Starship Asterisk*

by **Pete** » Tue Jan 08, 2008 7:53 pm

FieryIce wrote:
Thickening of the atmosphere is probably the best explanation.
So you are saying that in this train of particles the caboose is catching up with the engine. That does not add up.

That analogy doesn't capture what I meant. Here's my admittedly naive reasoning: assume that atmospheric density falls exponentially with height, or simply that it "gets thinner" with altitude. Incoming charged particles (mostly electrons, thanks Qev) would then have the opportunity to excite relatively few atoms at first. At lower altitudes, simply because more atoms are present, the auroral particles excite more atoms. Possibly, near the bottom of the auroral display, the Bragg peak becomes significant and the charged particles dump the rest of their energy into the air, dropping their K.E. below the ionization energy of atmospheric gas.

What's your take?

auroradude wrote: These graphs might add to the thread:

http://www.auroradude.com/auroralheight ... ution.html

Cool graphs! The sparse upper atmosphere seems to stretch the initial part of the Bragg curve:

(from Wikipedia)

by **FieryIce** » Tue Jan 08, 2008 3:44 pm

Try again bystander.

by **bystander** » Mon Jan 07, 2008 3:28 pm

FieryIce wrote:So you are saying that in this train of particles the caboose is catching up with the engine. That does not add up.

Sure it does. Happens all the time in train wrecks. Slow down or stop the front of the train, and the rear will catch up.

by **FieryIce** » Mon Jan 07, 2008 2:50 pm

Thickening of the atmosphere is probably the best explanation.

So you are saying that in this train of particles the caboose is catching up with the engine. That does not add up.

by **auroradude** » Fri Jan 04, 2008 10:57 pm

These graphs might add to the thread:

http://www.auroradude.com/auroralheight ... ution.html

The first plot is of several thousands of auroral height observations and the second is a graph based on these observations. I borrowed these from "The Polar Aurora" by Carl Stormer

Possibly the Bragg principle applies since a graph of it closely resembles the graph of height distribution of auroras. The fact that the atmosphere is denser at lower altitudes might only change the profile.

I might also point out the fact that ionized oxygen produces a red color at altitudes above about 100 miles and a green color below 100 miles is directly related to the density of the atoms as well as the energy of the particle stream.
The red is caused by less energetic particles that cannot penetrate the atmosphere below about 100 miles. This first excited state lasts about 110 seconds. Lower in the atmosphere a second excited state produced by higher energy particles produces the green. This process lasts only .74 seconds. Atoms here are also raised to the first excited state but are so dense that they collide within the 110 seconds required to release a red photon so the red emission is "quenched".
That there are red and green present at the same time would indicate a mixture of particles of different energies.

by **Qev** » Fri Jan 04, 2008 8:21 pm

I'm pretty certain aurora are caused by high energy electrons, but being charged particles, I think the Bragg peak still applies to them?

by **Pete** » Fri Jan 04, 2008 3:58 pm

craterchains wrote:this does not make a whole lot of sense

Sure it does! Read it again (carefully this time), or tell the proton therapy people at the Paul Scherrer Institut and at TRIUMF and elsewhere who save cancer patients' lives that the Bragg peak does not make a whole lot of sense.

That being said, the Bragg peak could be totally unrelated to the brightening at the bottom of auroral displays. It depends on whether the characteristic path length in atmosphere of aurora-producing particles (50 keV protons, maybe?) is comparable to the scale height of an average auroral display. Thickening of the atmosphere is probably the best explanation. craterchains, I'd love to hear yours!

by **Qev** » Fri Jan 04, 2008 5:51 am

I'm not sure what you're trying to say. The graphs are consistent with what the text is describing.

by **craterchains** » Thu Jan 03, 2008 10:11 pm

ahhh yes, the wikipee, and this does not make a whole lot of sense,

The interaction cross-section of nuclei passing through matter increases with decreasing kinetic energy.

Re-read what it says again, carefuly this time.
Note the graphs, , , ,

Damndest thing that wikkipee, but not the truth, any other guesses?

by **Qev** » Thu Jan 03, 2008 6:21 am

Pete wrote:Speculation: could the brightening at the bottom of the green rays be a manifestation of the Bragg peak? The interaction cross-section of nuclei passing through matter increases with decreasing kinetic energy.

EDIT: happy new year and happy perihelion day!

That's an interesting insight, Pete! It certainly makes sense!

by **iamlucky13** » Thu Jan 03, 2008 12:44 am

craterchains wrote:Note the brightest part is on the bottom.

Just a point of intresting physics, if you care to concider, , , why.

I think, although I'm not certain, it's because the brightest part of the aurora is furthest north. Consider the fact that the camera is looking over a curved horizon. If you visualize it, you'll see that for an object at a given altitude, the angle you view it at decreases with greater latitude difference.

by **Pete** » Wed Jan 02, 2008 4:46 pm

Speculation: could the brightening at the bottom of the green rays be a manifestation of the Bragg peak? The interaction cross-section of nuclei passing through matter increases with decreasing kinetic energy.

EDIT: happy new year and happy perihelion day!

by **Qev** » Wed Jan 02, 2008 5:42 am

Oxygen is actually responsible for both the red (high altitude) and green (low altitude) auroras, IIRC. Blues and purples are produced by ionized nitrogen.

by **BMAONE23** » Wed Jan 02, 2008 4:05 am

I think you might be correct about the density of the elemental hydrogen that is glowing red and the elemental oxygen that is glowing green. The lower the gases lie within the atmosphere, the denser the quantity per cm3.

by **FieryIce** » Wed Jan 02, 2008 3:07 am

Let me have a guess, uhmmm, more excited charged particles, hotter charged particles, different kinds of particles display a different colour, density of particles....

by **craterchains** » Wed Jan 02, 2008 1:32 am

Note the brightest part is on the bottom.

Just a point of intresting physics, if you care to concider, , , why.

by **FieryIce** » Tue Jan 01, 2008 2:23 pm

Rays from an Unexpected Aurora, Jan 1/08

captured from North Dakota

Damned, then here on Vancouver Island we should see these amazing lights. People are just going to have to turn off their lights at night and turn off the street lights for a few hours every night so humanity can view these amazing displays.

The unusal structure is very interesting.

Oh, btw, Happy New Year, looking forward to all new events.

Starship Asterisk*

Rays from an Unexpected Aurora (APOD 01 Jan 2008)

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