Ti I lines at 2.2 μm as probes of the cooler regions of sunspots

Abstract

Context. The sunspot umbra harbours the coolest plasma on the solar surface due to the presence of strong magnetic fields. The atomic lines that are routinely used to observe the photosphere have weak signals in the umbra and are often swamped by molecular lines. This makes it harder to infer the properties of the umbra, especially in the darkest regions. Aims. The lines of the Ti I multiplet at 2.2 μm are formed mainly at temperatures ≤4500 K and are not known to be affected by molecular blends in sunspots. Since the first systematic observations in the 1990s, these lines have been seldom observed due to the instrumental challenges involved at these longer wavelengths. We revisit these lines and investigate their formation in different solar features. Methods. We synthesized the Ti I multiplet using a snapshot from 3D magnetohydrodynamic (MHD) simulations of a sunspot and explored the properties of two of its lines in comparison with two commonly used iron lines, at 6302.5 Å and 1.5648 μm. Results. We find that the Ti I lines have stronger signals than the Fe I lines in both intensity and polarization in the sunspot umbra and in penumbral spines. They have little to no signal in the penumbral filaments and the quiet Sun, at μ = 1. Their strong and well-split profiles in the dark umbra are less affected by stray light. Consequently, inside the sunspot, it is easier to invert these lines and to infer the atmospheric properties as compared to the iron lines. Conclusions. The Cryo-NIRSP instrument at the DKIST will provide the first-ever high-resolution observations in this wavelength range. In this preparatory study, we demonstrate the unique temperature and magnetic sensitivities of the Ti multiplet by probing the Sun’s coolest regions, which are not favourable for the formation of other commonly used spectral lines. We thus expect such observations to advance our understanding of sunspot properties.