Discussion of the electromotive force terms in the model of Parker-unstable galactic disks with cosmic rays and shear

We analyze the electromotive force (EMF) terms and basic assumptions of the linear and nonlinear dynamo theories in our three-dimensional numerical model of the Parker instability with cosmic rays and shear in a galactic disk. We also apply the well-known prescriptions of the EMF obtained by the nonlinear dynamo theory of Blackman & Field and Kleeorin et al. to check if the EMF reconstructed from their prescriptions corresponds to the EMF obtained directly from our numerical models. We show that our modeled EMF is fully nonlinear, and it is not possible to apply any of the considered nonlinear dynamo approximations due to the fact that the conditions for the scale separation are not fulfilled.