Talk:Magnetic diffusivity

Do we need to put the definition in Gaussian units as well (eta = c^2 / (4*pi*sigma)), since those units are frequently used in MHD? Kishore G (talk) 09:25, 7 July 2018 (UTC)

Lead Sentence

@Kishore G: Regarding your edit summary (dif)

Intro: modify a cryptic reference to 'rate of magnetic flux transfer' to refer to the flux freezing theorem. Emphasize the more fundamental definition of the magnetic diffusivity (being cryptic does not make a definition general)..

for the edit changing the lead paragraph from

Magnetic diffusivity, or magnetic viscosity, is a measure of the rate of magnetic flux transfer in an electrically conducting medium. It controls the rate of magnetic field diffusion and dissipation, and appears in the magnetohydrodynamic induction equation and in the definition of the magnetic Reynolds number.

to

The magnetic diffusivity controls the rate of magnetic field diffusion. Since its role in the evolution equation for the magnetic field is analogous to that of the viscosity for the velocity field, some authors refer to it as the 'magnetic viscosity'. The magnetic diffusivity appears in the definition of the magnetic Reynolds number. A finite value of the magnetic Reynolds number (i.e. a nonzero magnetic diffusivity) is associated with violation of Alfvén's theorem.

in what way is "rate of magnetic flux transfer" cryptic? Furthermore, what is the "fundamental definition" of magnetic diffusivity you are referring to? CoronalMassAffection 𝛿 ^talk_contribs 00:24, 3 April 2024 (UTC)

For that definition to be easily understood, one would additionally have to mention that one is thinking in the Lagrangian picture, and not the Eulerian one (e.g., in the latter, even the advection term describes transport of the magnetic field). One could indeed make this change (e.g., by saying '... is a measure of the rate of magnetic flux transfer between fluid elements in an...') and preserve the sense of your lead sentence, but defining it that way seems a bit convoluted to me (especially when we already have a widely used word, 'diffusion', that carries a similar connotation). It feels like defining the kinematic viscosity in terms of the non-conservation of the Lagrangian surface integral of the vorticity.

I now see that my use of 'fundamental' was not entirely appropriate. What I meant is that to me, the most direct way of describing what the 'magnetic diffusivity' is, is that it is the diffusion coefficient for the magnetic field.

Nevertheless, flux freezing is an important concept, and so I also added a sentence mentioning it at the end of the first paragraph.

I wasn't sure how much detail was called for there (especially since it has its own dedicated article), so I restricted myself to a single sentence. Kishore G (talk) 04:43, 3 April 2024 (UTC)

@Kishore178: Thanks for this explanation. I see what you mean and now agree that using this direct way is probably the best option. I also agree that flux freezing should be mentioned. I am not sure why I didn't mention it in my edit... CoronalMassAffection 𝛿 ^talk_contribs 03:44, 5 April 2024 (UTC)