  Chromospheric evaporation is observed as Doppler blueshift during solar
flares. It plays one of key roles in dynamics and energetics of solar flares,
however, its mechanism is still unknown. In this paper we present a detailed
analysis of spatially-resolved multi-wavelength observations of chromospheric
evaporation during an M 1.0 class solar flare (SOL2014-06-12T21:12) using data
from the NASA's IRIS (Interface Region Imaging Spectrograph) and HMI/SDO
(Helioseismic and Magnetic Imager onboard Solar Dynamics Observatory)
telescopes, and VIS/NST (Visible Imaging Spectrometer at New Solar Telescope)
high-resolution observations, covering the temperature range from 10^4 K to
10^7 K. The results show that the averaged over the region Fe XXI blueshift of
the hot evaporating plasma is delayed relative to the C II redshift of the
relatively cold chromospheric plasma by about 1 min. The spatial distribution
of the delays is not uniform across the region and can be as long as 2 min in
several zones. Using vector magnetograms from HMI we reconstruct the magnetic
field topology and the quasi-separatrix layer (QSL) and find that the blueshift
delay regions as well as the H-alpha flare ribbons are connected to the region
of magnetic polarity inversion line (PIL) and an expanding flux rope via a
system of low-lying loop arcades with height < ~4.5 Mm. This allows us to
propose an interpretation of the chromospheric evaporation based on the
geometry of local magnetic fields, and the primary energy source associated
with the PIL.
