Northern Lights

:D :D :D
Weatherman! The Aurora Borealis was out tonight! I've never seen them before, as I moved to this Southern clime as a child....

What are they, and does this mean anything in the bigger weather picture? Also, very unusual cloud formation that just sat in one spot just north of the big moutain today.

Does this mean a good and cold winter? Exceptional skiing perhaps?
The Aurorae (in the north and south) are essentially damned big flourescent light bulbs. Energetic particles in the Solar Wind are 'channeled' by the Earth's magnetic field and they descend into the polar regions. When the particles collide with atoms in the atmosphere they increase the energy of some of the electrons in atom. These, after a short time, return to their original energy level and emit light. A similar process is also used in lasers.

It is unusual to see an Aurora this far south, it is usually indicative of high levels of activity on the Sun's 'surface', such as large flares, sun spots, etc. There is an 11 year cycle to sun-spot activity, but AFAIK no-one knows why its 11 years and not some other number.

It does tend to mean that there are problems with long wave radio reception


HellonWheels said:
Does this mean a good and cold winter? Exceptional skiing perhaps?
It means "interesting" HF propagation and fluctuating earth potentials! (What do you expect from a Signaller?)

I remember the first time I saw the Northern Lights properly from Northern Canada. Awesome.
As the lights should be visible tonight, I think I'll head up to the moutains to see if I can get a more dramatic show without the perpetual twighlight.

Off to fill a thermos....... :D


Kit Reviewer
Sorry I didn't see your post till now HellonWheels, glad MikeMCC was there with the splan'in. I've been busy 'looking up'.

the view from coyoteville:

If any effect on weather...increased solar radiation...would seemingly enhance global warming...a chilling thought!

No way does it look like that???!!

I thought I saw it on a night move in BATUS this year, looked a bit green and wavey that was all, massively disappointed.

Why does it get better when the weather is colder? It was about freezing point the night we saw it.
What makes the color of the aurora?

The composition and density of the atmosphere and the altitude of the aurora determine the possible light emissions.

When an excited atom or molecule returns to the ground state, it sends out a photon with a specific energy. This energy depends on the type of atom and on the level of excitement, and we perceive the energy of a photon as color. The upper atmosphere consists of air just like the air we breathe. At very high altitudes there is atomic oxygen in addition to normal air, which is made up of molecular nitrogen and molecular oxygen. The energetic electrons in aurora are strong enough to occasionally split the molecules of the air into nitrogen and oxygen atoms. The photons that come out of aurora have therefore the signature colors of nitrogen and oxygen molecules and atoms. Oxygen atoms, for example, strongly emit photons in two typical colors: green and red. The red is a brownish red that is at the limit of what the human eye can see, and although the red auroral emission is often very bright, we can barely see it.

Photo by Jan Curtis
Photographic film has a different sensitivity to colors than the eye, therefore you often see more red aurora on photos than with the unaided eye. Since there is more atomic oxygen at high altitudes, the red aurora tends to be on top of the regular green aurora. The colors that we see are a mixture of all the auroral emissions. Just like the white sunlight is a mixture of the colors of the rainbow, the aurora is a mixture of colors. The overall impression is a greenish-whitish glow. Very intense aurora gets a purple edge at the bottom. The purple is a mixture of blue and red emissions from nitrogen molecules.

The green emission from oxygen atoms has a peculiar thing about it: usually an excited atom or molecule returns to the ground state right away, and the emission of a photon is a matter of microseconds or less. The oxygen atom, however, takes its time. Only after about a 3/4 second does the excited atom return to the ground state to emit the green photon. For the red photon it takes almost 2 minutes! If the atom happens to collide with another air particle during this time, it might just turn its excitation energy over to the collision partner, and thus never radiate the photon. Collisions are more likely when the atmospheric gas is dense, so they happen more often the lower down we go. This is why the red color of oxygen only appears at the very top of an aurora, where collisions between air molecules and atoms are rare. Below about 100 km (60 miles) altitude even the green color doesn't get a chance. This happens when we see a purple lower border: the green emission gets quenched by collisions, and all that is left is the blue/red mixture of the molecular nitrogen emission.

That explained it thanks.

Thought i was gonna get lots of witty replies along the lines of "try not looking at it through your night sight" or something.
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