acceleration in the sense that a drag coefficient depending upon the fluid density emerges from collective behavior. This system is found to be overstable. It is unclear, however, in what sense the streaming instability conforms to this notion of collective drag since it is local in nature: the equations are not vertically integrated and the drag acceleration does not depend [*ab initio*]{} on the density of the fluid acted upon.

We are therefore motivated to reconsider the origins of dust clumping. In this paper, we revisit the linear streaming instability. In §2, we review the basic equations leading to the YG05 results and propose, in an appropriate limit, a markedly reduced system in which the instability arises. We make the point in §3 that rotational dynamics are essential to understand the streaming instability. In §4, we propose an interpretation of the streaming instability. In §5, we summarise our conclusions.

Streaming instability and reduced systems
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In this section, by way of establishing notation (largely that of YG05), we briefly review the fundamental equations along with some of their more important results. We then propose a significant reduction of the system of equations that involves two successive levels of approximation, yet 