Scientists Listened To The Sun’s Inner Vibrations And Found A Solar Cycle Hiding In Plain Sight

Something strange is happening inside the Sun. Its current activity cycle looks relatively quiet by the standards scientists have used for over a century. But a new study suggests that picture is misleading, and that the real story requires listening to the Sun from the inside out.

For decades, researchers have tracked the Sun’s roughly 11-year activity cycles by counting sunspots and measuring radio signals from the solar surface. By those standard measures, the Sun’s current cycle, Cycle 25, looks noticeably weaker than cycles from the 1980s and early 1990s. A team of researchers has now found that when measured through the Sun’s own internal vibrations, Cycle 25 appears just as strong as those earlier, more intense cycles. The mismatch points to something traditional gauges simply cannot see.

That something is a gradual shift in where the Sun’s internal activity is concentrated. Over the past several cycles, those changes appear to have crept steadily toward the surface, becoming so shallow that traditional measurements no longer capture the full picture.

Listening to the Sun Like a Bell

To reach these findings, the research team used data collected by the Birmingham Solar-Oscillations Network, known as BiSON, a collection of telescopes positioned around the world that has been continuously watching the Sun since the 1970s. The network measures tiny ripples in the Sun’s surface caused by sound waves bouncing around inside it, a technique called helioseismology. It works a bit like tapping a bell and learning about the metal from the sound it produces, except the “bell” is the entire Sun and the “taps” are generated by turbulence deep within it.

These internal sound waves cause the Sun’s surface to pulse at specific frequencies, similar to musical notes. When the Sun becomes more magnetically active, those frequencies shift slightly, allowing scientists to probe structural changes inside the Sun that no telescope can photograph directly.

BiSON data for this study span from 1987 through 2025, covering the rising phase of Cycle 22 through the peak of Cycle 25. Researchers sorted the frequency readings into three bands, low, middle, and high, each sensitive to changes at different depths beneath the solar surface. High-frequency vibrations respond most strongly to changes within the outermost 1,000 kilometers of the Sun; low-frequency vibrations are less responsive to changes shallower than about 3,000 kilometers, making them better probes of deeper layers.

A Solar Cycle That Looks Weak but Isn’t

When the team compared the frequency shifts in the high-frequency band to well-known activity indicators, sunspot counts and a standard measure of solar radio output, the results were telling. In Cycle 25, the high-frequency acoustic shifts were far stronger than would be expected based on the historical relationship between those vibrations and surface activity seen in Cycle 22. Through this acoustic lens, Cycle 25 looks as strong as Cycles 22 and 23, cycles that, by sunspot counts, were considerably more intense.

A separate but related trend has been unfolding in the middle frequency band. That band’s sensitivity to solar activity has been consistently lower across Cycles 23, 24, and 25 compared to Cycle 22. Statistical analysis found the probability of that three-cycle decline happening by chance was less than two-hundredths of a percent. Together, these trends paint a coherent picture: the Sun’s activity-driven internal changes are becoming progressively more confined to a thin layer just below the surface.

A Trend Decades in the Making

Earlier BiSON work had already identified a version of this pattern, starting around 2005 during the declining phase of Cycle 23, and concluded that structural changes had moved to within about 3,000 kilometers of the surface, shallower than in Cycle 22. New data from Cycle 25 confirm the trend has continued, and the migration toward the surface has gone even further.

Crucially, the researchers note that a simple change in the strength of magnetic fields below the surface cannot explain what they’re seeing. Stronger or weaker fields would change the size of the acoustic shifts, but they wouldn’t change how sensitive those shifts are to the activity measures. Only a change in how deep, or how shallow, those magnetic structures are confined can account for the pattern.

Misreading Solar Activity Could Affect Space Weather Forecasts

If the Sun is in a period of genuine, decades-long transition that conventional sunspot-watching has been underestimating, the study, published in Monthly Notices of the Royal Astronomical Society, matters for how scientists model solar cycles. Those cycles drive space weather capable of disrupting satellites, power grids, and communications systems on Earth. Cycle 25 may look quiet from the outside, but inside, the Sun is telling a different story, and the latest BiSON data are helping scientists hear that difference more clearly.

---