cold for significant thermal ionization, except in the very inner disk regions within about ten stellar radii [e.g. @Gammie:96].

@Gammie:96 originally appealed to the ionizing powers of cosmic rays to ionize the disk sufficiently to become MRI “active” in the outer layers, so that accretion proceeds more rapidly with increasing vertical height in a layered fashion. More recent work has found that the MRI is suppressed by ambipolar diffusion in the inner disk region ($\leq 15$ AU; @Bai:13 [@Bai.Stone:13]) and that a layered accretion flow is more applicable to the outer rather than the inner disk. Accretion is instead most likely driven by a magnetized disk wind [@Bai:14]. @Bai:17 has emphasised that the disk wind kinematics and the disk magnetic field geometry and evolution are key to understanding global protoplanetary disk evolution. The ionization state of the disk gas remains a central ingredient for each of these problems. Since numerical models of planet formation in a protoplanetary disk have demonstrated the importance of disk gas dynamics in controlling the types of planets that can be formed [e.g., @Coleman.Nelson:16], disk ionization and its spatial and temporal variation then plays a vital role in both determining the radial structure of the disk, 