  The recently discovered resonant drag instability of dust settling in
protoplanetary disc is considered as the mode coupling of subsonic gas-dust
mixture perturbations. This mode coupling is coalescence of two modes with
nearly equal phase velocities: the first mode is inertial wave having positive
energy, while the second mode is a settling dust wave (SDW) having negative
energy as measured in the frame of gas environment being at rest in vertical
hydrostatic equilibrium. SDW is a trivial mode produced by the bulk settling of
dust, which transports perturbations of dust density. The phase velocity of SDW
is equal to the bulk settling velocity times the cosine of the angle formed by
the wave vector and the rotation axis. In this way, the bulk settling of dust
makes possible the coupling of SDW with the inertial wave and the onset of the
instability. In accordance with the concept of the mode coupling, the
instability growth rate is proportional to the square root of the dispersion
equation coupling term, which itself contains the small mass fraction of dust
in gas-dust mixture, the squared radial wavenumber of the modes, and the
squared bulk settling velocity. Thus, the higher is the bulk settling velocity,
the heavier clumps of dust can be aggregated by the instability of the same
rate.
