2020/06/30 by Riddhi Mehta, Maxim Barkov, Lorenzo Sironi +1
Physics and Astronomy · #Current sheet #Instability #Ionosphere and magnetosphere dynamics #Magnetar #Magnetic dipole #Magnetic field #Magnetic reconnection #Magnetohydrodynamic drive #Magnetohydrodynamics #Pulsars and Gravitational Waves Research #Solar and Space Plasma Dynamics #Spheromak #astro-ph.HE #physics.plasm-ph
paper · pdf · doi:10.1017/s0022377820000768
published as J. Plasma Phys. 86 (2020) 905860407 · 20 pages, 10 figures, accepted for publication in Journal of Plasma Physics
openalex created_date 2020/07/02 · arxiv created 2020/07/09 · openalex publication_date 2020/08/01 · arxiv updated 2020/08/26 · openalex updated_date 2026/08/06
We consider the tilting instability of a magnetically confined spheromak using three-dimensional magnetohydrodynamic and relativistic particle-in-cell 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 a few Alfvén time scales. In the case of a 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 Alfvén waves, violating the assumptions of the Taylor relaxation theorem. In applications to magnetar 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.