Forty years of data give unique insight into Sun’s inner life

Scientists have analysed more than 40 years of astronomical data to uncover evidence that the Sun’s internal structure subtly changes from one solar cycle minimum to the next.

Publishing their findings in Monthly Notices of the Royal Astronomical Society, researchers from the University of Birmingham and Yale University reveal that even small differences in solar magnetic activity produce detectable changes inside the Sun.

Every 11 years, the Sun goes through a cycle of magnetic activity and is at its calmest during ‘solar minimum’ – few sunspots, weaker magnetic fields, and a more uniform surface.

Using observations from six telescopes sited around the world known as the Birmingham Solar-Oscillations Network (BiSON), the astrophysicists looked at what happens inside the Sun during four of its quietest periods between solar cycles 21–25.

Researchers analysed tiny vibrations inside the Sun – formed by trapped sound waves which make the Sun gently oscillate – to infer what was happening below the Sun’s surface. This is the first ever study to compare four successive solar minima by looking inside the Sun using these oscillations (the field of study called helioseismology).

Researchers looked for a distinctive sound wave ‘glitch’ created when helium becomes doubly ionized, changes in sound speed, as well as comparing observations against predictions from solar models with slightly altered internal conditions.

They found that the minimum which occurred in 2008/2009 between cycles 23 and 24, known to be one of the quietest and longest on record, showed measurably different internal conditions compared to the other three minima.

The helium ‘glitch’ was significantly larger than in the other three minima – indicating a real structural difference. The Sun exhibited a higher sound speed in its outer layers, suggesting higher gas pressures and temperatures, and lower magnetic fields.

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.”

Space weather

The researchers' findings could prove useful for forecasting future activity cycles. This is important because the Sun’s activity gives rise to space weather – energetic outbursts that can have significant impacts on Earth. Space weather can cause radio communication blackouts, GPS errors, power grid failures, and damage communications satellites.

Professor Sarbani Basu, from 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”.

Professor Chaplin added: “Our work demonstrates the power of long-term stellar seismic observations. With upcoming missions such as the European Space Agency’s PLATO, the techniques used in this study could be applied to other Sun-like stars, helping us to better understand how their activity changes and how they influence their local environments, including any planets they may host.”

BiSON is operated by the Sun, Stars, and Exoplanets Group at the University of Birmingham and funded by the UK Science and Technology Facilities Council (STFC). This world-wide network of remotely operated ground-based telescopes provides round-the-clock monitoring of the Sun’s oscillations and a truly global probe of the solar cycle.