Solar transition region

The solar transition region is a region of the Sun's atmosphere, between the chromosphere and corona.
It is visible from space using telescopes that can sense ultraviolet. It is important because it is the site of several unrelated but important transitions in the physics of the solar atmosphere:
Helium ionization is important because it is a critical part of the formation of the corona: when solar material is cool enough that the helium within it is only partially ionized (i.e. retains one of its two electrons), the material cools by radiation very effectively via both black-body radiation and direct coupling to the helium Lyman continuum. This condition holds at the top of the chromosphere, where the equilibrium temperature is a few tens of thousands of kelvins.
Applying slightly more heat causes the helium to ionize fully, at which point it ceases to couple well to the Lyman continuum and does not radiate nearly as effectively. The temperature jumps up rapidly to nearly one million kelvin, the temperature of the solar corona. This phenomenon is called the "temperature catastrophe" and is a phase transition analogous to boiling water to make steam; in fact, solar physicists refer to the process as "evaporation" by analogy to the more familiar process with water. Likewise, if the amount of heat being applied to coronal material is slightly reduced, the material very rapidly cools down past the temperature catastrophe to around one hundred thousand kelvin, and is said to have "condensed". The transition region consists of material at or around this temperature catastrophe.
The transition region is visible in far-ultraviolet (FUV) images from the TRACE spacecraft, as a faint nimbus above the dark (in FUV) surface of the Sun and the corona. The nimbus also surrounds FUV-dark features such as solar prominences, which consist of condensed material that is suspended at coronal altitudes by the magnetic field.