Laboratory study of the stability of solar-relevant, arched, line-tied magnetic flux ropes

Coronal mass ejections occur when long-lived magnetic flux ropes (MFR) anchored to the solar surface destabilize and erupt away from the Sun. One potential cause for these eruptions is an ideal MHD instability such as the kink or torus instability. These instabilities have long been studied in axisymmetric fusion devices where the instability criteria are given in terms of the edge safety factor and confining magnetic field decay index. Laboratory experiments have been performed in the Magnetic Reconnection Experiment (MRX), where the stability properties of arched, line-tied MFRs were controlled via the external fields. Previous experiments revealed a class of MFRs that were torus-unstable but kink-stable, which failed to erupt. These `failed-tori' went through a process like Taylor relaxation where the toroidal current was redistributed before the eruption failed. In more recent experiments, we have investigated this behavior via additional diagnostics that measure the current distribution at the foot points and the energy breakdown before and after an event. Measurements have revealed that the current redistribution during a failed torus event can be explained by ideal MHD without the need for non-ideal effects such as reconnection which is necessary for Taylor relaxation.