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Accreting neutron stars can either gain or loose angular momentum via interaction with their accretion disks. This leads to either spin-up or spin-down. Which of the two is the case, depends on the position at which the plasma couples to the magnetic field lines (see image). So far I kinda get it.

My question is: How exactly is the matter accelerated/decelerated when it "couples" to the magnetic field lines and how does this lead to transfer of angular momentum with the NS?

I guess, some magnetic fields lead to some Lorentz forces on some currents and eventually some torque will transfers angular momentum from the NS to the plasma (and the other way around); but how exactly?

I suspect some of the answers are included in the original work by Ghosh & Lamb 1979 (image below), but I have to admit some parts of their papers go a bit over my head.

enter image description here

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  • $\begingroup$ The explanation I had for myself is the following (but I am not sure whether others would agree): Magnetic fields accelerate charged particles in the accretion disk. If strong enough, magnetic fields may even strip of electrons from atoms and generate charged ions which are in turn accelerated as well. Moving charged particles in dense accretion disks will bounce onto non-charged matter and transfer momentum. $\endgroup$
    – B--rian
    Mar 2, 2021 at 11:21

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