Aurora

An aurora is a luminous glow in the sky, most frequently found in the polar regions. It varies in brightness from a faint glow at quiet times to approaching that of the full moon during active periods. The aurora is a permanent optical feature of the upper atmosphere, and appears as an oval encircling the Earth at a height of about 100 km or more. Its position varies with geomagnetic activity. During moderate activity, the aurora is located about 23° from the magnetic pole on the nightside of the Earth and 15° on the dayside. This belt runs over Alaska, across Hudson Bay and southern Greenland, and over northern Norway and Siberia. During magnetically quiet times, the oval shrinks poleward by as much as 5°, significantly reducing the size of the "polar cap," the region enclosed by the auroral oval.

The aurora is caused by particles, mainly electrons, bombarding the gases of the Earth's upper atmosphere. These gases become excited and lose energy by emission of light. The visible spectrum of auroral emissions is characteristic of the particular gases present at that atmospheric height. Higher altitude auroras, above about 150 km, appear red due to radiation from atomic oxygen. Auroras more commonly occur in the 100-150 km height region and tend to be mainly due to emissions from oxygen (green and red) and nitrogen (violet and pink). To observe these colors the aurora must be bright, as during auroral substorms. More commonly, auroras are faint and look gray or colorless.

While spectacular displays have been recorded throughout history as early as 500 BC, it has been through major research efforts, such as the International Geophysical Year, 1957-1958, and following in situ rocket and satellite investigations, that most understanding of the phenomenon has emerged. The auroral emission spectrum extends over a wide wavelength range extending from X-rays to radio emissions. Some major emissions are in the extreme ultraviolet region and are absorbed by the atmosphere, but can be detected from above. Orbiting satellites such as the Dynamics Explorer, Viking, and Polar have been used routinely since 1981 to photograph the aurora globally, even in the presence of full sunlight. They have verified that the aurora is a permanent, full halo encircling the Earth. Viewed around this "24-hour oval," there are typically quiet arcs in the evening sector, dynamic brighter auroras in the midnight sector, diffuse auroral remnants in the morning sector, and faint, red auroras throughout the noon sector.

Auroral activity is controlled to a major degree by solar activity and the solar wind, a continuous stream of electrons and protons emanating from the Sun trapped by the Earth's magnetosphere. The energy of these particles determines the depth to which they can penetrate the Earth's atmosphere and thus the nature (and color) of the aurora produced. Major auroras are due to coronal mass ejections (CMEs) from the Sun, while auroral substorms are usually triggered by changes in the solar wind. Auroral substorms occur periodically and typically last for about 3 h. The first sign of a substorm is the sudden brightening of the

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