Speaker
Description
Type II spicules often fade rapidly from cool chromospheric lines, commonly interpreted as their heating to transition-region temperatures. Here, we test whether apparent fading can also arise from a primarily kinematic visibility effect. Using high-resolution 2.5D ideal-MHD simulations, we show that transverse oscillations of a super-Alfvénic magnetized jet trigger the Kelvin–Helmholtz instability (KHI), which fragments the initially coherent jet into fine-scale strands. The internal-energy evolution is used only as a control diagnostic; the thermal-energy gain remains small ($\Delta E_{\text{th}}/E_{\text{kin},0} \simeq 5.18\%$), indicating that the modeled fading is not produced by imposed thermodynamic heating. Forward modeling with an optically thin emission proxy ($I \propto \rho^2$) shows that the finite spatial resolution of observations strongly affects detectability. It is found that KHI begins at the jet boundary before the emission-dominating core is sufficiently fragmented to fade observationally. This leads to a visibility delay, $\Delta t = t_{\text{obs}} - t_{\text{phys}}$, where the coherent transverse motion weakens while unresolved line-of-sight velocity dispersion increases. Non-thermal line broadening ($v_{\text{rms}}$) peaks at $\simeq 2.65\text{ km s}^{-1}$ during the simulations, while $\sigma_{\text{nonth}}$ reaches $\simeq 3.20\text{ km s}^{-1}$. We predict that DKIST-like diffraction-limited spatial resolution might recover some apparently faded events as fragmented fine-scale strands, whereas IRIS-like resolution will fail to resolve them.
