Four Decades of Solar Data Unveil the Sun’s Hidden Interior

For the first time in the history of helioseismology, scientists have compared four consecutive solar cycle minima by peering directly inside the Sun — and what they found is reshaping our understanding of solar dynamics.

A research team from the University of Birmingham and Yale University has published a landmark study in Monthly Notices of the Royal Astronomical Society, drawing on more than 40 years of observational data collected by six ground-based telescopes operating as part of the Birmingham Solar-Oscillations Network (BiSON). Their analysis reveals that the Sun’s internal structure does not remain static between solar minima — it changes, subtly but measurably, from one cycle to the next.

The technique behind this discovery is helioseismology: the study of acoustic waves trapped inside the Sun that cause it to oscillate like a ringing bell. By analyzing these vibrations, scientists can reconstruct what is happening beneath the solar surface — much like how seismologists use earthquakes to probe Earth’s interior.

The team focused specifically on solar minimum periods, the roughly 11-year intervals when the Sun reaches its quietest phase: fewer sunspots, weaker magnetic fields, and a generally calmer surface. During each minimum, the researchers looked for two key signatures: a characteristic acoustic «glitch» produced when helium undergoes double ionization in the Sun’s outer layers, and variations in sound speed at different depths.

The most striking finding centers on the solar minimum of 2008–2009, the deepest and most prolonged quiet period in nearly a century, which fell between solar cycles 23 and 24. Compared to the other three minima analyzed, this particular interval stood out sharply. The helium ionization glitch was significantly larger, and the sound speed in the outer solar layers was measurably higher — pointing to elevated gas pressure and temperature, alongside unusually suppressed 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 of the University of Birmingham. «The Sun’s outer layers subtly change across activity cycles, and we found that deep quiet minima can leave a measurable internal fingerprint.»

Professor Sarbani Basu of Yale University emphasized the broader implications: «Revealing how the Sun behaves beneath its surface during these quiet periods is significant because this behavior has consequences for predicting future cycles.»

And those predictions matter. Solar activity drives space weather — energetic bursts of radiation and charged particles that can knock out radio communications, corrupt GPS signals, trigger power grid failures, and destroy satellites in orbit. The better scientists understand the internal mechanisms driving solar cycles, the better equipped humanity will be to anticipate and mitigate those effects.

This study represents a rare achievement: four decades of continuous, high-quality data combined with cutting-edge seismological analysis, producing a measurable map of the Sun’s hidden interior across multiple complete cycles. It is, in every sense, a window into a star’s inner life.

More information in arXiv