# Northern lights - How?

1. Apr 19, 2006

### gem0688

Hi,

I have allways loved the look of the northern lights, but have never quite understood what actually causes them!

Could anyone possibly explain it to me?

Thanx

2. Apr 19, 2006

### EL

3. Apr 19, 2006

### gem0688

Ok , i understand a bit, but i am still a bit confused.

They are caused by electrons of a certain energy which collide with atoms. Yes or no? And then how are the colours produced?

Thanx

4. Apr 19, 2006

### Hootenanny

Staff Emeritus
Have you heard about quantum physics? Light spectroscopy for example?

~H

5. Apr 19, 2006

### gem0688

yea, i did quantum physics a few years ago so a bit rusty!! (all that photeletric effect and E=hf. Yea?!)

6. Apr 19, 2006

### Hootenanny

Staff Emeritus
Yep, the formula you gave, $E = hf$ is important here. Now if there are free electrons in the atmosphere, what would happen if one of these electrons were to collide inelastically with an electron of, say an oxygen atom in the atmosphere?

~H

7. Apr 19, 2006

### gem0688

Ok, well in an inelastic collision part of the kinetic energy will be changed to other forms of energy and so the electron will slow down. If it was to collide with an oxygen atom, the oxygen's electrons will become excited and so realease energy as light.

Ohhh i think i am getting there now. So then the colour of light will depend on the energy, frequency (because of E=hf) and the atom it has just collided with?

Still confused though on what makes them move around?

8. Apr 19, 2006

### Hootenanny

Staff Emeritus
Almost there, they will release energy when they drop back down energy levels.

It will depend on the difference between the energy levels.

The aura of the northern lights is caused by the solar winds (a stream of plasma carrying ionised positve ions and electrons), they interact with the earth's magnetic field.

~H

9. Apr 19, 2006

### gem0688

Ok, I was nearly there!

so does the colour of light emmited depend on how big the drop back down between energy levels? Does it also depend on how high the collision takes place?

So they seem to 'dance' in the sky because the earths magnetic field generated by the earths core interacts with the solar winds which pulls thems in different directions?

Thanx again

10. Apr 19, 2006

### Hootenanny

Staff Emeritus
Yes and yes. RE the second point, I'm not sure why this happens, perhaps do to the different composition of the atmosphere at different altitudes.

From thinkquest;
"As the wind is thrown thousands of kilometers out into space where it later collides with the earth's magnetic field, it will produce a powerful force while buffeting and distorting the shape of the earth's field. The gusts of the solar wind and the ripples they cause in the earth's magnetic field lead to the common brightening, fading, and dancing of the northern lights. Therefore, the greater the solar wind is, the more active the lights will be."

~H

11. Apr 19, 2006

### scott1

Isn't the Northen also caused by how the earth is titled and the angle of light?

12. Apr 19, 2006

### gem0688

Ohh ok, so it all depends with what it is colliding with at different levels. So for example, at a lower level there may more oxygen collisions compared to another part.

I think i am getting there at last!!

13. Apr 19, 2006

### gem0688

Would this be beacuse of the light gets split up when it hits parts of the earth's atmosphere at different angles?

14. Apr 19, 2006

### scott1

It might but I'am not sure.

15. Apr 19, 2006

### Labguy

The most "favorable" times of year is explained http://science.nasa.gov/headlines/y2001/ast26oct_1.htm" [Broken].

Ant http://www.atoptics.co.uk/" [Broken] is cool for about every other type of atmospheric phenonenom.

Last edited by a moderator: May 2, 2017
16. Apr 19, 2006

### gem0688

Thanx, had a look at that site, was good. The pictures of the northern lights are really good. Also found the animation of how the magnetosphere is squashed helpful.

So it is because the electrons escape from the magnetosphere into our atmosphere, where the collisions with atoms occur.

Thanx

Last edited by a moderator: May 2, 2017
17. Apr 19, 2006

### gem0688

ohhh and the collisions cause energy to be released when they drop back down energy levels, which causes the colour! ;)

18. Apr 21, 2006

### Schrodinger's Dog

The colour is due to the composition of gases in the atmosphere, the green yellowish hues come from the emission spectrum of oxygen where as the blues and reds are the result of the emission spectrums of Nitrogen ions.

19. Apr 21, 2006

### gem0688

Ohh now i see, so each different gas gives off a different colour.

Thnx Schrodinger's Dog

Gema

20. Apr 21, 2006

### BornLogician79

When I first thought about the northern lights, for some reason I was under the impression that it was gravity deflecting light relative to how a prism bends/divides light, in combination with the gas difference. Gravity of north pole deflect (c) and gas breaks light up???
Very nice display!