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Aurora Borealis / Aurora Australis

In astronomy, an aurora is an optical phenomenon characterised by colourful displays of light in the night sky, caused by the interaction of charged particles from the solar wind with the upper atmosphere of a planet. The most powerful aurorae tend to occur after coronal mass ejections. On Earth, Jupiter, Saturn, Uranus and Neptune, aurorae are caused by the interaction of solar wind particles with the planet's magnetic field, and are therefore most prominent in higher latitudes near the magnetic poles. For this reason, the aurora occurring in Earth's Northern Hemisphere is called the aurora borealis, or northern lights; and in the Southern Hemisphere the aurora australis. However, aurorae also occur on Venus and Mars, which lack planetary magnetic fields. On Venus, atmospheric molecules are energised directly by the solar wind; on Mars, aurorae occur near localised magnetic anomalies in the planetary crust which are remnants of a presumed former planetary magnetic field which is now long extinct. On Earth, aurorae occur when the Van Allen radiation belts become "overloaded" with energetic particles, which then cascade down magnetic field lines and collide with Earth's upper atmosphere.

The origin of the aurorae is 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 Earth's magnetic field to create the comet-shaped, plasma-filled magnetosphere. The terrestrial magnetic shield acts as a barrier, protecting Earth from energetic particles and radiation in the hot solar wind. Particle energy and momentum are transferred from the solar wind to the magnetosphere through a process known as "magnetic reconnection". In this process, interplanetary magnetic field lines (originating from the Sun) are coupled to Earth's magnetic field. Particles in the solar wind can enter this newly created magnetic field line. Auroral physicists call this an "open magnetic field line" (the field line is open into the solar wind). Due to the dynamic pressure of the solar wind, this newly opened magnetic field line will be convected over the polar cap, and into the tail of Earth's magnetosphere. Here, a new magnetic reconnection can occur, creating a new, closed magnetic field line. The convecting field line will contain solar wind particles. Some of these particles will be able to reach the ionosphere before the field line has reached the magnetospheric tail. These particles will create dayside aurorae. Nightside aurorae are created from particles accelerated from the magnetospheric tail towards Earth. These particles will be trapped on the closed field line.

Auroral features come in many shapes and sizes. Tall arcs and rays start brightly 100 km above Earth's surface and extend upward along its magnetic field for hundreds of kilometres. 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, begin to dance and turn. After magnetic midnight, aurorae 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) aurorae 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.

The Sun is a star with some features that are highly variable on time scales of hours to hundreds of years. The interplanetary magnetic field direction and solar wind speed and density are driven by the activity on the Sun. They can change drastically and influence the geomagnetic activity. As geomagnetic activity increases, the lower edge of the auroral ovals usually move to lower latitudes. Similarly, solar mass ejections coincide with larger auroral ovals. If the interplanetary magnetic field is in the opposite direction of the Earth's magnetic field, there can be increased energy flow into the magnetosphere and thus, increased energy flow into the polar regions of the Earth. This will result in an intensification of the auroral displays.

Disturbances in the Earth's magnetosphere are called geomagnetic storms. These, in turn, can produce sudden changes in the brightness and motion of the aurorae called auroral substorms. The magnetic fluctuations of these storms and substorms may cause surges in electric power lines and occasional equipment failures in the power grid, resulting in widespread power outages. They can also impact the performance of satellite-to-ground radio communications and navigation systems. Magnetospheric storms can last several hours or even days, and auroral substorms can occur several times a day. Each substorm can deliver several hundred terajoules of energy, as much as the electrical energy consumed in the entire United States over 10 hours.

External Links

Nordlys - Northern Lights
Of all naturally occuring heavenly phenomena, few come close to a night with a magnificent northern lights display. Flickering curtains of dancing light against the dark skies, northern lights is certainly one of the most spectacular of nature's phenomena.

Aurora Webcam
Aurorawebcam.com is located at Mt. Aurora Fairbanks Creek Lodge in Fairbanks, Alaska. The premier aurora viewing location in the world. The aurora webcam will start broadcasting at 19:00 Alaska Time. The webcam will run until about 06:30 hours Alaska time. Unless otherwise noted on the news page.

Santa Television - Northern Lights
The northern lights are a typical phenomenon appearing in the Arctic, night sky in wintertime. They hold a certain mystery, as well as many scientific explanations, of their existence. At santatelevision.com you don't need to travel around the world to see the colourful display of the Aurora Borealis: these video clips bring them right to you.