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So I'm trying to gain an intuitive grasp of plasmas and magnetohydrodynamics.

There's a couple of questions I have.

What does it means magnetic field lines are frozen into "plasmas"? Is it that the field lines will follow the bulk motion of the fluid? I know this follows from the idea that ideal plasmas have no resistivity. What is the qualitative physical explanation for the connection between frozen field lines and no resistivity?

-What are the microscopic arguments to the geometry of magnetic field lines? Naively, I would think if a plasma is in thermodynamic equilibrium and the particles have a tiny mean free path, there wouldn't be a discernible charge current because the charges would move around in random motion. Yet, the Magnetic field lines induced by these charges are not random at all.

-What is the geometry of field lines in a plasma in thermodynamic equilibrium with no bulk motion? For example, let's say we have a 3D cube of plasma, how would the field lines look like?

mathdummy
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  • Some book or another -- maybe Kulsrud? -- has a nice discussion about separating out the microscopic field from the bulk field. I'll see if I can dig it up at some point. –  Jul 18 '15 at 17:20
  • Your last two bullets probably should be posted together as a separate question. – Kyle Kanos Jul 18 '15 at 18:53
  • The answer to the 1st part of your 1st question can be found here. The answer to the 2nd part of the 1st question can be found here. – honeste_vivere Jul 18 '15 at 21:19
  • Regarding your 2nd question, plasmas are almost never in thermodynamic equilibrium. This is largely because most plasmas are at most weakly collisional. Those that are dominated by collisions have other issues. You need to be careful about separating background (i.e., created by an external source) and self-generated (i.e., generated by local charged particles) fields here too. – honeste_vivere Jul 18 '15 at 21:30
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    Regarding your 3rd question, see my last comment about your 2nd question. If there is no bulk motion, no external magnetic fields, and the plasma is at rest (with only thermal fluctuations) then it should not produce any self-generated magnetic fields unless an instability becomes unstable and radiates a wave. – honeste_vivere Jul 18 '15 at 21:32
  • So, when dealing with MHD problems, usually the magnetic field is initially assumed? For example, in our slab of plasma if we can assume initially that the magnetic field points in the z direction. Once making such an assumption, we say that magnetic flux is conserved and therefore if, let's say, a part of the slab is "wrinkled" the field will wrinkle too because it's frozen in. – mathdummy Jul 18 '15 at 21:35

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