Listening to the sun reveals previously hidden changes to solar cycle

Interpreting what exactly has been, or will be, happening with the physics of the sun in its variable ’11-year’ cycles has always been challenging. The relevance to Earth and the climate obviously adds to the interest. According to this research certain changes evident in recent cycles need to be included in the puzzle, but as usual further work is needed to try and iron out some of the uncertainties.
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Internal changes due to the sun’s “active biorhythm” have become increasingly “skin-deep” over the past four solar activity cycles, according to a new study, says Phys.org.

Publishing its findings in Monthly Notices of the Royal Astronomical Society, an international team led by the University of Birmingham reveals solar magnetic activity is being squeezed into an increasingly shallow layer just below the visible surface, signposting long-term changes to the sun’s active behavior.

Solar activity rises and falls in 11‑year cycles, producing solar flares, and ejections of highly charged particles and coronal mass ejections that give rise to space weather.

This activity, and its cyclic variation, has its origins in the sun’s interior, in processes that regenerate and reorganize the sun’s magnetic field.

Understanding what drives the solar cycle is therefore crucial for making predictions of space weather, which can disrupt satellites, communications, GPS systems and power grids on Earth.

Traditional measures of solar activity track these emissions and other surface phenomena like sunspots, but they do not look under the solar surface. However, by “listening” to tiny sound waves inside the sun—a technique known as helioseismology—it is possible to do just that.

By tracking changes in the otherwise hidden solar interior, the team found a different picture emerged of the sun’s activity over the past few cycles to the one given by the traditional measures.

Using almost 40 years of helioseismic data from six telescopes around the world in the Birmingham Solar Oscillations Network (BiSON), the researchers uncovered a gradual change in structure just beneath the surface that has spanned multiple cycles, with the current solar cycle 25 showing particularly strong signatures of these changes.
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The researchers analyzed the p-mode oscillations—formed by global sound waves inside the sun—whose frequencies shift in response to solar magnetic activity. This allowed them to determine how the sun’s internal structure changed across solar cycles 22–25, from 1987 to 2025.

They grouped oscillations into low-, mid-, and high-frequency bands to probe different depths beneath the solar surface. The team then compared these frequency shifts with traditional measures of solar activity to reach three main conclusions:

— Evidence of changing behavior—the link between oscillation frequencies and traditional activity measures has shifted significantly since Cycle 23, indicating long-term evolution in the sun’s internal processes.

— Surface confinement of structural changes—the combined behavior of low-, mid-, and high-frequency modes shows that solar-cycle-driven structural changes are becoming increasingly confined to shallow layers, within 1,000km of the sun’s surface.

— Reinterpreting the strength of the latest cycle—Cycle 25 appears weaker in traditional surface indicators but comparably strong when seen in the high-frequency helioseismic data.

Full article here.
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Image: Sun and Earth (not to scale)