Solar Flares
The most violent events on the surface of the Sun (indeed, in the entire Solar System)
are sudden eruptions called
solar flares. A flare typically lasts a few minutes and can release
energy equivalent to the explosion of
a billion average hydrogen bombs. Flares become frequent near
sunspot maximum, when smaller flares can occur daily and large flares can occur
about once a week. They are always associated with sunspots and active regions. The adjacent image
shows a flare eruption in H-alpha light
on July 14, 1996, recorded at the Big Bear Solar Observatory.
A Flare Event
During a flare as much as 1025
joules of energy may be released
(a joule, which is pronounced "jewel," is a standard unit of energy in physics; a
human being running carries about 1000 joules of kinetic energy).
This can heat the material in the flare to temperatures as high as ten to one hundred
million K. Matter at these temperatures emits copious amounts of UV and X-ray
light. Flares also eject matter (primarily in the
form of protons and electrons) into space at velocities that can approach a thousand
kilometers per second.
Magnetic Fields in Flare Regions
The adjacent left figure shows a TRACE satellite image of a region of the solar surface after the
eruption of a flare. The image is in the light of Fe IX and Fe X ions at 171
Å, and the loops
of gas are estimated to have temperatures of about one million degrees. The larger loops
are ten to fifteen times the size of the Earth. These well-defined loops
extending far above the solar surface are clear indications of strong magnetic fields in this region.
