The Sun’s fast solar wind may be powered by a hidden population of waves that has been difficult to observe until now. Using high-resolution observations from the European Space Agency’s Solar Orbiter spacecraft, researchers have detected thousands of rapidly propagating transverse waves in the Sun’s polar corona. Their findings provide some of the clearest observational evidence yet that high-frequency magnetohydrodynamic waves could transport significant energy outward, helping heat the corona and accelerate the solar wind to hundreds of kilometers per second.
The fast solar wind is one of the most persistent mysteries in solar physics. It streams continuously from regions known as coronal holes, where the Sun’s magnetic field opens into space rather than looping back into the solar surface. These open magnetic field lines create channels through which plasma can escape. Yet the mechanism that supplies enough energy to heat this plasma and propel it outward remains uncertain. For decades, scientists have proposed that Alfvénic waves—oscillations involving both plasma motion and magnetic-field fluctuations—could carry energy from the lower solar atmosphere into the corona.
The new study, led by Prof. Hui Tian and Dr. Yuhang Gao of Peking University, focuses on a part of this process that has remained particularly elusive: high-frequency transverse waves. In these waves, plasma moves sideways relative to the direction of propagation, while the magnetic field oscillates along with it. Because their periods are short and their structures can be extremely narrow, such waves are easily blurred or averaged out by instruments with limited resolution. Previous observations generally detected longer-period motions, leaving open the question of whether a substantial high-frequency wave population exists.
To search for these signals, the researchers examined observations obtained by Solar Orbiter’s Extreme Ultraviolet Imager on September 14, 2021. The spacecraft was observing the Sun’s north polar coronal hole, an environment filled with bright, elongated structures called coronal plumes. These plumes are formed by concentrated plasma and magnetic-field structures and can act as natural guides for disturbances traveling through the corona. Solar Orbiter’s imaging system recorded the region at five-second intervals, with a pixel scale of approximately 210 kilometers, allowing the researchers to follow changes that would be invisible or poorly resolved in many earlier datasets.
The team transformed the image sequence into time–distance maps, a technique that reveals moving features as slanted tracks across space and time. They then used an automated wave-tracking method to identify transverse disturbances and measure their speeds, periods, and propagation behavior. The analysis uncovered 2,318 propagating wave events in the Solar Orbiter observations. The waves appeared as repeated sideways displacements of plume structures, indicating that the corona was filled with coordinated motions rather than isolated, random fluctuations.
The contrast became striking when the researchers analyzed simultaneous observations from NASA’s Solar Dynamics Observatory, whose Atmospheric Imaging Assembly has a lower spatial and temporal resolution for this purpose. Applying the same analysis to those data produced only 560 identifiable wave events. The discrepancy was greatest for the shortest periods. Waves with periods below 100 seconds represented 38 percent of the Solar Orbiter detections, but only 9 percent of the events found in the Solar Dynamics Observatory observations. This suggests that many high-frequency waves have not been absent from earlier observations; they have simply been hidden by limited resolution and cadence.
The researchers also investigated whether the newly revealed waves carried meaningful amounts of energy. Their power-spectrum analysis showed that the Solar Orbiter data contained substantially stronger wave power above approximately 10 millihertz. In the 10–30 millihertz band, the wave power was more than twice that measured in the lower-frequency range of 2–10 millihertz. When the team estimated the associated energy flux—the rate at which wave energy passes through a given area—the value derived from Solar Orbiter was about 2.6 times higher than the estimate based on the Solar Dynamics Observatory data.
That result matters because high-frequency waves can dissipate their energy more efficiently under several theoretical models. As they travel outward, interactions with the structured and magnetized coronal plasma may convert wave energy into thermal energy or bulk motion. Processes such as turbulent cascade, resonant absorption, phase mixing, and ion–cyclotron interactions have all been proposed as possible routes by which Alfvénic waves could heat the corona or transfer momentum to the solar wind. The observations do not identify which mechanism dominates, but they demonstrate that the high-frequency portion of the wave spectrum may contain far more energy than previously recognized.
The study also highlights why the Sun’s polar regions are so important—and so difficult to observe. Telescopes positioned near Earth’s orbital plane view the polar corona from an oblique angle, causing multiple plume structures to overlap along the line of sight. That projection effect can obscure the location where waves originate and make their direction of travel difficult to determine. Future missions designed to obtain a more direct view of the poles, including China’s planned Solar Polar-orbit Observatory, could combine ultraviolet imaging with magnetic-field measurements and in-situ sampling of the solar wind. Such observations may reveal how these waves are generated, how they evolve as they move through the corona, and whether they provide the energy required to launch the fast solar wind into interplanetary space.
Subject of Research: High-frequency transverse magnetohydrodynamic waves in coronal plumes and their possible role in coronal heating and fast solar-wind acceleration.
Web References: https://doi.org/10.1093/nsr/nwag370
References: National Science Review, DOI: 10.1093/nsr/nwag370
Image Credits: © Science China Press
Keywords
Solar Orbiter, fast solar wind, Alfvénic waves, magnetohydrodynamic waves, coronal plumes, polar corona, coronal holes, solar physics, coronal heating, Solar Dynamics Observatory
