The corona is the collection of immediate gases around the Sun. It is extremely hot, much hotter than the surface of the Sun. Like the chromosphere, it can only be seen during a solar eclipse with the naked eye, as this is the only time that the light from the photosphere is blocked out enough so that anything else can be seen. It can also be observed with a coronagraph, which is an instrument that can produce an artificial eclipse which blocks out light from the photosphere. Its white light is just scattered light from the photosphere, which is why its color is the same of that of the photosphere. Below is a very clear image of the corona around the Sun, taken during a solar eclipse. Next to it is an animation of a coronal mass ejection. (For an explanation of coronal mass ejections, see the Solar Activity page.) (For an animation of some recent coronal mass ejections, see the Recent Solar Events page.)
Many different solar events are seen within the corona:
Helmet streamers emit from the Sun in long pointed peaks. The usually arise from sunspots and active regions, so at the base of a helmet streamer one will often find a prominence. They form magnetic loops that connect the sunspots and suspend material above the surface of the Sun. The magnetic field lines trap the material to form the streamers. The action of the solar wind is what forms the peaks that appear. Helmet streamers are seen coming off of the Sun in the picture below.
You can see the pointed "peaks" form the helmet streamers. An especially prominent one can be seen coming from the bottom of the Sun.
These are regions where the corona is dark. They are often found at the Sun's poles, and are associated with open magnetic field lines. Most of the solar wind originates from these holes in the corona. They can only be seen by looking at the Sun through an X-ray telescope.
Coronagraphs allow us to see the corona on the limb, but in order to see it on the disk, it has to be looked at through an X-ray telescope. X-rays allow us to see things with high temperatures, and since the photosphere is cool, the X-ray telescope blocks out the light from it so that the corona can be seen around the disk.
Here is an image of the Sun looked at through an X-ray telescope. The corona is visible all around the disk.
Being able to look at the corona in this way reveals its structure. It shows that it consists of loops and arches of material that originate from the chromosphere and photosphere. Some of these loops are associated with solar flares, while others last longer. These loops are denser than their surroundings.
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