IRIS Movie of the Day
At least once a week a movie of the Sun taken by NASA's Interface Region Imaging Spectrograph (IRIS) is posted by one of the scientists operating the instrument.
Ripples in a Sunspot
Credit: IRIS, LMSAL/NASA, Chad Madsen
Sunspots appear dark, stoic, and unlively when viewed in the visible wavelengths of light our eyes can detect. However, when viewed in the ultraviolet, we find a whole zoo of strange, energetic phenomena. This movie shows a particularly bizarre example known as "running sunspot waves". These appear as circular ripples emanating from the center of the sunspot, much like the ripples that form when rain drops strike the surface of a puddle. Although they appear to be moving across the sunspot, these waves are actually moving upward through the sunspot. Running waves are actually a modified version of sound waves known as "slow magneto-acoustic waves" which, in this case, are formed by vibrations on the surface of the Sun. These waves are stuck to the magnetic field lines running upward through the sunspot. As the magnetic field fans out and becomes more inclined in the outer regions of the sunspot, the waves near the edge take longer to reach the same altitude as those near the center, producing the illusion of a ripple moving outward across the sunspot.