The Sun’s magnetic activity is quite complex. Rapid, large fluctuations occur in numerous strong local magnetic fields that are threaded through the Sun’s atmosphere. Magnetic activity shapes the atmosphere and causes disturbances there called solar activity. This activity includes sunspots and violent eruptions. Overall, solar activity follows about an 11-year cycle, in which the numbers of sunspots and other disturbances increase to a maximum and then decrease again. The Sun seems to have a weak global magnetic field. Once each 11-year cycle, the north and south poles of the field switch polarity.
Periodically, darker cooler blotches called sunspots temporarily appear on the Sun’s surface. Sunspots are areas where very strong local magnetic fields interfere with the normal convection activity that brings heat to the surface. The spots usually appear in pairs or groups of pairs. Each spot typically has a dark, circular center, called the umbra, surrounded by a lighter area, the penumbra. The umbras are about 2,000 K (3,100° F) cooler than the photosphere around them (which means that they are still very hot). Sunspots vary greatly in size but are always small compared to the size of the Sun. When they appear in groups, they may extend over tens of thousands of miles. They last from tens of minutes to a few days or even months.
Photosphere of the Sun with sunspots, image taken by the Solar and Heliospheric Observatory (SOHO) satellite on Oct. 29, 2003. SOHO/NASA
Regular observations of sunspots have been made since 1750. They reveal that the spots appear and disappear in a cycle and that they are limited to the two zones of the Sun contained between about latitudes 40° and 5° of its northern and southern hemispheres. As mentioned above, the cycle lasts an average of about 11 years. At the beginning of a cycle a few spots appear at around 35° latitudes. Then they rapidly increase in number, reaching a maximum in the course of around five years. At the same time, the spots get closer and closer to the equator. During the next six years their number decreases while they continue to approach the equator. The cycle then ends, and another cycle starts.
In the early 20th century George E. Hale observed that certain photographs of sunspots showed structures that seemed to follow magnetic lines of force. Often a pair of sunspots appeared to form the north and south poles of a magnetic field. Hale was finally able to establish that sunspots are indeed seats of magnetic fields. In addition, from one 11-year cycle to the next, a total reversal of the sunspots’ polarity occurs in the two solar hemispheres. In other words, the north pole of a magnetic field associated with a sunspot pair becomes the south pole, and vice versa. A magnetic cycle of sunspots lasts an average of 22 years, since it encompasses two approximately 11-year cycles.
A more violent phenomenon is the solar flare, a sudden eruption in the chromosphere above or near sunspot regions. Flares release magnetic energy that builds up along the boundaries between negative and positive magnetic fields that become twisted. The flares usually form very rapidly, reaching their maximum brilliance within minutes and then slowly dying out. They emit huge amounts of radiation at many different wavelengths, including X rays and gamma rays, as well as highly energetic charged particles.
One of the strongest solar flares ever detected appears at right in an extreme ultraviolet (false-color) image of the Sun taken by the Solar and Heliospheric Observatory (SOHO) orbiting spacecraft. Such powerful flares, called X-class flares, release intense radiation that can temporarily cause blackouts in radio communications all over Earth. The flare occurred on Nov. 4, 2003. SOHO/ESA/NASA
Features called prominences also form along sharp transitions between positive and negative magnetic fields. Early astronomers noticed huge red loops and streamers around the black disk of solar eclipses. These prominences are areas of relatively cooler, denser plasma suspended like clouds through the hot, low-density corona. Magnetic lines of force hold the plasma in place. Prominences appear as bright regions when seen extending from the solar disk but as long, dark, threadlike areas when seen against the disk. The dark areas are also called filaments.
An image taken in extreme ultraviolet light reveals a solar prominence lifting off the Sun. The false-color image was captured by the Transition Region and Coronal Explorer (TRACE) orbiting satellite. TRACE/NASA
Long-lived, or quiescent, prominences may keep their shape for months. They form at the boundaries between large-scale magnetic fields. Prominences in active regions associated with sunspots are short-lived, lasting only several minutes to a few hours. When prominences become unstable, they may erupt upward. These eruptions are significantly cooler and less violent than solar flares.
A type of violent eruption called coronal mass ejections also occurs in the corona. The corona sometimes releases enormous clouds of hot plasma into space. Like solar flares, these coronal mass ejections release energy built up in solar magnetic fields. They usually last hours, however, while the rapid eruptions from flares typically last only minutes.
Like other kinds of solar activity, coronal mass ejections are most common during the solar maximum. Scientists believe that flares, prominence eruptions, and coronal mass ejections are related phenomena. Their relationship is complex, however, and not yet fully understood.
A very large coronal mass ejection, at upper right. The red disk in the center is part of the coronagraph used to take the image. The white circle indicates the size and position of the Sun’s disk. The false-color image was taken by the Solar and Heliospheric Observatory (SOHO) orbiting spacecraft. SOHO/ESA/NASA
