The Sun Celebrates Bastille Day!
The most violent events on the surface of the Sun are sudden eruptions called solar flares. Flares typically last a few minutes and can release energies equivalent to millions of hydrogen bombs. Flares become frequent near sunspot maximum, when smaller flares can occur daily and large flares can occur about once a week. The adjacent image shows shows a flare eruption in H-alpha light at 15:00 UT on July 14th, 1996, recorded at the Big Bear Solar Observatory (Ref).

Coronal Mass Ejections

During a flare the material in the flare may be heated to temperatures of 10 million K; matter at these temperatures emits copious amounts of UV and X-Ray, as well as visible light. In addition, flares tend to eject matter, primarily in the form or protons and electrons, into space at velocities that can approach 1000 km/second. These events are called coronal mass ejections, and produce bursts in the solar wind that influence much of the rest of the Solar System, including the Earth (However, there is controversy within the astrophysics community about whether coronal mass ejections and flares should be classified together; see this discussion). Thus, the observation of a large flare on the surface of the Sun is usually a signal for increased auroras and related activity several days hence when the ejected burst reaches Earth.

The Cause of Solar Flares

Although the cause of flares is not completely understood, they are known to be associated with the magnetic field of the Sun. One favored explanation is that they occur when magnetic fields in the Sun pointing in opposite directions interact strongly with each other. Such a situation can be brought about by the churning motion of solar material near the surface, and is more likely during periods of the active sun. Thus, there typically is a correlation between the frequency of flares and the number of sunspots.