Guided Waves Along a Density Interface in Partially Ionised Solar Plasmas

This study investigates the properties of waves that propagate along a density interface in partially ionised plasmas, separating two regions of different properties, including ionisation degree. Our analysis covers frequencies that are much smaller than the collisional frequency of particles, so we are using a single-fluid approximation, where the partial ionisation aspect of the plasma appears through the ambipolar diffusion in the generalised Ohm's law. The derived dispersion relation is solved numerically. Our results show that guided waves along a density interface undergo very little change in their propagation speed (frequency); however, their damping rate shows variation with the ionisation degree and plasma- $\beta$ parameter. We find that waves can only propagate when plasma- $\beta >1.2$ , indicating pressure-driven dynamics relevant to photospheric structures with moderate magnetic fields. The damping rate increases with higher neutral particle content but decreases with higher plasma- $\beta$ values. For ionisation degrees close to fully ionised plasma, the damping is minimal but becomes more significant as the neutral particle concentration increases. These findings provide important insights into wave behaviour in partially ionised plasma interfaces and lay the groundwork for future studies of wave propagation in partially ionised plasma slab waveguides.