On the modelling of kink oscillations in fine-structured coronal loops with field-aligned nonlinear longitudinal disturbances.

We investigate the influence of nonlinear longitudinal disturbances, triggered by an initial field-aligned velocity flow, on the damping of standing kink oscillations in fine-structured, cool coronal loop strands under isobaric conditions. Using two-dimensional ideal magnetohydrodynamic (MHD) simulations, we model different realistic flow geometries, bounded, unbounded and external, and quantify their impact on wave excitation, damping and energy leakage. Our modelled loop strand incorporates strong density contrasts (d=5 and d=20), providing a configuration consistent with cool-loop observations from Hinode/SOT and Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA). Our results show that while the nonlinear disturbances exert mild influence on the oscillation period, they substantially modify the damping time. Unbounded flows yield the strongest damping, reducing the damping time by up to approximately 25% compared to bounded flows, caused by enhanced wave-flow coupling and scattering. Longitudinally inhomogeneous flows further intensify the damping, with high-density strands exhibiting faster convergence towards the uniform-flow limit. In cases with supersonic internal flow, slow sausage-mode harmonics and weak slow shocks are additionally excited, indicating the generation of mixed-mode wave responses in flow-dominated loops. These findings demonstrate that realistic, spatially extended nonlinear disturbances play a significant role in the damping of kink oscillations and should be incorporated into forward modelling and coronal seismology diagnostics. This article is part of the theme issue 'Transferable mechanisms in the oscillatory processes of solar and stellar flare'.