# Why are soft x-ray images of plasmas considered to represent magnetic field line configurations?

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.

The charged particles are constrained to move in helical paths along the magnetic field lines. The gyroradius is given by $$r_{\rm gyro} = \frac{mv_{\perp}}{|q| B}$$ and is largest for electrons.
For $$kT \sim 10^{6}$$ K (soft X-ray emitting plasma) then $$v_{\perp} \sim \sqrt{kT/m} = 4 \times 10^{6}$$ m/s and if B-fields are of order $$10^{-7}$$ T (reasonable for the solar corona), then $$r_{\rm gyro} \sim 200$$ m. This small gyroradius is why the charged particles, and hence the X-ray emission, trace the B-field lines - they spiral around the field lines on radii that are much too small to be resolvable with X-ray telescopes.