Solar Activity

The Sun is not a quiet place, but one that exhibits sudden releases of energy. One of the most frequently observed events are solar flares: sudden, localized, transient increases in brightness that occur in active regions near sunspots. They are usually most easily seen in H-alpha and X-rays, but may have effects in the entire elecromagnetic spectrum. The X-ray brightness from a large flare often exceeds the X-ray output from the rest of the Sun. Another type of event, the coronal mass ejection, typically disrupt helmet streamers in the solar corona. As much as 1e13 (10,000,000,000,000) kilograms of material can be ejected into the solar wind. Coronal mass ejections propagate out in the solar wind, where they may encounter the Earth and influence geomagnetic activity. Coronal mass ejections are often (but not always) accompanied by prominence eruptions, where the cool, dense prominence material also erupts outward.

All of these forms of solar activity are believed to be driven by energy release from the solar magnetic field. How this energy release occurs, and the relationship between different types of solar activity, is one of the many puzzles facing solar physicists today. The amount of solar activity on the Sun is not constant, and is closely related to the typical number of sunspots that are visible. The number of sunspots and the levels of solar activity vary with an 11 year period known as the solar cycle.

Click on image for full size (125K JPEG)
A coronal mass ejection and prominence eruption observed in white light from the SMM (Solar Maximum Mission) spacecraft, courtesy of the High Altitude Observatory. The time of each panel increases from left to right. The dashed inner circle in each panel is the solar radius, the occulting radius is at 1.6 solar radii.

A movie of a prominence eruption in H-alpha, courtesy of the High Altitude Observatory (169K MPEG) Movie credit

A movie of a coronal mass ejection in white light, courtesy of the High Altitude Observatory (180K MPEG) Movie credit

A book about solar activity called The 23rd Cycle: Learning to Live with a Stormy Star

Last modified May 16, 2001 by the Windows Team

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