Expert: Philip Stahl - 1/6/2005

Question
hi. i think i read somewhere that the northern lights were the result of a solar jet stream or something and i was wondering... what are the northern lights, and why can you only see them closer to the north pole? is it a result of something from the sun? why do they actually make the light they do, and why are they the colors that they are? i really want to know why they are what they are. you said you could help with questions about the sun and its effect on the earth, so i thought you might be the right person to ask. thanks in advance.

Answer
Hello.

You can visualize the Earth as a giant spherical magnet, with magnetic field lines extending from its north to south magnetic poles. These magnetic field lines, have the property that any charged particles (+ protons, - electron or ions) that approach, will spiral along them.

The Earth itself, is "bathed" in the solar wind, a stream of high speed charged particles that flows into space, originating from the Sun's corona. Around the Earth the speed of these particles can reach 400- 500 km/second. (Because of its high temperature, over a million degrees, the corona gas is *ionized* so must consist of charged particles, mainly (+) protons, and (-) electrons).

During high solar activity (e.g. near sunspot maximums) a higher flux of these charged particles inundates the solar wind, and the region around the Earth.

The Earth's magnetic field traps these charged particles, and the highest density is around the polar regions - which we refer to as the "auroral ovals". In these regions, very high electric currents are set up, as the charged particles start moving in unison about the magnetic field lines. These currents can easily reach a few MILLION amperes.

Now the atoms of the upper atmosphere, though normally UNCHARGED or neutral, can become discharged when exposed to the incoming currents- for example once a power of a million or more MEGA-WATTS is attained. (1 mega -watt = 1 million watts, so 1 million mega-watts = 1 MILLION MILLION watts)

As this discharge occurs, one or more outer elecrons is stripped from the atoms, for example from oxygen in the atmosphere - then RECOMBINES again - to form new )e.g. oxygen) atoms.

With this RECOMBINATION - there is EMISSION of light, for a certain part of the visible spectrum.

For example, in the case of recombination of oxygen atoms - their emitted light is in the GREEN region of the spectrum. The aurora or northern lights we see displays a kind of green curtain-like shimmering. Other colors (e.g. red) can be produced when other atoms in the atmosphere undergo recombination in the way I already described for oxygen).

Thus, multiple colors of the aurorae can be explained by emissions from different atoms in the upper atmosphere, mainly in the region of the magnetic poles. This is also why, of course, they are more often seen in the vicinity of the N, S magnetic poles. (Though there have been reports of N. lights being seen as far south as northern Florida, especially during periods of esxceptional sunspot activity or LARGE SOLAR FLARES, massive explosions on the Sun)

This is only the most basic of explanations, since a more detailed one requires some knowledge of atomic physics to understand. Including the basic principles of atomic spectra, and specifically how absorption and emission (spectral) lines are produced.

The preceding is also deliberately MORE detailed than most "explanations" for the northern lights - which inevitably border more on cartoons, than proper accounts!