  A well known behavior of EUV light curves of discrete coronal loops is that
the peak intensities of cooler channels or spectral lines are reached at
progressively later times than hotter channels. This time lag is understood to
be the result of hot coronal loop plasma cooling through these lower respective
temperatures. However, loops typically comprise only a minority of the total
emission in active regions. Is this cooling pattern a common property of active
region coronal plasma, or does it only occur in unique circumstances,
locations, and times? The new SDO/AIA data provide a wonderful opportunity to
answer this question systematically for an entire active region. We measure the
time lag between pairs of SDO/AIA EUV channels using 24 hours of images of AR
11082 observed on 19 June 2010. We find that there is a time-lag signal
consistent with cooling plasma, just as is usually found for loops, throughout
the active region including the diffuse emission between loops for the entire
24 hour duration. The pattern persists consistently for all channel pairs and
choice of window length within the 24 hour time period, giving us confidence
that the plasma is cooling from temperatures of greater than 3 MK, and
sometimes exceeding 7 MK, down to temperatures lower than ~ 0.8 MK. This
suggests that the bulk of the emitting coronal plasma in this active region is
not steady; rather, it is dynamic and constantly evolving. These measurements
provide crucial constraints on any model which seeks to describe coronal
heating.
