Transverse coronal loop oscillations excited by homologous circular-ribbon flares

Abstract

Aims. We report our multiwavelength observations of two homologous circular-ribbon flares in active region 11991 on 2014 March 5, focusing on the transverse oscillations of an extreme-ultraviolet (EUV) loop excited by the flares. Methods. The flares were observed in ultraviolet and EUV wavelengths by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory spacecraft. These flares were also observed in Hα line center by the 1 m New Vacuum Solar Telescope. Soft X-ray fluxes of the flares in 0.5–4 and 1–8 Å were recorded by the GOES spacecraft. Results. The transverse oscillations are of fast standing kink mode. The first-stage oscillation triggered by the C2.8 flare is decayless with lower amplitudes (310–510 km). The periods (115–118 s) in different wavelengths are nearly the same, indicating coherent oscillations. The magnetic field of the loop is estimated to be 65–78 G. The second-stage oscillation triggered by the M1.0 flare is decaying with larger amplitudes (1250–1280 km). The periods decrease from 117 s in 211 Å to 70 s in 171 Å, implying a decrease of loop length or an implosion after a gradual expansion. The damping time, which is 147–315 s, increases with the period, so that the values of τ/P are close to each other in different wavelengths. The thickness of the inhomogeneous layer is estimated to be ∼0″​​​.45 under the assumption of resonant absorption. Conclusions. This is the first observation of the excitation of two kink-mode loop oscillations by two sympathetic flares. The results are important to understand the excitation of kink oscillations of coronal loops and hence the energy balance in the solar corona. Our findings also validate the prevalence of significantly amplified amplitudes of oscillations by successive drivers.