A solar flare is a sudden, localized release of energy in the sun's outer atmosphere. This energy, in the form of radiation, is distributed throughout the electromagnetic spectrum, allowing flares to be seen at many different wavelengths, from the x ray to the radio regions.
The first recorded observation of a solar flare was in 1859 by Richard Carrington, who saw a sudden brightening in white light while observing sunspots. Most flares, however, are detectable only with a filter which passes wavelengths of light corresponding to certain spectral lines. The most common filter used is hydrogen-alpha (Ha), the first line of the hydrogen Balmer series, at 6,563 Å. Flares are also detected at x ray, ultraviolet, and radio wavelengths. X ray and ultraviolet observations are done from above the earth's atmosphere, using sounding rockets and satellites.
Flares are believed to be caused when magnetic reconnection occurs in a solar active region. The flares are associated with the magnetic fields accompanying sunspots in the sun's photosphere. Since flares are correlated with sunspots, their occurrence follows the eleven-year solar cycle. The sun's magnetic field lines connect the north and south magnetic poles, but are filled with kinks, causing them to emerge through the solar surface at the locations of
sunspots. Bundles of field lines, called magnetic flux tubes, occasionally become twisted, trapping excess magnetic energy. These twists may suddenly straighten out, returning the magnetic field lines to a more orderly form, and releasing enormous quantities of energy in the process. When this happens, huge quantities of charged particles are ejected into space, and radiation is emitted, particularly at x-ray wavelengths. Typical flares only cover a tiny fraction of the Sun, and last for only a few minutes.
Because the largest solar flares can produce substantial amounts of radiation and particles, their effects can be seen on the earth. Solar flares whose charged particles travel towards and collide with the earth (called a solar storm) affect radio transmissions, produce beautiful auroras (or the northern and southern lights), and can cause disruption of power transmission. Flares can also be a danger to spacecraft electronics, which must be shielded or radiation hardened to protect them, and astronauts, who could be exposed to lethal doses of radiation if not protected. Because of these effects, scientists hope to be able to predict when flares will occur, but they are not able to do so at this time. However, they do know that large solar flares are more likely near the peak of the sun's 11-year cycle. The next peak will occur between 1999 and 2004.
- Solar Illumination: Seasonal and Diurnal Patterns
- Solar Activity Cycle - Discovery Of The Activity Cycle, Cause Of The Activity Cycle
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