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

¹ Sterrekundig Instituut, Utrecht University, Postbus 80 000, 3508 TA Utrecht, The Netherlands
e-mail: R.J.Rutten@uu.nl
² Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Blindern, 0315 Oslo, Norway
³ High Altitude Observatory, NCAR, PO Box 3000, Boulder, CO 80307–3000, USA
⁴ Center of Mathematics for Applications, University of Oslo, PO Box 1053, Blindern, 0316 Oslo, Norway

Received: 30 March 2011
Accepted: 21 April 2011

Abstract

Imaging spectroscopy of the solar atmosphere using the Na I 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 Mg I b₂ line of comparable strength does not show this asymmetry, nor does the stronger Ca II 8542 Å line. We demonstrate the phenomenon with near-simultaneous spectral images in Na I D₁, Mg I b₂, and Ca II 8542 Å 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 Na I D₁ line. The Mg I 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 Ca II 8542 Å profile.