Sharpest Images Yet of the Sun's Corona [VIDEOS]

Astronomers learned how to overcome the blurring effects of Earth's unruly atmosphere decades ago. Adaptive optics correct for that effect in images of everything from stars and nebulae to the Sun's visible surface. Now, a new system has brought the power of adaptive optics to images of the Sun's atmosphere, the solar corona. The technology has the potential to explain a persistent and vexing question: Why can the corona be heated to millions of degrees while underlying regions closer to the Sun are up to 1,000 times cooler?

Temperature changes and motions in the air cells above Earth's surface slightly distort light as it passes through. An adaptive optics system changes the shape of the telescope's mirror to correct for those distortions, but that requires knowing what the distortions are. Nighttime imaging uses bright nearby stars for reference, or even artificial stars created by sodium lasers if no reference stars are near enough. Adaptive-optic images of the solar disk are corrected using the omnipresent granulation, or cells of boiling plasma on the Sun's visible surface.

But structures in the corona were too faint and transient to use as references until the development of the Cona system. The team designed and tested Cona on the Goode Solar Telescope at Big Bear Solar Observatory near Los Angeles, California.

Dirk Schmidt (National Solar Observatory) and colleagues present the Cona system on May 27th in Nature Astronomy. Cona reshapes the telescope's mirror 2,200 times per second. On the Goode Solar Telescope, Cona results in hydrogen-alpha images that resolve structures just 63 kilometers (39 miles) across — more than 10 times sharper than previously possible. The images capture fine details in the hot plasma of the solar corona, where the high temperature splits neutral atoms into their constituent charged particles, which speed along magnetic field lines like beads on a string.

The roiling magnetic field shapes coronal plasma into features such as loops and arcs over the visible surface, called prominences (scroll down to watch two videos). The pull of gravity may also trap plasma, creating narrow threads of coronal rain that fall back onto the Sun (first video, above). Sprouting like grass from all over the surface are spicules, narrow jets of plasma traveling as fast as 100 kilometers per second (more than 200,000 mph). Spicules climb as high as 10,000 kilometers from the visible surface.

The researchers also spotted a fast-evolving feature they'd never seen before. "Hoping to capture tiny structures that demonstrate the performance of the adaptive optics, we became astounded witnesses of the strange, fine-structured, and fast-evolving plasma feature," Schmidt and colleagues write.

The images are sharp enough to resolve twists in the short-lived phenomenon, which indicates its origin in the sudden snap and realignment of field lines known as magnetic reconnection. More work is needed to see how this feature, and the underlying physics it represents, can explain why the corona is so hot compared to regions closer to the Sun.

This new adaptive optics technology stands to be implemented at solar telescopes worldwide, with the team already at work building a similar system for the Daniel K. Inouye Telescope in Hawai'i.