The Sun is about 93 million miles away, and for most of human history, all we could do was watch its surface. Count the dark spots. Measure the brightness. Watch for sudden flares. It was a bit like trying to diagnose a heart condition by only looking at someone’s skin. You’d see some things. You’d miss a lot.
What a team of researchers at the University of Birmingham has just pulled off is something rather different. By listening to sound waves reverberating through the Sun’s interior, they’ve picked up a change in the star’s internal structure that no surface instrument would ever have caught – a shift that’s been building across four decades and has real consequences for how well we can predict when the Sun is about to throw something dangerous our way.
The technique at the heart of this is called sun helioseismology waves, and what those waves have just revealed changes the picture considerably.
What Helioseismology Actually Does
Helioseismology works by tracking tiny sound waves that reverberate around inside the Sun. Just as geologists use seismic waves to map Earth’s interior, solar physicists use these oscillations to peer beneath the Sun’s visible surface. No instrument can physically reach down there. But sound can.
The Sun constantly generates pressure waves, known as p-modes, which echo through its layers and cause its surface to shimmer in velocity by a fraction of a millimeter per second. The frequencies of those waves shift slightly with changes in magnetic activity, and those shifts, measured with extraordinary precision, carry information about temperatures, densities, and magnetic conditions at depths no telescope can directly observe.
Think of it as tapping on a wall to find a hollow space. You can’t see through the wall, but the sound tells you something is different in there. The Sun’s p-modes are doing something similar on a stellar scale, and scientists have been sitting with their ears pressed against it for a very long time.
The 40-Year Recording
Using almost 40 years of helioseismic data from six telescopes around the world in a global network called BiSON, the researchers uncovered a gradual change in structure just beneath the surface that has spanned multiple cycles.
The network is operated by the Sun, Stars, and Exoplanets Group at the University of Birmingham and funded by the UK Science and Technology Facilities Council. Six telescopes spread across the globe means round-the-clock coverage. When the Sun sets on one, another picks it up. The dataset that has accumulated since the late 1980s is, by any reasonable measure, one of the most sustained listening projects in the history of science.
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 through 25, from 1987 to 2025. Four complete solar cycles. Four rises and falls of the Sun’s eleven-year magnetic heartbeat. Enough data to see patterns that a shorter study simply could not.
That long view was crucial. A single cycle, or even two, might have looked like noise or a temporary irregularity. With nearly 40 years of data, the team could track changes across cycles 22, 23, 24, and 25 and see that the pattern was not going away.
What They Found
The headline result is striking in its simplicity: the Sun’s internal magnetic activity has been migrating upward, getting closer to the surface with each passing cycle.
The research showed that the Sun’s internal magnetic activity has become progressively confined to a shallower subsurface layer, within about 1,000 kilometers of the visible surface, across Solar Cycles 22 through 25. The shift has not been captured by traditional surface measures like sunspot counts.
That 1,000-kilometer figure deserves a moment. The Sun’s radius is about 696,000 kilometers. What’s being described here is the Sun’s magnetic engine concentrating itself into a layer roughly 0.1% as thick as the star itself. A skin, in solar terms.
Professor Sarbani Basu, from Yale University, said: “We discovered that the relationship between internal solar oscillations and surface activity has evolved over the past few cycles. This trend cannot be explained simply by weaker magnetic fields. Instead, it indicates a structural reorganisation of how the Sun’s magnetic activity is stored beneath the surface.”
The current solar cycle 25 is showing particularly strong signatures of these changes. Lead author Professor Bill Chaplin, from the University of Birmingham, said the Sun has its own “active biorhythm” creating rising and falling magnetic activity that shapes space weather.
The researchers found that cycle 25 is showing particularly strong signatures of these changes, meaning what’s happening right now, in the current cycle, is the most pronounced expression of this trend yet.
Why Sunspots Aren’t Enough
For centuries, sunspots were the primary way humans tracked solar activity. Dark blotches on the solar surface, they correlate broadly with the Sun’s magnetic intensity. When there are lots of them, the Sun is active. When they fade, things calm down. The count has been kept continuously since 1749, making it the longest running instrumental record in astrophysics.
The problem is that sunspots are a surface phenomenon. Solar activity is usually tracked through surface signs, including sunspot counts and the 10.7-centimeter radio flux. Those measures remain useful, but they don’t show what is happening beneath the surface, where magnetic fields are regenerated and rearranged.
The deeper story – what’s happening in the layers below, where the Sun’s magnetic dynamo actually operates – can diverge significantly from what sunspot counts show. The new research is precisely a case study in that divergence. Cycles that looked relatively subdued from the surface have internal signatures that look quite different when you examine the p-mode frequencies probing deeper layers.
If you want to understand how solar cycles affect our daily lives, this gap between surface appearance and subsurface reality is where things get complicated. Conventional surface indicators suggested Cycle 25 was relatively weak, consistent with the subdued Cycle 24 that preceded it. The helioseismic data, however, indicate that Cycle 25 looks comparably vigorous to earlier cycles when examined in the high-frequency band sensitive to near-surface structure, suggesting traditional metrics have been underrepresenting its actual activity level.
In other words, the Sun may be more active than it appears. And that gap between appearance and reality is exactly the kind of thing that gets people into trouble when a solar storm arrives unexpectedly.
The Space Weather Problem
The stakes of getting solar forecasting wrong aren’t abstract. Solar activity and its cyclic variation has its origins in the Sun’s interior, in processes that regenerate and reorganise 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.
In May 2024, that theoretical risk became very concrete. A G5 geomagnetic storm caused a GPS outage during a crucial planting period, costing American farmers over $500 million in potential profit, according to NOAA’s NESDIS. That was a single storm, relatively contained in its effects. A larger one, arriving with less warning, directed at a more vulnerable constellation of satellites, could do considerably more.
Every operational forecasting system relies on models of how the Sun’s internal dynamics behave across the solar cycle. Those models have already struggled. A comprehensive independent review of over 100 predictions for Cycle 24 and over 130 for Cycle 25 found that most methods failed to predict each peak correctly: Cycle 24 was widely forecast to be strong and turned out to be the weakest in a century, while Cycle 25 was widely forecast to be weak and surged well beyond initial projections.
If the models are calibrated against surface observations that are missing what’s happening in the subsurface, then every forecast built on those observations has an unquantified blind spot. The Birmingham team’s findings suggest that blind spot has been growing.
A Different Mode of Behaviour
The researchers are careful about how far to take the conclusion. They say more observations through the rest of Cycle 25 and into Cycle 26 will be needed to determine whether this is a sustained long-term transition.
But Professor Chaplin’s description of what they may be seeing carries real weight: he describes it as the Sun potentially entering “a different mode of behaviour.”
Ongoing collection and analysis of solar data over what remains of Cycle 25 and into the upcoming Cycle 26 will be crucial in determining whether the changes discovered in the Sun’s activity point to a sustained, systematic change in solar magnetic behaviour.
The study was published in a June 2026 paper in the Monthly Notices of the Royal Astronomical Society, one of the oldest and most respected journals in astrophysics. It is the first study of this kind – not the first to use helioseismology, and not the first to study the solar cycle, but the first to have enough data across enough consecutive cycles to actually see this particular structural shift.
Magnetic activity is becoming more tightly confined near the surface with each cycle. This is the first such discovery and would have been impossible without the long BiSON observations.
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The Part That Stays With You
Science moves in two speeds. There’s the slow kind – the decades of data collection that nobody writes headlines about, the six telescopes pointed at the Sun through decades of nights and seasons and budget reviews and everything else that happens to a long-running project. And then there’s the moment when all that patience produces something nobody expected.
What the Birmingham team has found is that the Sun has been changing in a way we didn’t know to look for. Not changing in some mild sense. Changing in a way that shifts the structural foundations of how its magnetic activity organises itself, in a way that our forecasting models haven’t accounted for, and in a way that has gotten stronger with each passing cycle.
Whether this is a blip or a lasting reorganisation of something fundamental is genuinely unknown. The researchers will keep listening, through Cycle 25 and into Cycle 26, and at some point there will be enough data to say something more definitive. In the meantime, we’re left with a more complicated picture of a star that most of us never thought about as complicated at all.
The Sun has been doing something for forty years, something deeper than sunspots, something the surface couldn’t show us. We’re only now beginning to understand what it is.
AI Disclaimer: This article was created with the assistance of AI tools and reviewed by a human editor.