What Causes The Aurora Borealis?


2 Answers

Stella Reardon Profile
Stella Reardon answered
Auroras result from 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 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. Green is the most common of all auroras. Behind it is pink, a mixture of light green and red, followed by pure red, yellow (a mixture of red and blue), and lastly pure blue.
E Jacobson Profile
E Jacobson answered
The Aurora Borealis are often referred to as the Northern Lights. They are caused by the sun throwing solar flares which thrust billions of tiny particles through space. Every particle is electrically charged and any that come in the vicinity of the North Pole are drawn towards it through the magnetic field of the North Pole.
The particles then mingle with the air and this results in the oxygen in the air turning green and shimmering, following an electro-magnetic reaction with the particles. From Earth we see this as the Aurora Borealis or Northern Lights.
In Britain the best places to see the Northern Lights are in the north of Scotland (although they are sometimes visible in southern regions).
Both the Orkney and Shetland Isles can enjoy spectacular displays of the Norther Lights, with the whole sky seemingly draped with shimmering green curtains which almost seem to dance in the sky.

Answer Question