Upper panel: a meridional section of the stellar and planetary magnetic field lines in a magnetohydrostatic regime in which the stellar field is force-free and the planetary field is potential. In this case the flux systems of the two bodies are topologically separated (see Sect. 3.1). The field lines of the star and the planet can release energy by reconnecting at the boundary between their magnetospheres that is highlighted with a red dashed contour. Lower panel: the magnetic field of the star-planet system in a magnetohydrostatic regime in which the stellar field is potential. In this case, field lines interconnecting the two bodies exist. Interconnecting magnetic loops can be stressed by moving their footpoints and accumulate energy that can be successively released by a relaxation to the minimum-energy potential field configuration (see Sect. 3.1 for details).