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.
