Quiet-Sun imaging asymmetries in NaI D1 compared with other strong Fraunhofer lines

TL;DR

This study investigates the asymmetry observed in NaI D1 line imaging of the quiet Sun, revealing its cause as a result of line formation physics and magnetic effects, with comparisons to other spectral lines.

Contribution

It provides a detailed explanation of the NaI D1 line asymmetry using both 1D modeling and 3D MHD simulations, highlighting the role of line sensitivity and magnetic concentrations.

Findings

NaI D1 line shows reversed granulation in blue wing and normal in red wing.

The asymmetry is caused by intensity-Dopplershift correlations and line sensitivity.

MgI b2 and CaII 8542 lines do not exhibit the same asymmetry.

Abstract

Imaging spectroscopy of the solar atmosphere using the NaI D1 line yields marked asymmetry between the blue and red line wings: sampling a quiet-Sun area in the blue wing displays reversed granulation, whereas sampling in the red wing displays normal granulation. The MgI b2 line of comparable strength does not show this asymmetry, nor does the stronger CaII 8542 line. We demonstrate the phenomenon with near-simultaneous spectral images in NaI D1, MgI b2, and CaII 8542 from the Swedish 1-m Solar Telescope. We then explain it with line-formation insights from classical 1D modeling and with a 3D magnetohydrodynamical simulation combined with NLTE spectral line synthesis that permits detailed comparison with the observations in a common format. The cause of the imaging asymmetry is the combination of correlations between intensity and Dopplershift modulation in granular overshoot and the sensitivity to these of the steep profile flanks of the NaI D1 line. The MgI b2 line has similar core formation but much wider wings due to larger opacity buildup and damping in the photosphere. Both lines obtain marked core asymmetry from photospheric shocks in or near strong magnetic concentrations, less from higher-up internetwork shocks that produce similar asymmetry in the spatially averaged CaII 8542 profile.