Forty years of data reveal subtle shifts inside the Sun
Researchers analyzing more than four decades of solar observations have discovered that the internal structure of the Sun changes subtly between periods of low magnetic activity, a finding that could improve predictions of solar storms and their effects on Earth.
A team from the University of Birmingham and Yale University examined over 40 years of data collected by the Birmingham Solar Oscillations Network, known as BiSON. The network operates six telescopes positioned around the world to continuously monitor vibrations on the Sun’s surface. The scientists compared conditions inside the Sun during four consecutive periods of low magnetic activity, known as solar minima.
The study, published in Monthly Notices of the Royal Astronomical Society, marks the first time researchers have used helioseismology to directly compare four successive solar minima. Helioseismology analyzes sound waves trapped inside the Sun to reveal details about its internal structure.
By tracking tiny surface vibrations, the researchers gathered information about conditions deep within the star. They measured variations in the speed of sound inside the Sun as well as an acoustic signal linked to helium atoms losing two electrons simultaneously in the Sun’s interior.
One period stood out clearly. The solar minimum of 2008 and 2009, which occurred between solar cycles 23 and 24 and is widely regarded as one of the quietest and longest on record, showed distinct characteristics compared with the other three minima.
During this period, the helium related acoustic signal was significantly stronger. The Sun’s outer layers also showed higher sound speeds, indicating increased gas pressure and temperature combined with weaker magnetic fields.
Professor Bill Chaplin from the University of Birmingham said the research provided the first clear measurement of how the Sun’s internal structure evolves from one solar minimum to another. According to Chaplin, the outer layers of the Sun change subtly between activity cycles, and unusually quiet periods can leave a measurable imprint deep within the star.
The discovery could have practical implications for space weather forecasting. Scientists believe that the depth of a solar minimum may influence the strength of the activity cycle that follows.
Solar cycle 24, which began after the unusually deep minimum of 2008 and 2009, was notably weak. Understanding the mechanisms behind this pattern could improve forecasts of solar eruptions that disrupt radio communications, affect GPS signals, damage satellites and trigger power grid failures on Earth.
Professor Sarbani Basu of Yale University said studying the Sun’s behavior beneath its surface during quiet periods is crucial because these processes strongly influence how activity builds up during the next solar cycle.
Researchers also noted that the techniques used in the study could eventually be applied to other Sun like stars. Future space missions such as the European Space Agency’s PLATO mission may allow scientists to study stellar interiors in similar ways, expanding understanding of how stars evolve and how their activity affects surrounding planets.
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