  Coronal mass ejections (CMEs) are often associated with coronal dimmings,
i.e. transient dark regions that are most distinctly observed in Extreme
Ultra-violet (EUV) wavelengths. Using Atmospheric Imaging Assembly (AIA) data,
we apply Differential Emission Measure (DEM) diagnostics to study the plasma
characteristics of six coronal dimming events. In the core dimming region, we
find a steep and impulsive decrease of density with values up to 50-70%. Five
of the events also reveal an associated drop in temperature of 5-25%. The
secondary dimming regions also show a distinct decrease in density, but less
strong, decreasing by 10-45%. In both the core and the secondary dimming the
density changes are much larger than the temperature changes, confirming that
the dimming regions are mainly caused by plasma evacuation. In the core
dimming, the plasma density reduces rapidly within the first 20-30 min after
the flare start, and does not recover for at least 10 hrs later, whereas the
secondary dimming tends to be more gradual and starts to replenish after 1-2
hrs. The pre-event temperatures are higher in the core dimming (1.7-2.6 MK)
than in the secondary dimming regions (1.6-2.0 MK). Both core and secondary
dimmings are best observed in the AIA 211 \AA\ and 193 \AA\ filters. These
findings suggest that the core dimming corresponds to the footpoints of the
erupting flux rope rooted in the AR, while the secondary dimming represents
plasma from overlying coronal structures that expand during the CME eruption.
