40 Years of Listening to the Sun Reveals Its Interior Changes Between Cycles

The Sun looks, to an unaided eye, like a constant thing. But it breathes. Every 11 years it swings from magnetic maximum - sunspots crowding its surface, coronal loops arcing high into space - to minimum, when things go quiet. Most people assume those quiet periods are roughly identical. Forty years of data from six telescopes scattered across the globe suggest they are not.

Listening to the Sun from the inside

The technique is helioseismology, and it works on the same principle as seismology on Earth. Sound waves bounce around inside the Sun, making its surface oscillate in ways that carry information about the interior - pressure, temperature, composition. Measure the oscillations precisely enough and you can infer conditions thousands of kilometers below the photosphere.

The Birmingham Solar-Oscillations Network, or BiSON, has been doing exactly that since the 1970s. Its six remotely operated telescopes, positioned around the world to provide continuous coverage, record the Sun's subtle pulsations around the clock. The dataset now spans more than four complete activity cycles.

A team from the University of Birmingham and Yale University used that archive to do something no one had done before: compare the internal conditions of the Sun across four successive solar minima, the quiet troughs between cycles 21 and 25. The findings were published in Monthly Notices of the Royal Astronomical Society.

One minimum that stood apart

The minimum between cycles 23 and 24, which fell in 2008 and 2009, was already notable to observers. It was the longest and deepest quiet period in nearly a century - sunspot counts stayed near zero for months on end, the solar wind thinned, and some measures of magnetic activity hit record lows for the space age.

The helioseismology data now reveal that something was different inside the Sun too. The team looked at a distinctive acoustic signature produced when helium becomes doubly ionized in the Sun's outer layers - a kind of sound-wave glitch that acts as a structural tracer. During the 2008-2009 minimum, that glitch was measurably larger than in the other three minima. The outer layers also showed higher sound speeds, consistent with elevated gas pressure and temperature, alongside weaker magnetic fields.

"For the first time, we've been able to clearly quantify how the Sun's internal structure shifts from one cycle minimum to the next," said Professor Bill Chaplin from Birmingham. "The Sun's outer layers subtly change across activity cycles, and we found that deep quiet minima can leave a measurable internal fingerprint."

Why space weather forecasters care

The practical stakes are real. Space weather - the energetic outbursts the Sun generates at solar maximum - can knock out radio communications, corrupt GPS signals, trigger power grid failures, and damage satellites. Forecasting the strength of the next cycle depends on understanding what sets the baseline from which activity builds.

Professor Sarbani Basu from Yale, a co-author on the study, put it directly: "Revealing how the Sun behaves beneath its surface during these quiet periods is significant because this behaviour has a strong bearing on how the activity levels build up in the cycles that follow."

That connection - between the depth of a minimum and the vigor of the maximum that follows - has been suspected for years. The new internal measurements give forecasters a physical mechanism to point to rather than a statistical correlation.

The next step: other stars

Chaplin sees the BiSON results as a proof of concept for work that will eventually extend beyond our solar system. The European Space Agency's PLATO mission, scheduled for launch in late 2026, will monitor more than 200,000 bright stars with 26 cameras, looking for planetary transits and stellar oscillations. The same helioseismology techniques developed at Birmingham could then be applied to Sun-like stars, testing whether their activity cycles leave internal fingerprints too - and what that means for planets in their habitable zones.

Forty years is a long time to listen. The reward is that the Sun is finally beginning to reveal what it sounds like from the inside when it's at its most peaceful.