  We study the coronal dimming caused by the fast halo CME (deprojected speed v
=1250 km s $^{-1})$ associated with the C3.7 two-ribbon flare on 2012 September
27, using Hinode/EIS spectroscopy and SDO/AIA Differential Emission Measure
(DEM) analysis. The event reveals bipolar core dimmings encompassed by
hook-shaped flare ribbons located at the ends of the flare-related polarity
inversion line, and marking the footpoints of the erupting filament. In coronal
emission lines of $\log T \, [{\rm K}] = 5.8-6.3$, distinct double component
spectra indicative of the superposition of a stationary and a fast up-flowing
plasma component with velocities up to 130 km s$^{-1}$ are observed at regions,
which were mapped by the scanning EIS slit close in time of their impulsive
dimming onset. The outflowing plasma component is found to be of the same order
and even dominant over the stationary one, with electron densities in the
upflowing component of $2\times 10^{9}$cm$^{-3}$ at $\log T \, [{\rm K}] =
6.2$. The density evolution in core dimming regions derived from SDO/AIA DEM
analysis reveals impulsive reductions by $40 - 50\%$ within $\lesssim$10 min,
and remains at these reduced levels for hours. The mass loss rate derived from
the EIS spectroscopy in the dimming regions is of the same order than the mass
increase rate observed in the associated white light CME ($1 \times 10^{12}
{\rm \; g \; s}^{-1}$), indicative that the CME mass increase in the
coronagraphic field-of-view results from plasma flows from below and not from
material piled-up ahead of the outward moving and expanding CME front.

Solar and Stellar Astrophysics