  SDO/AIA images the full solar disk in several EUV bands that are each
sensitive to coronal plasma emissions of one or more specific temperatures. We
observe that when isolated active regions (ARs) are on the disk, full-disk
images in some of the coronal EUV channels show the outskirts of the AR as a
dark moat surrounding the AR. Here we present seven specific examples, selected
from time periods when there was only a single AR present on the disk.
Visually, we observe the moat to be most prominent in the AIA 171 Angstrom
band, which has the most sensitivity to emission from plasma at log10 T = 5.8.
By examining the 1D line-of-sight emission measure temperature distribution
found from six AIA EUV channels, we find the intensity of the moat to be most
depressed over the temperature range log10 T ~ 5.7 - 6.2 for most of the cases.
We argue that the dark moat exists because the pressure from the strong
magnetic field that splays out from the AR presses down on underlying magnetic
loops, flattening those loops -- along with the lowest of the AR's own loops
over the moat -- to a low altitude. Those loops, which would normally emit the
bulk of the 171 Angstrom emission, are restricted to heights above the surface
that are too low to have 171 Angstrom-emitting plasmas sustained in them,
according to Antiochos & Noci (1986), while hotter EUV-emitting plasmas are
sustained in the overlying higher-altitude long AR-rooted coronal loops. This
potentially explains the low-coronal-temperature dark moats surrounding the
ARs.
