Is it generally accepted that Field Aligned Currents are caused by Force-Free Fields? I am currently an undergraduate working on a project about FAC (Birkeland Currents) and it seems that most of the sources on the subject are very technical and hard for me to read (particularily because I am not at all familiar with plasma physics). My understanding is that FACs are caused by a Force-Free magnetic field but it is not at all clear to me what exactly causes them.
I looked into Perrat's Physics of the Plasma Universe, Donald Scott's paper about force-free FAC model (not too sure Scott is a good source tbh), D. Southwood and M. Kivelson's paper "An approximate Description of Field-Aligned Currents in a Planetary Magnetic Field".
 A: 
Is it generally accepted that Field Aligned Currents are caused by Force-Free Fields?

In a plasma, a force-free field is one that satisfies $\mathbf{j} \times \mathbf{B} = 0$, where $\mathbf{j}$ is the electric current density and $\mathbf{B}$ is the magnetic field.  One way to guarantee that the system is force-free would be to only allow $\mathbf{j}$ to be parallel to $\mathbf{B}$, i.e., field-aligned.

My understanding is that FACs are caused by a Force-Free magnetic field but it is not at all clear to me what exactly causes them.

No, the field-aligned currents (FACs) are not caused by the force-free magnetic field, at least not in the sense I think you are implying.  A FAC corresponds to a force-free configuration of the magnetic field but that doesn't mean it causes itself (see the circular reasoning there if it did?).  FACs can be caused by all sorts of things from just the simple Lorentz force making it easier to flow along $\mathbf{B}$ than across it, to large-scale kinetic Alfven waves differentially accelerating particles along $\mathbf{B}$ (e.g., electrons can be accelerated better than ions in some cases resulting in a differential flow between oppositely charged particles, i.e., a current).
