Researchers from the University of Birmingham and Yale University have discovered changes in the Sun’s internal structure between solar cycle minima, the quietest points of its 11-year magnetic activity cycle.
The findings, published in the Monthly Notices of the Royal Astronomical Society, reveal that even small variations in solar magnetic activity leave an internal ‘fingerprint’ beneath the surface.
Their analysis drew on more than 40 years of data collected by the Birmingham Solar-Oscillations Network (BiSON), a network of six telescopes worldwide that monitors the Sun’s natural vibrations.
Professor Bill Chaplin from the University of Birmingham said: “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.
“The Sun’s outer layers subtly change across activity cycles, and we found that deep quiet minima can leave a measurable internal fingerprint.”
By studying sound waves trapped inside the Sun – a technique known as helioseismology – the team compared four recent solar minima spanning cycles 21 to 25.
They focused on a specific ‘glitch’ in the sound waves caused by doubly ionised helium, which reveals changes in temperature, pressure, and magnetic field.
The 2008 to 2009 minimum, one of the quietest and longest on record, showed notably different interior conditions compared to the others, with higher temperatures and pressures but lower magnetic fields in the outer layers.
Professor Sarbani Basu of Yale University said: “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.”
The results may improve long-term solar forecasting and highlight the value of continuous stellar monitoring.
Professor Chaplin said: “Our work demonstrates the power of long-term stellar seismic observations.”
