  Coronal loops trace out bipolar, arch-like magnetic fields above the Sun's
surface. Recent measurements that combine rotational tomography, extreme
ultraviolet imaging, and potential-field extrapolation have shown the existence
of large loops with inverted temperature profiles; i.e., loops for which the
apex temperature is a local minimum, not a maximum. These "down loops" appear
to exist primarily in equatorial quiet regions near solar minimum. We simulate
both these and the more prevalent large-scale "up loops" by modeling coronal
heating as a time-steady superposition of: (1) dissipation of incompressible
Alfven-wave turbulence, and (2) dissipation of compressive waves formed by mode
conversion from the initial population of Alfven waves. We found that when a
large percentage (> 99%) of the Alfven waves undergo this conversion, heating
is greatly concentrated at the footpoints and stable "down loops" are created.
In some cases we found loops with three maxima that are also gravitationally
stable. Models that agree with the tomographic temperature data exhibit higher
gas pressures for "down loops" than for "up loops," which is consistent with
observations. These models also show a narrow range of Alfven wave amplitudes:
3 to 6 km/s at the coronal base. This is low in comparison to typical observed
amplitudes of 20 to 30 km/s in bright X-ray loops. However, the large-scale
loops we model are believed to comprise a weaker diffuse background that fills
much of the volume of the corona. By constraining the physics of loops that
underlie quiescent streamers, we hope to better understand the formation of the
slow solar wind.
