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In the highly structured solar corona, resonant absorption is an unavoidablemechanism of energy transfer from global transverse MHD waves to localazimuthal Alfv\'en waves. Due to its localised nature, a direct detection ofthis mechanism is extremely difficult. Yet, it is the leading theory explainingthe observed fast damping of the global transverse waves. However, at odds withthis theoretical prediction, recent observations indicate that in the lowamplitude regime such transverse MHD waves can also appear decay-less, a yetunsolved phenomenon. Recent numerical work has shown that Kelvin-Helmholtzinstabilities (KHI) often accompany transverse MHD waves. In this work, wecombine 3D MHD simulations and forward modelling to show that for currentlyachieved spatial resolution and observed small amplitudes, an apparentdecay-less oscillation is obtained. This effect results from the combination ofperiodic brightenings produced by the KHI and the coherent motion of the KHIvortices amplified by resonant absorption. Such effect is especially clear inemission lines forming at temperatures that capture the boundary dynamicsrather than the core, and reflects the low damping character of the localazimuthal Alfv\'en waves resonantly coupled to the kink mode. Due to phasemixing, the detected period can vary depending on the emission line, with thosesensitive to the boundary having shorter periods than those sensitive to theloop core. This allows to estimate the density contrast at the boundary.

the astrophysical journal. letters

Modelling observed decay-less oscillations as resonantly enhanced  Kelvin-Helmholtz vortices from transverse MHD waves and their seismological  application