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Whether protoplanetary disks accrete at observationally significant rates bythe magnetorotational instability (MRI) depends on how well ionized they are.Disk surface layers ionized by stellar X-rays are susceptible to chargeneutralization by small condensates, ranging from ~0.01-micron-sized grains toangstrom-sized polycyclic aromatic hydrocarbons (PAHs). Ion densities inX-ray-irradiated surfaces are so low that ambipolar diffusion weakens the MRI.Here we show that ionization by stellar far-ultraviolet (FUV) radiation enablesfull-blown MRI turbulence in disk surface layers. Far-UV ionization of atomiccarbon and sulfur produces a plasma so dense that it is immune to ionrecombination on grains and PAHs. The FUV-ionized layer, of thickness 0.01--0.1g/cm^2, behaves in the ideal magnetohydrodynamic limit and can accrete atobservationally significant rates at radii > 1--10 AU. Surface layer accretiondriven by FUV ionization can reproduce the trend of increasing accretion ratewith increasing hole size seen in transitional disks. At radii < 1--10 AU,FUV-ionized surface layers cannot sustain the accretion rates generated atlarger distance, and unless turbulent mixing of plasma can thicken theMRI-active layer, an additional means of transport is needed. In the case oftransitional disks, it could be provided by planets.

Surface Layer Accretion in Conventional and Transitional Disks Driven by Far-Ultraviolet Ionization