The quiet solar atmosphere observed and simulated in NaD1

TL;DR

This study investigates the formation of the NaD1 line in the quiet Sun, revealing it primarily samples photospheric magnetic concentrations rather than the chromosphere, with implications for understanding solar atmospheric heating.

Contribution

It combines high-resolution observations with 3D NLTE modeling to clarify the NaD1 line's formation and its relation to magnetic structures in the quiet Sun.

Findings

NaD1 brightness mainly samples photospheric magnetic concentrations

Line core sensitive to magneto-acoustic shocks in magnetic regions

Shocks occur deeper in magnetic concentrations, indicating deep heating

Abstract

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.