Kink Oscillation of a Flux Rope During a Solar Failed Eruption

We report a decaying kink oscillation of a flux rope during a confined eruptive flare, observed off the solar limb by SDO/AIA. The erupting flux rope underwent kinking, rotation, and apparent leg–leg interaction during the failed eruption. The oscillations were observed simultaneously in multiple AIA channels, 304, 171, and 193 Å, indicating that multithermal plasma was entrained in the rope. After reaching the overlying loops in the active region and failing to erupt as a CME, the flux rope exhibited large-amplitude decaying kink oscillations with an apparent initial amplitude of 30 Mm, period of ≈16 min, and decay time about 17 min. We interpret these oscillations as a fundamental standing kink mode of the flux rope. The oscillation polarization has a clear vertical component, while the departure of the detected wave form from a sinusoidal signal suggests that the oscillation could be circularly or elliptically polarized. The estimated kink speed is 1080 km s^-1, corresponding to an Alfvén speed of ≈760 km s^-1. This speed and the estimated electron density in the rope from our DEM analysis, n_e ≈ (1.5–2.0) ×10⁹ cm^-3, yield a magnetic field strength of about 15 G. To the best of our knowledge, the decaying kink oscillations of a flux rope with a non-horizontal polarization during a confined eruptive flare have not been reported before. These oscillations provide a unique opportunity for indirect measurements of magnetic-field strength in flux ropes in the low corona during failed eruptions.