Abstract
Observations have revealed two regimes of kink oscillations of coronal loops. Large amplitude oscillations excited by impulsive energy releases such as coronal mass ejections are characterised by their strong damping by resonant absorption. Lower amplitude oscillations may also be excited and sustained by the ubiquitous motions present in the corona and so are characterised as being decayless. We perform numerical simulations to study the oscillation and evolution of coronal loops in a dynamical environment. We investigate the observational signatures of kink oscillations and the Kelvin-Helmholtz instability in terms of high-resolution seismological and spatial data analysis techniques. We find that low amplitude kink oscillations are capable of generating significant changes in the loop profile which can affect estimates of the transverse loop inhomogeneity based on seismological and forward modelling methods. The disparity between methods may be indicative of nonlinear evolution of coronal loops. The influence on forward modelling estimates could also account for previous observational evidence favouring loops having wider inhomogeneous layers.
Highlights
Kink oscillations of coronal loops are periodic displacements of the loop axis
In this paper we have simulated the non-linear evolution of a coronal loop due to Kelvin-Helmholtz instability (KHI) and investigated the effect on the corresponding observational signatures
The low amplitude of our driver corresponds to the decayless regime of kink oscillations and the repetitive perturbations may be considered as an approximation of some intermittent driving mechanism, for example, random buffeting of loop footpoints by photospheric motions
Summary
Kink oscillations of coronal loops are periodic displacements of the loop axis. They were first observed using the Transition Region And Coronal Explorer (TRACE; Handy et al, 1999) in an active region perturbed by a solar flare (Aschwanden et al, 1999; Nakariakov et al, 1999). The inhomogeneous layer width (commonly normalized by the loop radius to produce the parameter ǫ) represents a transverse spatial scale which is important for other physical processes such as phase mixing (Heyvaerts and Priest, 1983) and the Kelvin-Helmholtz instability It is a key physical parameter but difficult to observe directly, which motivates its seismological inference through the damping of kink oscillations. The azimuthal motions will typically appear as unresolved Doppler velocity perturbations due to line-of-sight (LOS) integration of multiple waves and structures (e.g., De Moortel and Pascoe, 2012; Pant et al, 2019) This transfer is a linear and ideal process, subsequent phase-mixing of the Alfvén waves in the inhomogeneous layer can generate small spatial scales which enhance dissipative processes such as viscosity and resistivity (e.g., Pagano and De Moortel, 2017; Pagano et al, 2018).
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