Forces behind the Northern and Southern Lights
The beautiful sky show known as aurora borealis (or australis, in the Southern Hemisphere) is, in fact, a manifestation of molecular violence. Aurorae are produced by the solar wind, a stream of charged particles from the Sun. Because Earth's magnetic field is a very good plasma shield, the solar wind usually flows around it much like a stream flows around a rock. At certain times the magnetism in the solar wind is especially 'geoeffective” – it's especially intense and oriented so that it clashes with Earth's magnetic field. When a batch of geoeffective plasma arrives, electrons trapped on the dark side of Earth are energized. They travel downward toward the pole along magnetic field lines and strike atoms and molecules at heights between 80-1000km/50-600 miles. Aurorae develop as each particle struck emits a characteristic hue by element: red and yellow-green from oxygen, blue from nitrogen, etc. Aurorae can occur anytime, but they're especially frequent in a one- or two-year span close to the eleven-year peaks in the solar magnetic cycle; this millennium's first peak occurred in 2001 .The most intense aurorae can be seen as far away from the poles as 30 N or S (in 1909, one aurora was spotted near the equator).
Aurorae are just one consequence of the high-altitude magnetic and electrical paroxyms known as geomagnetic storms. Most of these are caused by solar storms – giant ejections of as much as a billion tons of magnetized gas from the Sun. Occasionally, a geomagnetic storm is induced by a strong solar wind emerging from a quiet region on the Sun called a coronal hole. Despite the beauty of their auroral calling cards, geomagnetic storms can scramble radio waves and corrupt power grids on Earth. Six million people in the Montreal area lost power for up to nine hours following a geomagnetic storm on March 13, 1989. By heating the upper atmosphere, geomagnetism also exerts a drag on low-orbiting satellites, pulling them downward until a reboost can be accomplished. Space-weather experts rely on satellites to monitor the Sun's action; they can now provide a day or two of advance warning to help industry and government prepare for the worst storms, and aurora lovers to get into position.