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Is Our Sun Slowing Down in Its Middle Age?

The Sun, now halfway through its life, might be slowing its magnetic activity, researchers say, which could lead to permanent changes in the sunspots and auroras we see.

The Sun has changed its figure, researchers say, and might keep it that way.

Spotless Sun

The spotless Sun of July 21, 2017.

The structure of the Sun’s surface, where sunspots live, appears to have changed markedly 23 years ago. That’s when solar magnetic activity might have started slowing down, Rachel Howe (University of Birmingham, UK, and Aarhaus University, Denmark) and collaborators speculate in paper to appear in the Monthly Notices of the Royal Astronomical Society (full text here). Such a structural change might help explain the Sun’s mysteriously weak cycles in recent years.

The interior of the Sun pulsates as rhythmically as a human heart. But while the heart pulses at one fairly steady frequency, the Sun reverberates at thousands of different frequencies. Pressure changes inside the Sun create these reverberations, just like pressure changes in the air create sound. The sound waves inside the Sun are outside the range of human hearing — they’re too low frequency — but if we sped them up, we could hear them just like any other sound.

Some of these sound waves come from deep within the Sun, while others come from shallower layers. Since these sound waves can tell us about the structure of the solar interior, scientists measure them constantly using instruments like the Birmingham Solar-Oscillations Network.

Acoustic waves in the Sun

A computer model of sound waves resonating in the interior of the Sun.
Stanford University

Howe and colleagues collected 29 years of data on these sound waves, and they measured how much the waves’ frequency changed over time relative to a four-year period spanning 1988 to 1992. If the Sun were to stay pretty much the same, researchers would find no real difference between this four-year period and any other period. Instead, they were surprised to find that, since 1994, low-frequency sound waves have changed quite a bit compared with their behavior during the four-year benchmark. While higher-frequency sound waves changed too, it was by a much smaller amount. This confirms previous research, which has indicated a possible structural change in the surface layer of the Sun.

Then, the researchers compared the change in higher-frequency sound waves, which reverberate in the shallowest layer of the Sun, with the number of sunspots. In general, the two tracked each other pretty well over the years: the change in high-frequency sound waves went up and down with the number of spots during the Sun’s 11-year cycle of magnetic activity.

But over the last two solar cycles, the change in high-frequency sound waves exceeded the number of sunspots. This shift, the researchers reasoned, could mean that the waves were picking up on sunspots so small that they weren’t even recorded. If there are a lot more tiny sunspots than there used to be, and they’re all confined to the shallowest layer of the Sun — well, maybe the Sun’s surface is thinning and its magnetic activity is slowing down.