Extended outer atmosphere of the Sun. The glow of the corona is a million times less bright than that of the photosphere; it can only be seen when the disk of the Sun is blocked during a total solar eclipse, or by using a coronagraph, which artificially blocks the disk of the Sun so that it can image the regions surrounding the Sun. The corona is very hot as indicated by emission lines corresponding to very highly ionized atoms (e.g., Fe16+). Such highly ionized atoms can only be produced at temperatures in the 106  degree range. The extremely high temperature of the corona is thought to due to the solar magnetic field, which can store and transport energy from lower regions of the Sun to the corona. However, the details of how this heating takes  place are not yet fully understood.

Some or all content above used with permission from J. H. Wittke.