On August 21, if you have the good fortune to be along the path of totality, stretching from Oregon to South Carolina, you’ll get to see one of the rarest sights in nature — the wispy solar corona. The corona is the sun’s atmosphere, and it’s constantly changing shape in response to magnetic fields reaching up from the surface of the sun.
Dr. Gordon Petrie from the National Solar Observatory (NSO) explains: “The corona might look like it’s a fuzzy halo around the Sun, but it actually has quite a lot of structure to it. The Sun has a magnetic field that, at first glance, might remind us of the middle-school experiment where you sprinkle iron filings over a bar magnet to get a butterfly shape. However, on closer inspection, it is far more complicated than that.”
Petrie and team use a special coronagraph and other instruments to subtract skylight from the region around the sun to reveal the shape and extent of the corona. We’re now only about a half a solar rotation from eclipse day, and according to Petrie, the corona’s not likely to change much between now and then.
“We expect to see faint, straight structures protruding from the north and south poles of the Sun – these are the polar plumes,” he said. “We will be able to see brighter bulbs of material closer to the equator – these are called helmet streamers.”
The Sun’s magnetic field is generated inside of the Sun; when it breaks through to the surface, it cools its surroundings. The cooling produces dark spots we see as sunspots where magnetic energy is concentrated. You can’t see a magnetic field directly, but its effects include the sunspots and super-heated gases in the sun’s atmosphere. Astronomers can trace out the sun’s magnetic field lines in the corona by mapping those hot gases and “see” the corona outside of an eclipse.
“The corona changes its shape over time, and looks drastically different during solar maximum compared to solar minimum,” explains Dr. David Boboltz, the National Science Foundation’s program officer for the NSO. “During solar maximum, such as the 2012 eclipse, the corona looks like a spiky ring around the entire sun. In contrast, a solar minimum eclipse such as the one this month, will have lots of complexity near the equator but will be drastically different near the north and south poles of the Sun.”
The predicted view above gives us an idea of what observers will see during the Aug. 21 totality. The solar wind, a stream of high-speed electrons and protons, flows outward from the sun along open field lines. Closed field lines, where the lines loop back onto the Sun, trap coronal gases. As a result, the corona appears brighter there than the open corona in eclipse images. Closed and open lines are shown white and dark grey in the plot. The largest closed structures represent the helmet streamers that we see during the total solar eclipse.
Also in anticipation of the upcoming eclipse, I wanted to suggest a couple apps you’ll find useful whether you’re traveling or not. My fave is the free Smithsonian Eclipse 2017 available for both iPhone and Android. You get a eclipse countdown, an interactive eclipse map where you can zoom into your site or find your way to other locations along the centerline in the event of bad weather, an eclipse guide, recent sun images and a link to NASA’s live stream of the eclipse.
On the interactive map, you can click anywhere and get a complete list of times and a diagram for each phase of the eclipse at that location whether you’re on the centerline or anywhere else on the continent. What more could you want? An ironclad guarantee of clear skies from my app, that’s all 🙂
I also like the Exploratorium’s free Total Solar Eclipse 2017 app for iPhone and Android mostly because you have four options on the interactive map: streets, satellite images, National Geographic map and World Street Map. There are at least a dozen apps out there, some free and some costing a few bucks. I think you’ll be very happy with these … and they’re free!