Astronomy Picture of the Day

Explanation: This ominous, dark shape sprawling across the face of the active Sun is a coronal hole -- a low density region extending above the surface where the solar magnetic field opens freely into interplanetary space. Studied extensively from space since the 1960s in ultraviolet and x-ray light, coronal holes are known to be the source of the high-speed solar wind, atoms and electrons which flow outward along the open magnetic field lines. During periods of low activity, coronal holes typically cover regions just above the Sun's poles. But this coronal hole, one of the largest seen so far in the current solar activity cycle, extends from the south pole (bottom) well into northern hemisphere. Coronal holes like this one may last for a few solar rotations before the magnetic fields shift and change configurations. Shown in false-color, this picture of the Sun on January 8th was made in extreme ultraviolet light by the EIT instrument on board the space-based SOHO observatory.