Corona
Written by Fraser Cain
The surface of the Sun is called the photosphere; it's the point where light from inside the Sun reaches space. It's the familiar white part of the Sun that we can see. But rising above the surface of the Sun is the atmosphere of the Sun, and one of the most prominent layers of this atmosphere is the corona.
The corona begins at an altitude of about 20,000 km above the surface of the Sun; where the chromosphere ends. Although it's normally invisible from here on Earth, the corona is visible as a white halo surrounding the disk of the Sun during solar eclipses.
Perhaps the most amazing aspect of the corona is its high temperature. You would expect temperatures to decrease as it moves away from the Sun, but the temperature of the corona actually increases, reaching a high end of 1-3 million degrees K. The source of this temperature is a puzzle. Obviously it's coming from the Sun, but astronomers don't understand the mechanisms involved.
There are many interesting features in the Sun's corona. Helmet streamers are large cap-like coronal structures with long, pointed peaks. They're named because of their similarity to helmets worn by armies in World War 1. When the Sun is eclipsed, these are the wispy halo that we can see. The pointed peaks in the helmet streamers are caused by the Sun's solar wind.
Polar plumes are long thin streamers that project outward from the Sun's poles. They're often associated with small magnetic regions on the solar surface.
Coronal loops are fast arches created as hot plasma follows the Sun's magnetic field lines. These lines will pierce out of the surface of the Sun as sunspots, which almost always go in pairs. The loops can last for days or even weeks as the long as the sunspots are on the surface of the Sun.
Coronal holes are regions where the corona is dark. These were first seen with X-ray telescopes. This high-speed solar wind is known to originate from the coronal holes.
We have written many articles about the corona for Universe Today. Here's an article that shows how solar scientists predicted the shape of the corona during a recent eclipse, and another that explains how temperatures change in the corona, from hot to hotter.
Here's an article with photographs of the Sun's corona, and another from NASA's Imagine the Universe.
We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.
Filed under: Astronomy
