  Magnetic reconnection is believed to be the prime mechanism to trigger solar
flares and accelerate electrons up to energies of MeV. In the classical
two-dimensional reconnection model, the separation motion of chromospheric
ribbons manifests the successive reconnection that takes place higher up in the
corona. Meanwhile, downward traveling energetic electrons bombard the dense
chromosphere and create hard X-ray (HXR) emissions, which provide a valuable
diagnostic of electron acceleration. Analyses of ribbon dynamics and HXR
spectrum have been carried out separately. In this Letter, we report a study of
the comparison of reconnection electric field measured from ribbon motion and
hardness (spectral index) of X-ray emission derived from X-ray spectrum. Our
survey of the maximum average reconnection electric field and the minimum
overall spectral index for 13 two-ribbon flares show that they are strongly
anti-correlated. The former is also strongly correlated with flare magnitude
measured using the peak flux of soft X-ray emissions. These provide strong
support for electron acceleration models based on the electric field generated
at reconnecting current sheet during flares.
