and would evolve towards the stable configuration releasing the respective energy difference. This instability is a possible scenario for the flare activity observed in magnetars.

The lowest energy solutions are the ones that are most similar to the vacuum solutions, with all field lines connected to the surface, while the higher energy solutions are more radially extended, and can contain disconnected field lines.

![image](images/figure3.pdf){width="80.00000%"}

Magnetar interior {#sec:interior}
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The initial models described above provide solutions only for the magnetosphere. For each possible magnetospheric model one can build infinite solutions to describe the neutron star interior. The magnetospheric (exterior) values of $P$ and $T$ determine the magnetic field ${\boldsymbol{B}}$ at the exterior side of the surface (equations \[eq:Br\] to \[eq:Bphi\]). To match this solution to the interior, one has to ensure the continuity of $B^r$ at the surface. This is valid if $P$ is continuous and, hence, $T$ and $B^\varphi$ are continuous as well. However, $B^\theta$ does not necessarily match continuously to the neutron star interior because current sheets (thin current-carrying layers across which the magnetic field changes either direction or magnitude) in the $\varphi$ direction may occur. Even if all components of ${\boldsymbol{B}}$ are continuous at the surface, the magnetic 