We may finally know what causes sunquakes. The cause of these strange rumbles within the sun has divided solar physicists for decades, but a new study has found that they may come from beams of high-energy electrons burrowing through the outer layers of the sun.
Sunquakes are waves in the sun’s photosphere – the surface from which its light shines – that ripple across the star like the waves from a pebble tossed in a lake. They are usually associated with strong solar flares, which are powerful eruptions of energy that sometimes fling plasma from the sun into huge tendrils and loops in the solar corona, its tenuous outermost layer.
Despite this apparent connection, it has long been debated whether flares could actually cause sunquakes. “The origin of sunquakes is located deep in the photosphere, while solar flares usually occur in the corona,” says  at Nanjing University in China.
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It makes sense that it would be extremely hard for an energy release in the corona to produce a disturbance in the photosphere because the photosphere is about 1 trillion times denser than the corona, says Ding. “Just as a tail cannot wag a dog.”
Ding and his colleagues examined data from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) space observatory on 20 flares that occurred during the most recent solar cycle, lasting from 2008 through 2019. The researchers focused their analysis on 12 of them that occurred at the same time as sunquakes, and which had enough RHESSI data to examine in detail.
The researchers found that 11 of the 12 flares also coincided with blasts of X-rays that hinted at the presence of high-energy electrons, far more of them than accompany solar flares without sunquakes. This supports the idea that the magnetic activity thought to cause solar flares also creates beams of electrons that plunge into the photosphere of the sun.
Understanding these sunquakes is important because if we can figure out what causes them, we can use them to probe the areas of the sun through which they propagate. “When there are earthquakes we can study the interior of the earth, and when there are sunquakes we can measure the interior of the sun, we can look at things that we can’t otherwise see,” says at the New Jersey Institute of Technology.
But it’s not all wrapped up yet: researchers still struggle to explain exactly how the electrons transfer their energy into the sun’s plasma to cause a ripple. We also don’t know whether any of these mechanisms apply to the quakes we’ve observed without strong X-rays.
There are most likely multiple mechanisms behind these quakes, the researchers say – the electrons might need some help penetrating the photosphere, and some tremors don’t seem to coincide with high energy electrons at all. “Our work did not provide a full explanation for all kinds of sunquakes, but only a possible scenario in triggering sunquakes,” says at Nanjing University. “The mystery requires more accurate observation for further validation.”
It also requires detailed modelling of how the sun’s plasma, its magnetic fields and its electrons interact with one another. There are several spacecraft observing the sun now, so the new data they provide should help researchers finish unravelling the mystery.
The Astrophysical Journal Letters
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