Tilting Instability of Magnetically Confined Spheromaks. (arXiv:2006.14656v1 [physics.plasm-ph])
<a href="http://arxiv.org/find/physics/1/au:+Mehta_R/0/1/0/all/0/1">Riddhi Mehta</a>, <a href="http://arxiv.org/find/physics/1/au:+Barkov_M/0/1/0/all/0/1">Maxim Barkov</a>, <a href="http://arxiv.org/find/physics/1/au:+Sironi_L/0/1/0/all/0/1">Lorenzo Sironi</a>, <a href="http://arxiv.org/find/physics/1/au:+Lyutikov_M/0/1/0/all/0/1">Maxim Lyutikov</a>
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.
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.
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