Such a Magnetic Personality Because the Earth is solid, all parts of it must spin around the axis at the same rate. Regions near the poles must spin around once a day, the same as regions near the equator, or else the planet would fly apart. The fact that the Sun is a gas, however, lets different regions pass by one another fluidly. In this way, the equator can spin around faster than the poles, and in fact it does. A point on the solar equator takes only 26 days to travel all the way around the Sun, whereas a point near the poles takes about 10 days longer. This differential rotation has interesting ramifications for the Sun's magnetic field. Imagine the Sun's magnetic field lines as ropes that get stretched and kinked and tangled as different latitudes of the Sun move around at different rates. This bizarre distortion of the magnetic field results in active phenomena on the surface of the Sun. Sometimes the magnetic field restricts the energy flow to the surface, resulting in cooler, darker, roughly Earth-sized patches called sunspots. Sunspots look dark only compared to the surface of the Sun; if we could somehow float a sunspot in chilly outer space we'd see the spot glow red hot. Giving up the ghost. The Sun occasionally throws off large amounts of hot, electrically charged gas in events referred to as coronal mass ejections. Thought to be due to instabilities in the solar magnetic field, CMEs can contain tens of trillions of kilograms of gas. This particular CME was observed in December 1996, as the Sun lay in the constellation Sagittarius, by one of the instruments on the SoHO spacecraft. The circular mask in the center of the image covers the Sun's bright photosphere to permit study of the tenuous corona. Image courtesy of SoHO/LASCO consortium. SoHO is a project of international cooperation between ESA and NASA. More violent phenomena also result from the Sun's warped magnetic field. If field lines poke an elbow out the surface of the Sun, material can travel along them in streams, creating a prominence. These huge loops of gas are typically ten times the diameter of the Earth and can last for weeks. Magnetic instabilities in the solar field cause short, violent ejections of gas called solar flares-the most explosive events on the Sun. The Sun's atmospheric activity peaks about every 11 years, a pattern known as the sunspot cycle. At the low point in the cycle, called solar minimum, there are very few sunspots, and those that do appear are located far from the Sun's equator. During the next solar maximum in the year 2001, however, there may be hundreds of sunspots on the Sun at once, and several prominences or flares. Just when you think you've got our star figured out, out leaps another provoking magnetic characteristic: orientations of the magnetic poles reverse for every solar maximum. For example, if at one maximum the magnetic north pole is "up" and the south pole "down," at the next maximum the magnetic north pole will be "down" and the south "up." So the entire solar cycle lasts twice as long as the sunspot cycle, 22 years! This is the period of time needed for the Sun to return to the exact same configuration (the same point in the sunspot cycle with the same orientation of the poles).

Continue...