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FU Ori is the prototype of FU Orionis systems which are outburstingprotoplanetary disks. Magnetic fields in FU Ori's accretion disks havepreviously been detected using spectropolarimetry observations for Zeemaneffects. We carry out global radiation ideal MHD simulations to study FU Ori'sinner accretion disk. We find that (1) when the disk is threaded by verticalmagnetic fields, most accretion occurs in the magnetically dominated atmosphereat z$\sim$R, similar to the "surface accretion" mechanism in previouslocally-isothermal MHD simulations. (2) A moderate disk wind is launched in thevertical field simulations with a terminal speed of $\sim$300-500 km/s and amass loss rate of 1-10\% the disk accretion rate, which is consistent withobservations. Disk wind fails to be launched in simulations with net toroidalmagnetic fields. (3) The disk photosphere at the unit optical depth can beeither in the wind launching region or the accreting surface region. Magneticfields have drastically different directions and magnitudes between these tworegions. Our fiducial model agrees with previous optical Zeeman observationsregarding both the field directions and magnitudes. On the other hand,simulations indicate that future Zeeman observations at near-IR wavelengths ortowards other FU Orionis systems may reveal very different magnetic fieldstructures. (4) Due to energy loss by the disk wind, the disk photospheretemperature is lower than that predicted by the thin disk theory, and thepreviously inferred disk accretion rate may be lower than the real accretionrate by a factor of $\sim$2-3.

Global 3-D Radiation Magnetohydrodynamic Simulations for FU Ori's Accretion Disk and Observational Signatures of Magnetic Fields