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.

Simultaneous transverse oscillations of a coronal loop and a filament excited by a circular-ribbon flare

“…The amplitude of loop oscillation in this study is slightly lower than that of the EUV loop during its decayless oscillation on 2014 March 5 (Zhang et al 2020a). Terradas et al (2007) examined the energy that an initial disturbance stores in the eigenmodes of coronal loops.…”

“…1 and 5). It is noted that the start times of both decayless and decaying loop oscillations were also coincident with the HXR peaks on 2014 March 5 (Zhang et al 2020a), suggesting that the excitation of kink oscillations is related to the most impulsive release of magnetic energy rather than the very beginning of energy release. Hence, the real speeds of the driver can potentially reach ∼1000 km s −1 , the typical speed of a blast wave (Tothova et al 2011).…”

“…5a. Likewise, it is fitted with a decayless sine function using the standard SSW program mpfit.pro (Zhang et al 2020a):…”

“…Like typical two-ribbon flares, CRFs are also capable of triggering transverse loop oscillations (Li et al 2018b). Zhang et al (2020a) investigated the transverse oscillations of an EUV loop excited by two homologous CRFs on 2014 March 5. Large-amplitude prominence or filament oscillations are divided into two categories according to their directions: transverse and longitudinal oscillations (Hyder 1966;Ramsey & Smith 1966;Kleczek & Kuperus 1969;Tripathi et al 2009;Luna et al 2018).…”

Mapping the magnetic field in the solar corona through magnetoseismology

“…This restriction could be eliminated if we apply magnetoseismology to more ubiquitous and continuous oscillations or waves in the corona. At least two types of such oscillations/waves are known to exist in the corona: the decayless/persistent standing transverse waves in coronal loops observed through extreme-ultraviolet (EUV) imaging and spectroscopic observations [43][44][45][46], and the pervasive propagating transverse waves observed with the Coronal Multi-channel Polarimeter (CoMP, [47]) [48][49][50][51][52][53]. These ubiquitous oscillations/waves, especially the latter, are potentially important for continuous diagnostics of coronal magnetic field [51,54].…”