(Sorry if I'm using the wrong terms, but I've yet to find any search terms that seem related to what I'm thinking of. If I did know what to search for, I wouldn't be asking this question.)

  • Would an accretion disk of plasma around a compact body see enough difference between how the electrons and protons/nuclei orbit to produce a significant net current?
  • How much difference in orbits would be needed?
  • Is there a name for that (hypothetical) effect?
  • Is this a thing anyone has even looked into?

The motivating idea is that if the plasma around a compact body interacted in some way so that the electrons were on average in slightly smaller/faster orbits than the protons (or the other way around) that this would result in a very high effective current around the body (the mobile charge density is way higher than metal and the velocity differences could be a significant fraction of $c$) and should create a powerful magnetic field.

The fact that electrons and protons/nuclei have very large differences in there charge/mass ratio (and several other parameters) suggests that it's not unreasonable expect them to react differently to the environment in an accretion disk.

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    $\begingroup$ It's a good question! The dynamics in an accretion disk is described by relativistic electrohydrodynamics and this is notoriously complicated. Far too complicated for me so I cannot post an authoritative answer. However it seems very unlikely there would be any large scale charge separation as the energy involved would be prohibitively large. $\endgroup$ Nov 25, 2023 at 9:46
  • $\begingroup$ @JohnRennie How much separation would be needed to be significant? Close to the ISCO, stuff moves a significant fraction of c so I'd think even a few mm of difference would create significant net current. $\endgroup$
    – BCS
    Nov 25, 2023 at 17:21
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    $\begingroup$ There's a neat idea/paper on accretion disks that discusses some of the questions raised here at: ui.adsabs.harvard.edu/abs/2016MNRAS.458.4400B/abstract. Just a note, you won't get a net charge separation for long (i.e., time scales of the inverse of the plasma frequency come to mind) in any plasma unless some significant external forces do work on the particles in somewhat unphysical ways. Net currents? Sure, all over the place... $\endgroup$ Nov 27, 2023 at 22:11
  • $\begingroup$ @honeste_vivere Would gravity and photon pressures potentially be strong enough external forces? At a first guess, separation could only happen up to the point where electrostatic effects are of the same order of magnitude as tidal effects but those could get rather strong in some setups. $\endgroup$
    – BCS
    Nov 27, 2023 at 23:16
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    $\begingroup$ @BCS - Generally near neutron stars the plasma is a pair-plasma, i.e., positrons and electrons. In that case, the constituents have equal masses. This does not mean currents are absent. On the contrary, there are rather large currents due to pair-production and the fast rotation rate of the star core. Near large black holes, the accretion disk can be partially ionized or nearly fully ionized, depending on the heating rates and ionization rates etc. Regardless, there are tons of instabilities that pop up and many of them self-generate currents or feed off of currents... $\endgroup$ Nov 28, 2023 at 14:26


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