Four decades of data give unique insight into the Sun’s inner life

(Press-News.org) Embargoed copy of the research paper available on request

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 today (London - 00.01hrs on 4 March) 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.”

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.

ENDS

For more information, please contact the Press Office on +44 (0) 121 414 2772 or pressoffice@contacts.bham.ac.uk

IMAGE CAPTION – please credit NASA/SDO/Joy Ng:

The Sun in ultraviolet, as seen by the NASA Solar Dynamics Observatory, during two periods: one the recent solar minimum in around 2019, and the solar maximum that preceded it, when the Sun was much more active. ‘The seismic diversity of four successive solar cycle minima as observed by the Birmingham Solar-Oscillations Network (BiSON)’ - Sarbani Basu, William J. Chaplin, Rachel Howe, Yvonne Elsworth, Steven J. Hale, and Eleanor Murray is published in Monthly Notices of the Royal Astronomical Society.

Notes for editors:

As well as being ranked among the world’s top 100 institutions, the University of Birmingham is the most targeted UK university by top graduate employers. Its work brings people from across the world to Birmingham, including researchers, educators and more than 40,000 students from over 150 countries. The European Space Agency (ESA) PLATO (PLAnetary Transits and Oscillations of stars) mission, scheduled for launch in late 2026, aims to detect and characterize Earth-sized exoplanets orbiting Sun-like stars. Using 26 cameras to monitor over 200,000 bright stars, it will identify planets in habitable zones and determine their density, mass, and age to assess potential suitability for life.   Professor Bill Chaplin will join acclaimed British artist Luke Jerram when Helios, a breathtaking, large-scale illuminated sculpture of the Sun, goes on free public display on Friday 20 March at The Exchange – the University of Birmingham’s community engagement hub in Birmingham city centre. Professor Chaplin will take part in a special Q&A event with the artist. The striking sculpture will also be enhanced by a specially created surround sound composition by Duncan Speakman and Sarah Anderson. As well as the sculpture, visitors will be able to explore some of the climate and space research carried out at the University in a wrap-around exhibition called Seeing the Sun. Helios and Seeing the Sun will be on display until 1 November 2026.  

END