tokens: transverse MHD wave derived from 3D MHD numerical simulations and forward modeling. The main factors affecting the intensity modulation in the host loops are the KHI and resonant absorption. Importantly, the KHI vortices (TWIKH rolls) carry over characteristics of the resonant absorption and phase mixing mechanisms, allowing these to be detected with current instrumentation. We have found that the KHI produces significant intensity changes and that emission lines with different temperature sensitivity to either the loop core or the external medium provide insights into different wave modes. At a high spatial resolution and especially in the hot line, the fine strand-like structure generated by the TWIKH rolls can be observed, as described in Antolin et al. ( 2014 ). At a lower spatial resolution corresponding to AIA, we have shown that the detected damping and period can vary depending on the emission line, which can lead to an out-of-phase behavior between the core and boundary lines. These results can be understood from the fact that the boundary line is more sensitive to the KHI dynamics, to resonant absorption and phase mixing. A decay-less oscillation is obtained in the boundary line for any LOS angle and this is the result of periodic brightenings and the coherent motion of the vortices. The vortices, in turn, result from unstable azimuthal Alfvén waves. Therefore, this decay-less oscillation is more reflective of the Alfvén waves coupled resonantly to the kink mode than the global kink mode itself. Resonant absorption transfers the energy of the kink mode to the Alfvén waves, whose dampings are expected to be much lower than that of the kink mode, since they rely on phase mixing and the turbulence resulting from the instability. The Alfvén waves therefore act as an energy reservoir for the TWIKH rolls, which persist over time, leading to the decay-less oscillation. A shorter period is found for the boundary line and is due to phase mixing. Indeed, the azimuthal Alfvén waves in the boundary layer have an increasingly higher phase speed the farther they are from the loop core. Since it is these waves that are enhanced due to resonant absorption and that become K-H unstable, their signal becomes dominant in the boundary line. The fit to the density from the numerical model ends up with a shorter period than that obtained from the core line. This is because the density profile is influenced by the KHI dynamics, which broaden the flux tube, even though the small-scale structure produced by the instability is unresolved. Accordingly, when fitting only the central section of the flux tube along the oscillation axis, which is minimally influenced by resonant absorption and by the KHI, we obtain a period of 256 s, closer to that obtained from the core line. The out-of-phase behavior between the core and boundary lines is ultimately due to the same effect that leads to the out-of-phase behavior between the LOS velocity and the POS motion seen in
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