The total solar eclipse is less than three weeks away, but researchers already have a good indication of how the Sun’s corona will look when it comes into view.
Updated 8/16/17 to include Predictive Science, Inc.'s predictions of the corona's appearance.
As seen from Earth, the Sun takes 27.2753 days to rotate once, so the perspective of the Sun apparent on July 25th was approximately the perspective viewers will see on Eclipse Day — August 21st.
This image (right) is a solar coronal magnetic model based on measurements from the National Solar Observatory Integrated Synoptic Program (NSO/NISP) taken one solar rotation before eclipse day. So, barring the appearance of a large active region, this is the approximate structure of the corona that eclipsophiles will see from the path of totality.
What will that look like? The solar wind flows outward from the Sun along open field lines (white lines) that don’t appear to connect back to the Sun. Closed field lines (dark grey), the lines that loop back onto the Sun, trap coronal plasma. As a result, the corona in closed structures appears brighter than the open corona.
Gordon Petrie (NSO) says, “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.”
Scientists can’t observe the Sun’s magnetic field lines directly. What they can study are the super-heated gases present in the Sun’s atmosphere that spiral around those magnetic field lines, much as iron filings are used to trace the field lines around bar magnets in elementary-school science experiments. Those observations then guide models of the Sun’s magnetic field.
Under normal circumstances, Earthbound viewers can’t see the Sun’s ephemeral outer atmosphere at all — the Sun’s brilliance outshines it. But as totality passes from coast to coast on August 21st, scientists will be able to observe the solar corona for 90 minutes, collectively.
An upcoming project is seeking to change the way we can look at the corona — and not just during an eclipse. Using the NSF-funded Daniel K. Inouye Solar Telescope (DKIST) in Maui, Hawaii, scientists would be able to consistently measure the solar corona’s magnetic fields directly. According to NSO director Valentin Pillet, “This will be revolutionary in the field of solar physics.”
Of course, we’re just eagerly waiting until the end of the current rotation of the Sun — not its revolution — for August 21st.
To view the press release, click here.
On August 16, Predictive Science, Inc., released a projection of what the solar corona should look like on August 21st to the human eye, as well as with several filters. Their prediction is based on a magnetohydrodynamic model of the solar corona with improved energy transport. For more information on how the prediction was made, and to see the predicted corona in other filters, visit their page.