Four decades of acoustic data from six telescopes planted around the globe have delivered a finding that upends assumptions embedded in every modern space weather forecast: the Sun's internal magnetic reorganization has been quietly retreating toward the surface — cycle by cycle, since at least 1987 — in ways that standard sunspot measurements never captured.
The study, published Thursday in Monthly Notices of the Royal Astronomical Society by an international team led by Professor Bill Chaplin of the University of Birmingham, draws on nearly 40 years of helioseismic data from the Birmingham Solar Oscillations Network — a ring of six remotely operated ground-based observatories designed to eavesdrop on the Sun around the clock. Its core finding: the solar-cycle-driven structural changes detected beneath the Sun's surface are becoming progressively confined to a shallow skin within approximately 1,000 kilometers of the photosphere, leaving existing forecasting models calibrated against a Sun that may no longer behave as it once did.
The result matters because space weather forecasting — the discipline responsible for warning satellite operators, power grid engineers, GPS network administrators, and astronaut mission planners about incoming solar storms — depends on models of the solar dynamo: the interior engine that generates the Sun's magnetic field. If that engine is restructuring itself in ways the models have not accounted for, those warnings could arrive late, underestimate severity, or reflect conditions that no longer match the Sun's actual behavior.
Listening Deep Into Solar Interior
The technique underpinning the discovery is helioseismology — a discipline that treats the Sun the way geologists treat the Earth: by listening to waves that propagate through its hidden interior. 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.
BiSON has been collecting exactly that kind of data since 1976, making it the longest-running helioseismology network on Earth. 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.
For this study, Chaplin's team analyzed p-mode oscillation data spanning Solar Cycles 22 through 25 — a window running from 1987 to 2025. They sorted the oscillations into low-, mid-, and high-frequency bands, each probing successively shallower depths beneath the solar surface, and compared how the frequency shifts in each band tracked against traditional surface indicators of activity, principally sunspot counts and 10.7-centimeter radio flux.
Sun Magnetic Activity Confined Near Surface Since Cycle 23
The comparison produced three linked findings that, taken together, indicate a structural shift in solar dynamics that existing models do not explain.
First, the correspondence between helioseismic frequency shifts and surface activity measures has drifted markedly since the declining phase of Cycle 23. Where the two measures once moved together, they have since been diverging — meaning the surface is increasingly telling a different story from the interior.
Second, and most consequentially, the divergence tracks with a specific geometric change: the solar-cycle-driven structural changes detectable inside the Sun are becoming increasingly confined to a shallow layer within approximately 1,000 kilometers of the visible photosphere. In earlier cycles, the measured internal changes reached deeper. "We have uncovered evidence of systematic changes in the solar activity cycle," Professor Chaplin said. "Crucially, 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."
Third, the apparent weakness of Solar Cycle 25 — which looked subdued by conventional surface metrics — turns out to be partially an illusion generated by that same divergence. When examined through the high-frequency helioseismic band, which is sensitive to near-surface layers, Cycle 25 appears comparably vigorous to earlier cycles. The surface indicators were simply not measuring what they appeared to be measuring.
Professor Sarbani Basu of Yale University, a co-author, said the findings point to something more fundamental than a simple variation in field strength. "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."
Why Space Weather Forecasting Depends on Solar Interior Models
The implications for infrastructure operators are concrete. Space weather events — solar flares, coronal mass ejections, and the geomagnetic storms they trigger on Earth — originate in the Sun's interior magnetic dynamics. Every operational forecasting system, including the one operated by the National Oceanic and Atmospheric Administration's Space Weather Prediction Center, relies on models of how those 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. A panel convened by NOAA, NASA, and the International Space Environment Services predicted in 2019 that Cycle 25 would peak with a maximum sunspot number of roughly 115; actual activity exceeded that threshold well before the predicted maximum date.
If the structural shift documented by Chaplin's team is an ongoing reconfiguration of the Sun's internal dynamo rather than a transient fluctuation, both categories of forecast error may reflect the same root cause: models built on data from an era when the Sun's interior behaved differently from how it behaves now.
The research also carries direct implications for long-duration space missions. Radiation hardening for spacecraft and exposure planning for astronauts depend on estimates of the solar radiation environment over mission timescales that span years to decades. A Sun whose interior dynamics are in systematic transition represents a moving target for that kind of planning.
What 40 Years of BiSON Data Made Possible
What makes this discovery achievable at all is the unusual duration of the BiSON dataset. Continuous helioseismic monitoring covering nearly five full solar cycles is not available from any other instrument or network. Space-based helioseismology missions such as the Solar and Heliospheric Observatory and the Solar Dynamics Observatory have contributed high-quality data, but their operational windows are shorter. BiSON's ground-based observations, running uninterrupted since 1976, provide the long baseline that multi-cycle structural comparisons require. The researchers are explicit: the finding would have been impossible without it.
How Does Helioseismology Work to Detect Solar Interior Changes?
The analogy to earthquake seismology is precise. Both fields use waves that propagate through an opaque medium to infer internal structure. Earthquake seismologists listen for waves generated by tectonic events; helioseismologists listen for acoustic waves generated by turbulent convection in the Sun's outer layers — a continuous, low-level roar that drives the five-minute oscillation visible at the solar surface.
Each mode of oscillation probes a different depth: low-frequency modes resonate through the deep interior; high-frequency modes are trapped in shallower layers near the surface. By measuring how the frequencies of those modes shift in response to magnetic activity across a full solar cycle, researchers can construct a picture of where in the solar interior the magnetic reorganization is actually occurring — and, crucially, whether that picture changes from one cycle to the next.
What Chaplin's team found is that it has changed. The structural fingerprint of magnetic activity inside the Sun is being pressed progressively closer to the surface with each successive cycle, in a trend that current dynamo models do not reproduce.
What Comes Next for Solar Cycle 25 Research
The team plans to continue monitoring BiSON data through the remainder of Solar Cycle 25 and into Solar Cycle 26, which NOAA projects will begin sometime between 2029 and 2032. The central question is whether what the current data reveal represents a sustained, systematic shift in the Sun's interior behavior — possibly connected to the known multi-decade modulation of solar activity — or a transient that will reverse as the Sun enters its next cycle.
That answer has practical consequences. Space weather forecasting is increasingly viewed, including by NOAA and the U.S. Space Force, as a strategic infrastructure priority in the same category as weather prediction for aviation and agriculture. If the solar dynamo is entering a new mode of operation, the observational record that existing forecast models were trained on may no longer represent the Sun that currently exists.
Frequently Asked Questions
What did the new solar cycle study find?
Researchers at the University of Birmingham used nearly 40 years of helioseismology data from the BiSON network to show 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.
How does helioseismology reveal what is inside the Sun?
Helioseismology measures tiny shifts in the frequencies of acoustic waves — pressure waves similar to sound — that resonate through the Sun's interior. Different frequency modes probe different depths, allowing researchers to infer conditions beneath the solar surface without any direct observation. The technique is analogous to using earthquake waves to map the Earth's interior structure.
Why does the solar interior change matter for space weather forecasting?
Space weather events — including solar flares and the coronal mass ejections that trigger geomagnetic storms — originate in the Sun's internal magnetic dynamics. Operational forecast models depend on assumptions about how those dynamics behave across the solar cycle. If the interior structure is shifting in ways those models have not accounted for, forecast accuracy for events that can disrupt satellites, power grids, and GPS systems may be affected.
How does Solar Cycle 25 compare to its predecessors based on this research?
Conventional surface indicators suggested Cycle 25 was relatively weak, consistent with the subdued Cycle 24 that preceded it. The new 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.
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