What Causes the Different Colors of the Aurora?

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In summary: But I will probably read it sooner or later!Thank you!In summary, the colors of the Aurora are caused by the interaction between charged particles ejected from the sun and the Earth's magnetic field. These particles excite elements in the air, mainly nitrogen and oxygen, which then emit light in the form of different colors. The colors are not uniform due to variations in altitude and the irradiation not being consistent. The process of ionization and recombination also plays a role in the different colors seen. Additionally, solar radiation is not all the same kind, with lower energy radiation being stopped higher in the atmosphere and higher energy radiation being stopped lower. The phenomenon of gas discharge tubes also helps to explain the separation of colors in the Aurora
  • #1
rogerk8
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Hi!

I wonder what makes the Aurora colors.

This is what I naively believe I know:

Charged particles are ejected from the Sun and hits our protective magnetic field eight minutes later.

At the poles, the magnetic flux density, B, is the greatest.

Particles coming from the Sun are intercepted by B and gyrates around the lines of force.

Close to the poles B is the greatest, which means a higher radial frequency of the particles.

This excites the elements of our air (mainly Nitrogen and Oxygen).

The ejected particles excites them in such a way that when they first hit them, they are excited but later on they lose this excited energy and sends out the difference according to hf (=hc/λ) which is the Aurora light we see.

The Aurora is however not uniform in color.

And gasous vacuum tubes, where air has leaked inside, glows pink.

I therefore wonder two things:

1) Are the diffent colors (red and green, as far as I understand) due to Nitrogen and Oxygen excitation accordig to above?
2) How come the colors are separated in the sky (and not pink or some mixture)? This obviously means that there is some kind of gradient or separate distribution in the sky while ordinary air seems to be quite homogenous.

On the other hand I have heard something about an ionsphere up there too...

Roger
PS
Please give me a physical explanation and not too much of a layman explanation.
 
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  • #2
Please see:
http://ffden-2.phys.uaf.edu/211.fall2000.web.projects/Christina Shaw/AuroraColors.html
You have the basics - but the devil is in the details.
i.e. the colors depend on altitude as well as atom, and the separation of the colors is because they happen at different altitudes and the irradiation is not uniform.

As for the discharge tube:
Oxygen suppresses long electron cascades in the closed environment of a discharge tube - which is why the spectrum gets weaker when there's a leak - and tends to the red end (shorter - low energy - transitions).
 
  • #3
Simon Bridge said:
Please see:
http://ffden-2.phys.uaf.edu/211.fall2000.web.projects/Christina Shaw/AuroraColors.html
You have the basics - but the devil is in the details.
i.e. the colors depend on altitude as well as atom, and the separation of the colors is because they happen at different altitudes and the irradiation is not uniform.

Hi Simon!

Thank you for your reply!

Viewing this beautiful picture

110600_1.jpg


while reading the interesting information in the same nicely provided link, I come to the conclusion that Oxygen at high altitudes is responsible for the red glow and Oxygen at lower altitudes is responsible for the green glow.

But how can this be?

How can the same molecule/atom be responsible for two different colors?

Can the ionization/recombination be in several steps?

Let's think of this in layman terms. Oxygen is barely excited at higher altitudes for some reason (say that only one electron is leaving the "atom"). But at lower altitudes more energetic particles exists somehow and excites the Oxygen even more (say that two electrons leave the "atom").

By the miracle of recombination (which is frequently used but not really understood) the Oxygen residing in the upper altitude sends out red light and the Oxygen in the lower altitude sends out green light.

Might it perhaps be something like this?

The only thing I then do not understand is how an atom missing one or more electrons has a higher energy state than a neutral atom of the same type.

Because the energy of light (hf) sent out, and this is one of the few things I know, is the difference between the excited (=ionized?) energy level and its neutral/rest energy level.

To me an excited energy level of an atom would mean that the electron still hangs in there but at a higher "mgh". Which is rediculous, right?

I really do not understand this recombination part that "makes light".

So what I would like here is an equation for the excited/ionized energy level as well as the rest energy or at least the difference.

This would make my day :smile:

As for the discharge tube:
Oxygen suppresses long electron cascades in the closed environment of a discharge tube - which is why the spectrum gets weaker when there's a leak - and tends to the red end (shorter - low energy - transitions).

Here I really do not understand what you are referring to. I have never spoken of any "discharge tube". I have only spoken of the fact that when vacuum tubes are leaking they glow pink.

But perhaps this is what you mean with a "discharge tube"?

Anyway, your nice explanation of the fenomena told me absolutelly nothing :wink:

Take care!

Roger
 
  • #4
rogerk8 said:
How can the same molecule/atom be responsible for two different colors?
...they are not exactly the same
[displaying] Spectrum of oxygen O (two lines in the red region) and O+ (the blue and purple lines).
-- http://www.itp.uni-hannover.de/~zawischa/ITP/atoms.html

... add to that: solar radiation is not all the same kind.
Low energy radiation gets stopped high in the atmosphere, higher energy gets stopped lower.

Here I really do not understand what you are referring to. I have never spoken of any "discharge tube". I have only spoken of the fact that when vacuum tubes are leaking they glow pink.
It's an example - the effect is obvious in gas discharge tubes: look it up.

I may have misunderstood you though:
What are you calling "vacuum tubes" - I'm thinking either valves or CRTs.
How do you see the pinkish glow and where?
 
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  • #5
Simon Bridge said:
Low energy radiation gets stopped high in the atmosphere, higher energy gets stopped lower.

Very interesting information. This explains why the more high energic photons are emitted at lower altitude.

It's an example - the effect is obvious in gas discharge tubes: look it up.

I will.

I may have misunderstood you though:
What are you calling "vacuum tubes" - I'm thinking either valves or CRTs.
How do you see the pinkish glow and where?

I was thinking of this phenomena:

purpleglow.jpg


which is visible in leaking vacuum tubes. The picture is by the way fetched from another thread of mine namely:

https://www.physicsforums.com/showthread.php?p=4631652&posted=1#post4631652

You may want to look it up :wink:

Finally, I loved the link you so thankfully have supplied. Lots of interesting things to read!

However, I am a quite lazy guy who likes these "social" discussions more than reading.

So I like trying to understand things without reading too much "useless" information because there is lots of "noise" out there.

You could spend a lifetime reading stuff but never feel that you grasp it all.

And that's the fun of living.

But I am mostly interested in plasma physics.

And I need to focus on that before I get old :wink:

Thanks for your reply!

Roger
 
  • #6
The 6L6 GC (pictured) is a guitar-amp valve
... it is not a vacuum tube as it contains gas (mercury vapor iirc).
Purple-pink and orange glows are part of normal operation - but I think it's supposed to be bluer than that.

A bad seal can produce a muddier pinkish-orange due to trace oxygen and nitrogen, as described for gas discharge tubes.
 
  • #7
Simon Bridge said:
The 6L6 GC (pictured) is a guitar-amp valve
... it is not a vacuum tube as it contains gas (mercury vapor iirc).
Purple-pink and orange glows are part of normal operation - but I think it's supposed to be bluer than that.

A bad seal can produce a muddier pinkish-orange due to trace oxygen and nitrogen, as described for gas discharge tubes.

A 6L6 is a normal vacuum tube, the color in his tube picture is abnormal.
http://www.radiomuseum.org/forum/beam_power_tetrode.html
 
  • #8
Link reference needs to be more specific
... don't know what it is supposed to show or which part you are referring to.

I don't think we are actually disagreeing though: if you've got a better answer to OPs question, I'd love to hear it... it's why I'm here after all...

A 6L6 is a normal vacuum tube...
I'll take your word for it.

I wanted to make a distinction between a "vacuum tube" (I know them as "thermionic valve"s) and a "tube designed to have a vacuum in it" - reason: see context above. I don't know what you mean by "normal" in this context.

Maybe I'm just not being clear:
The 6L6GC I am thinking of is an audio-frequency power pentode - commonly found, these days, in electric guitar amp boxes. (see below)

You'll find a lot of chatter on the guitar amp groups about their blue/purple glow in operation.
The blue glow is characteristic of tubes using mercury vapor as opposed to a simple vacuum action.
Without the vapor, the only part that should glow at all is the heater (on the cathode).

But I don't have one handy to check - so I could be mistaken.

I do have my own refs though:

A FAQ: http://www.rru.com/~meo/Guitar/Tubes/blue_glow.html

... also see specs.
http://www.tubeampdoctor.com/images/File/TAD-Datasheet_6L6GC-STR_Version1.3%2008.07.11.pdf
... notice the pic shows a clear orange glow from the heater? The purple tinge is not evident though.

I believe the Sylvana Engineering Data Service reference in FAQ is available in docstoc:
http://www.docstoc.com/docs/81418143/SYLVANIA-engineering-data-service

... the color in his tube picture is abnormal.
I agree - it should be bluer - you wouldn't normally get the 6L6GC with argon or neon, which is the other way you see pinky or reddish glows in the gas.

There are plenty of pics of the tube in normal operation - you'll see that the exact color of the glow varies with manufacturer and use.

I don't see a disagreement.
 
  • #9
Simon Bridge said:
Link reference needs to be more specific
... don't know what it is supposed to show or which part you are referring to.

I don't think we are actually disagreeing though: if you've got a better answer to OPs question, I'd love to hear it... it's why I'm here after all...


I'll take your word for it.

I wanted to make a distinction between a "vacuum tube" (I know them as "thermionic valve"s) and a "tube designed to have a vacuum in it" - reason: see context above. I don't know what you mean by "normal" in this context.

I don't see a disagreement.

Not much of a disagreement really but a clarification.

Normal in this case would be a "6L6 type thermionic valve" with very little residual gas. When I was younger I rebuilt a quite a few tube audio amplifiers. (some were even for guitars :wink:) Mercury vapor tubes were normally only seen in the rectifier sections or as power switches as they are very non-linear.

As I was explaining in the other thread the 'normal' blue glow in receiving type tubes is not from gas ionization, it's from FLUORESCENCE per your link:

A FAQ: http://www.rru.com/~meo/Guitar/Tubes/blue_glow.html

Q: "Does the blue glow mean my tubes are 'gassy'?"

A: No. Gassy tubes are characterized by excessive grid currents and other poor performance characteristics. Extremely gassy tubes may glow with a pinkish cast throughout the entire tube. A faint blue glow on the glass, or inside the plate structure is completely normal. In fact tubes exhibiting this glow are frequently lower in residual gas.

Sorry for side-tracking the thread on Aurora, your information on them was pretty cool.
 

1. What causes the different colors in the Aurora?

The colors in the Aurora are caused by charged particles from the sun colliding with gases in the Earth's atmosphere. Different gases emit different colors when they are energized by these collisions, resulting in the variety of colors seen in the Aurora.

2. Why do Auroras sometimes appear as green and other times as red or purple?

The color of the Aurora depends on the type of gas involved in the collision. Oxygen molecules produce green and red light, while nitrogen molecules produce purple and blue light. The altitude at which the collision occurs also affects the color, with green and red appearing at lower altitudes and purple and blue at higher altitudes.

3. What is the role of the Earth's magnetic field in the formation of Auroras?

The Earth's magnetic field plays a crucial role in the formation of Auroras. It acts as a shield, protecting the Earth's atmosphere from the solar wind. However, in areas where the field is weaker, such as near the poles, some of the charged particles from the sun can enter the atmosphere and collide with gases, creating the Auroras.

4. Can the Auroras be seen from other planets?

Yes, the Auroras can be seen on other planets as well. On Jupiter, for example, the Auroras are caused by particles from its own moon, Io, instead of the solar wind. On Saturn, the Auroras are created by particles from its moon, Enceladus. Other planets with Auroras include Uranus and Neptune.

5. How often do Auroras occur and where is the best place to see them?

Auroras occur regularly, but they are not always visible from Earth. They are most commonly seen in the polar regions, especially during periods of high solar activity. The best places to see Auroras are in Alaska, Canada, Norway, and other locations near the Arctic and Antarctic circles. However, they can also occasionally be seen in more southern locations during particularly strong solar storms.

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