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The NaD1 line in the solar spectrum is sometimes attributed to the solarchromosphere. We study its formation in quiet-Sun network and internetwork. Wefirst present high-resolution profile-resolved images taken in this line withthe imaging spectrometer IBIS at the Dunn Solar Telescope and compare these tosimultaneous chromospheric images taken in Ca 8542 and Halpha. We then modelNaD1 formation by performing 3D NLTE profile synthesis for a snapshot from a 3Dradiation-magnetohydrodynamics simulation. We find that most NaD1 brightness isnot chromospheric but samples the magnetic concentrations that make up thequiet-Sun network in the photosphere, well below the height where they mergeinto chromospheric canopies, with aureoles from 3D resonance scattering. Theline core is sensitive to magneto-acoustic shocks in and near magneticconcentrations, where shocks occur deeper than elsewhere, and may provideevidence of heating sited deep within magnetic concentrations.

The quiet solar atmosphere observed and simulated in NaD1