  The structure of the inner region of an advection-dominated accretion disk
around a nonrotating black hole is explored by applying asymptotic analysis in
the region just outside the event horizon. We assume that the viscous transport
is described by the standard Shakura-Sunyaev prescription throughout the disk,
including the inner region close to the horizon. One of our goals is to explore
the self-consistency of this assumption by analyzing the causality of the
viscous transport near the black hole. The effects of general relativity are
incorporated in an approximate manner by utilizing a pseudo-Newtonian
gravitational potential. Analysis of the conservation equations yields unique
asymptotic forms for the behaviors of the radial inflow velocity, the density,
the sound speed, and the angular velocity. The specific behaviors are
determined by three quantities; namely, the accreted specific energy, the
accreted specific angular momentum, and the accreted specific entropy. The
additional requirement of passage through a sonic point further constrains the
problem, leaving only two free parameters. Our detailed results confirm that
the Shakura-Sunyaev viscosity yields a well-behaved flow structure in the inner
region that satisfies the causality constraint. We also show that the velocity
distribution predicted by our pseudo-Newtonian model agrees with general
relativity in the vicinity of the horizon. The asymptotic expressions we derive
provide convenient boundary conditions for the development of global models via
numerical integration of the conservation equations.
