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

The Sun’s corona has mystified astronomers ever since it became visible to human eyes during total solar eclipses. Its temperature soars to millions of degrees Kelvin—dramatically hotter than the solar surface—yet the mechanisms behind this intense heating remain one of the most profound puzzles in astrophysics. Observing the corona at high resolution has been notoriously difficult, primarily because Earth’s atmosphere disrupts the incoming light, causing significant image blur. Until now, no ground-based telescopic system could achieve enough stability and resolution to reveal coronal features at scales smaller than about 1000 kilometers. This crude limitation has sharply constrained scientists’ ability to probe the microphysics driving solar eruptions and space weather.

Cona’s design represents an innovative leap in adaptive optics technology, a technique originally developed to compensate for atmospheric distortions when imaging the solar surface and distant astronomical objects. Adaptive optics employs deformable mirrors that adjust thousands of times per second to correct optical aberrations in real-time. While standard adaptive optics have transformed solar surface imaging, extending their capabilities to the faint and dynamic corona was deemed nearly impossible due to lower brightness and complex optical conditions. The Cona system overcomes these challenges by utilizing a wavefront sensor finely attuned to detect and correct turbulence affecting the coronal light, allowing the GST to push its theoretical resolution limits to mere 63 kilometers.

The new imagery captured by Cona reveals an astonishing level of internal detail within solar prominences—massive, glowing plasma loops anchored to the Sun’s surface by magnetic fields. Time-lapse videos document rapid and intricate changes within these structures, highlighting previously unseen turbulent flows. This granular view into solar plasma dynamics is not only thrilling from a scientific standpoint but also critical for understanding how solar storms evolve and potentially impact Earth’s space environment. The team’s observations suggest complex magnetic reconnections and plasma interactions occurring on minute spatial and temporal scales, insights that were impossible before this adaptive optics breakthrough.

Beyond prominences, Cona’s observations offer novel perspectives on “coronal rain,” a phenomenon wherein cooling plasma condenses and descends along magnetic field lines back toward the solar surface. Remarkably, the system can resolve such strands of plasma that are narrower than 20 kilometers, a scale that had never been attainable. Understanding coronal rain at this resolution informs models of heat transport and plasma behavior in the corona, enhancing predictions of solar activity. Coronal rain is not merely a solar curiosity but a cornerstone for testing theories about the Sun’s atmospheric heating and magnetic field structuring.

Cona’s rapid mirror adjustments occur at an extraordinary frequency of 2,200 times per second. This rapid reshaping combats the distortion caused by turbulent air in Earth’s troposphere, effectively “cleaning” the solar image before it reaches detectors. Such technology parallels the autofocus and image stabilization systems commonplace in today’s smartphones, except here, the system corrects distortions on a scale and speed that surpasses any consumer device. This capability is crucial because the quality of astronomical data fundamentally depends on the steadiness and clarity of the captured light.

Prior to this innovation, scientists relied heavily on space-based observatories or indirect methods for studying the corona’s intricate structures, both of which come with compromises in temporal resolution, cost, and accessibility. Ground-based solar telescopes equipped with conventional adaptive optics had succeeded in imaging the solar photosphere and chromosphere with remarkable clarity but hit a resolution wall when venturing into coronal observations. Cona bridges this gap, delivering imagery that matches the theoretical diffraction limit of the 1.6-meter aperture, effectively making the Earth’s atmosphere a much less formidable barrier.

The implications for future solar research are profound. The team is actively preparing to deploy coronal adaptive optics at the 4-meter Daniel K. Inouye Solar Telescope (DKIST) in Hawaii—the world’s largest solar telescope—which promises even finer resolution and more precise studies of the Sun’s outer atmosphere. Such enhancements will dramatically deepen our understanding of solar magnetic phenomena, energy release mechanisms, and the genesis of solar storms that can disrupt satellite operations, communications, and power grids on Earth.

This breakthrough also affords a rare opportunity to link observational data with sophisticated computer models of coronal plasma physics and magnetohydrodynamics. The ability to resolve smaller features and faster dynamics means that theoretical predictions can now be tested and refined with unprecedented empirical rigor. Consequently, this advancement could accelerate progress toward solving enduring questions about coronal heating, solar wind acceleration, and space weather forecasting.

The team behind Cona—composed of NSO researchers Dirk Schmidt, Thomas Schad, and Thomas Rimmele, along with NJIT experts Vasyl Yurchyshyn, Nicolas Gorceix, and Philip Goode—highlights that this work is the product of decades of development in solar adaptive optics. Their efforts have combined cutting-edge optical engineering, state-of-the-art sensors, and intricate computational algorithms to transform how we view our star’s most elusive layer. This breakthrough is not merely incremental but transformative, laying the groundwork for a new era in solar physics.

In summary, the advent of coronal adaptive optics has finally lifted a curtain that has limited humanity’s view of the Sun’s outer atmosphere for nearly a century. By enabling direct imaging of fine-scale structures and dynamics within solar prominences, coronal rain, and other phenomena, this technology stands poised to revolutionize our scientific understanding of processes governing space weather and stellar atmospheres. As this adaptive optics approach is deployed across major solar telescopes worldwide, the coming years promise a cascade of discoveries that will illuminate the fundamental physics of our closest star and its interaction with the solar system.

Subject of Research: Imaging and analysis of fine coronal structures using high-order solar adaptive optics.

Article Title: Observations of fine coronal structures with high-order solar adaptive optics

News Publication Date: 27-May-2025

Web References:

• Nature Astronomy article: https://www.nature.com/articles/s41550-025-02564-0
• NSF National Solar Observatory: http://nso.edu
• Big Bear Solar Observatory: http://bbso.njit.edu

References:
Schmidt, D., Schad, T.A., Yurchyshyn, V., Gorceix, N., Rimmele, T.R., & Goode, P.R. Observations of fine coronal structures with high-order solar adaptive optics. Nature Astronomy, May 2025. DOI: 10.1038/s41550-025-02564-0

Image Credits: Schmidt et al./NJIT/NSO/AURA/NSF

Keywords

Solar Corona; Adaptive Optics; Goode Solar Telescope; Coronal Rain; Solar Prominences; Space Weather; High-Resolution Solar Imaging; Atmospheric Turbulence Correction; NSF National Solar Observatory; Big Bear Solar Observatory; Solar Magnetic Fields; Coronal Heating Mystery