Quiz solved? The sun rotates faster at its equator than at the poles – and this difference is larger than it should be. But why? Astronomers may now have found the answer to this. Accordingly, in the high latitudes of the sun there are huge, fast oscillating vortices that regulate the temperature distribution of the solar plasma: They transport heat from the poles to the equator and then drive the faster rotation of the plasma there, as the researchers report in “Science Advances”.
The sun is not just a glowing fireball; quite complicated processes take place on it. Two large-scale circulation currents in the solar plasma set the pace for the eleven-year solar cycle, while other convection currents influence the solar magnetic field and the ebb and flow of the bubbling solar plasma. The Sun’s massive plasma eruptions are also shaped by plasma vibrations. In 2018, solar researchers also discovered a type of Rossby waves on the sun’s surface – huge vortex-shaped plasma waves that extend across half the star.
Contradiction to theory
Now one of these processes provides the possible answer to a puzzle that has existed for decades. The interior of the sun does not rotate at the same speed everywhere, but shows a differential rotation depending on the latitude: At the poles of the sun, the plasma needs around 34 days to complete one revolution, but at the equator it only takes 24 days. These differences in rotation speed are not limited to the superficial layers, but extend down to around 200,000 kilometers deep into the sun’s interior. They are considered to be influential for the solar magnetic field and many other processes.
The problem, however, is that the extent of the sun’s differential rotation contradicts common models. Why is still unclear. According to one theory, small temperature differences between the solar poles and the equator could play a role. “Unfortunately, this latitude-dependent temperature difference is too small to be measurable through direct observation,” explain Yuto Bekki and his colleagues from the Max Planck Institute for Solar System Research in Göttingen. Because the view into the deep interior of the sun is made difficult by the sun’s corona, which is millions of degrees hot.
Long-period but fast oscillations
But Bekki and his team have now solved this problem. During their analyzes of data from NASA’s solar satellite Solar Dynamics Observatory (SDO), they discovered a new phenomenon in the flow pattern of the solar plasma: In addition to the already known Rossby waves in the equatorial region of the sun, there are also long-period oscillations at the solar poles. They become visible as large eddy currents on the surface of the sun.
As Bekki and his team have now discovered, these long-period flows play a crucial role in the temperature distribution and thus also the differential rotation of the solar plasma. When they recreated the observation data in astrophysical models, it became apparent that the fast currents, which reach around 70 kilometers per hour at the poles, have far-reaching effects.
“Comparing the nonlinear simulations with the observations allowed us to understand the physics of long-period oscillations and their role in controlling the differential rotation of the Sun,” explains Bekki.
This is what the long-period oscillations occurring in the sun’s polar latitudes would sound like.© MPI for Solar System Research
Temperature gradient drives rotation patterns
It turned out that these long-period oscillations transport heat from the solar poles to the equator. Because unlike on Earth, the sun’s poles are hotter than its equator. The newly discovered fast current ensures that the temperature difference between the solar poles and the equator always remains at just under seven degrees. The highlight: According to the models, this gradient is optimal for driving the differential rotation of the solar plasma.
“The 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,” explains senior author Laurent Gizon from the MPI for Solar System Research. In the simulations, the rotation patterns caused by the long-period oscillations and the temperature gradient corresponded almost exactly to those observed on the Sun.
“We conclude that the high-latitude oscillation modes explain the differential rotation of the Sun,” the researchers state. Whether this also applies to other, even faster rotating stars is one of the questions that needs to be clarified next. (Science Advances, 2024; doi: 10.1126/sciadv.adk5643)
Source: Max Planck Institute for Solar System Research
April 3, 2024 – Nadja Podbregar