Ti I lines at 2.2 ${\mu}$m as probes of the cool parts of sunspots

• Main Authors: Smitha, H. N, Durán, J. S. Castellanos, Solanki, S. K, Tiwari, S. K
• Format: Journal Article
• Language: English
• Published: 02-07-2021
• Subjects: Physics - Solar and Stellar Astrophysics
• DOI: 10.48550/arxiv.2107.01247

A&A 653, A91 (2021) The sunspot umbra harbors the coolest plasma on the solar surface due to the
presence of strong magnetic fields. The routinely used atomic lines 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. The lines of the Ti I multiplet at 2.2
$\mu$m are formed mainly at temperatures $\le\!4500$ K and are not known to be
affected by molecular blends in sunspots. Since the first systematic
observations in the 1990's, 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. We
synthesize the Ti I multiplet using a snapshot from 3D MHD simulation of a
sunspot and explore the properties of two of its lines in comparison with two
commonly used iron lines at 630.25 nm and $1.5648\,\mu$m. 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 $\mu=1$. 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, compared to the iron lines. 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.