Solar Activity

The constant release of tremendous amounts of energy keeps the sun's surface in a state of seething, churning activity. Telescopic observations show that the surface is covered with fine granules, or cells, which are the tops of rising columns of hot gas from the interior. The granules are usually about 500 to 1,000 miles (800 to 1,600 km) in diameter and have a lifespan of only a few minutes. Spikelike spicules of gas, apparently associated with the surface granules, form constantly. They typically shoot upward to a height of 3,000 to 4,500 miles (4,800 to 7,200 km) and fall back within several minutes.

From time to time, irregular and seemingly dark features known as sunspots appear on the sun's surface. These are the most conspicuous and best-known of the sun's markings. The dark appearance of sunspots is due to their lower temperature—as much as 3,000 °F. (1,670 °C.) cooler than the surrounding surface. However, the darkness is only relative, since sunspots are actually very bright.

Sunspots appear both individually and in groups, and may range in diameter from 3,000 miles (4,800 km) to more than 100,000 miles (160,000 km). Their occurrence varies in a cycle averaging approximately 11 years between periods of maximum activity. At the peak period, hundreds of sunspots may be observed at one time. Although their precise causes and mechanisms are not clearly understood, sunspots apparently are associated with local magnetic disturbances beneath the surface.

Several other kinds of solar activity are related to sunspots and the sun's magnetic field. Appearing frequently near sunspot groups are bright, irregular spots called faculae. Resembling luminous clouds, faculae seem to float just above the surface and often occur shortly before the appearance of a new sunspot group.

Occasionally, a sunspot group shows a sudden and rapid increase in activity that results in a solar flare. Such an occurrence is one of the most dramatic of all solar events; within a few seconds, a tongue of atomic particles and glowing gas is violently ejected from the sun at speeds high enough to escape the sun's gravitational pull. Small flares are relatively common occurrences, but large ones are not; even during a year of peak sunspot activity only a few are seen.

Streamer-like prominences often arc through the sun's atmosphere, sometimes extending 100,000 miles (160,000 km) or more above the surface. When seen at the edge of the sun's disk, prominences may appear as long plumes or arches. Observed on the face of the sun, a prominence usually appears as a dark thread or filament. Prominences may last days or weeks before sinking back beneath the lower atmosphere.