The physical mechanism of the streaming instability

TL;DR

This paper elucidates the physical mechanism behind the streaming instability, a key process in planet formation, by framing it as a Resonant Drag Instability involving inertial waves and feedback loops, with detailed mathematical analysis.

Contribution

It clarifies the physical and mathematical basis of the streaming instability as a Resonant Drag Instability, detailing the feedback mechanisms and conditions for growth.

Findings

The SI is a Resonant Drag Instability involving inertial waves.

The instability operates through coupled feedback loops, with specific fast and slow mechanisms.

The paper provides a detailed mathematical framework for understanding SI development and saturation.

Abstract

The main hurdle of planet formation theory is the metre-scale barrier. One of the most promising ways to overcome it is via the streaming instability (SI). Unfortunately, the mechanism responsible for the onset of this instability remains mysterious. It has recently been shown that the SI is a Resonant Drag Instability (RDI) involving inertial waves. We build on this insight and clarify the physical picture of how the SI develops, while bolstering this picture with transparent mathematics. Like all RDIs, the SI is built on a feedback loop: in the `forward action', an inertial wave concentrates dust into clumps; in the `backward reaction', those drifting dust clumps excite an inertial wave. Each process breaks into two mechanisms, a fast one and a slow one. At resonance, each forward mechanism can couple with a backward mechanism to close a feedback loop. Unfortunately, the fast-fast loop is stable, so the SI uses the fast-slow and slow-fast loops. Despite this last layer of complexity, we hope that our explanation will help understand how the SI works, in which conditions it can grow, how it manifests itself, and how it saturates.