Safety first: Stability and dissipation of line-tied force-free flux tubes in magnetized coronae

Abstract

Magnetized plasma columns and extended magnetic structures with both footpoints anchored to a surface layer are an important building block of astrophysical dissipation models. Current loops shining in X-rays during the growth of plasma instabilities are observed in the corona of the Sun and are expected to exist in highly magnetized neutron star magnetospheres and accretion disk coronae. For varying twist and system sizes, we investigate the stability of line-tied force-free flux tubes and the dissipation of twist energy during instabilities using linear analysis and time-dependent force-free electrodynamics simulations. Kink modes ($m=1$) and efficient magnetic energy dissipation develop for plasma safety factors $q\lesssim 1$, where $q$ is the inverse of the number of magnetic field line windings per column length. Higher-order fluting modes ($m>1$) can distort equilibrium flux tubes for $q>1$ but induce significantly less dissipation. In our analysis, the characteristic pitch $\tilde{\mu}_0$ of flux tube field lines determines the growth rate ($\propto \tilde{\mu}_0^3$) and minimum wavelength of the kink instability ($\propto \tilde{\mu}_0^{-1}$). We use these scalings to determine a minimum flux tube length for the growth of the kink instability for any given $\tilde{\mu}_0$. By drawing analogies to idealized magnetar magnetospheres with varying regimes of boundary shearing rates, we discuss the expected impact of the pitch-dependent growth rates for magnetospheric dissipation in magnetar conditions.