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The sun's corona—the outermost layer of its atmosphere, visible only during a total solar eclipse—has long intrigued scientists due to its extreme temperatures, violent eruptions, and large prominences. However, turbulence in Earth's atmosphere has caused image blur and hindered observations of the corona. A recent development by scientists from the U.S. National Science Foundation (NSF) National Solar Observatory (NSO), and New Jersey Institute of Technology (NJIT), is changing that by using adaptive optics to remove the blur.

As published in , this pioneering "coronal adaptive optics" technology has produced the most astonishing, clearest images and videos of fine-structure in the corona to date. This development will open the door for deeper insights into the corona's enigmatic behavior and the processes driving space weather.

Most detailed coronal images to date revealed

Installed at the 1.6-meter Goode Solar Telescope (GST), operated by NJIT's Center for Solar-Terrestrial Research (CSTR) at Big Bear Solar Observatory (BBSO) in California, "Cona"—the adaptive optics system responsible for these new images—compensates for the blur caused by air turbulence in Earth's atmosphere —similar to the bumpy air passengers feel during a flight.

"The turbulence in the air severely degrades images of objects in space, like our sun, seen through our telescopes. But we can correct for that," says Dirk Schmidt, NSO Adaptive Optics Scientist who led the development.

Among the team's remarkable observations is a movie of a quickly restructuring solar prominence unveiling fine, turbulent internal flows. Solar prominences are large, bright features, often appearing as arches or loops, extending outward from the sun's surface.

A second movie replays the rapid formation and collapse of a finely structured plasma stream. "These are by far the most detailed observations of this kind, showing features not previously observed, and it's not quite clear what they are," says Vasyl Yurchyshyn, co-author of the study and NJIT-CSTR research professor. "It is super exciting to build an instrument that shows us the sun like never before," Schmidt adds.

A third movie shows fine strands of coronal rain—a phenomenon where cooling plasma condenses and falls back toward the sun's surface. "Raindrops in the sun's corona can be narrower than 20 kilometers," NSO Astronomer Thomas Schad concludes from the most detailed images of coronal rain to date, "These findings offer new invaluable observational insight that is vital to test computer models of coronal processes."

Another movie shows the dramatic motion of a solar prominence being shaped by the sun's magnetism.

A breakthrough in solar adaptive optics

The corona is heated to millions of degrees—much hotter than the sun's surface—by mechanisms unknown to scientists. It is also home to dynamic phenomena of much cooler solar plasma that appears reddish-pink during eclipses.

Scientists believe that resolving the structure and dynamics of the cooler plasma at small scales holds a key to answering the coronal heating mystery and improving our understanding of eruptions that eject plasma into space driving space weather—i.e., the conditions in Earth's near-space environment primarily influenced by the sun's activity (e.g., solar flares, coronal mass ejections, and the solar wind) that can impact technology and systems on Earth and in space.

The precision required demands large telescopes and adaptive optics systems like the one developed by this team.

The GST system Cona uses a mirror that continuously reshapes itself 2,200 times per second to counteract the image degradation caused by turbulent air. "Adaptive optics is like a pumped-up autofocus and optical image stabilization in your smartphone camera, but correcting for the errors in the atmosphere rather than the user's shaky hands," says BBSO Optical Engineer and Chief Observer, Nicolas Gorceix.

Since the early 2000s, adaptive optics have been used in large solar telescopes to restore images of the sun's surface to their full potential, enabling telescopes to reach their theoretical diffraction limits—i.e., the theoretical maximum resolution of an optical system. These systems have since revolutionized observing the sun's surface, but until now, have not been useful for observations in the corona; and the resolution of features beyond the solar limb stagnated at an order of 1,000 kilometers or worse—levels achieved 80 years ago.

"The new coronal adaptive optics system closes this decades-old gap and delivers images of coronal features at 63 kilometers resolution—the theoretical limit of the 1.6-meter Goode Solar Telescope," says Thomas Rimmele, NSO Chief Technologist who built the first operational adaptive optics for the sun's surface, and motivated the development.

Implications for the future

The coronal adaptive optics system is now available at the GST. "This technological advancement is a game-changer, there is a lot to discover when you boost your resolution by a factor of 10," Schmidt says.

The team now knows how to overcome the resolution limit imposed by Earth's lowest region of the atmosphere—i.e., the troposphere—on observations beyond the solar limb and is working to apply the technology at the 4-meter NSF Daniel K. Inouye Solar Telescope, built and operated by the NSO in Maui, Hawaiʻi. The world's largest solar telescope would see even smaller details in the sun's atmosphere.

"This transformative technology, which is likely to be adopted at observatories world-wide, is poised to reshape ground-based solar astronomy," says Philip R. Goode, distinguished research professor of physics at NJIT-CSTR and former director at BBSO, who co-authored the study. "With coronal adaptive optics now in operation, this marks the beginning of a new era in solar physics, promising many more discoveries in the years and decades to come."