tom thinks

Am I solid, obdurate, unmoved?
Or liquid that flows glittering in the sun?
Am I a gas, diffuse, the wind itself?
Yet none of these: a plasma I
Burning hot, fully ionized --
Radiating words
Unstable yet by your magnetic field
date 2000-10-29:21:27
Physics Following up on yesterday's comments, how come the aurora has different colors sometimes?

What you are about to see is speculation. It may be true. Then again, it may not. The world is full of stuff that sounds really plausible (name an "ism" -- it seems plausible to somebody. Really!) and is completely false.

I wrote yesterday that the aurora was caused by atoms emitting light when they decayed from excited states. The question is: what color of light do they emit? Most often, the aurora is white. Sometimes it's green, and rarely (and faintly) red.

To understand how this comes about, you need to understand a bit about how the quantum world works. The first thing is that unlike classical waves, light waves carry energy in proportion to their frequency:

E = h*f

where E = energy, f = frequency and h = Planck's constant, which is a really small number. Unlike classical waves, where the energy of the wave depends on its amplitude, quantum waves have an energy that depends on frequency alone. Frequency and wavelength are reciprocally related, so:

l = 2*pi*c/f

where c = the speed of light and l = the wavelength, so:

E = (2*pi*c*h)/l

That is, the larger the wavelength (l) the smaller the energy (E).

So fairly highly excited states emit light with middling wavelengths, which is green light, and states with only a little energy emit light with long wavelengths, which is red. If all sorts of states, from very high to very low enegies, were present, you'd get all sorts of wavelengths, and the light would be white.

This is starting to sound interesting, because we can now see that the colors of the aurora may have something to do with the excited states of the atoms that emit the light. But how come we don't get white light all the time?

This has to do with the rate of collisional de-excitation versus de-excitation by emission of light. All else being equal, atomic states with high excitation energy don't live very long compared to ones with low excitation energy [I managed to get this backward in the original copy, which means the following probably didn't make much sense.] That means if an atom is in a low-lying excited state in a moderately poor vacuum, the odds are good it'll live long enough to run into something before it can emit any light. A highly excited atom in the same poor vacuum is much more likely to emit light before it collides with anything.

The thinner the gas, the more likely it is we will see light from low-lying excited states -- that is, red light. But the thinner the gas, the less stuff there is to excite, so we expect red aurora to be dimmer, which they are.

But how come the aurora is sometimes red, sometimes green and sometimes white? How come sometimes we see red light from the thin gas in the very upper reaches of the atmosphere, sometimes green light from where the gas is denser and sometimes white light from still denser regions?

The answer lies in how energetic the particles are that have come from the sun in the first place. Sometimes we get a burst of particles that are only energetic enough to penetrate the very upper reaches -- this is a rare and finely tuned phenomenon, because only a fairly narrow range of particle energies will do. More energetic particles will reach more deeply, producing green light that swamps the red from the thinner regions above. And most often, the particles have energy and to spare, enough to slice deep enough into the atmosphere that atoms in the the long-lived red-emitting states never get a chance to decay, but are always collisionally de-excited, so we only see light from short-lived, high-energy states.

All of this is consistent with the observed behavior of the aurora. But it's all speculation. To make it otherwise would require a deeper look, which might include considering a model of an exponential atmosphere, it's stopping power for fast electrons and protons, the lifetimes of excited states in oxygen and nitrogen, and the mean time between collisions between atoms in an ideal gas of the composition found in the high atmosphere (which doesn't have quite the proportions found at lower altitudes, and indeed, an exponential atmosphere might not be a good model at such heights.) All of this is within the grasp of an undergraduate in physics, if any are interested.

An alternative would be to look the answers up in a book, but I've often found that working things out for myself, while it sometimes means barking up entirely the wrong tree, puts me in a much better state to understand the books when I get to them.
Creatures Caro asked how big my owl was, and I replied it was smaller than a 747 but bigger than a DC-3.

It was at dusk a double-decade ago, in late August or early September, and my brother and I had spent the whole day haying. We'd gotten the last of the bales in, using an old station-wagon with the roof removed as a make-shift pickup truck, and I was walking down to get something from the far field, when I saw it.

A great horned owl, gliding silently across the field in the half-light, looking so huge I immediately understood the stories of witches riding on their backs. Just then and there, a healthy teenage boy, I wouldn't have minded meeting one.

The owl flew on, taking no notice of me, surveying the field for mice, or possibly bears. It really did look big enough to take one. It only made one pass that I saw, then flew to richer pastures. Haying was going on all over the district, so the pickings must have been rich. A bird that size would need a lot of mice to keep itself going, even without a passenger.
Reading Started on Tennyson's Idyll's of the King and am enjoying it a great deal. I don't have a lot of time for Arthurian legend, although I treasure Steinbeck's The Act's of King Arthur and His Noble Knights despite of -- or possibly because of, given the way the story comes out -- it's unfinished state. That the same person could have written this as wrote The Pearl is one more monument to the flexibility and power of the individual.

Someone -- I can't remember who -- said that Tennyson knew everything about melancholy and nothing about anything else, and I'm not finding that with his Arthur at all. The weak iambic pentameter is remarkably readable as well -- I've been reviewing some of my early poetry and am a tad dismayed at my over-use of strong meter. But hey, I was just learnin'. Besides, it's fun.

I finally broke down and bought a real copy of Eliot's "The Waste Land", which I've been avoiding doing for a while -- it's such a lot of money to pay for a book that essentially only contains one or two poems. But it's such an unavoidable poem, and it's full of metrical sophistocation, to the point of sometimes getting beyond itself and losing its way. "Prufrock" is probably a better poem considered as a sustained effort, but the peaks of the "The Waste Land" are higher: "Gentile or Jew/O you who turn the wheel and look to windward/Consider Phlebas, who was once handsome and as tall as you."

I dislike the poem's dismal view of life, but can appreciate Eliot's motives. He was living in a pretty dismal time, at the beginning of the war that dominated the past century, fought from 1914 to 1989, beginning in Sarajevo, ending with the fall of the Berlin wall. But he missed the hope of individual redemption, even in the midst of collective dispair. I've often wondered if John Enright's "What Child" was intended to answer "Death By Water": "For feet that take such giant steps/Once were soft and small." Eliot missed out on the tenacity of beauty and life amidst the mid-industrial decay.

I was going to write more about The Count of Monte Cristo so I shall. The real fantasy Monte Cristo is living is that of independence from societal rules. His servant in the chapter XLIV (The Vendetta) says that once you've given up your own life your strength increases tenfold because of that, and Monte Cristo agrees with him. This is what these men have done, and what no other characters in the novel have done -- they have treated themselves as dead to the world, and pursued only the goals that matter to them.

We live in a society of faux rebels -- where rebellion is given lip service, but anyone who dares actually rebel is either reviled or ignored. I think there's probably less tolerance for diversity and eccentricity than there was a hundred years ago, although probably more than there was fifty years ago.

Conformity is the product of war, and the War Is Over. We need to grow into the space and freedom that that allows us.

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