Different magnetic field profiles for a moderately expanding tube are plotted in the upper panel with dashed lines. The first profile is constant, the second one decreases by a factor of 25. The corresponding Alfvén speeds are plotted with dotted lines. The flow profile is fixed. The lower two panels display the eigenmode frequencies as functions of the expansion factor which is reciprocal to the magnetic field strength. An unstable mode appears as the tube begins to expand.