The solar activity sensitivity of Mn I lines

Abstract. The Mn I 5394.7 A ̊ line is the only spectral line originating in the solar photosphere for which it has been established that it tracks solar magnetic activity in its line-center brightness in the disk-averaged “Sun-as-a-star ” solar irradiance spectrum. This activity sensitivity has been attributed to spectral interlocking to the chromospheric Mg II h&k lines in the ultraviolet via overlap with the Mn I resonance multiplet. However, this explanation does not hold when one accounts properly for partial frequency redistribution in the Mg II h&k lines. A detailed radiation-magnetohydrodynamics simulation shows that the reason for the activity sensitivity of Mn I 5394.7 A ̊ and other Mn I lines is rather that all other photospheric lines show the non-magnetic granulation too bright. The Mn I lines do not suffer such brightening because their cores are widened by large hyperfine structure, so that they are less sensitive to the granular Dopplershifts. This is a purely photospheric effect