For decades, solar astronomers have grappled with one of the Sun’s greatest enigmas: the coronal heating problem. The Sun’s outermost layer—the corona—blazes at temperatures exceeding one million kelvins, far hotter than the 6,000 K photosphere beneath it. Until now, high-resolution observations of the corona have been fleeting events—total solar eclipses or spaceborne coronagraphs like those aboard the Parker Solar Probe. However, a pioneering team at the National Solar Observatory (NSO), in collaboration with the New Jersey Institute of Technology (NJIT), has broken new ground by employing adaptive optics (AO) to capture the fine structure of the corona from a ground-based facility.
Their paper, “Observations of Fine Coronal Structures with High-Order Solar Adaptive Optics,” published in Nature Astronomy, details how AO on the 1.6-meter Goode Solar Telescope (GST) has finally resolved coronal features at scales as small as 63 kilometers—a tenfold improvement over previous ground-based capabilities. These images unveil plasma strands, twisted prominences, and rapid “coronal rain” downflows with unprecedented clarity. Researchers hope these observations will provide critical insights into the physics of coronal heating and the triggers of solar eruptions.
The Challenge of Observing the Corona from Earth
The corona extends millions of kilometers into space, dominated by intricate magnetic field lines. It is the source of powerful phenomena—coronal mass ejections (CMEs)—which, when they collide with Earth’s magnetosphere, spawn dazzling aurorae but also threaten satellites, power grids, and communications. Yet, observing the corona from Earth is notoriously difficult:
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- Extreme Brightness Contrast: The photosphere’s intense glare overwhelms the much fainter corona. Only during a total solar eclipse—when the Moon blocks the photosphere—does the corona briefly shine out against a dark sky.
- Atmospheric Turbulence: Earth’s atmosphere blurs fine details in all celestial images, a problem known as “seeing.” Even with specialized solar filters, imaging the tenuous coronal plasma remains a struggle.
- Limited Coronagraph Access: Space missions such as NASA’s Parker Solar Probe and ESA’s Solar Orbiter carry onboard coronagraphs that block the photosphere, but these instruments cannot continuously monitor the corona at the highest spatial resolutions.
To study the coronal heating problem—how and where plasma is heated to millions of degrees—scientists must observe structures on the order of tens of kilometers. Until now, adaptive optics systems have been used only for imaging the bright photosphere and sunspots, where sufficient light exists for real-time wavefront sensing. The corona’s low photon flux stymied attempts to apply AO.
Adaptive Optics: Correcting the Blur
Adaptive optics technology combats atmospheric distortion by using a deformable mirror and wavefront sensor to measure and correct turbulence in real time. For solar telescopes, wavefront sensors track granulation patterns on the photosphere or sunspot edges, then compute the mirror adjustments necessary to sharpen the image. Over the past two decades, AO has revolutionized ground-based solar imaging—allowing telescopes such as the GST on Haleakalā, Maui to achieve diffraction-limited resolution (down to 100 km on the solar surface).
However, extending AO into the corona required overcoming two main hurdles:
- Lack of Reference Features: The corona has few bright reference points for wavefront sensors to lock onto.
- Low Photon Flux: Even the most sensitive detectors struggle to gather enough light from coronal emission lines for real-time corrections.
The NSO–NJIT team solved these challenges by developing a “high-order coronal AO system” that uses specialized sensors tuned to strong coronal emission lines—such as Fe XIV (530.3 nm, the “green line”)—and optimized algorithms capable of working with low-light signals. By integrating these wavefront measurements with deformable-mirror actuators operating at kilohertz speeds, they achieved corrections at the theoretical diffraction limit of the 1.6-meter Goode Solar Telescope—resolving structures as small as 63 kilometers across at the Sun’s distance.
First Light: Resolving Filamentary Loops and Coronal Rain
During a series of observing runs in late 2024 and early 2025, Schmidt and colleagues directed the GST toward a quiescent active region just beyond the solar limb. With the new AO system fully engaged, the telescope captured video sequences of dynamic coronal features that had, until now, only been hinted at in space-based or eclipse images.
Twisted Prominences in Exquisite Detail
One of the most striking observations was of a quiescent prominence—a cool, dense structure suspended above the solar surface by magnetic fields. In the high-resolution AO footage, individual magnetic threads within the prominence become visible, swirling in a slow-motion dance. The video shows tangled bundles of plasma, sometimes only a few dozen kilometers across, twisting and merging along their supporting field lines.
“These are by far the most detailed observations of this kind,” explained Vasyl Yurchyshyn, co-author and professor at NJIT–Center for Solar–Terrestrial Research. “Previously, we could only guess at the fine-scale structure of prominences. Now we see filament threads as narrow as 100 km, with complex internal flows that we can track in real time.”
Scientists believe that magnetic reconnection—or the process by which magnetic field lines break and reconnect—plays a crucial role in heating plasma and triggering eruptions. The clarity of these prominence videos will allow researchers to observe small-scale reconnection events as they unfold, providing direct tests of theoretical models.
Coronal Rain: Plasma Showers Along Magnetic Highways
Another mesmerizing phenomenon revealed by the AO system is coronal rain: cool, dense plasma that condenses in the hot corona and then cascades along magnetic loops back toward the photosphere. Traditional observations showed coronal rain as fuzzy streaks spanning thousands of kilometers. With AO, these “raindrops” appear as discrete blobs—sometimes narrower than 20 kilometers—sliding along parabolic trajectories dictated by magnetic field geometry.
“Watching coronal rain at 63 km resolution is like seeing individual drops in a torrential downpour instead of a muddy waterfall,” said Thomas Schad, NSO astronomer and co-author. “These coronal rain blobs let us measure cooling rates, densities, and magnetic field shapes with precision. It’s a game-changer for understanding energy transport in the corona.”
Post-Flare Rain: The Aftermath of Solar Eruptions
When a solar flare erupts, it rapidly heats coronal plasma to tens of millions of degrees. As the flare dissipates, plasma cools and condenses, forming post-flare coronal rain that plummets along newly reconfigured field lines. The AO-enabled videos of this process reveal intricate, fan-like patterns—thousands of mini–“rain channels” funneling plasma to the surface. Each channel traces a unique magnetic loop, allowing scientists to map magnetic connectivity in unprecedented detail.
“When you see post-flare coronal rain at this resolution, you realize how braided and multithreaded the loops really are,” Schmidt said. “Instead of thinking of one big loop, we see that dozens, maybe hundreds, of sub–loops coexist within a single magnetic structure. Each of these is a conduit for energy loss and mass transport after a flare.”
Implications for the Coronal Heating Problem
The million-dollar question in solar physics is: Why is the corona orders of magnitude hotter than the photosphere? Two leading theories—magnetic reconnection and Alfvén wave heating—attempt to explain this paradox:
- Magnetic Reconnection: Tangled magnetic fields break and reconnect, releasing energy that heats nearby plasma.
- Alfvén Waves: Magnetohydrodynamic waves generated by convective motions at the solar surface propagate upward, dissipating energy in the corona.
High-resolution observations are crucial to distinguish between these mechanisms. If heating is concentrated along small-scale reconnection sites, we should observe tiny brightenings and rapid plasma flows at scales below 100 km. If wave heating dominates, we might see oscillatory motions and periodic intensity enhancements.
By resolving features as small as 63 km, the new AO system offers the first real possibility of capturing either signature in action. For instance, Schmidt’s team has already identified transient bright points—sub-100 km hot spots—that flicker for a few seconds before fading, suggesting micro-reconnection events. Similarly, preliminary Doppler measurements of coronal rain speeds hint at wave-induced oscillations along loops.
“For the first time, we’re watching coronal heating processes unfold at their true spatial scale,” said Schmidt. “It’s akin to transitioning from a grainy photo to an ultra-HD video. Everything we see challenges our assumptions about how energy is deposited and dissipated in the corona.”
Technical Breakthroughs: Building a Coronal Adaptive Optics System
Creating a functional AO system for coronal imaging required multiple innovations:
- Focal-Plane Wavefront Sensing: Instead of using photospheric granulation as a reference, the team developed “intensity-differencing wavefront sensors” optimized for coronal emission lines such as Fe XIV (530.3 nm) and Ca XIII (408.6 nm). By modulating deformable-mirror actuators at high frequencies and analyzing speckle patterns, these sensors extract wavefront distortions from faint coronal light.
- High-Order Deformable Mirrors: The GOde Solar Telescope’s AO bench was upgraded with a 2,048-actuator deformable mirror, enabling correction of high-spatial-frequency turbulence. Such a mirror can counteract atmospheric aberrations down to 10–15 centimeters on the telescope aperture, critical for near-diffraction-limited performance at visible wavelengths.
- Real-Time Control Algorithms: Previously, standard AO control software struggled with low signal-to-noise ratios. The NSO–NJIT team implemented machine-learning-enhanced reconstructors that predict turbulence-induced phase errors using both current and prior wavefront measurements. These algorithms run on custom GPUs at over 2,000 updates per second, ensuring rapid mirror adjustments even with noisy coronal signals.
Collectively, these advances closed a decades-long gap in solar instrumentation. As NSO Chief Technologist Thomas Rimmele—who pioneered visible-wavelength AO for photospheric imaging—remarked, “We had plateaued at about 1,000 km resolution in the corona. This new coronal adaptive optics system pushes us down to 63 km. It’s the theoretical limit for a 1.6-meter aperture, and now we have it.”
Future Prospects: From Maui to Hawaii
While the Goode Solar Telescope is currently the only ground-based facility equipped with coronal AO, the next ambitious step is to adapt this technology to the Daniel K. Inouye Solar Telescope (DKIST) on Haleakalā, Maui. With its 4-meter primary mirror, DKIST boasts four times the light-collecting area and a diffraction limit near 25 km on the solar disk.
“Imagine seeing coronal loops as narrow as 25 km,” said Philip Goode, NJIT research professor and co-author. “The amount of detail we’ll unlock at DKIST with coronal adaptive optics is staggering. We anticipate discoveries about nano-flare heating, wave dissipation, and small-scale eruptive events that are currently beyond our reach.”
Implementing AO on DKIST presents its own challenges: more actuators (often ~5,000), faster computational cycles, and specialized wavefront sensors for extremely faint coronal emission. However, the groundwork laid at GST provides a blueprint. NJIT scientists are already designing a five-millimeter subaperture Shack-Hartmann sensor for DKIST’s coronal observations, coupled with next-generation GPUs and real-time neural-net controllers.
Another future avenue involves deploying similar AO systems at upcoming large-aperture solar telescopes such as the European Solar Telescope (EST) in the Canary Islands and the National Large Solar Telescope (NLST) in India. As Goode notes, “This transformative technology will likely be adopted by observatories worldwide and reshape ground-based solar astronomy.”
Broader Impacts: Space Weather, Climate, and Fundamental Physics
The practical implications of unveiling the corona’s fine structure extend far beyond academic curiosity. Improved understanding of coronal heating and eruption triggers will enhance space weather forecasting—critical for protecting satellites, power grids, and communication networks. Early detection of micro-instabilities or small-scale reconnection events could provide valuable lead times for issuing geomagnetic storm warnings.
Moreover, these observations feed directly into magnetohydrodynamic (MHD) simulations that predict solar cycle dynamics and long-term irradiance variations—factors that influence Earth’s climate. By anchoring theoretical models with high-fidelity data, scientists can refine predictions of solar wind properties, ultraviolet flux variations, and the behavior of magnetic storms.
“Our climate models rely on accurate solar irradiance inputs,” said Vasyl Yurchyshyn. “Coronal physics underlies those inputs. With this level of detail, we can better constrain how much energy the Sun emits in high-energy wavelengths—and that has downstream effects on upper-atmosphere chemistry, satellite drag, and even terrestrial weather patterns.”
At a deeper level, revealing plasma dynamics at fine scales addresses fundamental plasma physics questions applicable across the universe—from accretion disks around black holes to astrophysical jets in active galactic nuclei. Coronal loops serve as a natural laboratory for studying magnetic reconnection, Alfvén wave dissipation, and plasma instabilities in low-collisionality regimes. Lessons learned on our own star can inform models of far-distant, high-energy environments.
Conclusion: A New Dawn in Solar Observation
The deployment of high-order AO on the Goode Solar Telescope marks a milestone in solar astronomy. For the first time, scientists can watch coronal phenomena at tens-of-kilometers resolution, capturing the Sun’s dynamic outer atmosphere with clarity once reserved for the Hubble Space Telescope or specialized satellite missions.
As Dirk Schmidt—lead author and NSO AO scientist—places it, “This technological advancement is a game-changer. There is a lot to discover when you boost your resolution by a factor of ten. We’re poised to answer long-standing questions about coronal heating, eruption triggers, and the fundamental nature of magnetized plasma.”
With coronal AO now operational at GST and plans underway to scale the system to DKIST and other large-aperture solar telescopes, a new era dawns. Over the coming years, we can anticipate a cascade of discoveries: detailed mapping of nanoflares, direct observation of wave–particle interactions, and refined models of solar wind acceleration. Each achievement will unravel pieces of the puzzle that is our star, shedding light on phenomena that shape not only our space environment but the broader cosmos.
