The aurora is a luminous glow that occurs in the Earth’s upper atmosphere. The diffuse aurora cannot be seen from the ground because it is faint. It is visible from the ground if it is discrete aurora. Discrete aurora can be so bright that reading a book is possible. Some of the types of discrete aurora that occur at night are auroral arcs, spirals, curls, folds, auroral bulge, omega bands. These types are more intense before midnight than after midnight. Other types include Sun-aligned arcs, afternoon auroral spots, and mantle aurora.
The aurora borealis (northern lights) is the aurora effect in northern latitudes. The aurora is most frequent in high latitudes (Arctic and Antarctic or in other words North Pole and South Pole). Most auroras occur in the auroral zone, which is ten to twenty degrees from each magnetic pole. During a geomagnetic storm, the auroras move into lower latitudes. The aurora effect is caused by the collision of charged particles.
The aurora borealis were named after Aurora, the Roman goddess of dawn and Boreas, the Greek name for the north wind by Pierre Gassendi in 1621. It occurs most often near the equinoxes. The two equinoxes occur when the tilt of the Earth’s axis is even with the Sun. The center if the Sun is even with the Earth’s equator. The Spring Equinox is March 20/21 and the Vernal Equinox is September 22/23. The lengths of day and night are approximately the same during the equinoxes. This year (2012), the Spring Equinox is 5:14 a.m. March 20 and the Vernal Equinox is 2:49 p.m. September 22. Next year (2013) the Spring Equinox is 11:02 a.m. March 20 and the Vernal Equinox is 8:44 p.m. September 22. The year after next (2014), the Spring Equinox is 4:57 p.m. March 20 and the Vernal Equinox is 2:29 a.m. September 23.
Auroral arcs are the most common kind of discrete auroral before midnight. They are luminous bands pointing from the east to the west. The quiet auroral arcs become more active during increased geomagnetic activity. This causes them to change their shapes to spirals, curls, and folds. Auroral bulge form from when a quiet aurora is deformed to a large scale. Three types of auroral bulge are westward termination, north south aligned, and eastward propagating. Omega bands are large scale folds or wavy structures.
Sun aligned polar arc discrete aurora occur within the polar cap. They are one hundred kilometers or more in width and can last for several hours. Afternoon auroral spots occur for about 1.5 hours.
