Spotless Sun
The spotless Sun of July 21, 2017.
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.

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.

Solar Sound Waves
© Stanford University
A computer model of sound waves resonating in the interior of the Sun.
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.

There is evidence that Sun-like stars slow their magnetic activity after reaching middle age. And the Sun is, in fact, at just that age. But while we might be seeing some evidence of a slow-down, the process will likely happen over thousands, if not millions, of years. Furthermore, this is not the first time that the Sun has deviated from previous behavior. During a 70-year period in the 17th century, the Sun shed all but a few of its sunspots, only to right its course again.
Sunspot Groups
This graph shows the number of sunspot groups over the past 400 years. The Maunder Minimum, when sunspots were scarce between 1645 and 1715, is clearly visible, and the more regular changes of the 11-year solar cycle are also clearly seen.
And maybe we're not seeing a slow-down at all. While the strongest sunspot cycle rises and falls over the course of 11 years, there are other sunspot cycles that rise and fall over longer timespans. These other cycles could affect the 11-year cycle. So we can't yet conclude that the Sun's relatively recent changes are permanent. The best thing to do is just keep on looking: there are many more clues buried in the Sun's many heartbeats.