Observations of the Structure and Dynamics of Coronal Loops

The most striking thing about the SXT images is the range of loop sizes and shapes. Active regions are bright tangles of magnetic field lines, surrounded by a network of large-scale quiet coronal loops stretching between regions or to the quiet-Sun areas over distances in excess of 100,000 km. Often quite exotic loop geometries are seen, particularly cusped, sheared, and twisted loops. All of these magnetic structures show changes on timescales from seconds to months, depending on the nature of the coronal structure. The question of how these structures become bright and fill with hot plasma is still open. While we see the propagation of brightenings along the length of loops with velocities of several hundred km/s in active regions, particularly during flares, much higher velocities are seen in the quiet Sun. In X-ray bright point flares velocities of over 1000 km/s have been observed. X-ray jets with complex structure are also seen. The active-region loops themselves see! m to be in constant motion, moving slowly out, carrying plasma with them.