Magnetoacoustic–Gravity Surface Waves with Evershed flow – A model for Running Penumbral Waves

The penumbral region of a sunspot is modelled as a two-layer plasma. The upper layer with magnetic field is taken with Evershed flow and the static lower layer is assumed to be field-free. The magnetoacoustic–gravity surface wave (MAGSW) propagation along this interface is studied. Our results show that the flow suppresses the fast MAGSW and allows only slow MAGSW. More importantly, we suggest that the running penumbral waves are more likely to be slow MAGSW.