Propagation of Longitudinal Waves in Super-Radially Expanding Solar Plumes

Recent observations indicate that coronal plumes are the preferred channels for the propagation of slow magnetosonic waves from the Sun’s poles to the corona. This problem is of relevance in solar physics because polar plumes are well observed exactly at the heights of the solar wind acceleration. In this chapter, we study the effects of the basal geometric spreading of polar plumes on the propagation of slow-mode waves up to \(r=5R_{\odot }\) by means of a non-linear analysis of the equations of hydrodynamics. We find that super-radial expansion at the base of the flux tube induces a strong dilution of the wave energy flux close to the solar surface, implying a steep decrease of the wave amplitude from the very beginning. Slow waves with periods of 7–25 min diffuse out at heights between \({\approx } {1.6}\) and \(2.4R_{\odot }\) owing to dissipation. This result is in good agreement with recent observations.