Tilting Instability of Magnetically Confined Spheromaks

TL;DR

This paper investigates the tilting instability of magnetically confined spheromaks using 3D MHD and relativistic PIC simulations, with implications for astrophysical plasmas like magnetar magnetospheres.

Contribution

It introduces a detailed analysis of spheromak tilting instability and its rapid magnetic reconnection dynamics, especially in higher order configurations, relevant to astrophysical phenomena.

Findings

Tilting instability occurs on a few Alfvén time scales.

Higher order spheromaks develop internal core ejection and form independent structures.

Helicity is carried away by torsional Alfvén waves during dissipation.

Abstract

We consider the tilting instability of a magnetically confined spheromak using 3D MHD and relativistic PIC calculations with an application to astrophysical plasmas, specifically those occurring in magnetar magnetospheres. The instability is driven by the counter alignment of the spheromak's intrinsic magnetic dipole with the external magnetic field. Initially the spheromak rotates - tilts - trying to lower its magnetic potential energy. As a result a current sheet forms between the internal magnetic field of a spheromak and the confining field. Magnetic reconnection sets in; this leads to the annihilation of the newly counter-aligned magnetic flux of the spheromak. This occurs on few Alfv\'en time scales. In the case of higher order (second order) spheromak, the internal core is first pushed out of the envelope, resulting in formation of two nearly independent tilting spheromaks. Thus, the magnetically twisted outer shell cannot stabilize the inner core. During dissipation, helicity of the initial spheromak is carried away by torsional Alfv\'en waves, violating the assumptions of the Taylor relaxation theorem. In applications to magnetars' giant flares, fast development of tilting instabilities, and no stabilization of the higher order spheromaks, make it unlikely that trapped spheromaks are responsible for the tail emission lasting hundreds of seconds.