coronal loop

A coronal loop is a visible arch-shaped bright prominence on the surface of the Sun. They can be made more distinct through choice of wavelength observation-regime, showing up more distinctly in extreme ultraviolet (EUV) and X-ray observation. They are sometimes associated with sunspots, one or both ends of the loops coinciding with spots and are most common around solar maximum. They last anywhere from seconds to days. Their size varies, the largest extending hundreds of thousands of kilometers above the photosphere.

They are presumed to be a brightening effect on the plasma of the corona and transition region by field lines of the Sun's magnetic field that coincide with the loop and are taken as clues to the field lines and their evolution. Various visible phenomena, such as magnetic arcades are presumed to be coronal loops in a pattern, and they are a component of the standard model of a flare.

A recent notion is that an observed loop can be a double fold in a sheet-shaped region of the stellar atmosphere's plasma, which from some angles would resemble a slender, bright loop. This would be an entirely different mechanism, motivating research into how this would affect previous assumptions, whether both mechanisms occur and if so, ways to distinguish them.