  The Alfven wave is analyzed in case of one quasineutral plasma propagating
with some constant speed $v_0$ through another static quasineutral plasma. A
dispersion equation is derived describing the Alfven wave coupled with the flow
driven mode $\omega= k v_0$ and solutions are discussed analytically and
numerically. The usual solutions for two oppositely propagating Alfv\'en waves
are substantially modified due to the flowing plasma. More profound is
modification of the solution propagating in the negative direction with respect
to the magnetic field and the plasma flow. For a large enough flow speed
(exceeding the Alfven speed in the static plasma), this negative solution may
become non-propagating, with frequency equal to zero. In this case it
represents a spatial variation of the electromagnetic field. For greater flow
speed it becomes a forward mode, and it may merge with the positive one. This
merging of the two modes represents the starting point for a flow-driven
instability, with two complex-conjugate solutions. The Alfven wave in
interpenetrating plasmas is thus modified and coupled with the flow-driven mode
and this coupled mode is shown to be growing when the flow speed is large
enough. The energy for the instability is macroscopic kinetic energy of the
flowing plasma. The dynamics of plasma particles caused by such a coupled wave
still remains similar to the ordinary Alfven wave. This means that well-known
stochastic heating by the Alfv\'en wave may work, and this should additionally
support the potential role of the Alfven wave in the coronal heating.
