Spectral line width decrease in the solar corona: resonant energy conversion from Alfvén to acoustic waves

Context. Observations reveal an increase with height of the line width of several coronal spectral lines probably caused by outwardly propagating Alfvén waves. However, the spectral line width sometimes shows a sudden decrease at a height ∼ 0.1–0.2 R⊙, where the ratio of sound to Alfvén speeds may approach unity. Aims. To explain the observed line width reduction in terms of the energy conversion from Alfvén to another type of wave motion. Methods. Weakly non-linear wave-wave interaction in ideal MHD. Results. Qualitative analysis shows that the resonant energy conversion from Alfvén to acoustic waves near the region of the corona where the plasma β approaches unity may explain the observed spectral line width reduction.