Self‐generation of magnetic fields by sheared flows in weakly ionized plasmas

A sheared, relative ion–neutral flow can generate a magnetic field in an unmagnetized, weakly ionized plasma. The field generation term is ∂B/∂t=(mec/e)∇×νen(Vi−Vn) where νen is the electron–neutral collision frequency, Vi is the ion fluid velocity, and Vn is the neutral fluid velocity. The time period over which the field grows is limited by diffusion, convection, or collisional relaxation of the relative drift. Since the field generation term scales as νen/Ωe relative to the other terms in the field induction equation, the maximum field generated is found from Ωe≂few νen so that Bmax≂few (mec/e)νen. Both analytical and numerical results are presented. The computational results are based upon a two‐dimensional (2‐D) magnetohydrodynamic (MHD) code which includes the following terms: ion–neutral drag, gravity, resistivity, recombination, the Hall term, and the shear‐driven source term. The theory is applied to the generation of magnetic fields in an unmagnetized planetary ionosphere, such as Venus, and to ...