Streaming instabilities in weakly ionized protoplanetary discs: The ambipolar streaming instability

The regions of protoplanetary disks in which planets can form are thought to be weakly ionized, which suggests that nonideal magnetohydrodynamics (MHD) effects play an important role in the disk dynamics and in the planet formation process. In particular, the combined effect of ohmic resistivity and ambipolar diffusion might cause the launch of MHD-driven disk winds. In this context, we focus on the effect of ambipolar diffusion (AD) and examine the stability of a dusty, magnetized disk by employing linear stability analyses and numerical simulations. We show that dust feedback tends to stabilize the MRI oblique modes of the magnetorotational instability (MRI) involved in the ambipolar shear instability. We also find that ambipolar diffusion leads to the onset of a strong resonant drag instability, in which an Alfvén wave is destabilized by the relative drift between the gas and dust components. The main effect of AD is to modify the Alfvén wave frequency, resulting in a large resonance width. The instability is found to have significant growth rates even in dust-poor discs and for tightly coupled particles, which may help to bridge the gap between the growth of dust grains through coagulation and planetesimal formation.