The Sun is subtly changes in ways we have not previously fully understood, according to new research.
Astronomers looked at data on the Sun taken over the course of more than 40 years and found that its internal structure changes in small ways between its cycles.
Every 11 years, the Sun goes through a cycle that takes it from a busy maximum to a quieter minimum. Those quiet periods have fewer sunspots, weaker magnetic fields and a more uniform surface.
But in comparing those different periods of minimum, scientists found that the Sun is actually behaving slightly differently each time. Even small differences in the activity on the Sun produce important changes inside of it, they found it.
In the research, astronomers looked at tiny vibrations inside the Sun, which are produced by trapped sound waves. Those vibrations can be used to understand what is going on in the surface.
They were looking for a particular “glitch” that is created in the sound waves when helium inside the Sun becomes double ionised, as well as changes in the speed of the sound.
They examined four different cycles and found that during the quietest of them – which came in 2008 and 2009, between cycles 23 and 24 – the conditions inside the Sun were notably different.
The helium “glitch” was larger than in the other three periods, they found, and the Sun also had a higher sound speed as well as lower 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 Bill Chaplin, from 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.”
Scientists hope that the research will prove useful in better understanding the Sun and its contribution to solar weather. That can bring energetic outbursts that can be potentially disastrous on Earth, bringing failures in the power grid and communications systems and damage to satellites.
“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,” said Sarbani Basu, from Yale University.
The work is reported in a new paper, ‘The seismic diversity of four successive solar cycle minima as observed by the Birmingham Solar-Oscillations Network (BiSON)’, published in the journal Monthly Notices of the Royal Astronomical Society.
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