Three‐dimensional Magnetohydrodynamic Modeling of the Gaseous Structure of the Galaxy: Setup and Initial Results

We show the initial results of our 3D MHD simulations of the flow of theGalactic atmosphere as it responds to a spiral perturbation in the potential.In our standard case, as the gas approaches the arm, there is a downwardconverging flow that terminates in a complex of shocks just ahead of themidplane density peak. The density maximum slants forward at high z, preceededby a similarly leaning shock. The latter diverts the flow upward and over thearm, as in a hydraulic jump. Behind the gaseous arm, the flow falls again,generating further secondary shocks as it approaches the lower z material.Structures similar to the high z part of the gaseous arms are found in theinterarm region of our two-armed case, while broken arms and low column densitybridges are present in the four-armed case. We present three examples of what can be learned from these models.