51动漫

The sun is growing. And shrinking, and growing again. Every 11 years, the sun鈥檚 radius oscillates by up to two kilometres, shrinking when its magnetic activity is high and expanding again as the activity decreases.

We already know that the sun is not a static object. Its surface is regularly covered with darker areas known as sun spots and brighter areas known as flares. The first precise measurements of the sun鈥檚 size in the 18th and 19th centuries showed that it appeared to be larger when there were fewer of these features.

Researchers聽later realised that these surface changes are side effects of an 11-year variation in the strength of the sun鈥檚 magnetic field. When the field is strongest, a period that we call solar maximum, there are more sunspots than when it is weakest, at solar minimum.

Plasma wave

Now, Alexander Kosovichev at the New Jersey Institute of Technology and Jean-Pierre Rozelot at the University of C么te d鈥橝zur in France have used 21 years of solar observations from two space observatories to pin down how the sun鈥檚 size has changed through the last two solar cycles.

The pair looked at waves of plasma that travel over the sun鈥檚 surface. Just like how the sound waves moving through a bigger instrument have a lower pitch, the frequencies of these solar waves depend on the sun鈥檚 size. They are a much more precise way of measuring the sun than just looking at it, and they indicate that its average radius is 695,000 kilometres.

By examining these waves over nearly two full solar cycles, Kosovichev and Rozelot found that this radius shrinks by one to two kilometres during solar maximum, and grows to its normal size again by solar minimum. The waves slightly change frequency depending on how deep into the sun they penetrate, which allowed the researchers to determine that most of the shrinkage happens about five million metres under the surface.

This reduction in size is likely caused by a strong magnetic field compressing the plasma under the sun鈥檚 surface, but the details are still hazy. We don鈥檛 fully understand how the sun鈥檚 magnetic field is created or what its structure is like beneath the surface 鈥� that鈥檚 part of the mission of NASA鈥檚 Parker Solar Probe, set to launch in August.

The work is an important stepping stone in understanding solar activity says Michael Thompson at the US National Center for Atmospheric Research in Colorado. 鈥淭he magnetic activity cycle is important because it has impacts on the Earth, on satellites, on high-altitude aircraft, on the power grid, even on humans, particularly in space.鈥�

It does that by blasting high-energy charged particles towards Earth in coronal mass ejections, which occur primarily when magnetic activity is high. But the sun鈥檚 size itself probably doesn鈥檛 affect much on Earth. In fact, we get slightly more sunlight during solar maximum, when the sun is smaller, because of tiny bright spots called faculae.

鈥淲e don鈥檛 know if the changes in the sun鈥檚 size affect the climate on Earth, but if they do those changes would be very small,鈥� says Kosovichev. 鈥淥bviously, human activity causes significantly stronger climate changes.鈥�