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Active Region longitudes


hamateur 1953
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Since I pay little attention to these myself and presently there is obviously some  disagreement regarding two active regions currently at South 18-19 as to which one is really old 3664 I realised that other than tracking them via their current location on the disc and seeing “ returning longitudes “ in SWPC synoptic maps and being even more confused once when someone asked me the approximate degree location of an AR west we had lost track of that some simplification sure would be nice, and am betting I am not the only one here.   🤣 If @Jesterface23 or maybe @3gMike could illustrate for us an easy way to check an AR arrival at CL when first numbered it would be interesting, if nothing else.  I think there was quite a lot of discussion regarding “ hot longitudes “ some time back also. 

Edited by hamateur 1953
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Posted (edited)

Thank You Mike!   I will reference this page in my history.  The torsion effects can even be viewed sometimes depending on what is presently on the surface.   I am sure others have noticed the “ arrow” effects also of dissimilar speeds causing these effects.  Great post.  and TU again for taking the time.  Mike/Hagrid 🐈‍⬛

Edited by hamateur 1953
Acknowledging
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All I can add is JHelioviewer makes things a lot earlier, along with using the Carrington coordinates. It can take a 2D HMI image and stretch it to 3D where you can then rotate.

I could make a video on how to use it if wanted. Though, one issue being loading a lot of images won't be best for slow internet connections.

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8 hours ago, 3gMike said:

It does get a little complicated.The preferred method is to use the Carrington longitude. A Carrington Rotation is based on a period of 27.2753 days, and it is worth noting that this is a figure based on sunspots appearing at a latitude of 26 degrees. At this latitude the surface of the sun completes a rotation in 25.38 days. The additional period to complete a Carrington rotation accounts for the distance that the Earth has travelled during that rotation, and therefore the time it will take for the exact same point (at 26 degrees latitude) to be seen at the exact same location when viewed from Earth. 

The suns rotation rate at the Equator is faster at 24.47 days resulting in an object in a fixed location on the Sun appearing to return to that point after 26.24 days.

The Active Regions under discussion are located at 18 degrees latitude so will apparently complete a rotation somewhere between 26.24 and 27.2753 days. That can be calculated exactly but I will not go into that here.

For simplicity Icaptured HMIBC images from 2nd May, 29th May and today, 25th June (all at 13:00)

Results here...  It can be seen that even with the growth between 2nd and 29th May the centre remained in same position. By today that same line divides the two ARs

CompositeHMIBC.thumb.jpg.da316aa0fdd28b5c5a4808a6dac70488.jpg

This is great analysis; I'd almost forgotten that the synodic Carrington rotation is useful precisely because it makes it easier to compare images like that. I tried to replicate it (using exactly 27 days instead to approximate the rotation at that latitude, as you did), and got more or less the same result:

merged.jpg

Since you mention the different rotational rates, I also wanted to check what it'd look like if we were to use the expected rotation rate at that latitude instead, assuming 18°. Using the formula provided here, which gives a sidereal period of 25.37 days at a latitude of 26° (close enough to the definition at 25.38), I found the period at 18° to be 24.88 days, and calculating the synodic period I got almost exactly 26.7 days; indeed right around where you estimated it.

Since it however is a bit shorter than 27 days, I did the same as above, but subtracting those 0.3 days for each image back in time, resulting in:

merged.jpg

I've kept the line marked at the dividing line between the polarities as observed in 3664 (first image) for comparison. Interestingly here we see something I found when checking the Carrington longitudes in the SHARPs as well, i.e. that 3697 (second image) definitely appeared further "east" than expected. This to me seems to indicate that it's either not so easy to place the regions exactly, or maybe that it's not flowing as fast as expected. In the latter case perhaps it could be something related to how the magnetic field blocks convection there, or maybe just natural variation in the flow rate (would be very interested to know more about the details of this). If we assume the former, then it wouldn't be too unreasonable for the delta we see straddling 3723 and 3727 to be part of the same underlying complex, although I'd still say 3723 would really be the rightmost part that we saw pop up in the EUI imagery.

If on the other hand we assume the latter, i.e. that it's not actually rotating as fast as the photosphere is expected to at that latitude, then things are a bit different. If we e.g. do the same, but use a full synodic Carrington rotation of 27.2753 days instead (so that the lines will essentially match Carrington longitudes almost exactly), we find this:

merged.jpg

Maybe it's not rotating exactly that much slower, but the way the most prominent parts of the regions match up is fairly convincing, especially right at a major polarity division line in all cases. What makes it more convincing is also how this seems to match up with the data from the SHARPs, where the flux-weighted longitude of 3697 probably being displaced more "east" due to the temporary flux emergence on that side.

All in all I don't know exactly what the case is here. I think there's definitely a case to be made for the last one being closest to right, assuming slower rotation of the AR than is expected of the photosphere at that latitude, and that 3727 is the best match; maybe someone can dig something up in the literature about it. Another minor thing that could be accounted for without too much problem would be the different orbital angular speed at aphelion (which we're close to) vs. perihelion, as the resulting difference in synodic period for the Carrington rotation would just be ~0.08 days in each direction from the average; small, but on the order of some of the other differences.

In either of the cases it's still not unthinkable that the flux emergence around it could be part of the same underlying mess of tangly twisty tubes, seen on both sides of the main part during the successive rotations. The SHARPs still have the entire thing detected as one region, although they might get separated out eventually.

Edited by Philalethes
wrong region number
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Thank you so much to everyone who explained everything in such detail. This is so interesting and informative for me and probably for a lot of people who are interested in ways to identify active regions. Can I have some fun today, when we were finally able to recognize the old region 3664 in a new guise?  Extempore poem  from me to AR3727: 

  Jingle bells, jingle bells,

 coming he again!

 O what fun it is to watch:

when he wants to X-flare! 😄

 
 
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