"The aurora is a bright glow observed in the night sky, usually in the polar zone. For this reason some scientists call it a "polar aurora" (or "aurora polaris"). In northern latitudes it is known as the aurora borealis, which is named after the Roman goddess of the dawn, Aurora, and the Greek name for north wind, Boreas, since in Europe especially it often appears as a reddish glow on the northern horizon as if the sun were rising from an unusual direction. The aurora borealis is also called the northern lights since it is only visible in the North sky from the Northern Hemisphere. The aurora borealis most often occurs from September to October and from March to April. Its southern counterpart, aurora australis, has similar properties. Auroras are now known to be caused by the collision of charged particles (e.g. electrons), found in the Sun's Solar wind, with atoms in the Earth's upper atmosphere (at altitudes above 80 km). These charged particles are typically energized to levels between 1 thousand and 15 thousand electronvolts and, as they collide with atoms of gases in the atmosphere, the atoms become energized. Shortly afterwards, the atoms emit their gained energy as light."