# Axisymmetric force-free magnetosphere in the exterior of a neutron star

## TL;DR

This paper models axisymmetric force-free magnetospheres of neutron stars, incorporating general relativistic effects, revealing how magnetic energy and helicity accumulate and lead to flux rope detachment and potential giant flares.

## Contribution

It provides new solutions for twisted magnetospheres considering relativistic effects and explores the conditions leading to magnetic eruptions.

## Key findings

- Relativistic effects enhance magnetic flux rope formation.
- Magnetic energy and helicity have upper bounds in the magnetosphere.
- Catastrophic events like giant flares may occur when helicity exceeds limits.

## Abstract

Magnetar magnetospheres gradually become twisted due to shearing by footpoint motion. The axisymmetric solutions for the force-free field with a power-law current model are calculated by taking into account general relativistic effects. Here we show how the magnetic energy and helicity are accumulated along a sequence of equilibria. In a strongly twisted case, a magnetic flux rope, in which a large amount of toroidal field is confined, detaches in the vicinity of the star. It is found this kind of magnetic field line structure is easily produced due to strong relativistic effects compared with the results in flat spacetime. Although the structure is not inherent to the relativity, the confinement is possible even for a smaller power-law index. There is an upper bound on the energy and helicity stored in the magnetosphere. When the helicity is further accumulated, a catastrophic event such as a giant flare may occur.