Protoplanetary Disk Turbulence Driven by the Streaming Instability: Linear Evolution and Numerical Methods

We present local simulations that verify the linear streaming instability that arises from aerodynamic coupling between solids and gas in protoplanetary disks. This robust instability creates enhancements in the particle density in order to tap the free energy of the relative drift between solids and gas, generated by the radial pressure gradient of the disk. We confirm the analytic growth rates found by Youdin and Goodman using grid hydrodynamics to simulate the gas and, alternatively, particle and grid representations of the solids. Since the analytic derivation approximates particles as a fluid, this work corroborates the streaming instability when solids are treated as particles. The idealized physical conditions—axisymmetry, uniform particle size, and the neglect of vertical stratification and collisions—provide a rigorous, well-defined test of any numerical algorithm for coupled particle-gas dynamics in protoplanetary disks. We describe a numerical particle-mesh implementation of the drag force, which is crucial for resolving the coupled oscillations. Finally, we comment on the balance of energy and angular momentum in two-component disks with frictional coupling. A companion paper details the nonlinear evolution of the streaming instability into saturated turbulence with dense particle clumps.