The origin of the fast solar wind — a stream of charged particles flowing outward from the Sun at hundreds of kilometers per second — remains one of the long-standing puzzles in solar and space physics. A leading class of theories suggests that high-frequency magnetohydrodynamic waves, especially transverse Alfvénic waves, may transport energy from the lower solar atmosphere into the corona, where it can contribute to coronal heating and solar wind acceleration. Yet direct observational evidence for such high-frequency waves carrying substantial energy has remained limited.
A new study led by Prof. Hui Tian and Dr. Yuhang Gao from Peking University now provides important observational evidence for this missing wave population. Using high-cadence, high-resolution extreme-ultraviolet observations from the Extreme Ultraviolet Imager (EUI) onboard Solar Orbiter, the team detected abundant high-frequency magnetohydrodynamic waves in coronal plumes above the Sun’s north polar coronal hole. The results were published in National Science Review.
In the new work, the researchers analyzed Solar Orbiter/EUI observations taken on September 14, 2021. The data targeted a north polar coronal hole, a region where open magnetic field lines allow plasma and waves to travel outward into interplanetary space. The researchers focused on coronal plumes, bright ray-like structures that can act as natural waveguides.
Thanks to Solar Orbiter’s 5-second cadence and pixel scale of about 210 kilometers, the team was able to resolve fine and rapidly evolving plume structures that are difficult to capture with earlier instruments. By constructing time-distance maps and applying an automated wave-tracking method, the researchers identified 2,318 propagating transverse wave events in the Solar Orbiter data.
For comparison, the team applied the same analysis to simultaneous observations from the Atmospheric Imaging Assembly onboard NASA’s Solar Dynamics Observatory. These data yielded 560 wave events. The difference was especially clear at short periods: 38% of the waves detected by Solar Orbiter had periods shorter than 100 seconds, while the corresponding fraction in the Solar Dynamics Observatory data was only 9%.
The energy contribution of these waves was further confirmed by power-spectrum analysis. In the Solar Orbiter data, wave power above about 10 millihertz was much stronger than that inferred from the lower-resolution observations. The power in the high-frequency range of 10–30 millihertz was more than twice that in the lower-frequency range of 2–10 millihertz. The estimated wave energy flux from Solar Orbiter was also about 2.6 times larger than that inferred from the Solar Dynamics Observatory data.
High-frequency waves are especially important because they can dissipate more efficiently than low-frequency waves in several theoretical models. Dr. Yuhang Gao said, “The new findings help bridge the gap between models that require high-frequency waves and previous observations that mainly detected longer-period waves.”
The study also points to the importance of future polar observations. From the ecliptic plane, polar coronal structures are affected by line-of-sight superposition, making it difficult to identify wave origins and follow their propagation. “Future missions such as China’s planned Solar Polar-orbit Observatory will provide a more direct view of the Sun’s polar regions, together with imaging, magnetic-field measurements, and in-situ solar wind observations,” said Professor Hui Tian. “Such observations will be crucial for understanding where these high-frequency waves are generated, how they propagate and dissipate, and how they contribute to the acceleration of the fast solar wind.”
National Science Review
Observational study