High-resolution EUV observations from SDO/AIA and Hinode/EIS
are used, together with updated atomic data and EIS calibration,
to measure the physical characteristics of active region (AR) cores,
and their relation to the other AR structures.
Quiescent AR cores are found to have nearly isothermal
distributions, with very little emission above 3 MK, and with
a strongly peaked distribution in the 1-3 MK range.
This is generally consistent with high frequency nanoflare heating.
The temperature distributions do not change
significantly during the AR evolution. The hot loops
have chemical abundances that are consistent with the Laming
model, where the ponderomotive force of the Alfven waves
produces the observed fractionation.