solar particles to escape.

The sun is in a constant state of flux. Last week, NASA put out a press release on how the "solar minimum is coming." In it, the space agency explains how sunspots—highly magnetized regions that appear on the sun—come and go, over an 11-year period. It peaked in 2014, when sunspot counts were high.

The solar minimum also has an effect on Earth's upper atmosphere and satellites in low Earth orbit, as there is less ultraviolet radiation from the sun. The number of galactic cosmic rays that reach the upper atmosphere also increases as the sun's magnetic field is weaker, meaning it provides less of a shield from them.

The last solar minimum was of particular interest as it displayed some highly unusual behavior in 2010. For example, sunspots were almost completely missing for a period of two years—an absence not recorded for almost 100 years. Understanding whether this solar minimum was an anomaly, or if it is part of a longer fluctuation, is hugely important to our understanding of the sun.

Elsworth's findings show the magnetic field distribution (where a lot of the sun's magnetic activity is located) in the outer layers has become thinner over recent years—this thinning was first observed during the last cycle and appears to be continuing into this solar minimum. "The acoustic properties have as such failed to re-set to their pre-1994 state," the researchers write in the study.

The data also indicates the rotation rate of the sun has changed.
"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. However, we are beginning to detect some features belonging to the next cycle and we can suggest that the next minimum will be in about two years."

She continues: "Recent activity maxima have actually been rather quiet and the last cycle had a long, extended minimum. 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."
Concluding, the team say it appears there are periods where activity on the sun is "altered significantly," and that this unusual behavior could be a "radical transition suggested by data on other stars," indicating "a change in cycle behavior."