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Our Sun is about five times less magnetically active than other sunlike stars – effectively a special case. The reason for this could reside in the planets in our solar system, say researchers at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR). In the last ten years, they have developed a model that derives virtually all the Sun’s known activity cycles from the cyclical influence of the planets’ tidal forces. Now they have also been able to demonstrate that this external synchronization automatically curbs solar activity (DOI: 10.1007/s11207-025-02521-0).
At the moment, the Sun is actually reaching a maximum level of activity only seen roughly every eleven years. That is the reason why we on Earth observe more polar lights and solar storms as well as turbulent space weather in general. This has an impact on satellites in space right down to technological infrastructure on Earth. Despite this, in comparison with other sunlike stars, the strongest radiation eruptions from our Sun are 10 to 100 times weaker. This relatively quiet environment could be an important precondition for Earth being inhabitable. Not least for this reason, solar physicists want to understand what precisely drives solar activity.
Many cycles – one model
It is known that solar activity has many patterns – both shorter and longer periodic fluctuations that range from a few hundred days to several thousand years. But researchers have very different ways of explaining the underlying physical mechanisms. The model developed by the team led by Frank Stefani at HZDR’s Institute of Fluid Dynamics views the planets as pacemakers: on this understanding, approximately every eleven years, Venus, Earth and Jupiter focus their combined tidal forces on the Sun. Via a complex physical mechanism, each time they give the Sun’s inner magnetic drive a little nudge. In combination with the rosette-shaped orbital motion of the Sun, this leads to overlapping periodic fluctuations of varying lengths – exactly as observed in the Sun.
“All the solar cycles identified are a logical consequence of our model; its explanatory power and internal consistency are really astounding. Each time we have refined our model we have discovered additional correlations with the periods observed,” says Stefani. In the work now published, this is QBO – Quasi Biennial Oscillation – a roughly bi-annual fluctuation in various aspects of solar activity. The special point here is that in Stefani's model, QBO cannot only be assigned to a precise period, but it also automatically leads to subdued solar activity.
Cyclical events
Up to now, solar data have usually reported on QBO periods of 1.5 to 1.8 years. In earlier work, some researchers had suggested a connection between QBO and so-called Ground Level Enhancement events. They are sporadic occurrences during which energy-rich solar particles trigger a sudden increase in cosmic radiation on the Earth’s surface. “A study conducted in 2018 shows that radiation events measured close to the ground occurred more in the positive phase of an oscillation with a period of 1.73 years. Contrary to the usual assumption that these solar particle eruptions are random phenomena, this observation indicates a fundamental, cyclical process,” says Stefani. This is why he and his colleagues revisited the chronology once again. They discovered the greatest correlation for a period of 1.724 years. “This value is remarkably close to the value of 1.723 years which occurs in our model as a completely natural activity cycle,” says Stefani. “We assume that it is QBO.”
QBO subdues overall activity
While the Sun’s magnetic field oscillates between minimum and maximum over a period of eleven years, QBO imposes an additional short-period pattern on the field strength. This subdues the field strength overall because the Sun’s magnetic field does not maintain its maximal value for so long. A frequency diagram reveals two peaks: one at maximum field strength and the other when QBO swings back. This effect is known as bimodality of the solar magnetic field. In Stefani’s model, the two peaks cause the average strength of the solar magnetic field to be reduced – a logical consequence of QBO.
“This effect is so important because the Sun is most active during the highest field strengths. This is when the most intense events occur with huge geomagnetic storms like the Carrington event of 1859 when polar lights could even be seen in Rome and Havanna, and high voltages damaged telegraph lines. If the Sun's magnetic field remains at lower field strengths for a significantly longer period of time, however, this reduces the likelihood of very violent events,” Stefani explains.
Publication:
F. Stefani, G. M. Horstmann, G. Mamatsashvili, T. Weier, Adding Further Pieces to the Synchronization Puzzle: QBO, Bimodality, and Phase Jumps, in Solar Physics, 2025 (DOI: 10.1007/s11207-025-02521-0)
Additional information:
Dr. Frank Stefani
Institute of Fluid Dynamics at HZDR
Phone: +49 351 260 3069 | Email: f.stefani@hzdr.de
Media contact:
Simon Schmitt | Head
Communications and Media Relations at HZDR
Phone: +49 351 260 3400 | Mob.: +49 175 874 2865 | Email: s.schmitt@hzdr.de
The Helmholtz-Zentrum Dresden-Rossendorf (HZDR) performs – as an independent German research center – research in the fields of energy, health, and matter. We focus on answering the following questions:
• How can energy and resources be utilized in an efficient, safe, and sustainable way?
• How can malignant tumors be more precisely visualized, characterized, and more effectively treated?
• How do matter and materials behave under the influence of strong fields and in smallest dimensions?
To help answer these research questions, HZDR operates large-scale facilities, which are also used by visiting researchers: the Ion Beam Center, the Dresden High Magnetic Field Laboratory and the ELBE Center for High-Power Radiation Sources.
HZDR is a member of the Helmholtz Association and has six sites (Dresden, Freiberg, Görlitz, Grenoble, Leipzig, Schenefeld near Hamburg) with almost 1,500 members of staff, of whom about 680 are scientists, including 200 Ph.D. candidates.
Dr. Frank Stefani
Institute of Fluid Dynamics at HZDR
Phone: +49 351 260 3069 | Email: f.stefani@hzdr.de
F. Stefani, G. M. Horstmann, G. Mamatsashvili, T. Weier, Adding Further Pieces to the Synchronization Puzzle: QBO, Bimodality, and Phase Jumps, in Solar Physics, 2025 (DOI: 10.1007/s11207-025-02521-0)
https://www.hzdr.de/presse/qbo
Coronal mass ejections are closely linked to the Sun's magnetic activity.
Source: NASA/GSFC/SDO
Copyright: NASA/GSFC/SDO
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Coronal mass ejections are closely linked to the Sun's magnetic activity.
Source: NASA/GSFC/SDO
Copyright: NASA/GSFC/SDO
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