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The magnetic latitude problem


libmar96

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Hello,

I'm preparing an aurora visibility map for mid and low latitudes for Europe based on "magnetic latitude". The concept is simple: based on calculated dependence between plasma and IMF data, a maximum visbility line for certain aurora type (red glow/pillars height/green showing over horizon/green reaching zenith etc.) for a given moment is drawn, keeping in account that's for a potential pillars outburst moment. A certain distance from the north magnetic pole is calculated and that circle is drawn.

This is a bit different for currently known 'aurora visibility maps', when the red/green reaches as far south as France, but observers in Canary Islands still can watch the show. I would say, more usable for a mid-latitude observer. I rather use current oval maps only to see if there's some yellow or red area indicating that "something is happening" rather than where it reaches.

It is clear that I need to use geomagnetic north pole instead of magnetic north pole as a '0 km' distance. A proof is that nowdays America still sees auroras further south despite magnetic pole being now closer to Severnaya Zemlya (Russia) than northernmost islands of Canada. And here comes the first inconsistency: why do we use MLATs in describing how far May 2024 storm reached in "magnetic latitudes" instead of "geomagnetic latitudes" (GMLAT)?

Talking about the May 2024 storm, you probably remember the map/photo below. The pole, the '0 km point' should be pointed exactly in the center of pole.

May%2011%202024%20aurora%20.png

And for reference, here's the position for geomagnetic and magnetic pole... for 2017. The position of north geomagnetic pole hasn't really changed, maybe one pixel at most. The north magnetic pole goes in direction of those islands right to 'Arctic Ocean' sign. What I want to note here, the center of aurora oval from graphic above fits the position of north geomagnetic pole. So I use it.

North_Magnetic_Poles.svg

Okay, so. Let me show you an example map I got generated. Lines are testing visibility values. It's important to note, all places in one of lines are laying at the same distance of geomagnetic north pole. Lines are curved due to projection, so that it's also taken into account.

442454306_420898337443465_64345131868178

And there are tons of "aurora KP map limit" to be searched in Google. Let me paste two maps.

The first one is found by seraching for "magnetic latitude map".

maplats.gif

This one below, "aurora kp Europe map".

Captura-de-pantalla-2018-02-17-a-las-23.\\

There is only some small inconsistency in the second type of maps, can be skipped. But when you scroll up to my map, you will notice that Paris and Warsaw are in the same line. Or even Berlin a little closer than the north geomagnetic pole than Moscow. Or more "extreme", north-western Spain as close as Slovakia. This is certainly not the case in any KP map that can be found in the internet. Okay, so I might be drawing the map wrong, but you can try calculating yourself the distance of certain city to the north geomagnetic pole, which is 72.68°W in longitude and 80.37°N in latitude. Berlin is about 4225 km from the north geomagnetic pole and the distance for Moscow is 4299 km.

Okay, so maybe it's that fast moving magnetic pole? Well, not everything is right there too. Probably if I made "0 km point" as a half distance between magnetic and geomagnetic pole would be roughly correct from my sight.

I'm looking for some support there, so I can create more adequate maps for aurora visibility in Europe. If all above I've written is unclear, I'll just leave one question to answer: do you think north-western coast of Spain has the same chances of aurora like Austria, Slovakia or Kyiv? For me no. I can't find beautiful aurora pictures from Spain that are comparable to those taken in Austria or Slovakia. KP maps say no. May 2024 aurora graphic... I think no, but unclear (Spain is outside FOV). But from the distance to pole, the answer becomes yes.

Edited by libmar96
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  • 2 weeks later...

Thanks! Actually since writting that post, I found out that the center of pole isn't certainly at the geomagnetic north pole (for some it could be "how surprising"). I matched the IR photo with other aurora oval graphs and it clearly shown some shift, towards the magnetic north pole, approximately at 1/3 distance between geomagnetic and the magnetic one (probably coincidential due to recent magnetic pole movement and small difference between now and the oval shape in 20th century). Unfortunately, I could not find the coordinates in the Internet by searching "aurora oval center latitude longitude", so I calculated it manually approximately at 85°31'35"N 80°40'39"W. After applying the correction, I got more appropriate results for Europe, consistent with KP maps. I'm looking forward if someone knows more exact coordinates for that one, because I might be 100 km off... or more.

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I missed this thread entirely, glad hamateur bumped it.

On 5/31/2024 at 12:17 AM, libmar96 said:

And here comes the first inconsistency: why do we use MLATs in describing how far May 2024 storm reached in "magnetic latitudes" instead of "geomagnetic latitudes" (GMLAT)?

I think that's more a matter of nomenclature, as I believe MLAT refers to geomagnetic latitude, with "magnetic latitude" simply being synonymous with that in that context, even though you're right that it can easily get confused with a latitude based on the magnetic north pole instead.

On 5/31/2024 at 12:17 AM, libmar96 said:

The position of north geomagnetic pole hasn't really changed, maybe one pixel at most. The north magnetic pole goes in direction of those islands right to 'Arctic Ocean' sign. What I want to note here, the center of aurora oval from graphic above fits the position of north geomagnetic pole. So I use it.

Yep, that's exactly right, and I guess you've probably seen the difference be discussed here from time to time by now as well. Empirically it's indeed clear that the auroral oval follows the geomagnetic pole, which is also what's to be expected.

On 5/31/2024 at 12:17 AM, libmar96 said:

There is only some small inconsistency in the second type of maps, can be skipped. But when you scroll up to my map, you will notice that Paris and Warsaw are in the same line. Or even Berlin a little closer than the north geomagnetic pole than Moscow. Or more "extreme", north-western Spain as close as Slovakia. This is certainly not the case in any KP map that can be found in the internet. Okay, so I might be drawing the map wrong, but you can try calculating yourself the distance of certain city to the north geomagnetic pole, which is 72.68°W in longitude and 80.37°N in latitude. Berlin is about 4225 km from the north geomagnetic pole and the distance for Moscow is 4299 km.

I'm fairly sure this problem arises because you're simply using the geomagnetic pole as a reference and assuming a perfect dipole, whereas in reality the field has a number of higher-order spherical harmonics, which while significantly smaller still matter quite a bit in this context. This is why the geomagnetic map you see above (the 350 km one) looks a bit less orderly than you'd expect from a perfect dipole. I think it'll also look different depending on altitude, and I'm not sure what altitude is ideal, nor do I know to what extent the auroral oval adheres to the differences, it's possible that there are forces at play that keeps it in more of an oval shape despite those differences.

To calculate the closest approximation of a geomagnetic latitude I would think something like this calculator would be the best; there's no specification for altitude, so I suspect that's possibly at sea level, but it should give some idea. Note that it provides both dipole coordinates under the assumption of a simple dipole, as well as the quasi-dipole coordinates that are what you're probably looking for, where a lot of those extra harmonics have been taken into account. Plugging in the coordinates for Paris and Warsaw, the dipole latitudes do indeed seem to be roughly the same, as in your map, but the quasi-dipole latitudes, which would be more realistic, have Paris at 44.22 and Warsaw at 48.21, which is a clear difference. The geomagnetic map I could find with a quick search that seems to correspond the best to this appears to be this one:

Geomagnetic-latitude-bin-limits-used-in-

That's from this paper (which is somewhat related to space weather, but not exactly to this). Not sure how it was generated, doesn't say as far as I can tell, although presumably it was with reference to IGRF-13 like the calculator above, and the paper is from 2020.

On 5/31/2024 at 12:17 AM, libmar96 said:

If all above I've written is unclear, I'll just leave one question to answer: do you think north-western coast of Spain has the same chances of aurora like Austria, Slovakia or Kyiv? For me no. I can't find beautiful aurora pictures from Spain that are comparable to those taken in Austria or Slovakia. KP maps say no. May 2024 aurora graphic... I think no, but unclear (Spain is outside FOV). But from the distance to pole, the answer becomes yes.

Accounting for the above and checking the quasi-dipole latitude for the capitals of Austria (Wien) and Slovakia (Bratislava), as well as Kiev, I get 43.56 for Wien, 43.51 for Bratislava, and 46.52 for Kiev. For north-western Spain I used Santiago de Compostela for the city, with the latitude being 37.10. I used 2024.5 as the decimal date in all cases, just for reference.

Based on this, and from looking at the map above (which seems to reflect the above fairly well), you'd be correct in surmising that the chances of aurora there would be significantly lower, being roughly 6-10° lower geomagnetically than the other cities. That being said, it's not that far from Florida, which did get some nice aurora action if I'm not mistaken; and wasn't there someone from Spain who posted a few great pictures recently? So it's not out of the question for a massive storm like the one we saw, but there should be less aurora there in general indeed.

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