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.
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
Here is a spectacular MPEG movie (489 kB) of a coronal ejection event captured by
the LASCO instrument on SOHO. In the movie a comet first appears to encounter
the Sun and the coronal mass ejection then occurs almost as if in response to the
comet. However, this is just a coincidence; the two events are unrelated.
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
The Sun Celebrates Bastille Day!
Solar flares are huge outbursts of solar material, which are several miles long. If we had
some way of capturing all the energy emitted in one of the smallest solar flares, we would
have enough energy to power the Earth for one million years.