Coronal rain forms when hotter plasma in the sun’s corona cools down and becomes denser. Like raindrops on Earth, coronal rain is pulled down to the surface by gravity. Because the plasma is electrically charged, it follows the magnetic field lines, which make huge arches instead of falling in a straight line. [Image: Schmidt et al./ NJIT/ NSO/ AURA/ U.S. National Science Foundation]
A long-standing mystery of the sun has baffled scientists for decades: Why is the atmosphere of the sun so much hotter than its surface? While the temperature of the surface is about 6000 K, the sun’s outer atmosphere—known as the corona—can heat up to millions of kelvins. High–spatial resolution observations are critical for understanding the solar atmosphere and the physical processes that lead to coronal heating.
Now, researchers in the United States have pioneered a new adaptive optics technology that has produced the most detailed images and videos of coronal structures to date (Nat. Astron., doi: 10.1038/s41550-025-02564-0). The system, called Cona, reached the diffraction limit of the Goode Solar Telescope at Big Bear Solar Observatory (BBSO) in California to reveal previously unknown phenomena and fine-grained features of the corona.
Introducing Cona
Previously, high–spatial resolution observations at the diffraction limit of large aperture telescopes were only possible for the sun’s surface, called the photosphere, and not the corona. The corona is home to plasma phenomena like solar prominences, large loop-like structures shaped by the sun’s magnetic field, and coronal rain, which occurs when hot plasma cools and condenses in strong magnetic fields and falls to the photosphere.
Adaptive optics has long been applied to all major solar telescopes to correct for turbulence in the Earth’s atmosphere. But wavefront sensors used to adjust the shape of the adaptive mirror were designed only for photospheric structures. The current study has developed a new wavefront sensor that allows adaptive optics to be leveraged specifically for observations of coronal objects.
The current study has developed a new wavefront sensor that allows adaptive optics to be leveraged specifically for observations of coronal objects.
“With this new application of adaptive optics, we want to enable new diagnostic methods for small-scale plasma and magnetic field dynamics that may have a role in the mystery of coronal heating and in the triggering of eruptions,” said study author Dirk Schmidt, adaptive optics scientist at the National Solar Observatory (NSO). “The coronal adaptive optics system that we call Cona enables diffraction-limited observations through Earth’s turbulent atmosphere of cool plasma features in the sun’s corona, above the surface.”
A new view of the corona
Like most adaptive optics systems, Cona uses a wavefront sensor to measure optical aberrations and a deformable mirror with 357 actuators to correct them. The correlating Shack‒Hartmann wavefront sensor in Cona was specifically designed for dim features above the sun’s surface that emit hydrogen-alpha light.
“In contrast to wavefront sensors for the very bright and very low-contrast surface, where we needed fast cameras with large full well capacities, here we need a fast, low-dark-noise camera optimized for low-light applications,” he said. “In addition, the parameters in the wavefront sensor design are also specifically optimized.”
Schmidt and his colleagues developed the technology at the 1.6-m Goode Solar Telescope, the world’s second largest solar telescope, reaching the diffraction limit with resolution better than 70 km. The first few days of observation uncovered a strange fine-structured and fast-evolving plasma feature, which they named a twisted coronal plasmoid. Later, Cona revealed new insights into the smallest scales of coronal rain, a marker for coronal heating.
“The very next step is to apply this technology to the National Science Foundation’s 4-m Daniel K. Inouye Solar Telescope that the National Solar Observatory operates in Maui,” said Schmidt. “The aperture of the Inouye Telescope is almost three times bigger than the Goode Solar Telescope. Its much larger aperture poses a significantly harder challenge for adaptive optics and puts more demanding requirements on the hardware.”