Scientists Spot Solar Raindrops 20 Kilometers Wide

Scientists have achieved a major breakthrough in solar observation by developing the first adaptive optics system capable of studying the Sun’s corona—the mysterious outer atmosphere visible only during eclipses—at unprecedented resolution.

The new “coronal adaptive optics” technology has revealed previously unknown phenomena in the corona, including twisted plasma streams that form and collapse within minutes and coronal rain droplets narrower than 20 kilometers. Published in Nature Astronomy, this advance promises to unlock secrets about why the corona burns at millions of degrees while the Sun’s surface remains relatively cool.

The achievement represents a quantum leap in solar physics, bringing resolution improvements of nearly ten times better than previous corona observations.

This time-lapse video captures a solar prominence undergoing rapid, intricate, and turbulent restructuring—revealed here in unprecedented detail.

Breaking Through Atmospheric Barriers

“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.

For over two decades, adaptive optics systems have revolutionized observations of the Sun’s surface by compensating for atmospheric blur. However, these systems failed when pointed beyond the solar limb into the corona, where different physics and much dimmer features required entirely new approaches.

The team’s solution involved developing a specialized wavefront sensor that could track faint hydrogen-alpha structures in prominences and other coronal features. This sensor controls a mirror that reshapes itself 2,200 times per second to counteract atmospheric distortion.

Discovering the Unknown

Within the first days of operation, the system made an unexpected discovery—a twisted plasma stream that exhibited behavior unlike anything previously observed in solar physics.

“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.

The mysterious feature, which researchers termed a “coronal plasmoid,” formed during the decay phase of a solar flare and exhibited twisted strands less than 100 kilometers across. The plasma stream moved at speeds of 55-90 kilometers per second before suddenly stopping and collapsing in a collision that reminded researchers of “a spiral galaxy with two arms.”

Coronal Rain at the Diffraction Limit

The system also revealed new details about coronal rain—cooling plasma that condenses and falls back toward the Sun’s surface like precipitation in Earth’s atmosphere.

“Raindrops in the Sun’s corona can be narrower than 20 kilometers,” concludes NSO Astronomer Thomas Schad 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.”

The observations showed that approximately half of coronal rain strands were narrower than 100 kilometers, with structures extending down to the telescope’s diffraction limit of 64 kilometers. This finding challenges current computer simulations and suggests that three-dimensional effects may be necessary to accurately model coronal rain formation.

Key Technical Achievements:

• Resolution improvement from 1,000+ kilometers to 63 kilometers
• Detection of plasma features as narrow as 20 kilometers
• Discovery of twisted coronal plasmoids during flare decay
• Successful tracking of fast-moving plasma at 55-90 km/s
• First diffraction-limited coronal observations from ground-based telescopes

The Technical Challenge

“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.

The technical hurdles were immense. Unlike the Sun’s surface, which provides abundant bright features for tracking, the corona offers only sparse, dim structures scattered across vast areas. The team needed to develop new algorithms, optimize detector sensitivity, and solve complex calibration problems unique to off-limb observations.

A crucial detail not widely covered involves the sophisticated flat-fielding challenges the team overcame. Because hydrogen-alpha light appears as absorption against the Sun’s disk but emission in prominences, traditional calibration methods failed completely. The researchers had to develop novel techniques using diffuse prominence structures as calibration sources—a delicate process requiring precise matching of brightness levels and camera settings.

Implications for Coronal Heating

The corona’s extreme temperature—millions of degrees compared to the Sun’s 6,000-degree surface—represents one of solar physics’ greatest unsolved mysteries. The new observations provide crucial clues about potential heating mechanisms.

The discovered twisted plasmoid may represent direct evidence of magnetic reconnection processes occurring in current sheets following solar flares. The feature’s characteristics—fast motion, twisted structure, kink instabilities, and brief lifetime—align closely with theoretical predictions for plasmoids generated during three-dimensional magnetic reconnection.

These observations could help validate laboratory plasma experiments that have demonstrated nanoflare production in braided magnetic loops, though the scales involved differ by several orders of magnitude.

Looking Beyond Earth’s Atmosphere

The achievement required overcoming what scientists call “anisoplanatism”—the way atmospheric turbulence varies with viewing direction. This meant the wavefront sensor had to use actual coronal structures rather than the bright granulation patterns available on the Sun’s surface.

“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.

The system achieved Strehl ratios—a measure of optical quality—between 20% and 40%, sufficient for diffraction-limited observations when combined with post-processing techniques. This performance matched or exceeded systems used for solar surface observations.

Future Applications

The team is already working to implement similar technology on the 4-meter Daniel K. Inouye Solar Telescope in Hawaii, which would provide even finer resolution of coronal structures.

“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.

Beyond simple resolution improvements, the technology enables routine observations at previously impossible quality levels. This consistency factor may prove more valuable than peak performance, as it dramatically increases the chances of capturing rare solar events at high resolution.

The researchers envision future developments including laser guide star systems that could extend adaptive optics correction deeper into the corona, where natural features suitable for wavefront sensing become scarce.

What mysteries will these new eyes on the Sun reveal? With coronal adaptive optics now operational, solar physicists are poised to unlock secrets that have remained hidden in atmospheric blur for decades. The twisted plasmoid discovery within the first operational days suggests that many more surprises await in the Sun’s superheated outer atmosphere.