Solar coronal loops. Colored ultraviolet TRACE image of coronal loops on the Sun. The bases of the loops are the hottest areas (white). These heat the solar corona (atmosphere) to temperatures thousands of times that of the Sun's surface (dark, lower frame). The loops comprise electrically-charged gases (plasma) at millions of degrees Celsius. Gas is heated as it is ejected from the surface, and follows the arched paths of magnetic field lines before crashing back down to the surface at over 100 kilometers per second. TRACE (Transition Region and Coronal Explorer) is an observatory launched in 1998 to study the Sun at ultraviolet wavelengths.