Twisted magnetosphere with quadrupolar fields in the exterior of a neutron star

TL;DR

This paper models the twisted magnetosphere of magnetars with dipole and quadrupole fields, revealing how energy buildup can lead to magnetic eruptions and flares, with implications for understanding magnetar outbursts.

Contribution

It provides axisymmetric static solutions for relativistic force-free magnetospheres with mixed dipole-quadrupole fields, highlighting the role of small-scale irregularities and flux ropes in energy storage and release.

Findings

Energy increases along equilibrium sequences, enabling magnetic field opening.

Metastable states form flux ropes near the star, storing large magnetic energy.

General relativity helps confine flux ropes and supports energy storage.

Abstract

The magnetar magnetosphere is gradually twisted by shearing from footpoint motion, and stored magnetic energy also increases at the same time. When a state exceeds a threshold, flares/outbursts manifest themselves as a result of a catastrophic transition. Axisymmetric static solutions for a relativistic force-free magnetosphere with dipole--quadrupole mixed fields at the surface have been calculated. The quadrupole component represents a kind of magnetic-field irregularity at a small scale. Locally twisted models are constructed by limiting current flow regions, where the small part originates from a dipole--quadrupole mixture. The energy along a sequence of equilibria increases and becomes sufficient to open the magnetic field in some models. In energetically metastable states, a magnetic flux rope is formed in the vicinity of the star. The excess energy may be ejected as a magnetar flare/outburst. The general relativistic gravity is sufficient to confine the flux rope and to store huge magnetic energy, and the mechanism is also discussed.