The chromosphere is the second outermost most layer of the sun’s atmosphere, which is only visible to the naked eye during a solar eclipse. The chromosphere is notable for being hotter than the photosphere, the next layer towards the sun.
The chromosphere is located between the photosphere and the corona, which is the outermost part of the sun’s atmosphere. The chromosphere is around 1,250 miles (2,011 kilometers) deep. Its name, which means color sphere, comes from the fact that it is a reddish color. This is caused a particular type of hydrogen.
Despite this color, it is usually impossible to see the chromosphere from Earth without special equipment. The one exception is during a total solar eclipse, when the moon is directly in line between the earth and the sun. At this point, the chromosphere appears as a series of red blotches around a solid black circle.
Logic would suggest that the chromosphere would be cooler than other parts of the sun’s atmosphere because it is the furthest away. In reality, it is considerably hotter, and appears to get hotter further away from the sun. The next nearest layer, the photosphere, is around 7,250 degrees Fahrenheit (4,010 degrees Celsius), while parts of the chromosphere are almost 36,000 degrees Fahrenheit (19,982 degrees Celsius).
One theory for this apparent disparity is that it contains magnetic fields projected outwards from the photosphere. Electric currents flow through these fields from the photosphere to the corona. This process can lose some energy in the fields, which produces the higher temperature. It’s thought the energy may be lost through magnet field lines being disturbed and having to oscillate in an attempt to return to their original form.
When visible, the chromosphere appears to flow. This is because gases are emitted from it at varying wavelengths. During an eclipse in 1868, astronomers noted a bright yellow line in the chromosphere. At first they thought it was sodium, but the wavelength showed it must be a previously uncharted element. They named it helium, after the Greek name for the sun, Helios. It wasn’t until 1895 that scientists could isolate helium on Earth.
There is a considerable amount of movement of gases within the chromosphere. The most common are spicules, vertical plumes of gas which rise away from and then back towards the sun. Their counterparts are fibrils, which travel horizontally and last for around 20 minutes, twice as long as spicules.
The chromosphere can also produce filaments, which are made up of plasma which is cooler than the surrounding gases and thus easier to see. These can sometimes lead to coronal mass ejections, where plasma leaves the sun’s atmosphere completely. This can affect the solar system’s equivalent of a planet’s weather and may even have an effect on spacecraft and other satellites.