Why Do Torus-unstable Solar Filaments Experience Failed Eruptions?

We study the magnetic field and 3D configuration of 16 filament eruptions during 2010 July–2013 February in order to investigate the factors that control the success and/or failure of solar eruptions. All of these events, i.e., eruptions that failed to be ejected and become coronal mass ejections, have filament maximum heights exceeding 100 Mm. The magnetic field of filament source regions is approximated by a potential field extrapolation method. The filament 3D configuration is reconstructed from three vantage points by the observations of Solar Terrestrial Relations Observatory Ahead/Behind and Solar Dynamics Observatory spacecraft. We calculate the decay index at the apex of these failed filaments and find that in seven cases, their apex decay indexes exceed the theoretical threshold (ncrit = 1.5) of the torus instability (TI). We further determine the orientation change or rotation angle of each filament top during the eruption. Finally, the distribution of these events in the parameter space of rotation angle versus decay index is established. Four distinct regimes in the parameter space are empirically identified. We find that all the torus-unstable cases (decay index n > 1.5) have large rotation angles ranging from 50° to 130°. The possible mechanisms leading to the rotation and failed eruption are discussed. These results imply that, in addition to the TI, the rotation motion during the eruption may also play a significant role in solar eruptions.