ИСТИНА

The recently discovered resonant drag instability of dust settling/streaming in protoplanetary disc is considered as the mode coupling of subsonic gas-dust mix- ture 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/streaming 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.