Northern Lights | The Canadian Encyclopedia

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Northern Lights

Current theory states that the energy driving auroras is obtained from the solar wind, a gas composed primarily of protons and electrons, blowing away from the SUN at supersonic velocities of 300-1000 km/s (seePHYSICS).
Northern Lights
Northern lights are caused by energized electrons which accelerate towards the ionosphere from 5000-10 000 km above the Earth's surface (courtesy Canadian Tourism Commission & Ocean Images).
Aurora Borealis
The green or red light of the Aurora comes from excited atomic oxygen - purple light from excited molecular nitrogen (courtesy Canadian Space Agency).

Northern Lights

Northern Lights, or aurora borealis, dynamic displays of multicoloured luminosity appearing in the day or night sky in high latitudes in the Northern Hemisphere. At any instant of time, the auroras are arrayed along a band (the auroral oval) with the North MAGNETIC POLE near its centre. The band is normally about 500-1000 km wide, its average position in Canada being over Yellowknife, NWT, to the West, and Grande rivière de la Baleine, Qué, to the East. During intense activity the oval may expand as far south as Miami, Florida (August 1972), and beyond. The luminosity can fluctuate violently, particularly near local midnight when explosive bursts of activity called substorms are triggered. Auroras originate in the ionosphere, the upper atmosphere 100-300 km above the Earth's surface. They are caused primarily by energized electrons (1-20 kilo-electron volts) which are accelerated towards the ionosphere from a region 5000-20 000 km above the Earth's surface. These energetic electrons bombard the upper atmosphere and "excite" atmospheric constituent particles. When these particles return to less excited states, they give off light. Green or red light comes from excited atomic oxygen; purple light from excited molecular nitrogen.

Northern Lights
Astrophotography star trails on midsummer night sky with Aurora borealis or Northern Lights over shore willow bush at Lake Laberge, Yukon Territory.
Northern Lights
Intense bands of Northern lights or Aurora borealis or Polar lights dancing on night sky over boreal forest spruce trees of Yukon Territory. Photo taken on: August 28, 2013.
Northern Lights
Astrophotography star trails with green glowing display of Aurora borealis or Northern Lights over boreal forest taiga at Lake Laberge, Yukon Territory.
Intense display of Northern Lights Aurora borealis
Spectacular display of intense Northern Lights or Aurora borealis or polar lights over landscape of Yukon Territory. 31128908 \u00a9 Stephan Pietzko | Dreamstime.com
Northern Lights
Spectacular view from snow-covered bench: light clouds and Northern Lights or Aurora borealis on night sky over snowy winter landscape of Lake Laberge, Yukon Territory. 31128893 \u00a9 Stephan Pietzko | Dreamstime.com
Northern Lights
Intense Northern Lights in night sky over icy landscape of frozen Lake Laberge, Yukon Territory. 31128850 \u00a9 Stephan Pietzko | Dreamstime.com
Northern Lights
Northern Lights in Saskatchewan. Photo taken on: June 29, 2011. 23679284 \u00a9 Pictureguy66 | Dreamstime.com
Northern Lights
Northern Lights in Saskatchewan. Photo taken on: June 29, 2011. 23679273 \u00a9 Pictureguy66 | Dreamstime.com
Northern Lights
Northern Lights in Saskatchewan. Photo taken on: October 23, 2011. 23317129 \u00a9 Pictureguy66 | Dreamstime.com
Northern Lights, Yellowknife
The Aurora Borealis or Northern lights shine over Vee Lake near Yellowknife, Northwest Territories. Photo taken on: September 13, 2014.

Current theory states that the energy driving auroras is obtained from the solar wind, a gas composed primarily of protons and electrons, blowing away from the SUN at supersonic velocities of 300-1000 km/s (seePHYSICS). The solar magnetic field is embedded in the gas and is pulled by the wind deep into interplanetary space. The wind interacts with the Earth's magnetic field, distorting it to form the magnetosphere (ie, the comet-shaped cavity, blunt end towards the sun, carved in the interplanetary medium by the Earth's magnetic field). The magnetosphere is broken near each pole by a cleft region. Some solar-wind plasma, slowed and heated by interaction with the magnetosphere, enters the cleft region and penetrates directly to the ionosphere, causing the dayside auroras. However, most of the particles leaking into the magnetosphere are trapped behind the Earth, and through processes analogous to convection end up flowing towards the Earth. Some of these particles are accelerated into the nightside ionosphere, causing brilliant auroras.

The northern lights in the Yukon Territory.
Image: The Canadian Press/Phil Hart/Solent News / Rex Features.

The amount of energy leaking into the magnetosphere is regulated predominantly by the strength of the interplanetary magnetic field and its direction in relation to the Earth's magnetic field lines at the outer boundary of the magnetosphere. Activity also varies with sunspot and solar-flare occurrence. Parallel phenomena in the Southern Hemisphere are known as the aurora australis.

Auroras are the visible portion of the dissipation of the energy which has entered the magnetosphere from the solar wind. This energy is also dissipated through ohmic heating (ie, heat generated by electric current flow through a resistor). This process involves giant currents, sometimes in excess of one million amperes, which flow through the resistive ionosphere in the region of luminous auroras. These currents create magnetic fields which can make compass needles show direction incorrectly and which can cause surges in power lines resulting in electric-power outages.

The northern lights have haunted the imaginations of spectators for centuries. To the Inuit, the arsaniit are the sky people enjoying a ball game. Some Indigenous peoples view the lights as ancestral spirits dancing before the Great Spirit. Recently, Canadian researchers placed an ultraviolet imager aboard the Swedish Viking satellite and succeeded in obtaining a global view of the auroras and in following their rapid time variations.

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