Scientists obtain detailed images of the solar corona

Findings show corona is more complex and dynamic than previously thought.
By Laurel Kornfeld | Jul 19, 2018
Using sophisticated software techniques and long exposures of the COR-2 camera on NASA's Solar and Terrestrial Relations Observatory-A (STEREO-A), a team of scientists based at the Southwest Research Institute (SwRI) obtained the most detailed ever images of the Sun's outer atmosphere, known as the solar corona.

Clearly visible in these images are fine-grained structures never previously detected.

The solar wind, a stream of charged particles emitted by the Sun in all directions, originates in the outer corona.

Led by solar physicist Craig DeForest of SwRI, the researchers began their imaging by using STEREO-A's coronagraph, a special telescope that blocks out the Sun's light, making it possible for scientists to see the corona. Coronagraphs are capable of capturing highly detailed images of the corona, but these images are often contaminated by noise from space and from the instruments themselves.

This time, new technology enabled scientists to identify and separate this noise, improving the signal-to-noise ratio. As a result, they imaged the outer corona in a very high level of detail.

"Previous images showed the outer corona as a smooth structure, but in deep space, the solar wind is turbulent and gusty. Using new techniques to improve image fidelity, we realized that the corona is not smooth, but structured and dynamic. Every structure that we thought we understood turns out to be made of smaller ones, and to be more dynamic than we thought," DeForest said.

With the new software techniques, the scientists filtered out light, adjusted brightness, and even corrected for blurring caused by the solar wind.

"This technique adjusted images not just in space, not just in time, but in a moving coordinate system," he explained. "That allowed us to correct motion blur not just by the speed of the wind, but by how rapidly features changed in the wind."

One of the researchers' most significant discoveries is that coronal streamers, the magnetic loops that are the sources of coronal mass ejections, are highly defined structures.

"What we found is there is no such thing as a single streamer. The streamers themselves are composed of myriad fine strands that, together, average to produce a brighter feature," DeForest explained.

Additionally, the researchers found there is no one boundary where the solar wind disconnects from the Sun.

At a distance of about 10 solar radii, still within the outer corona, the solar wind abruptly changes its speed and characteristics for reasons scientists do not yet understand.

Data from this study will be used by NASA's Parker Solar Probe, set to launch this summer, which will enter the outer corona to measure it.

A paper on these findings has been published in the Astrophysical Journal.

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