This movie shows a simple loop flare observed with the Very Large Array radiotelescope in New Mexico. The 27 antennas of the array were divided into two subarrays in order to observe at two frequencies (5 and 15 GHz) simultaneously. At 15 GHz (colored in green) a sunspot source is present in the lower right throughout. During the flare, the two footpoints of a magnetic loop system filled with energetic electrons show up; the loop itself is outlined at 5 GHz, at which frequency it is probably optically thick. The reason that the two frequencies appear quite different is that the magnetic field is much stronger near the footpoints of the loop, which favors the higher frequency. The static image above shows the flaring loop (with colors reversed relative to the movie) overlaid on a magnetogram which shows the direction of the magnetic field at the surface of the Sun (white = upgoing, black = downgoing), in which you can see that the two footpoints lie in regions of opposite polarity.