  A hypothesis on the physical mechanism generating the heartbeat instability
in complex (dusty) plasmas is presented. It is suggested that the instability
occurs due to the periodically repeated critical transformation on the boundary
between the microparticle-free area (void) and the complex plasma. The critical
transformation is supposed to be analogous to the formation of the sheath in
the vicinity of an electrode. The origin of the transformation is the loss of
the electrons and ions on microparticles surrounding the void. We have shown
that this hypothesis is consistent with the experimentally measured stability
parameter range, with the evolution of the plasma glow intensity and
microparticle dynamics during the instability, as well as with the observed
excitation of the heartbeat instability by an intensity-modulated laser beam,
tuned to the atomic transition of a working gas.
