Does the Streaming Instability exist within the Terminal Velocity Approximation?
Terminal velocity approximation is appropriate to study the dynamics of gas-dust mixture with solids tightly coupled to the gas. This work reconsiders its compatibility with physical processes giving rise to the resonant Streaming Instability in the low dust density limit. It is shown that the linearised equations have been commonly used to study the Streaming Instability within the terminal velocity approximation actually exceed the accuracy of this approximation. For that reason, the corresponding dispersion equation recovers the long wavelength branch of the resonant Streaming Instability caused by the stationary azimuthal drift of the dust. However, the latter must remain beyond the terminal velocity approximation by its physical definition. The refined equations for gas-dust dynamics in the terminal velocity approximation does not lead to the resonant Streaming Instability. The work additionally elucidates the physical processes responsible for the instability.