I'm learning about plasma physics and I've seen a number of papers which state something like 'soft x-ray pictures reveal the magnetic field lines', and it is common to use a soft x-ray image of the sun to demonstrate the magnetic field lines of a flare. Here's an example: https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.82.603
In the earth's magnetosphere it is chalked up to charge exchange, eg: https://arxiv.org/abs/1107.0680
However, in the sun is charge exchange still a good explanation? Isn't everything ionized already?
I can't figure out why this bandwidth in particular reveals B field lines, and I haven't been able to find a citation which explains this assumption. Either it's something quite obvious or it's an empirical observation. I found this paper which delineates the empirical observation: https://iopscience.iop.org/article/10.1086/378944/, but I haven't been able to find a physical explanation for this emission on the sun.