What are the Northern Lights?

What are they, how are they caused, what is the magnetosphere

1 Answer
Nov 10, 2017


OK ... not simple, but here goes.


I’ll explain a bit about the Sun, then a bit about the Earth, then put it all together.

The sun, whilst it appears to be a gently warming & beneficial, yellowish orb that reappears regularly (unless you live where I do) is actually an unconstrained nuclear fusion reaction of screaming intensity. The power it puts out as radiation is literally unimaginable.

Along with electromagnetic radiation of all frequencies, it also continually emits a stream (doesn’t convey the intensity I’m after, try torrent) of charged particles out into space. These charged particles are particularly intense when there are many sunspots. [It seems to have an 11 year cycle, but I’ll leave that bit.]

Next we need to understand our side. The earth’s outer core is a molten liquid, rich in iron. Due to the heat released by nuclear decay (predominantly in the inner core) there are strong convection currents circulating it. Recent evidence (https://phys.org/news/2016-12-satellites-jet-stream-earth-core.html) suggests this motion is much more energetic than previously thought. The motion of this conducting fluid gives rise to our magnetic field, which extends outwards into space. This is the magnetosphere, the region of space where earth’s magnetic field dominates that of the Sun.

When these charged particles emitted by the Sun, moving at millions of metres per second, collide with the earth’s magnetic field they are made to spiral in towards the poles. As they descend, still moving at tremendous velocities, they collide with atoms and molecules in the atmosphere.

This enetgises the molecules (lifts their electrons into higher orbits) and as they tumble back down, the electrons’ energy is released as light. If the molecule happens to be nitrogen (it often is, as nitrogen makes up about 78% of the air) then a red, violet or blue colour is seen. Oxygen molecules (the majority of the remaining atmospheric particles) tend to produce green or yellow colours.

Here’s the mechanism:

enter image source here

Taken from: http://earthsky.org/earth/what-causes-the-aurora-borealis-or-northern-lights

The whole show appears from the surface of the earth like curtains wafting in a breeze. I’m told it is exceptionally beautiful, but despite years of trying, have yet to see it.