magnetic field is constant ${{\boldsymbol B}}=B_0 \hat{{{\boldsymbol e}}}_z$. The gas velocity field is set to be sub-Keplerian with $u_y=-\Delta v$, and we furthermore perturb all components of the velocity field by small random fluctuations with amplitude $\delta v=0.001$, to seed modes that are unstable to the magnetorotational instability. In simulations with particles we give particles random initial positions and zero velocity.

![image](rhomx_t.eps){width="0.8\linewidth"}

Gas evolution {#s:gas}
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We start by describing the evolution of gas without particles, since the large-scale geostrophic pressure bumps appearing in the gas controls particle concentration and thus the overall ability for planetesimals to form by self-gravity. The most important agent for driving gas dynamics is the magnetorotational instability [MRI, @BalbusHawley1991] which exhibits dynamical growth when the vertical component of the magnetic field is not too weak or too strong. The non-stratified MRI saturates to a state of non-linear subsonic fluctuations [e.g. @Hawley+etal1995]. In this state there is an outward angular momentum flux through hydrodynamical Reynolds stresses $\langle \rho u_x u_y \rangle$ and magnetohydrodynamical Maxwell stresses $\langle -\mu_0^{-1} B_x B_y \rangle$.

In [Fig. \[f:stress\_t\]]{} we show the Maxwell and Reynolds stresses as a function of time. Using a mean vertical field of $B_0=0.0015$ (corresponding to plasma-beta of 