October 25, 2003
Last month there was a remarkable series of spots on the Sun. The active region, called sunspot 464, is about as wide as 17 Earths lined up in a row! This means it was easy to see with the unaided eye, but never look directly at the Sun without suitable eye protection. Perhaps the most obvious features of the Sun are the sunspots that characterize the photosphere. As the above photo indicates, sunspots are characterized with a dark core, the "umbra," where the temperature is about 1600 °C less than the surrounding temperature of the photosphere. The penumbra, the lighter envelope typically encompasses the umbral region, is about only 500 °C less than the surrounding photospheric temperature.
Due to the differential rate of rotation of the solar disk (26 days at the equator and 36 days at the poles), there's a "twisting" of the magnetic fields, which acts to promote sunspot production. Typically, these spots and groups are found to lie + 30° of the solar equator.
See the Earth Science Picture of the Day for April 29, 2003.