The Quiet Solar Atmosphere Observed and Simulated in Na I D1

The Na I D₁ line in the solar spectrum is sometimes attributed to the solar chromosphere. We study its formation in quiet-Sun network and internetwork. We first present high-resolution profile-resolved images taken in this line with the imaging spectrometer Interferometric Bidimensional Spectrometer at the Dunn Solar Telescope and compare these to simultaneous chromospheric images taken in Ca II 8542 Å and Hα. We then model Na I D₁ formation by performing three-dimensional (3D) non-local thermodynamic equilibrium profile synthesis for a snapshot from a 3D radiation-magnetohydrodynamics simulation. We find that most Na I D₁ brightness is not chromospheric but samples the magnetic concentrations that make up the quiet-Sun network in the photosphere, well below the height where they merge into chromospheric canopies, with aureoles from 3D resonance scattering. The line core is sensitive to magneto-acoustic shocks in and near magnetic concentrations, where shocks occur deeper than elsewhere, and may provide evidence of heating deep within magnetic concentrations.