Captain's Cabin

Pirate Lord of the Platinum Coast
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Dogs have owners, Cats have staff.
Cleo on the Beach - Dianna
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Monday, November 30, 2009

Myth Monday - The Northern Lights

Northern Lights


Auroras, sometimes called the northern and southern (polar) lights or aurorae (singular: aurora), are natural light displays in the sky, usually observed at night, particularly in the polar regions. They typically occur in the ionosphere. They are also referred to as polar auroras. In northern latitudes, the effect is known as the aurora borealis, named after the Roman goddess of dawn, Aurora, and the Greek name for north wind, Boreas, by Pierre Gassendi in 1621.

The aurora borealis is also called the northern polar lights, as it is only visible in the sky from the Northern Hemisphere, the chance of visibility increasing with proximity to the North Magnetic Pole, which is currently in the arctic islands of northern Canada. Auroras seen near the magnetic pole may be high overhead, but from further away, they illuminate the northern horizon as a greenish glow or sometimes a faint red, as if the sun were rising from an unusual direction. The aurora borealis most often occurs near the equinoxes; from September to October and from March to April.

The northern lights have had a number of names throughout history. The Cree people call this phenomenon the "Dance of the Spirits." In the middle age the auroras has been called by sign of God (see Wilfried Schröder, Das Phänomen des Polarlichts, Darmstadt 1984). Auroras can be spotted throughout the world. It is most visible closer to the poles due to the longer periods of darkness and the magnetic field.

Auroras are the result of the emissions of photons in the Earth's upper atmosphere, above 80 km (50 miles), from ionized nitrogen atoms regaining an electron, and oxygen and nitrogen atoms returning from an excited state to ground state. They are ionized or excited by the collision of solar wind particles being funneled down, and accelerated along, the Earth's magnetic field lines; excitation energy is lost by the emission of a photon of light, or by collision with another atom or molecule:

oxygen emissions
Green or brownish-red, depending on the amount of energy absorbed.

nitrogen emissions
Blue or red. Blue if the atom regains an electron after it has been ionized. Red if returning to ground state from an excited state.

Oxygen is a little unusual in terms of its return to ground state, it can take three quarters of a second to emit green light, and up to two minutes to emit red. Collisions with other atoms or molecules will absorb the excitation energy and prevent emission. The very top of the atmosphere is both a higher percentage of oxygen, and so thin that such collisions are rare enough to allow time for oxygen to emit red. Collisions become more frequent progressing down into the atmosphere, so that red emissions do not have time to happen, and eventually even green light emissions are prevented.

This is why there is a colour differential with altitude; at high altitude oxygen red dominates, then oxygen green and nitrogen blue/red, then finally nitrogen blue/red when collisions prevent oxygen from emitting anything.

Auroras are associated with the solar wind, a flow of ions continuously flowing outward from the sun. The Earth's magnetic field traps these particles, many of which travel toward the poles where they are accelerated toward earth. Collisions between these ions and atmospheric atoms and molecules causes energy releases in the form of auroras appearing in large circles around the poles. Auroras are more frequent and brighter during the intense phase of the solar cycle when coronal mass ejections increase the intensity of the solar wind.

Seen from space, these fiery curtains form a thin ring in the shape of a monks tonsure.

(I wonder why you only see them by North Pole. Now I know.)

1 comment:

Khyra The Siberian Husky And Sometimes Her Mom said...

Mom gets all 'awwwwwwww' about those kholours!

Hugz&Khysses,
Khyra