  We investigate several key questions of plasma heating in open-field regions
of the corona that connect to the solar wind. We present results for a model of
Alfven-wave-driven turbulence for three typical open magnetic field structures:
a polar coronal hole, an open flux tube neighboring an equatorial streamer, and
an open flux tube near a strong-field active region. We compare time-steady,
one-dimensional turbulent heating models (Cranmer et al., 2007) against fully
time-dependent three-dimensional reduced-magnetohydrodynamics modeling of BRAID
(van Ballegooijen et al., 2011). We find that the time-steady results agree
well with time-averaged results from BRAID. The time-dependence allows us to
investigate the variability of the magnetic fluctuations and of the heating in
the corona. The high-frequency tail of the power spectrum of fluctuations forms
a power law whose exponent varies with height, and we discuss the possible
physical explanation for this behavior. The variability in the heating rate is
bursty and nanoflare-like in nature, and we analyze the amount of energy lost
via dissipative heating in transient events throughout the simulation. The
average energy in these events is 10^21.91 erg, within the "picoflare" range,
and many events reach classical "nanoflare" energies. We also estimated the
multithermal distribution of temperatures that would result from the
heating-rate variability, and found good agreement with observed widths of
coronal differential emission measure (DEM) distributions. The results of the
modeling presented in this paper provide compelling evidence that turbulent
heating in the solar atmosphere by Alfven waves accelerates the solar wind in
open flux tubes.
