Axisymmetric force-free magnetosphere in the exterior of a neutron star II: Maximum storage and open field energies

TL;DR

This paper models the magnetosphere of a magnetar, showing how energy storage can lead to magnetic field opening and potential outbursts, with implications for understanding magnetar activity.

Contribution

It provides numerical solutions for relativistic force-free magnetospheres with a power-law current model, highlighting conditions for maximum energy storage and magnetic field opening.

Findings

Stored energy can reach levels sufficient to open magnetic fields.

Formation of a magnetic flux rope near the star in high-energy states.

States exceeding open field energy are metastable and may trigger outbursts.

Abstract

A magnetar's magnetosphere gradually evolves by the injection of energy and helicity from the interior. Axisymmetric static solutions for a relativistic force-free magnetosphere with a power-law current model are numerically obtained. They provide information about the configurations in which the stored energy is large. The energy along a sequence of equilibria increases and becomes sufficient to open the magnetic field. A magnetic flux rope, in which a large amount of toroidal field is confined, is formed in the vicinity of the star, for states exceeding the open field energy. These states are energetically metastable, and the excess energy may be ejected as a magnetar outburst.