Investigating the behaviour of neutral hydrogen Lyα spectral line width in polar coronal holes at solar minimum

We investigate the behaviour of the HI Ly alpha spectral line widths measured by UVCS/SOHO in polar coronal holes at minimum of solar magnetic activity. The line widths are reported to significantly increase up to 3 R-circle dot, while above 3 R-circle dot there is observational evidence of either nearly constant or slightly decreasing values. We adopt empirical models of polar coronal holes at solar activity minimum reported in the literature and calculate the characteristic timescales relevant to different processes coupling neutral hydrogen atoms and protons, which are heated and accelerated in the outflowing plasma. This analysis leads us to believe that the progressive decoupling of the two sets of particles below 10 R-circle dot, caused by the decrease of the plasma density due to the rapid expansion of the wind, cannot explain the behaviour of the Ly alpha line profile observed in polar coronal holes. We also synthesise the intensity and profile of the Ly alpha line as a function of heliocentric distance from the coronal hole models, adopting HI densities computed in non-equilibrium ionisation with the aim of satisfactorily reproducing the UVCS Ly alpha observations reported in the literature. Our analysis shows that the coronal Ly alpha emission decreases with heliocentric distance, down to values below the interplanetary Ly alpha emission, owing to the decrease of the plasma density and to non-equilibrium ionisation effects in the expanding plasma. This can lead to the predominance of the interplanetary emission, which is characterised by HI velocity distributions corresponding to temperatures about one order of magnitude lower than the coronal temperatures, and to the narrowing of the resulting coronal profile at higher heliocentric distances. This scenario can be a plausible explanation for the behaviour of the Ly alpha line profile with height observed in polar coronal holes at solar activity minimum.