X-ray images of the Sun's corona show complex loop structures near sunspot groups, and near smaller X-ray bright points. The X-ray emission, plus emission lines due to highly ionized atoms (coronal lines), shows that the temperature is about 2 million K; even higher temperatures of 4 million K or more are found in active regions. Magnetic fields, with a strength of about 10−3 tesla, govern the corona's shape. The magnetic fields form closed loops in active regions and much of the quiet corona (i.e. non-active regions), but in coronal holes the magnetic field lines are open and stretch out into space, not returning to the Sun.
The energy that heats the corona is thought to come from motions in and below the solar photosphere. High-frequency photospheric motions create waves in the Sun's magnetic field that travel outwards into the corona. Conversion of the wave energy into heat can account for the high temperatures of the coronal gas. One method of dissipating the wave energy is if the wave resonates with the coronal gas. An alternative heating mechanism requires low-frequency photospheric motions to tangle up the magnetic lines of force in the corona. Eventually the stresses in the magnetic field are released through reconnection of field lines. The energy released in such events is expected to vary widely, from flares to so-called microflares and even smaller nanoflares.
The appearance of the corona changes during the solar cycle. At solar maximum it consists of many active-region loops and streamers around the disk, but at solar minimum it is dominated by large coronal holes at each pole and a sheet-like structure near the equator.
Main-sequence stars cooler than spectral type F0 often have coronae with active regions, as indicated by their X-ray emission. This is particularly true of the M-type dwarf flare stars. Coronae are also present in some interacting binary systems like the RS Canum Venaticorum stars.