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

Abstract

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.