Scientists have used a 40-year dataset from the Birmingham Solar-Oscillations Network (BiSON) to study the Sun's internal vibrations during its quiet solar minimum periods. Their analysis revealed a stronger acoustic "glitch" during the 2008-2009 minimum, indicating subtle structural changes, like a slightly hotter outer layer, during that time of low magnetic activity.
The findings confirm that the Sun does not return to an identical state after each 11-year cycle, with its activity varying over decades and centuries. This research provides new insights into the Sun's internal dynamics and is a first step in understanding the long-term changes in how the Sun generates its magnetic field.
The main topics covered are solar cycles and minima, asteroseismology (studying stellar interiors via oscillations), long-term solar behavior changes, and the analysis of specific acoustic glitches within the Sun.
Scientists have peered inside the Sun and observed subtle shifts and “glitches” that have occurred over four decades, shedding light on the enigmatic long-term vibrations of our star, reports a study published on Tuesday in Monthly Notices of the Royal Astronomical Society.
The Sun goes through a roughly 11-year cycle that includes a period of high and low activity, known as solar maximum and minimum. The past few cycles have revealed changes in solar behavior that could have implications for predicting space weather and unraveling the internal dynamics of our Sun, along with other Sun-like stars.
To drill down on this mystery, researchers with the Birmingham Solar-Oscillations Network (BiSON), a network of telescopes that have monitored the Sun since the 1970s, compared the last four solar minima using this unique 40-year dataset and focused on internal vibrations that make the sun subtly oscillate.
“The entire Sun oscillates in a globally coherent way, and the oscillations are formed by sound waves trapped inside the Sun that make it resonate just like a musical instrument,”said Bill Chaplin, a professor of astrophysics at the University of Birmingham who co-authored the study, in a call with 404 Media.
“For this particular study, we were interested in seeing whether there are differences in what the Sun is doing in its structure when you focus on the periods or epochs when the Sun is very quiet,” he continued. “The last few cycles have seen some quite marked changes in behavior.”
For example, scientists have been perplexed for years by an unusually long and quiet solar minimum between cycle 23 to 24, which occurred from 2008 to 2009. Chaplin and his colleagues were able to use BiSON’s long record of asteroseismology—the study of stellar interiors—to directly contrast the interior vibrations of the Sun during this minimum to others.
“There were hints that there were things that were different” about this cycle, said Chaplin. “But now that we have the cycle 24-25 minimum—the last one in about 2019—in the bag, then we thought, ‘okay, now's the time to actually go back and look at this.’”
The team specifically looked for an acoustic wave “glitch” caused by an interior layer in which helium atoms lose electrons, producing a detectable change in the Sun’s internal structure. This glitch was significantly stronger during the 2008–2009 minimum, suggesting that the Sun’s outer interior was slightly hotter and allowed sound waves to travel faster at that time of magnetic weakness.
“The ionizing helium affects the speed at which the sound waves move through that region,” explained Chaplin. “It leaves a characteristic imprint.”
“It's not just that there is a difference with the other cycles, but it's starting to tell us about what physically has really changed beneath the surface,” he added. “They're quite subtle changes, but it's nevertheless giving us clues as to what is actually happening beneath the Sun during this very quiet period.”
The results confirm that the Sun doesn’t return to the same minimum baseline at the end of every cycle, and its activity varies within timescales of decades and centuries. For example, Chaplin pointed to one bizarrely long quiet period from 1645 to 1715, known as the Maunder Minimum.
Astronomers during this time marvelled at the prolonged lack of visible sunspots on the Sun’s surface, a sign of extremely low solar activity. Centuries later, BiSON and other solar observatories are allowing scientists to study the interior dynamics behind these shifts in depth for the first time.
“This is the first step in actually demonstrating that there are changes,” Chaplin said. “Does this mean that there are systematic changes in the way that the Sun is generating its field? It's really only now, because we have this long dataset, that we can start to ask questions like that. Previously, we just didn't have enough data to say.”
Scientists hope to keep recording the long-term behavior of the Sun with projects like BiSON so that we can better understand its mercurial nature over time. This is interesting work on its own merits, but it is also useful for refining forecasts of space storms that can wreak havoc on power grids and space assets (while also producing pretty auroras).
Chaplin also nodded to the European space telescope PLAnetary Transits and Oscillations of stars (PLATO), due for launch in 2027. This mission will search for analogous oscillations in stars beyond the Sun, building on similar work conducted by NASA’s retired Kepler space telescope.
Studying the vibrations of the Sun and other similar stars is not only important for life here on Earth; it also has implications in the search for extraterrestrial life, because local solar activity is one key to assessing the habitability of star systems similar to our own.
“The data that we have on other stars from Kepler has really helped to understand and get a better picture of the cyclic variability of other stars, like the Sun,” Chaplin concluded. “But it's still not an entirely clear picture; let's put it that way. Seismology now enables you to do really detailed analysis of stars that you can't do by other means.”
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