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We study the temporal evolution of the Na I D1 line profiles in the M3.9flare SOL2014-06-11T21:03 UT, using high spectral resolution observationsobtained with the IBIS instrument on the Dunn Solar Telescope combined withradiative hydrodynamic simulations. Our results show a significant increase inline core and wing intensities during the flare. The analysis of the lineprofiles from the flare ribbons reveal that the Na I D1 line has a centralreversal with excess emission in the blue wing (blue asymmetry). We combineRADYN and RH simulations to synthesise Na I D1 line profiles of the flaringatmosphere and find good agreement with the observations. Heating with a beamof electrons modifies the radiation field in the flaring atmosphere and exciteselectrons from the ground state $\mathrm{3s~^2S}$ to the first excited state$\mathrm{3p~^2P}$, which in turn modifies relative population of the twostates. The change in temperature and the population density of the energystates make the sodium line profile revert from absorption into emission.Analysis of the simulated spectra also reveals that the Na I D1 flare profileasymmetries are produced by the velocity gradients generated %and opacityeffects in the lower solar atmosphere.

Observations and simulations of the Na I D1 line profiles in an M-class solar flare