Densities of protons and water ions for uniform and non-uniform magnetic fields. (a) Proton density in the x–z plane in the simulation with a uniform magnetic field (run A). (b) Proton density in the x–y plane in run A (c) Water-ion density in the x–z plane in run A. (d) Water-ion density in the x–y plane in run A. (e) Proton density in the x–z plane in the simulation with a non-uniform magnetic field (run B). (f) Proton density in the x–y plane in run B. (g) Water-ion density in the x–z plane in run B. (h) Water-ion density in the x–y plane in run B. The proton densities are shown on a linear colour scale, while the colour scale for the water ions is logarithmic. The arrows indicate the direction of the ion velocity.