Is the Sun's photosphere fluorescent?

Is the Sun photosphere fluorescent ? I have seen some beautiful images with sunspots etc, filtered to particular spectral lines eg green O[III] and violet Ca for better contrast. I am wondering if those lines are Fraunhofer line absorbances or is there extra luminance from fluorescence at those wavelengths ?
I gather sunspots are dark because they are cooler because of the concentrated magnetic field (?) It would be nice to think, some elements are being excluded in sunspots acting like large mass spectrometers [ no doubt implausible, I realize :( ]

No, not really. Not to my knowledge at least. Fluorescence arises due to electronic transitions (electrons moving to higher and then lower energy levels) in molecules and thus requires a predominantly non-ionized medium. The Sun's photosphere is highly ionized with relatively few ions and electrons bound into atoms or molecules. While you might have an occasional molecule form, the density is far too low for any sort of fluorescence to show up in images. Especially vs the overwhelmingly bright thermal radiation emitted by the Sun.

I thought ions in plasma could fluoresce ?

How can they if they can't combine with electrons?

synch said:

I have seen some beautiful images with sunspots etc, filtered to particular spectral lines eg green O[III] and violet Ca for better contrast.

Can you please give us a link to those images, so we can also enjoy them.

synch said:

I am wondering if those lines are Fraunhofer line absorbances or is there extra luminance from fluorescence at those wavelengths ?

The Fraunhofer lines give the game away. They tell us that there is significantly more absorption in the photosphere, than there is fluorescence of that line.

synch said:

I gather sunspots are dark because they are cooler because of the concentrated magnetic field (?) It would be nice to think, some elements are being excluded in sunspots acting like large mass spectrometers [ no doubt implausible, I realize :( ]

Not mass spectrometre effect, but stars slightly hotter than Sun routinely have odd photosphere compositions. This is thought to be because in the absence of turbulent mixing, light pressure causes various elements to sink or float. But while magnetic fields suppress turbulence in sunspots, I have not heard it is stable enough.

Stars definitely have "emission lines" and "absorption lines". Sun has mostly absorption ones. How is the mechanism of emission lines different from "fluorescence"?

snorkack said:

Stars definitely have "emission lines" and "absorption lines". Sun has mostly absorption ones. How is the mechanism of emission lines different from "fluorescence"?

My limited understanding:

If you have a free ion and a free electron in a plasma that recombine, emitting radiation as the electron takes one or more steps downward to its ground state, that's not fluorescence.

Fluorescence requires that an electron first be excited by the absorption of EM radiation. For example, shining UV light at some molecules excites certain electrons into a higher energy level. Those electrons then typically fall one or more small nonradiative steps downward before taking a larger radiative jump, emitting a photon when they do.

See this diagram:



Any kind of luminescence, of which fluorescence is but one of many, seems to require a non-plasma state, as the electrons need to be excited by something first before falling back down in their energy states to emit light. This obviously can't happen in a plasma as the electrons are already unbound.

Really this is just a matter of classification. Light may be emitted any time electrons transition to lower energy states. The distinction is really in how the electrons were excited to begin with.

Drakkith said:

Any kind of luminescence, of which fluorescence is but one of many, seems to require a non-plasma state, as the electrons need to be excited by something first before falling back down in their energy states to emit light. This obviously can't happen in a plasma as the electrons are already unbound.

Plasma is only defined by having some unbound electrons. Which is in no way incompatible with having a lot of bound electrons. Like Sun has the notorious spectrum of coronium. Which is an ion... Fe¹³⁺. With 13 bound electrons.

Sure. My statement shouldn't be taken to mean that no atoms or molecules can ever exist in a plasma.

snorkack said:

How is the mechanism of emission lines different from "fluorescence"?

I'd bet that fluorescence was a term, introduced very early on, to describe those colours (on flowers and insects, perhaps) which look unnaturally more vibrant than you'd expect with simple pigments. The observation could have come way before the explanation of 'frequency shifting' of high energy em waves which we use now.

Flourescence is observed in reflection nebulae too; not such high temperature conditions.

As far as I know :
Fluorescence is the phenomenon where a high energy radiation is absorbed, the energy creates an excited state which then collapses with a lower energy radiation being emitted. It is routinely seen with UV being absorbed and visible light given off, eg typically by some metal ions. And I am assuming there is enough UV and Xray radiation around the photosphere to be absorbed by relevant elements whatever their ionisation state is.

The images I remember were in a magazine ( "Sky and Telescope" maybe, some time back ) so will be copyright but there are many around. The images are fairly standard. They show sunspots as looking like holes in a bright layer. The light from the structure in the layer, ie the wall of the "hole" is clearly visible. So the layer/wall is emitting that light, and the "hole" is clearly transmitting that light, as opposed to the idea that it is absorbing it.

If the hole was absorbing the light (as in a element absorbing the element Fraunhofer line ) it would obscure the image of the wall structure. On the other hand if it was emitting at that line it would also confuse the image. So ... ?I was interested as it has been a long-long-term project of mine to take images taken solely in element line frequencies, eg to create an image as a map of element distribution. The filters would be good but they still include the thermal background at that wavelength. So, maybe by taking another image just outside the specific line (which would be the background) and subtracting it digitally, the background could be removed, and element distribution could then be seen directly. So the side question arose, whether fluorescence would be a significant contribution. ( So said from someone who just found a 10" Newtonian mirror making kit - in its box still - that I bought about 40 years ago....as I said, it's a long term project )

synch said:

The images I remember were in a magazine ( "Sky and Telescope" maybe, some time back ) so will be copyright but there are many around. The images are fairly standard. They show sunspots as looking like holes in a bright layer. The light from the structure in the layer, ie the wall of the "hole" is clearly visible. So the layer/wall is emitting that light, and the "hole" is clearly transmitting that light, as opposed to the idea that it is absorbing it.

While I don't know what exact images you saw, sunspots do not transmit light from underneath them. They are red-orange hot and only appear dark because they are cooler than the surrounding photosphere, making them emit somewhat less light.

synch said:

I was interested as it has been a long-long-term project of mine to take images taken solely in element line frequencies, eg to create an image as a map of element distribution. The filters would be good but they still include the thermal background at that wavelength. So, maybe by taking another image just outside the specific line (which would be the background) and subtracting it digitally, the background could be removed, and element distribution could then be seen directly. So the side question arose, whether fluorescence would be a significant contribution.

You could certainly try this. It makes me curious how they get those images of specific elements in the Sun to begin with, such as this image which supposedly shows light emitted from calcium in the chromosphere:



From here: https://solar.physics.montana.edu/YPOP/Classroom/Lessons/Filters/Calcium.html
From the website it appears they simply take it through a narrowband filter, but who knows what other processing might have gone on.

I suppose I must amend my first response to: I don't know. Given that there are atoms in the Sun's photosphere, it seems like there must be some degree of luminescence, I just don't know how much.

synch said:

Is the Sun photosphere fluorescent ? I have seen some beautiful images with sunspots etc, filtered to particular spectral lines eg green O[III] and violet Ca for better contrast. I am wondering if those lines are Fraunhofer line absorbances or is there extra luminance from fluorescence at those wavelengths ?
I gather sunspots are dark because they are cooler because of the concentrated magnetic field (?) It would be nice to think, some elements are being excluded in sunspots acting like large mass spectrometers [ no doubt implausible, I realize :( ]

If you're asking about how and why the contrast increases, when narrowing in on very narrow Frounhofer lines, the simple answer is: It's because of Doppler shifts. By that I mean the Doppler effect.

The sun's atmosphere immediately above the photosphere will absorb light at very narrow lines described by the Frounhofer spectrum. The same is true higher up in the Sun's atmosphere.

But if a small chunk of the photosphere -- and here , the word "small" is relative: it might be an Earth sized chunk -- is moving toward or away from us, then from our frame of references, it's going to be absorbing at a slightly different wavelength: a wavelength different than what's being absorbed by the Sun's atmosphere much higher above it. In other words, in that case, some of the light at that wavelength that would normally be absorbed gets through.

Assuming nothing is moving on a particular chunk of photosphere, its light in that wavelength is mostly absorbed: it appears darker; And if it's moving, a lot more of passes through to us: it appears brighter. But only at that very particular wavelength. If nothing more is done, this subtle difference at that wavelength is washed out by the swath of sunlight at other wavelengths.

Now imagine a filter on the telescope that only lets in a very, very narrow band of the sunlight, having a bandwidth of less than 7 nm, maybe closer to 3 nm.[Edit: Etalons are actually closer to ~0.7 angstrom bandwidth. That's 0.07 nm. Really, really expensive ones are even less than that.] And you can tune that wavelength (the actual wavelength, not the bandwidth) back and forth to maximize this effect. In other words, this filter, called an "etalon," gets rid of the broad spectrum of light emitted by the sun, only letting through the very narrow band of wavelengths that exhibit this velocity dependency on the contrast.

With that, we can "see" velocity differences on the sun since different patches will show up brighter or darker, depending on their velocities.

---------------------------------

Sunspots, by the way, are a different matter. They are actually cooler and appear darker because they are in fact darker. You don't need the above equipment/principles to see sunspots.

[Edit: corrected a mistake above regarding the typical bandwidth of Etalons.]

So images seen through a oxygen(III) filter are only seeing the photosphere hotter oxygen, with the bulk main emission being absorbed by the upper layers?
BTW The image I remember (maybe incorrectly !! ) apparently showed structure in the wall of the sunspot, ie the photosphere around the perforation, lots of column shapes . I guess they are the "granules" described in an older thread - Aha !
How do we see the main line emissions ? Is that light all from the upper layers ?

synch said:

How do we see the main line emissions ? Is that light all from the upper layers ?

The black body radiation from the extra-high temperature plasma photosphere, swamps the line emissions that come from cooler and less-well ionised plasmas deeper in the surface.

synch said:

So images seen through a oxygen(III) filter are only seeing the photosphere hotter oxygen, with the bulk main emission being absorbed by the upper layers?

Well, it's a combination of that, along with the very, very narrow bandwidth of the etalon.

By the way, I made a mistake in my last post regarding the bandwidth of a typical etalon. Originally I stated that they are typically a few nanometers in bandwidth, but what I should have said is that they are typically a fraction of an angstrom in bandwidth. That's very narrow.

As you know, if you shine white light through a cold gas you'll see absorption lines in the spectrum. We see this with the sun. But these absorption lines are smeared out somewhat due to the Doppler shift of the gas moving around at or near the "surface" of the sun (by "surface," of course I don't mean a solid surface, rather I mean the photosphere).

But if you observe the sun without an etalon, instead looking at the broad spectrum, you won't see much contrast since the black body spectrum totally dominates, washing out any effect of any Doppler shift. (@Baluncore mentions this in the above post.)


Figure 1. White light solar viewing. You'll still be able to see sunspots, but the contrast on the photosphere is nothing to write home about.

Now consider viewing the sun with an etalon that blocks out nearly all the sun's light with the exception of less than 1 angstrom of bandwidth, centered right around one of the absorption lines.

If a particular part of the sun's surface (and surrounding gas) produces an absorption line smack in the middle of the etalon's passband, that portion of the sun will appear a little bit dimmer. That's because that wavelength of light was absorbed in that portion of the sun, and etalon blocks out all the other wavelengths, so there isn't as much that gets through to the eyepiece or camera (relatively speaking).

Now consider a different portion of the sun's surface that's moving toward or away from you. That absorption line is now shifted, possibly outside the etalon's passband. That means that the light at the etalon's passband wavelength isn't blocked by the cooler gas. So that portion of the sun appears relatively brighter.


Figure 2. H-Alpha Solar Viewing. Here you can see a big boost in contrast. By the way, I'm sure the orange color in the photo was added in post processing; I'm guessing the image was obtained using a monochrome camera. The contrast is what's important here, not the orange color.

Etalons are commonly tunable, like with a dial or knob or whatnot, allowing you to tune into just the right Doppler shifted absorption line to get the most contrast of whatever feature on the sun you're interested in at that particular time. They're also very expensive.

(Images grabbed from Lunt Solar Systems, https://luntsolarsystems.com/)

Thanks for the fantastic explanations and help ! I was vaguely aware of etalon filters, but didn't realise how narrow the passband was. They are certainly precise and expensive.
Scratching around the net a bit, I can see there are (amateur level) oxygen(III) filters at ca 540nm/7nm width, and as it happens also some at about 520nm/20nm width. I was considering taking simultaneous images using the filters and using the 520 for the thermal background image and subtracting it from the 540 image. But now I gather the 520 frequency is relevant for the green laser pointers with the frequency-multiplying setup, (an erbium line from memory ?) so I hope there is not a lot of erbium in the Sun :) maybe there are better combinations near the other element lines, I haven't looked at them yet.

Edit - added -
BTW the images I saw are in "[Australian ed] Sky and Telescope" Jan/Feb 2023, a good article on solar photography by Sean Walker

I would guess that it's something about the Sun's abnormal behavior (sunspots, gravitational field, etc) but I may be wrong.
Definitely not the corona because it is made of plasma.
Could be solar wind interference or other still-not-understood phenomenon.

EventHorizon said:

I would guess ... but I may be wrong.

Then it is better not to guess.

EventHorizon said:

the Sun's abnormal behavior (sunspots, gravitational field)

Other stars have starspots and gravitational fields. Why is this abnormal?

collinsmark said:

They are actually cooler and appear darker because they are in fact darker.

No, they are darker because they are cooler than the surrounding photosphere, around 1000K cooler so the darker observations is purely a contrast between a hotter and cooler region

I think Drakkith mentioned earlier, if seen on their own, they wouldnt be black

Dave