of the gas must then give rise to vertical perturbations and gradients (see e.g. flow pattern in Fig. 5 of YG05).

Streaming instability as collective drag?
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The above demonstration leads us to question the correspondence suggested by CY10 between the streaming instability and the toy model of @GoodmanPindor2000, which was originally designed for a dust layer-scale instability. In this one-dimensional, single-fluid model, the acceleration is the sum of $g$, a proxy for gravity, pressure and inertial forces, and $-\nu_d(\Sigma)v$ a density-dependent drag acceleration proportional to the velocity $v$ of the fluid. As long as $d\nu_d/d\Sigma \neq 0$, linear analysis shows the system to be overstable (CY10).

The fact that this toy model is one-dimensional appears to prevent it from capturing the specificities of Coriolis forces, unlike other gravitational or inertial terms. It may also be questionable whether in the linear phase, and in the local treatment of the instability, the drag can be considered to be collective in the sense of the above toy model, and what would play the role of the (single) fluid of this model. Certainly, the latter cannot be the gas+dust fluid, as drag is internal to it (see equation \[\[Euler-COM\]\]). But even if we 