Are blue and red wings of hydrogen alpha basically equivalent?

[daniele_bonfiglio]

Dear all,
I have a Daystar Solar Scout and it looks that the Quark in it is in the opposite condition of what described in this wonderful test reported by Alexandra https://solarnutcase.livejournal.com/12257.html

I mean, my Quark is on band in the almost counter clock position of the dial (towards the blue) and therefore it would be possible to look at the red wing (instead of the blue wing as in Alexandra's test).

So my question is: are the red and blue wings equivalent in terms of the features that can be observed and the altitude above the photosphere of the observed layer? (I understand the H alpha wings allow to look at the lower chromosphere while the H alpha center shows the higher chromosphere layer). Of course regarding the Doppler effect the red and blue wings will not be equivalent but opposite.

Best, Daniele

[christian viladrich]

Hello Daniele,

The answer is yes :-)

Look for example slides 30 to 40 :

https://www.hao.ucar.edu/partnerships/h ... harmer.pdf

Best

Christian

[daniele_bonfiglio]

Dear Christian,
Thank you very much for your answer and the very interesting link!
By the way, I am really enjoying the Solar astronomy book edited by you, it is a wonderful resource!
Best, Daniele

[christian viladrich]

Thanks Daniele :-)

[DavidP]

Thanks for asking Daniele. I’ve wondered the same.
And thank you Christian for the link. Great info

[p_zetner]

Quote Daniele:

Are blue and red wings of hydrogen alpha basically equivalent?

Quote Christian:

The answer is yes :-)

I'll have to offer a somewhat dissenting opinion here. Depending on your use of the word "basically" I suppose Christian's answer is valid. For the purpose of tuning a quark or other filter, I suppose this would be a reasonably accurate answer. In my opinion, though, the answer of "yes" is a qualified one. Daniele is correct in mentioning that the Doppler effect is a consideration. Perhaps the degree to which the Doppler effect plays a role is not quite appreciated though. All structures in the chromosphere are in some state of motion so they are all affected by the Doppler shift. This is especially true in active regions. Here there can be dramatic differences between features observable in the red and blue wings. The easiest way to demonstrate this is by observing a H-alpha "dopplergram". This is a difference image in which a red wing image is subtracted from a blue wing image (or vice versa) where the red and blue wing offset wavelengths from line centre are equal and opposite. For a chromospheric line, like H alpha, the difference image allows one to visualize chromospheric plasma flow, mapping out the chromospheric velocity field. For a photospheric line, the photospheric velocity field is mapped out (as in the images produced by the Michelson Doppler Imager (MDI) instrument aboard the Solar and Heliospheric Observatory (SOHO) spacecraft).

I'm attaching a H-alpha dopplergram (14Sep2014) from the Coimbra Observatory (Portugal) taken from the BASS2000 site. If the features visible in the red and blue wings were equivalent, one would expect a uniformly gray image (a "zero" result from the red minus blue image subtraction). Instead there is a wealth of structure, mostly in active regions but, to some extent, visible over the entire solar disk. The spatial resolution of this full disk dopplergram is rather poor but my guess is that, at higher resolutions, even more differences would be visible. I make this guess even though the appearance of the high resolution but very narrow fov images from the CRisp Imaging SpectroPolarimeter that Christian linked to are very similar in the wings! Very few spicules are visible in these images so maybe this skews the observation somewhat.

Coimbra Halpha dopplergram 14Sep2014.jpg (49.4 KiB) Viewed 1896 times

To further prove the point, I'm attaching another image; figure 9.7 from the Zirin book "The Solar Atmosphere". This figure is a special type of spectroheliogram (in H-alpha) taken in such a way that the wavelength across the image varies in a uniform way from blue to red as you sweep from left to right in the photo. Although the original photo is not particularly crisp and my photo of the photo is even worse, you can still visually confirm the comment that Zirin makes in the figure caption: "The spicules are more prominent on the blue side, presumably because they are rising." In this photo, spicules are seen against the disk and are associated with the dark mottles that seem to be more numerous on the blue side.

Zirin figure 9.7.png (583.81 KiB) Viewed 1892 times

My two cents' worth!
Cheers.
Peter

[Bob Yoesle]

Hmmm...

I from my point of view, Daniele is asking if there is an observable difference in the east and west limb features of the Sun visible in the chromosphere via our commercial-grade H alpha filters.

Here I believe there would be no observable global difference in a properly tuned filter system as Christian states. The differences I have observed are due to tilting or other effects which change the CWL across the filter. And as we move off-band the chromosphere shows very definite changes in the contrast and visibility of certain features and intensified presence of mottles, etc. Of course, individual rapidly moving features such as a lone spicule or surge prominence will obviously show Doppler shifting effects. The aggregate of these smaller features appears to be the origin of the Dopplergram image posted by Peter. These images also appear to show the differences that occur at a very high resolution of the spectral line considered. So on an absolute scale there is an "observable" difference as Peter notes.

In one experiment, spectroscopic rotational H alpha line shifting shows only a 0.08 A shift between the east and west limbs of the Sun. An 8 one-hundredth Angstrom change would seem to be very difficult (if not impossible) to detect visually with even a double-stacked filter system of 0.5 A FWHM and skirts of several times that.

It is also questionable whether the human eye would be sensitive enough to discriminate such a small difference in wavelength. Studies show that it takes about a 4-8 nm (40-80 A) wavelength change at 650 nm to perceive a difference in color. What we are therefore seeing with H alpha doppler shifting apparently is differences in color intensity.

Additionally, high resolution images of +/- doppler shifted velocities of spicules shows no appreciable global differences:

Spicules SM.jpg (373.62 KiB) Viewed 1846 times

Note as one goes off-band in either direction, spicules and mottles begin to predominate, photosphere continuum increases, and "general chromosphere" features disappear. Astrophysical Journal

So the short answer to the question "are the red and blue wings equivalent in terms of the features that can be observed and the altitude above the photosphere of the observed layer?" appears to be that visually with a commercial-grade etalon system there is no significant difference in features other than for individual rapid velocity features as they approach or recede. With high-resolution spectral analysis, there appears to be a definite difference both globally and of individual features.

[daniele_bonfiglio]

Dear all, I have really appreciated that my somehow naive question got so interesting comments by some of the most experts in the forum! I have had a look at the only wavelength scan I did some months ago. In the line of what Bob wrote, looking at images in the red wing I did not noticed any differences between the proms in the East and West limbs. I naively expected on limb to enhance the proms and the other to weaken them. Instead, I just saw the proms to the overall much fainter than when imaged on band. I think this is simply due to the fact that they are mostly due to emission in the hydrogen alpha line, right. The features on disk were also less defined and there was a portion of the disk almost blank and featureless apart from mottling. Thank a lot!

[christian viladrich]

Hello,

Thanks Peter for your complements. As sumarized by Bob, I guess we can say that the blue/red wings are basically equivalent. But, but as you said Peter, if we have a better spectral resolution, it is possible to witness the impact of the Doppler effect.

Regarding the visibility of the spicules on the CRISP image, we can note that 27 km/s is about 0.6 A. On this image, the spicules looks indeed slightly more visible in the blue wing.