  The NaD1 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 IBIS at the Dunn Solar Telescope and compare these to
simultaneous chromospheric images taken in Ca 8542 and Halpha. We then model
NaD1 formation by performing 3D NLTE profile synthesis for a snapshot from a 3D
radiation-magnetohydrodynamics simulation. We find that most NaD1 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 sited deep within magnetic concentrations.
