One-dimensional, convective, vertical structure models and one-dimensional time-dependent, radial diffusion models are combined to create a self-consistent picture in which FU Orionis outbursts occur in young stellar objects (YSOs) as the result of a large-scale, self-regulated, thermal ionization instability in the surrounding protostellar accretion disk. Although active accretion disks have long been postulated to be ubiquitous among low-mass YSOs, few constraints have until now been imposed on physical conditions in these disks. By fitting the results of time-dependent disk models to observed timescales of FU Orionis events, we estimate the magnitude of the effective viscous stress in the inner disk (r less than or similar to 1 AU) to be, in accordance with an ad hoc ''alpha'' prescription, the product of the local sound speed, pressure scale height, and an efficiency factor alpha of 10(-4) where hydrogen is neutral and 10(-3) where hydrogen is ionized. We hypothesize that all YSOs receive infall onto their outer disks which is steady (or slowly declining with time) and that FU Orionis outbursts are self-regulated, disk outbursts which occur only in systems which transport matter inward at a rate sufficiently high to cause hydrogen to be ionized in the inner disk. We estimate a critical mass flux of M(crit) = 5 x 10(-7) M. yr-1 independent of the magnitude of alpha for systems with one solar mass, three solar radius central objects. Infall accretion rates in the range of M(in) = (1-10) x 10(-6) M. yr-1 produce observed FU Orionis timescales consistent with estimates of spherical molecular cloud core collapse rates. Modeled ionization fronts are typically initiated near the inner edge of the disk and propagate out to a disgtance of several tens of stellar radii. Beyond this region, the disk transports mass steadily inward at the supplied constant infall rate. Mass flowing through the innermost disk annulus is equal to M(in) only in a time-averaged sense and is regulated by the ionization of hydrogen in the inner disk such that long intervals (almost-equal-to 1000 yr) of low-mass flux: (1-30) x 10(-8) M. yr-1 are punctuated by short intervals (almost-equal-to 100 yr) of high-mass flux: (1-30) x 10(-5) M. yr-1. Timescales and mass fluxes derived for quiescent and outburst stages are consistent with estimates from observations of T Tauri and FU Orionis systems, respectively.
