  We explore the temporal relationship between microwave/HXR emission and
Doppler velocity during the impulsive phase of a solar flare on 2014 October 27
(SOL2014-10-27), which displays a pulse on the light curves in microwave (34
GHz) and hard X-ray (HXR, 25-50 keV) bands before the flare maximum. Imaging
observation shows that this pulse mainly comes from one footpoint of a solar
flare loop. The slit of Interface Region Imaging Spectrograph (IRIS) stays at
this footpoint during this solar flare. The Doppler velocities of Fe XXI
1354.09 A and Si IV 1402.77 A are extracted from the Gaussian fitting method.
We find that the hot line of Fe XXI 1354.09 A (logT~7.05) in corona exhibits
blue shift, while the cool line of Si IV 1402.77 A (logT~4.8) in transition
region exhibits red shift, indicating explosive chromospheric evaporation. The
evaporative upflows along the flare loop are also observed in the AIA 131 A
image. To our knowledge, this is the first report of chromospheric evaporation
evidence from both spectral and imaging observations in the same flare. Both
microwave and HXR pulses are well correlated with the Doppler velocities,
suggesting that the chromospheric evaporation is driven by nonthermal electrons
around this footpoint of a solar flare loop.
