The Influence of the Gravitational Acceleration on the Supernova-Driven P arker Instability

Abstract

Within a framework of 2D MHD simulations, we explore the dynamical regimes initi ated by a supernova explosion in a magnetized stratified ISM. We concentrate on the formation of large scale magnetic structures and outflows connected with the Parker instability. For the sake of simplicity we show only models with a fixed explosion energy corresponding to a single SN occuring in host galaxies with different fixed values of the gravitational acceleration $g$ and different ratios of specific heats. We show that in general depending on these two parameters, three different regimes are possible: a) a slowly growing Parker instability on time scales much longer than the galac tic rotation period for small $g$, b) the Parker instability growing at roughly the rotation period, which for ratios of specific heats larger than 1 is accompanied byan outflow resulting from the explosion for intermediate $g$, and c) a rapidly g rowing instability and a strong blowout flow for large $g$. By means of numerical simul ations and analytical estimates we show that the explosion energy and gravitational acceleration which separate the three regimes scale as $Eg^2\sim $const in the 2D case. We expect that in the 3D case this scaling law is $Eg^3\sim $const. Our simulations demonstrate furthermore, that a single SN explosion can lead to the growth of multiple Parker loops in the disc and large scale magnetic field loops in the halo, extending over 2-3 kpc horizontally and up to 3 kpc vertically abov e the midplane of the disc.