When I have an ionized fluid (plasma) and I describe it in a continuum approach I can use the MHD equations (the continuity equation, the equation of motion, the induction equation and the energy equation). However; I am modeling a certain model of a sunspot and there is also an unionized part (neutral part) present. How do I model that part? Can I just drop the induction equation and set B=0 in the other equations (the continuity equation, equation of motion and the induction equation?), because the magnetic field has no influence on the neutral part and so I need 1 equation less to calculate all variables?