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The Distinction between Plage Areas and Faculae


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  • 1 month later...

 

Howdy!

Very informative, thank you for that, Newbie! I would only add an historical footnote.
Way back in the day… faculae where known as “Calcium Faculae”.  In the early part of the 20th century, astronomers found that filtering with a very narrow band pass centered on 393nm, the Ca K-line (emission/absorption), would reveal a very detailed view of the faculae, hence the name “Calcium Faculae”.  There is another absorption line for calcium at 396nm, the Ca h-line.  Both provide similar views of the networks of faculae.  Below 400nm, we are moving more into the UV part of the spectrum, so it’s easier for our eyes to see the light, the closer it gets to 400nm. Once you go below about 390, you’ve entered the near-uv part of the spectrum and it becomes very difficult for our eyes to see. Continuing farther into the uv spectrum, one risks permanent eye damage. 
unlike many of the other solar filters out there, the Ca K/H lines are intense enough to be seen using a filter with 5-6nm band pass. 

Here are a couple of images of the sun using a Ca k-line filter. The k-line is a little better for imaging while the h-line is a little better for visual observation. 

5E4F9F9E-18D4-4745-ADD6-A60F7E262616.jpeg.6c296c894d47f249db4b43f63a63542a.jpeg
 

 

27E4E18C-28DD-4DBD-8CD6-16A2D03EEC54.jpeg.c19ed27a015329e72b2c65daa9e0505b.jpeg

2D2FE199-764F-4356-AFC8-F6D5F3D4CA5F.jpeg.4d834352037ad00ccdd702ab4ef91470.jpeg

 

pretty cool stuff if ya ask me!

Cheers!

WnA

Edited by WildWill
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  • 1 year later...
Posted (edited)

Maybe we could use this thread to discuss faculae (@MinYoongi, @Adohran); if any admin sees this, feel free to move the posts about plage in the 3697 thread over here too.

Another paper I found interesting when looking through some of the available literature is What are 'faculae'?; there they conclude:

Quote

[...] faculae are not depressions in the photosphere into which oblique lines-of-sight reveal deeper, hotter, layers in flux tubes – they are granules whose centerward walls are made relatively bright by the decrease in opacity caused by the magnetic fields directly “in front of” the granules. Thus it appears that the basic physics of the hot-wall model are correct in interpreting faculae as a relative opacity effect caused by the magnetic field. However the geometric interpretation of a Wilson depression into which oblique lines of sight penetrate into the flux tube appears outdated.

We can now see that the “hot wall” is not the interior of a flux tube depression but is instead the “granule wall” behind the flux element. Figure 4 of Keller et al. (2004) illustrates this (and the origin of the dark lane immediately centerward of many faculae) well. The large lateral extent, as well as the dynamic vertical striation, of faculae is explained by the fact that in plage regions, the intergranular lanes are typically filled with many small dynamic magnetic elements. Larger faculae are thus caused by looking through a group of several neighboring flux elements (sometimes coalesced into a small pore or micropore) at the granulation behind the group, as has recently been empirically modelled by Okunev and Kneer (2005). Thus viewing faculae is analogous to viewing a scene through a picket fence, with the magnetic field causing the gaps in the fence that allow us to see the scene behind the fence.

The distinction between the faculae being the interior of the flux tube itself and the actual wall of the granules is perhaps a bit academic, but according to their findings it does explain a few things, as the two are ultimately quite distinct. The part I highlighted above is a great analogy if you consider the flux tubes coming out of the granules as the picket fence, and that since they tend to cluster together you are then able to more easily see between them and right at underlying granule itself. The figure they refer to (from this little paper) illustrates the situation, although it might not be entirely clear without studying it a bit:

faculae.png

This doesn't necessarily explain everything, but it's definitely a very reasonable model that fits well together with everything we know, as well as the suggestion from the earlier paper by Dicke (and a paper published 6 years later where the "hot-wall" model they refer to was presented), and is thoroughly based in empirical observation, so it should be a valuable read for understanding the phenomenon better.

As a final note they refer back to how a similar characterization was made in the same time period as those other early papers in the 70s, which was thought to be incorrect, and how they've now actually reached more or less the same conclusion:

Quote

As a final note, it is interesting to recall that R. Muller proposed the term “facular granules” in a study of Pic du Midi images from the 1970s (Muller, 1977). Many of us believed that this was an misleading characterization – that faculae were not granules but were in fact magnetic flux tubes. It has been an educational experience to find that Muller’s characterization appears correct after all!

Edited by Philalethes
missing word, wrong word
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Posted (edited)

Although this article isn't specifically about the topic above, it does have some insight about Sunspot Pores, BiPoles, Flux Emergence, and how the Magnetic Field can be trapped in the Photosphere amongst other very fascinating processes. 

To me, it does a very good job explaining the formation and decay of Sunspots/Sunspot groups and how to determine which part of the cycle a Sunspot has reached. Hopefully this can be helpful understanding what Plage/Faculea/Pores/Granules/Magnetic Flux Tubes are and it's purpose it serves in the Solar Dynamo.

https://iopscience.iop.org/article/10.3847/1538-4357/ab4f84

Edited by Parabolic
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So basically we aren't looking directly down the flux tube into the deeper layers but rather to it's side into the granules. Interesting to know! Btw, maybe I missed it, but my research couldn't tell the heightened 10.7cm flux. Could you find something to it @Philalethes?

Would make the whole "where do the high 10.7 values come from" more exciting if those values are more correlated with faculae instead of sunspots directly.

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Posted (edited)
6 hours ago, Adohran said:

Btw, maybe I missed it, but my research couldn't tell the heightened 10.7cm flux. Could you find something to it @Philalethes?

Would make the whole "where do the high 10.7 values come from" more exciting if those values are more correlated with faculae instead of sunspots directly.

From looking through some papers on it it seems like such features tend to be associated with increased F10.7 indeed, as expected from the increased brightness of the underlying granules, but do not explain the F10.7 all on their own. This appears to be due to how F10.7 emission is also associated with other features directly, like sunspots and coronal loops, and that the relationship between these and the presence of faculae and plage varies in a lot of ways. In this paper (How faculae and network relate to sunspots, and the implications for solar and stellar brightness variations) they write:

Quote

As noted in the previous paragraph, chromospheric emission is strongly enhanced in plage and network features overlaying photospheric faculae and network. It follows that chromospheric indices such as the 10.7 cm radio flux (F10.7, Tapping 2013), Ca II K 1 Å emission index (Bertello et al. 2016), Lyman α irradiance (Woods et al. 2000), and Mg II index (Heath & Schlesinger 1986; Snow et al. 2014) are modulated with plage and chromospheric network emission, and their relation to sunspot indices offers another avenue to probe how faculae and network relate to sunspots.

[...]

It is worth pointing out that there are sources of variability in chromospheric and coronal emission other than their enhancement over faculae and network. For example, the enhancement of solar 10.7 cm emission in compact sources associated with sunspots and in coronal loops (Tapping 1987). The F10.7 and the various chromospheric indices are strongly, but not solely, modulated by faculae and network prevalence.

[...]

The observation here that the F10.7 departs from the other facular indices in terms how it compares to the S_A [total sunspot area] is at least partly due to the following. We noted in the introduction that while the various chromospheric indices are strongly modulated by faculae and network prevalence due to the enhancement of chromopheric emission over these photospheric magnetic features, there are other sources of variability. Solar 10.7 cm emission is enhanced in compact sources that are associated with sunspots (Tapping 1987). This is not the case for the Ca II H&K, Lyman α, and Mg II h&k lines, and while sunspots can still be darker or brighter in these lines, depending on height in the solar atmosphere, the effect is much weaker. Clearly, this would have contributed to the divergence between the F10.7 and the other chromospheric indices and Fφ noted here.

 

Edited by Philalethes
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So faculae play a large role when it comes to 10.7cm emissions but they are also caused by other solar features as well which make faculae and granules not the only contributor but a major one?

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24 minutes ago, Adohran said:

So faculae play a large role when it comes to 10.7cm emissions but they are also caused by other solar features as well which make faculae and granules not the only contributor but a major one?

Yeah, pretty much; and the same appears to be true for the other indices they mentioned, this part of the above in particular is probably the clearest statement:

Quote

The F10.7 and the various chromospheric indices are strongly, but not solely, modulated by faculae and network prevalence.

But the last paragraph mentioned above is also important, as it shows that F10.7 diverges from the other indices in how it compares to total sunspot area (which is what the paper was primarily focused on, comparing all those different chromospheric indices to total sunspot area); as they write they suspect that this is because of F10.7 enhancement from various sources associated with sunspots, which isn't the case for the other indices.

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6 hours ago, Philalethes said:

which isn't the case for the other indices.

What other indices do you, or they, mean? I'm fairly new to the in depth things related to solar behavior. 

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Posted (edited)
19 hours ago, Adohran said:

What other indices do you, or they, mean? I'm fairly new to the in depth things related to solar behavior. 

The indices referred to there would be what they refer to as facular indices in the abstract:

Quote

Chromospheric indices and the total magnetic flux enclosed in network and faculae, referred to here as “facular indices”, are modulated by the amount of facular and network present.

This is also mentioned in the introduction:

Quote

Let us refer to chromospheric indices and F_φ, which are both modulated by faculae and network prevalence, collectively as facular indices.

Here F_φ refers to "total magnetic flux enclosed in network and faculae", which is the only of those indices that isn't what they call a chromospheric index, as those relate to indices that have to do with emission of radiation; those chromospheric indices are those mentioned in one of the excerpts above:

Quote

As noted in the previous paragraph, chromospheric emission is strongly enhanced in plage and network features overlaying photospheric faculae and network. It follows that chromospheric indices such as the 10.7 cm radio flux (F10.7, Tapping 2013), Ca II K 1 Å emission index (Bertello et al. 2016), Lyman α irradiance (Woods et al. 2000), and Mg II index (Heath & Schlesinger 1986; Snow et al. 2014) are modulated with plage and chromospheric network emission, and their relation to sunspot indices offers another avenue to probe how faculae and network relate to sunspots.

The ones that aren't F10.7 refer to distinct spectral lines; while it's not treated in the paper Ca II K and Mg II indices refer to specific so-called Fraunhofer lines, which are essentially darker gaps in the otherwise relatively continuous spectrum due to various atoms (in this case singly ionized calcium and magnesium) in the chromosphere absorbing those specific frequencies, whereas Lyman-alpha (Ly-α) refers to an emission line of hydrogen instead, the first and simplest in the Lyman series, the transition from n = 2 to n = 1, very similar to the more familiar hydrogen-alpha (Hα) line in the visible range, which is the simplest in the Balmer series, the transition from n = 3 to n = 2.

Edited by Philalethes
typo, bad grandma
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