The corona is the tenuous plasma
envelope around the Sun. It is the outer layer of the atmosphere of the Sun. It has no clear outer boundary; the edges constantly are blown away into space
as the solar wind
. This writeup will start with a discussion on how the corona is observed. Then, an interesting and poorly understood physical phenomenon is discussed.
Observing the corona
In order to study the corona, it is nice if one can actually observe it. The major problem with observing the corona is that it is very close to the Sun. As such, the Sun will saturate your eyes, making it impossible for you to see the corona. In fact, if you stare at the Sun, it will soon be impossible to see anything, as it will blind you. The Sun is more than a million times more bright than the corona 1,2.
The only moment at which it is possible to see the corona without any equipment is during a total eclipse. It is visible as a fairly faint, whitish halo around the Sun, that looks most like a piece of fluff or a cloud with streaks it. In terms of size, it is much larger than the Sun, especially if we consider the spherical symmetry. It certainly does not look like an uniform sphere, like the Sun does. It is noted that eclipse glasses are recommended when seeing a solar eclipse, especially before it is total.
Astronomers observe the corona using a coronapgraph. This is an instrument that blocks out the direct light from the Sun (or another star) and, as such allows one to see the corona. A simple design that does this consists of a disk that blocks direct sunlight, but more complicated designs are possible as well2.
Astronomers have observed the corona using a spectrometer 3. A spectrometer measures the light emitted by atoms, molecules or ions. Because each kind of atom or molecule has its own spectrum, that is unique like a fingerprint is unique among humans, this allows identification of the ions in the corona. One of the most prominent species is an iron ion that is ionized 14 times 4. This requires an enormously high temperature of over 1 million kelvin.
Coronal temperature paradox
The surface of the Sun is not more than about 6000 kelvin, tops (this is about 5700 degrees Celcius). Having such a hot "envelope" around the Sun is akin to turning on an oven and noting that the outside is yellow-hot, while the pie in it is freezing. It isn't as much a problem of energy - indeed, the corona has a density that is 1012 times lower than the body of the Sun, so heating it requires relatively little energy - but more of a problem because heat flows from hot to cold an not vice versa. This is a corollary of the second law of thermodynamics.
So why does this happen? The short answer is: no one knows for sure. The most plausible theories involves magnetic reconnection and so-called wave heating, in which sonic waves compress the corona and as such create local increases in energy.
The corona is a very tenuous and very hot layer around the Sun. It is normally not visible, because the light from the rest of the Sun is so much brighter. The reason why it is so hot is currently not known, although there are theories that can partially explain it.