Modern ground-based telescopes rely on adaptive optics (AO) to deliver clear images. By correcting for atmospheric distortion, they give us exceptional pictures of planets, stars, and other celestial objects. Now, a team at the National Solar Observatory is using AO to examine the Sun's corona in unprecedented detail.
The corona is the Sun's outermost layer, extending into space for millions of kilometres. Unexpectedly, it's hotter than the layer beneath it, the photosphere. Scientists call this the 'coronal heating problem'. The corona is dominated by the Sun's powerful magnetic fields and is the source of coronal mass ejections (CMEs), which can collide with Earth's magnetosphere, causing aurorae and geomagnetic storms.
Since the corona is dimmer than the Sun's surface, it's challenging to observe. It's visible during total solar eclipses when the Moon blocks the Sun's photosphere, and space-based coronagraphs like the one on the Parker Solar Probe accomplish the same thing by mimicking an eclipse.
Observing the Sun's corona from Earth is challenging because of atmospheric interference. Adaptive Optics uses computer-controlled, deformable mirrors to counteract the interference and produce clear images. Researchers from the National Academy of Science's National Solar Observatory (NSO) and the New Jersey Institute of Technology have developed an AO system for the 1.6-meter Goode Solar Telescope to observe the corona in precise detail and reveal its fine structure.
Their work is presented in a new paper titled "Observations of fine coronal structures with high-order solar adaptive optics." It's published in Nature Astronomy, and Dirk Schmidt, an Adaptive Optics Scientist at the NSO, is the lead author.
"Resolving fine structures in the Sun's corona may provide key insights into rapid eruptions and the heating of the corona," the authors write in their research article. They point out that while AO has been used on large telescopes for two decades, none have been able to view the corona. "Here we present observations with coronal adaptive optics reaching the diffraction limit of a 1.6-m telescope to reveal very fine coronal details," they write.
"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." - Vasyl Yurchyshyn, NJIT-Center for Solar-Terrestrial Research.
Solar prominences, loops, and rain are all made of plasma. Understanding them and other unsolved problems relies on seeing their fine detail. "How is plasma in the corona heated to millions of kelvins when the Sun's surface is only 6,000 K?" the authors ask. "How and when are eruptions triggered?"
Adaptive optics relies on wavefront sensors and their enabling technologies and algorithms. These are available for the photosphere but haven't been for the corona, until now.
"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," said Dirk Schmidt, NSO Adaptive Optics Scientist, who led the development. "It is super exciting to build an instrument that shows us the Sun like never before," he said in a press release.
"This technological advancement is a game-changer, there is a lot to discover when you boost your resolution by a factor of 10." Dirk Schmidt, National Solar Observatory.
This video shows a dynamic prominence with a large-scale twist alongside raining coronal material.
Coronal rain is when strands of coronal plasma cool and fall back down to the surface. "Raindrops in the Sun’s corona can be narrower than 20 kilometers," said NSO Astronomer Thomas Schad. "These findings offer new invaluable observational insight that is vital to test computer models of coronal processes."
