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Coronal field lines
This image shows field lines of a solar coronal magnetic model based on measurements from the National Solar Observatory Integrated Synoptic Program, one solar rotation before the Aug. 21 total solar eclipse. (NSO / NSF Graphic)

Skywatchers will see a rare celestial sight during the Aug. 21 total solar eclipse: the sun’s shimmering outer atmosphere, known as the corona. What will it look like? Astronomers worked their magic to give us a glimpse.

The corona is more than just a fuzzy halo: The superheated gas that makes up the sun’s outermost layer tends to follow the patterns of magnetic force that arc around the sun.

To come up with their preview of the corona, researchers at the National Solar Observatory in Arizona modeled the sun’s magnetic field as of July 25, which was 27 days in advance of the solar eclipse. That’s important, because it takes the sun 27.2753 days to make a complete rotation.

“The corona is not likely to change too much between now and the eclipse, unless we get lucky and a large active region appears,” NSO solar scientist Gordon Petrie said in a news release. “We expect to see faint, straight structures protruding from the north and south poles of the sun – these are the polar plumes. We will be able to see brighter bulbs of material closer to the equator – these are called helmet streamers.”

The corona’s magnetic field changes shape over time, depending on where the sun is in its 11-year activity cycle.

“During solar maximum, such as the 2012 eclipse, the corona looks like a spiky ring around the entire sun,” said David Boboltz, the National Science Foundation’s program officer for the NSO. “In contrast, a solar minimum eclipse such as this one … will have lots of complexity near the equator, but will be drastically different near the north and south poles of the sun.”

Although the eclipse’s total phase will last no more than two minutes and 40 seconds in any one location, astronomers will be able to combine observations to study the corona’ over the course of 90 minutes. And there’s more to come.

NSO is playing a lead role in building the Daniel K. Inouye Solar Telescope, or DKIST, on the Hawaiian island of Maui. When DKIST goes into operation in 2019, scientists will be able to measure the corona’s magnetic fields directly for the first time.

“The solar corona is largely an enigma,” NSO Director Valentin Pillet said. “For now, the best we can do is compare high-resolution images of the solar corona, such as those we’ll obtain during the eclipse, to our theoretical models. But DKIST will allow us to actually measure the magnetic fields in the corona. This will be revolutionary in the field of solar physics.”

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