The Sun's internal 'biorhythm' – which plays a critical role in the space weather we experience on Earth – has mysteriously changed over the past 40 years, a new study suggests.

Listening to tiny sound waves inside our star's 'heart' led researchers to discover that it may be entering "a different mode of behaviour". They now need to explore what this means.

The research, published today in Monthly Notices of the Royal Astronomical Society, is of particular significance to space weather.

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 geomagnetic storms and aurorae.

This 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.

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 international team of 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.

They discovered that 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 behaviour.

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. However, traditional surface measures don't capture the full story – that the Sun may be entering a different mode of behaviour unfolding over decades.

"We have uncovered evidence of systematic changes in the solar activity cycle. 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."

The researchers analysed 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 behaviour – 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 behaviour 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.

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."

Ongoing collection and analysis of BiSON 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.

ENDS