Auroras Borealis and Aurora Australis are glows in the sky sometimes visible in the Northern and Southern Hemispheres, respectively. They are informally known as the northern lights and the southern lights. The glows are strongest near the poles, and originate in the Van Allen radiation belts, regions where high-energy charged particles of the solar wind that travel outward from the Sun are captured by the Earth's magnetic field. The outer Van Allen radiation belt consists mainly of protons, whereas the inner Van Allen belt consists mainly of electrons. At times electrons spiral down toward Earth near the poles along magnetic field lines and collide with ions in the thermosphere, emitting light in the process. Light in the aurora is emitted between a base level of about 50-65 miles (80-105 km), and an upper level of about 125 miles (200 km) above the Earth's surface.
The solar wind originates when violent collisions between gases in the Sun emit electrons and protons that escape the gravitational pull of the Sun and travel through space at about 250 miles per second
(A) Drawing of magnetosphere, showing asymmetric shape created by distortion of the Earth's magnetic field by the solar wind (B) Earth, showing typical auroral ring with the greatest intensity of auroral activity about 20-30° from the magnetic pole, where magnetic field lines are most intense
The Sun periodically experiences periods of high activity when many solar flares and sunspots form. During these periods the solar wind is emitted with increased intensity, and the plasma is emitted with greater velocity, in greater density, and with more energy than in its normal state. During these periods of high solar activity the extra energy of the solar wind distorts the magnetosphere and causes more electrons to enter the Van Allen belts, causing increased auroral activity.
When the electrons from the magnetosphere are injected into the upper atmosphere, they collide with atoms and molecules of gases there. The process involves the transfer of energy from the high-energy particles of the magnetosphere to the gas molecules from the atmosphere, which become excited and temporarily jump to a higher energy level. When the gas molecules return to their normal, regular energy level, they release radiation energy in the process. some of this radiation is in the visible spectrum, forming the aurora borealis in the Northern Hemisphere and the aurora australis in the Southern Hemisphere.
Auroras typically form waving sheets, streaks, and glows of different colors in polar latitudes. The colors originate because different gases in the atmosphere emit different characteristic colors when excited by charged particles from the magnetosphere, and the flickering and draperies are caused by variations in the magnetic field and incoming charged particles. The auroras often form rings around the magnetic poles, being most intense where the magnetic field lines enter and exit the Earth at 60-70° latitude.
See also magnetic field, magnetosphere; Sun.
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