Alfvén instability in coronal loops with siphon flows

A new magnetohydrodynamic (MHD) instability in coronal loops is presented. It is demonstrated that small-amplitude torsional Alfvénic disturbances generated by random photospheric motions are inevitably amplified in asymmetrically stratified loops with siphon flows. The loop asymmetry and the flow speeds can be arbitrarily small. The growth rates linked with the instability increase with increasing flow speeds. The instability is caused by over-reflection of Alfvén waves at the transition region where steep variations in density exist.