They are traveling at five kilometers per hour: Researchers report enormous vortex movements on the sun’s surface that can be traced back to very long-period oscillations of the star. Using computer simulations, they were able to make it clear that the newly discovered waveform is evidently caused by the so-called differential rotation of the sun. The scientists say that the findings could now be used for further research into the internal structure and dynamics of our mother star.
Streams of seething embers: the surface and the interior of the sun are constantly in motion. This dynamic can cause characteristic vibrations: It has been known since the 1960s that the sun produces “high sounds”. Plasma currents near the surface of the sun excite millions of so-called modes of acoustic waves with short periods of around five minutes. The term mode describes the temporal characteristics of waves.
These rapid oscillations of the sun have been recorded by earth-based telescopes and space observatories since the mid-1990s, because they can provide important information: They enable conclusions to be drawn about internal features of the sun – similar to how seismologists study the interior of the earth using the effects of earthquakes. Using what is known as helioseismology, researchers have already been able to document the rotation depending on the depth and the heliographic latitude: The matter of the sun therefore moves at different speeds in different latitudes – this is known as differential rotation.
On the trail of long-period oscillations
Already more than 40 years ago it was theoretically shown that stars should have vibrations with significantly longer periods in addition to short-period ones. So far, however, it has not been possible to clearly demonstrate these effects because they are comparatively elusive. As the international team led by Laurent Gizon from the Max Planck Institute for Solar System Research in Göttingen (MPS) explains, it was necessary to observe the horizontal movements of the sun’s surface over many years in order to prove the long-period vibrations. Exactly such data was now available: The researchers were able to evaluate information from NASA’s Solar Dynamics Observatory (SDO) space probe, which includes observations of the sun over a period of ten years.
The scientists have now been able to detect a few dozen vibrations that correspond to the predicted long-period versions. Some have their maximum flow velocities at the poles, others in the middle latitudes and still others near the equator of the sun. “The long-period oscillations manifest themselves as very slow vortex movements on the sun’s surface at speeds of around five kilometers per hour. It’s about as fast as a human goes, ”says co-author Zhi-Chao Liang from the MPS.
In order to characterize the newly discovered vibrations more precisely, the scientists compared their observation data with the results of computer models on the dynamics of the sun. The simulations included data on the internal structure of the sun, the characteristics of the convective currents in the upper layers, and information on the differential rotation obtained from helioseismological research. “The models allowed us to look inside the sun and reveal the full three-dimensional structure of the vibrations,” says co-author Yuto Bekki from MPS.
Potential for solar research
It became clear that the surface velocities in the model agree well with those of the observed vibrations. The simulations also showed that the wave formations extend to a depth of 200,000 kilometers below the sun’s surface. Above all, however, it became clear: “All of these new oscillations that we observe on the sun are strongly influenced by the differential rotation,” says co-author Damien Fournier from the MPS. The dependence of the sun’s rotation on the latitude determines where the speed of the modes is greatest. “The vibrations also depend sensitively on the properties of the interior of the sun. In particular, the strength of the turbulent movements and the associated viscosity of the solar medium as well as the strength of the convective drive, ”adds his colleague Robert Cameron from MPS.
This makes it clear: Research into long-period vibrations could also enable new insights into the interior of the sun. “Just as we use acoustic vibrations with helioseismology to learn more about the processes inside the sun, we can use the long-period vibrations to learn more about the turbulent processes,” says Cameron. First author Gizon adds: “The discovery of a new type of solar oscillation is very exciting. It allows us to infer properties such as the strength of the convective drive, which ultimately control the solar dynamo. “
Source: Max Planck Society, NASA, specialist article: Astronomy & Astrophysics, doi: 10.1051 / 0004-6361 / 202141462