Alfvén Amplifier in the Solar Atmosphere

Since their discovery, Alfvén waves have been studied in relation to the formation of spicules, the acceleration of the solar wind and the heating of the solar atmospehre. It is shown that under certain conditions magnetic flux tubes become amplifiers of Alfvénic disturbances that are generated at their footpoints through random convective motions. The amplification mechanism can be explained in terms of over-reflection of incompressible Alfvénic disturbances in compressible plasma flows. The process does not require flow shear or super-Alfvénic speeds. The implications of these results will be discussed in the context of recently observed large nonthermal velocities and associated blue-shifts.