The Sun’s corona (Latin for crown) is aptly named because it’s the faint halo we can see only during a total eclipse. One of the main goals of our expedition to view the eclipse in August is to study this ethereal region. The corona is extremely hot at 1,000,000 K, even hotter than the surface of the Sun, so one might think it would be bright. Unfortunately, this is not the case because while the corona is very hot, it is not particularly dense and so it’s too faint to see anytime other than during an eclipse.
Because the corona is so hot, its atoms are ripped apart into positive and negative particles, ions and electrons. This super heated gas of charged particles is called plasma, and its properties explain the beautiful filamentary structure seen in eclipse images.
The charged particles in a plasma are strongly affected by magnetic fields, and the Sun has lots of these. Like the Earth, the Sun has magnetic north and south poles, but unlike the Earth’s magnetic field, the Sun’s is highly variable. Loops and streamers of magnetic field emerge from the Sun’s surface and rise into the corona, carrying the plasma with them and creating the structure in the glowing halo.
The structure is easily seen in pictures of the corona. Every picture is different because the magnetic fields created by the Sun are so dynamic. We don’t know what the corona will look like on eclipse day, except that it will be spectacular.