User talk:CoronalMassAffection/Archive 2023

Alfven theorem

Hi there. I am not sure what you are trying to achieve with the edits to the Alfven's theorem page. But things are, in my opinion, going very badly awry, and a good page is deteriorating.

Firstly you seem to be removing references to magnetic field lines and expressing everything in terms of flux tubes. This is confusing to readers as flux tubes are just bundles of magnetic field lines and Alfven's theorem applies to flux tubes only because it applies to magnetic field lines.

Secondly. The point you have removed about Reynolds number is massively important one because although Alfven's theorem was envisaged in terms of infinite conductivity, we soon came to understand it applies to high magnetic Reynolds number which is a different thing. As it now reads (later on), it sounds like the convective limit (which means ideal MHD and the Alfven theorem) only applies because the conductivity is high. This is not the case as in much of space plasma science it applies because velocities are high and, even more importantly, because spatial scales are high. This crucial fact is now completely hidden from the reader.

On a much less important point: you are talking about a "co-moving surface" and it is not clear what this means. It actually means a surface moving with the plasma velocity which, as I have noted is the mass-weighted mean of the ion and electron velocities.

Incidentally, there is a much more easily-understood and quicker way of proving Alfven's theorem which is to assume apriori that it applies, think about magnetic flux continuity threading a closed loop and quickly arriving at Faraday's law in integral form, confirming the initial assumption.

I should say about who I am and why I feel that I can comment. I am a professor of space physics who has taught MHD and space physics for 30 years. I did in fact meet and discuss MHD with Alfven himself in later life (and learned why he abandoned frozen-in: it is somewhat bizarre), but it was only for one day when I was working for NASA and he was visiting. I do know almost all his former students in Stockholm and have examined PhDs for their students. I did also know well, and learned a lot from James Dungey, Eugene Parker, Ian Axford and Harry Petschek: sadly none of whom are with us any longer. MessageInABottle 20:06, 2 January 2023 (UTC) — Preceding unsigned comment added by MichaelLockwood (talk • contribs) MessageInABottle 20:33, 2 January 2023 (UTC)

Hello. Thanks for reaching out to discuss this.

Regarding your first point, I disagree; frozen field lines do not necessarily imply frozen flux (tubes), but the inverse is true. If a source term parallel to the magnetic field were to be added to the rhs of the ideal induction equation, the field lines would still be frozen to the fluid, but flux tubes would not. Therefore, I think it is important that flux tubes rather than field lines are specified at least in the lead. Most importantly, however, the majority of sources that I have come across present the theorem in terms of flux. I agree this might lead to some confusion, and I can look into making this more clear.

I agree with your second point. I initially removed the ideal MHD explanation, as I assumed the reader would know that a fluid can be treated as a perfect conductor in the limit of infinite magnetic Reynolds number. However, after thinking more about it, I agree that what is meant by a perfectly conducting fluid/ideal MHD should be elaborated upon.

On the less-important point, I did not write the "co-moving surface" sentence in the lead, and I agree that it is too vague.

I believe I have seen the derivation you mentioned using Faraday's law, but the derivation I used appears to be the most common.

And regarding credentials, I am aware of who you are, and I am thankful that someone with so much experience can comment on this. My experience with MHD is limited to books and papers I have read in my free time, and I have yet to even receive my high school diploma. Take that as you will, but I agree with the essay Wikipedia:Ignore all credentials—the sources presented are more important than the credibility of the editor. Thanks again for writing to me about this. CoronalMassAffection (talk) 03:52, 3 January 2023 (UTC)

Yes sorry! The point about credentials is a perfectly valid one. I guess the frustration for me though is that because I have taught MHD for many years and I have tried presenting it in lots of different ways to students, I have seen what sometimes confuses, what always confuses and what minimises (but sadly never eliminates) confusion. I have also kept notes on the many misconceptions that often crop up and how to counter them (which is why I mentioned, e.g. the importance of the definition of plasma velocity). The thing about MHD is you can approach it in lots of ways and at lots of different complexity levels and if you try to start with an all-singing and all-dancing comprehensive and fully rigorous approach then you don't get the reader through to the required level of understanding and knowledge that they have the context in which to place the complications. My old PhD supervisor used to say that teaching physics in general was a question of doing the same thing over and over, its just that you lie a little less each time. He was exaggerating for effect, but he had a point. In a wikipedia page you have to find a way to take the reader through the same process but without saying things that are wrong or simplistic, or at least flagging them as such when you have to. I guess what I am asking you to think about is a bit more of a layered approach so that the complications are left to later and not dealt with head on at the start. Hope that helps and gives food for thought. I'll "butt out" now. It's great you are making these pages. MessageInABottle 20:07, 4 January 2023 (UTC) — Preceding unsigned comment added by MichaelLockwood (talk • contribs)

Continuation

Moved from User talk:MichaelLockwood

Another quick point about the Alfven theorem page. The wording is perfectly correct (because velocities, electric fields and voltages all depend on what reference frame you are using). But the wording "magnetic flux tubes in a fluid with infinite electrical conductivity are frozen into the fluid and have to move along with it" may cause some confusion. A lot depends on the relative energy densities. The wording is perfectly descriptive (as well as valid) in the solar wind where the dominant energy density is in the bulk flow and the Interplanetary Magnetic Field is dragged along with the plasma velocity of the solar wind. However, by definition of the magnetosphere, the energy density in the magnetic field there dominates over both the thermal energy density and the bulk flow energy density of the particles: in this case the plasma is constrained by the magnetic field and the wording doesn't convey this. I realise it is all relative and depends on the adopted reference frame, but I would suggest wording to the effect that the plasma and the magnetic field are constrained to move together. I really do worry about expressing Alfven's theorem in terms of flux tubes - it applies to magnetic field lines. The present wording makes it sound like frozen-in is a peculiarity of flux tubes where it really applies to the quantum unit of flux tubes, i.e. field lines. On that, I am not sure about saying that a field line is a flux tube of zero cross sectional area: this is possibly meaningless and gets us into metaphysics about what can exist in zero spatial dimension. What one can say for sure is that flux tubes are collections of field lines. My point is that it is magnetic field lines that have a specific definition, not flux tubes and flux tubes are only defined from field lines, not vice-versa. Incidentally. I should tell you more about my conversation with Alfven. He objected to me invoking reconnection (as I knew he would: I was being provocative to try to get to the bottom of what he was thinking) because he objected to the concept of magnetic field lines moving. This raises an interesting point, if you do the reverse of what Einstein and Lorentz did and apply the equations of special relativity (the Lorentz-Fitzgerald equations for time, distance and force) to the Coulomb force law between two charges, one can the arrive at the Lorentz Force law F = q(E + vxB). What is interesting about that is it provides expressions for E and B. And what is interesting about them is one finds that E transforms when you move into a different frame (so there is always a de-Hoffman-Teller frame in which E is zero) but B does not depend on reference frame. If one thinks about it this, it makes sense from Ampere's law which relates B to currents which depend on the relative velocity of differently charged particles and those velocity differences are the same in all reference frames. Thus far, Alfven agreed with me. But he then said, "so it is meaningless to talk of field lines moving" - to which I replied, if they are not moving they are still which is just the zero speed limit of moving. At that point he stopped talking to me and moved on to somebody else. I would have pointed out (had he continued the conversation) that if you have a solanoid and increase/decrease the current through it the magnetic field lines (lines of force identified by, e.g., iron filings) expand outward/contract inwards so field lines moving is a good and valid description of what is happening. Interesting .... and that is the closest I ever got to understanding why he renounced his own theorem which was so phenomenally useful and predictive. I should add this was in 1984, when he was 76 and in answer to others he seemed to be saying that you cannot use a fluid theory at all because cosmic plasmas are collisionless!!! (not true when you include electromagnetic interactions between particles). So he seemed to be disowning MHD in totality (his great achievement) and yet he frequently invoked electromagnetic interactions on particles. MessageInABottle 13:27, 4 January 2023 (UTC)

@MichaelLockwood: Regarding I would suggest wording to the effect that the plasma and the magnetic field are constrained to move together: I agree that this would be a good improvement, although I cannot recall any sources presenting it in this way explicitly. I hesitate to make this change for that reason, but I can see what I can do. If you know of any sources that do present it in this way, I would appreciate it if you could cite them in the article along with making the change or let me know.

Regarding I am not sure about saying that a field line is a flux tube of zero cross sectional area: I did not write this and have never seen a source present the relationship between the two in this way. There are two supporting citations provided next to the statement in the article; however, I do not have access to either sources. If you have access to them, I would suggest checking them out to see if and/or how they support the claim. In the meantime, I have placed the tag Template:Verification needed following the two citations in the article.

Regarding I really do worry about expressing Alfven's theorem in terms of flux tubes - it applies to magnetic field lines. The present wording makes it sound like frozen-in is a peculiarity of flux tubes where it really applies to the quantum unit of flux tubes, i.e. field lines. [...] What one can say for sure is that flux tubes are collections of field lines. My point is that it is magnetic field lines that have a specific definition, not flux tubes and flux tubes are only defined from field lines, not vice-versa.: I have explained why it would be inadequate to present the theorem in terms of field lines—and only field lines—earlier in the discussion and have provided supporting references therein. In short, I mentioned that the property of frozen-in field lines is not equivalent to the property of frozen-in flux tubes (or any surface of constant flux) and that the former can occur without the latter (Helmholtz-Zorawski criterion).

And finally, thanks for detailing your discussion with Alfvén. It is very interesting to read. Just keep in mind that any changes made to articles on the basis of past experiences will need attribution to verifiable and reliable outside sources. CoronalMassAffection (talk) 02:19, 22 January 2023 (UTC)

References

Flares and aurorae

Sadly, I don't think you realize how widespread this old myth of flares and aurorae still is. I have added the note twice and you have reverted twice, for no good reason that I can see. It doesn't need a reference since the wikilinks support the statement.Skeptic2 (talk) 14:58, 20 January 2023 (UTC)

For future reference, this discussion pertains to edits A and B made to articles Solar flare and Solar cycle, respectively.

@Skeptic2 Your first statement is exactly right. I don't know how widespread this misconception is. And therein lies the problem. There needs to be a reference from a reputable source stating that this is, in fact, a misconception and that it is widespread. From personal experience, I understand that it is a misconception that some people hold, but personal experience is not an adequate source. I should have been more clear about what I meant needs a reference in the edit summary.

In addition to the reference issue, IMO the clarification would be unnecessary if we were to simply state outright that CMEs cause aurora. Also, the edits seemed out of place in their respective locations, and I think that they would fit better in Solar flare#Effects and Solar cycle#Effects where impacts on Earth are discussed. CoronalMassAffection (talk) 16:42, 20 January 2023 (UTC)

I do agree that one doesn't want to "knock down a strawman" (i.e. disprove things that no reasonable person believes anyway); howeverc the wording of the section "Impact on Earth" of the page "Coronal Mass Ejection" does make misleading statements about what flares cause - which does imply it is still quite a common misconception. MessageInABottle 21:18, 21 January 2023 (UTC) — Preceding unsigned comment added by MichaelLockwood (talk • contribs)

@Skeptic2: Coming back to this, I am reminded of John T. Gosling's 1993 paper outlining the "solar flare myth", what he believed to be a misconception at the time regarding the relationship between flares and other solar phenomena. The solar flare myth ended up becoming widely accepted as myth by heliophysicists, but the misconception clearly still persists to some degree. I think that this paradigm shift is notable enough to be mentioned in the history sections of Solar flare and Coronal mass ejection (perhaps MichaelLockwood can provide his thoughts on this), but I maintain my view that it would be inappropriate to mention elsewhere for the purposes of knock[ing] down a strawman. CoronalMassAffection (talk) 14:56, 27 January 2023 (UTC)

Well remembered. Believe me, the myth lives on in much popular literature. Personally, I blame Richard Carrington and his famous Event. Skeptic2 (talk) 16:04, 27 January 2023 (UTC)

References

Incorrect statements about solar energetic particles and misleading wording about the effect of flares

This topic was split off from § Flares and aurorae, above.

Hi Skeptic2, I'm not sure what page you are commenting on - but I think I am going to agree with you. Certainly the connection between CME and flares is much weaker that originally thought, but there is some relation and CMEs do tend to cause aurora to move to lower latitudes and intensify (it is CMEs that do that and certainly not flares) but aurora is present at some latitude all the time without flares or CMEs. On the page Coronal Mass Ejection page under "Impact on Earth" there are some seriously incorrect statements. The worst is: "Solar energetic particles can cause particularly strong aurorae in large regions around Earth's magnetic poles. These are also known as the Northern Lights (aurora borealis) in the northern hemisphere, and the Southern Lights (aurora australis) in the southern hemisphere. Coronal mass ejections, along with solar flares of other origin, can disrupt radio transmissions and cause damage to satellites and electrical transmission line facilities, resulting in potentially massive and long-lasting power outages." Solar energetic particles (SEP) do actually cause aurora but not the conventional kind of aurora that this statement invokes. They precipitate into Earth's atmoaphere INSIDE the polar cap (not in the auroral oval around the polar cap where we see electron aurora) where they generate "proton aurora", because the dominant species in an SEP event is protons (which are mainly generated in interplanetary space by the shock front ahead of CMEs but a weak flux of "impulsive" SEP can come from a flare) and proton aurora is generated by precipitating protons. This is quite different from the aurora called "aurora Australis" and "Aurora Borealis" which is caused by precipitating electrons (i.e not by SEPs at all as stated). It is frequently said that the electrons causing these aurora in the auroral oval come from the solar wind and this is also largely incorrect - most come from Earth's ionosphere (mainly generated by photoionization by solar EUV and X rays) but are energized inside the magnetosphere using energy extracted from solar wind. The wording "Coronal mass ejections, along with solar flares of other origin" is highly misleading as many CMEs are not associated with a flare at all and this wording implies they all are. The way that flares and CMES disrupt radio transmissions are completely different and on different timescales and flares DO NOT cause GICs (Geomagnetically Induced Currents) that disrupt power systems, but CMEs do. Hope this helps. The paragraph I have highlighted is seriously incorrect and must be corrected. I will leave it to someone-else to do it, but will edit if no-one else does. MessageInABottle 15:41, 20 January 2023 (UTC) — Preceding unsigned comment added by MichaelLockwood (talk • contribs)

Mike, you know far more about this than any of us, so please go ahead, If you can make it comprehensible to a non-specialist then even better. Your comment "many CMEs are not associated with a flare at all and this wording implies they all are" rather makes my original point.Skeptic2 (talk) 21:27, 23 January 2023 (UTC)

@MichaelLockwood: Thanks for pointing this out. I had noticed the errors in May 2022 and had placed a Template:Off topic banner to warn readers and indicate that the section needed work. As Skeptic2 mentions, it would be great if you could work on this section. CoronalMassAffection (talk) 16:27, 26 January 2023 (UTC)

The widespread myth that flares cause aurorae has just been repeated again, by someone who should know better

https://www.bbc.co.uk/news/uk-64782372

Even the BBC get it wrong. We really should knock this myth on the head.Skeptic2 (talk) 09:29, 27 February 2023 (UTC)

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Thanks

Thanks for all your great work in re the sun (and in all seriousness don't forget to include your Wikipedia contributions on the "what else should we know about you?" section of your college applications) Cheers jengod (talk) 01:31, 23 April 2023 (UTC)

Thank you! I really appreciate your comment. I actually have already applied to universities, and I did end up writing a little bit about working on Wikipedia in my applications. CoronalMassAffection (talk) 13:17, 24 April 2023 (UTC)

A barnstar for you!

	The Original Barnstar
	praise the sun! pony in a strange land (talk) 17:45, 19 August 2023 (UTC)

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