Spectral line width decrease in the solar corona: resonant energy conversion from Alfv{é}n to acoustic waves

Abstract

Observations reveal an increase with height of the line width of several coronal spectral lines probably caused by outwardly propagating Alfv{é}n 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 Alfven speeds may approach unity. Qualitative analysis shows that the resonant energy conversion from Alfven to acoustic waves near the region of the corona where the plasma $β$ approaches unity may explain the observed spectral line width reduction.