Thursday, January 26, 2012

Introduction into Aurorae

The beautiful and often eerie curtains of light in the night-time sky known as aurora have been enjoyed by people for millennia. Called the aurora borealis or “northern lights”, aurora also occur in the Southern Hemisphere and are called the aurora australis.

Picture Credit: Andy Keen, Inari Municipality, Northern Lapland, Finland. Taken on January 24, 2012
Many legends, myths and superstitions have revolved around the aurora throughout the history of mankind. The early dragon legends of China and Europe are said to have originated from the aurora. Some cultures have regarded the sighting of the aurora as a sign of royal birth; to others it suggests ghosts of the dead orthe precursor for war. The Eskimos of North America believed that if you whistled at the aurora it would sweep down and take you from the earth; by clapping your hands you could force it to retreat.

The origin of the aurora is 93 million miles (149 million km) from Earth at the Sun. Energetic particles from the Sun are carried out into space along with the ever-present hot solar wind. This wind sweeps supersonically toward Earth through interplanetary space at speeds ranging from 300 to over 1000 km per second, carrying with it the solar magnetic field. The solar wind distorts the Earth’s magnetic field to create the comet-shaped magnetosphere.

The terrestrial magnetic shield acts as a barrier, protecting the Earth from energetic particles and radiation in the hot solar wind. Most of these energetic particles are deflected around the Earth by the magnetosphere, but some get trapped. Electrons trapped in the Earth’s magnetic field are accelerated along the magnetic field toward the polar regions and then strike the atmosphere to form the aurora.

The particles, which stream down the magnetic field of the Earth, reach the neutral atmosphere in a rough circle called the auroral oval. This circle, or annulus, is centered over the magnetic pole and is around 3000 km in diameter during quiet times. The annulus grows larger when the magnetosphere is disturbed. The location of the auroral oval is generally found between 60 and 70 degrees north and south latitude.
Great education video about Aurorae
Auroral features come in many shapes and sizes. Tall arcs and rays start brightly 100 km above the Earth’s surface and extend upward along its magnetic field for hundreds of km. These arcs or curtains can be as thin as 100 meters while extending from horizon to horizon. Auroral arcs can nearly stand still and then, as though a hand has been run along a tall curtain, the aurora will begin to dance and turn. After midnight, the aurora can take on a patchy appearance and the patches often blink on and off once every 10 seconds or so until dawn.

Most of the auroral features are greenish yellow but sometimes the tall rays will turn red at their tops and along their lower edge. On rare occasions, sunlight will hit the top part of the auroral rays creating a faint blue color. On very rare occasions (once every 10 years or so) the aurora can be a deep blood red color from top to bottom. In addition to producing light, the energetic auroral particles deposit heat. The heat is dissipated by infrared radiation or transported away by strong winds in the upper atmosphere.
Image Credit: An all-red aurora captured in Independence, Mo., on October 24, 2011. Image Courtesy of Tobias Billings
The aurora is caused by the interaction of highenergy particles (usually electrons) with neutral atoms in the Earth’s upper atmosphere. These high energy particles can “excite” (by collisions) valence electrons that are bound to the neutral atom. The excited electrons can then return to their initial, lower energy state, and in the process release photons (light particles). This process is similar to the discharge in a neon lamp.

Any particular color of the aurora depends on a specific atmospheric gas and its electrical state, and on the energy of the particle that hits the atmospheric gas. Atomic oxygen is responsible for the two main colors of green (wavelength of 557.7 nm) and red (630.0 nm).

Credit: Andy Keen of Inari, Finland

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