Long-period oscillations and the rotation of the sun
Long-period oscillations appear to play a crucial role in controlling the Sun’s rotation pattern. Our central star does not rotate at the same speed at all latitudes. For the study now presented, observational data from the Solar Dynamics Observatory
combined with the most modern numerical simulations.
Three-dimensional visualization of the oscillations at maximum velocities at high latitudes of the sun. Snapshot of the streamlines of the long-period oscillations at high latitudes in the convection zone. The red and blue colors indicate the prograde (consistent with rotation) and retrograde (opposite rotation) zonal flows, respectively. [Groansicht] |
The Sun’s differential rotation pattern has puzzled scientists for decades: While the poles rotate with a period of about 34 days, the mid-latitudes rotate faster and the equatorial region only needs about 24 days to complete a full revolution. Furthermore, in recent years, advances in helioseismology, that is, the study of the Sun’s interior using solar sound waves, have shown that this rotation profile is almost constant throughout the convection zone. The sun’s convection zone extends from a depth of about 200,000 kilometers to the visible surface of the sun and is the scene of violent upheavals of the hot solar plasma. These play a crucial role in the magnetism and activity of the sun.
Theoretical models have long suggested that there must be a small temperature difference between the solar poles and the equator. This is the only way to maintain the sun’s well-known rotation pattern. However, measuring this temperature difference has proven to be extremely difficult. Finally, observations have to “see through” the background of the sun’s deep interior, which has a temperature of up to a million degrees. As the MPS researchers have now shown, it is possible to determine the temperature difference from observations of the long-period oscillations of the sun.
In their analysis of observational data, the Helioseismic and Magnetic Imagers (HMI) on board the Solar Dynamics Observatory NASA recorded from 2017 to 2021, the scientists turned to global solar oscillations with long periods, which can be seen as vortex movements on the sun’s surface. Three years ago, MPS researchers discovered these inertial oscillations. Of these observed vibration modes, those that reach their maximum speeds of up to 70 kilometers per hour at high latitudes proved to be particularly influential.
To investigate the nonlinear nature of these oscillations, a series of numerical three-dimensional simulations were carried out. The simulations of long-period oscillations with maximum velocities at high latitudes show that these oscillations transport heat from the solar poles to the equator. This limits the temperature difference between these areas to less than seven degrees. “The very small temperature difference between the poles and the equator controls the angular momentum balance in the sun and is therefore an important feedback mechanism for the global dynamics of the sun,” says MPS director Prof. Dr. Laurent Gizon.
With their simulations, the researchers have described the crucial processes in a completely three-dimensional model for the first time. Previous efforts had been limited to two-dimensional approaches that assumed symmetry around the sun’s rotation axis. “Comparing the nonlinear simulations with the observations allowed us to understand the physics of long-period oscillations and their role in controlling the Sun’s differential rotation,” says Dr. Yuto Bekki, MPS postdoc and lead author of the study now presented.
The oscillations with maximum speeds at high latitudes of the sun are driven by a temperature gradient, similar to extratropical eddies on Earth. The physics are similar, even if the details are different: “On the Sun, the solar pole is about seven degrees hotter than the equator, and that is enough to drive currents of about 70 kilometers per hour across a large part of the Sun. The process “In a certain way it is similar to the drive of eddy currents,” says MPS scientist Dr. Robert Cameron.
Exploring the physics of the sun’s deep interior is difficult. The current study is important because it shows that the sun’s long-period oscillations are not only useful “diagnostic tools” for the sun’s interior, but that they actively control the processes in the sun. Future work, carried out as part of the ERC Synergy Grant WHOLESUN and the Collaborative Research Center 1456 “Mathematics of Experiments”, aims to better understand the role of these oscillations and their diagnostic potential.
The results have now been published in the specialist journal Science Advances
published.
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