One of the two distinct types of tail developed by comets as they near the Sun. The ion tail, also known as the gas tail or plasma tail, consists of ionized atoms and molecules that are emitting light by the mechanism of resonance fluorescence. In this process, energy is absorbed from sunlight then re-emitted later. The ion tail lies in the plane of the comet’s orbit. It is nearly straight but curves away from the direct radial line to the Sun by a few degrees. The ion tail is blown away from the comet by the solar wind and the effects of its magnetic field. See also: disconnection event.