Ideal magnetohydrodynamic simulation of magnetic bubble expansion as a model for extragalactic radio lobes

TL;DR

This paper uses ideal magnetohydrodynamic simulations to model magnetic bubble expansion, providing insights into the dynamics relevant for understanding extragalactic radio lobes and guiding related experiments.

Contribution

It presents the first detailed MHD simulation of magnetic bubble expansion tailored to the PBEX experiment, revealing key features of bubble dynamics and asymmetries.

Findings

Expansion depends on the ratio of toroidal to poloidal magnetic fields.

An MHD shock and wavefront form ahead of the bubble.

Asymmetries arise due to reconnection and magnetic inhomogeneities.

Abstract

Nonlinear ideal magnetohydrodynamic (MHD) simulations of the propagation and expansion of a magnetic "bubble" plasma into a lower density, weakly-magnetized background plasma are presented. These simulations mimic the geometry and parameters of the Plasma Bubble Expansion Experiment (PBEX) [A. G. Lynn, Y. Zhang, S. C. Hsu, H. Li, W. Liu, M. Gilmore, and C. Watts, Bull. Amer. Phys. Soc. {\bf 52}, 53 (2007)], which is studying magnetic bubble expansion as a model for extra-galactic radio lobes. The simulations predict several key features of the bubble evolution. First, the direction of bubble expansion depends on the ratio of the bubble toroidal to poloidal magnetic field, with a higher ratio leading to expansion predominantly in the direction of propagation and a lower ratio leading to expansion predominantly normal to the direction of propagation. Second, an MHD shock and a trailing slow-mode compressible MHD wavefront are formed ahead of the bubble as it propagates into the background plasma. Third, the bubble expansion and propagation develop asymmetries about its propagation axis due to reconnection facilitated by numerical resistivity and to inhomogeneous angular momentum transport mainly due to the background magnetic field. These results will help guide the initial experiments and diagnostic measurements on PBEX.