Ti I lines at 2.2 m as probes of the cool parts of sunspots
arXiv:2107.01247 · doi:10.1051/0004-6361/202141447
Abstract
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 m are formed mainly at temperatures 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 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 . 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.
arXiv commentAccepted for publication in A&A
References in corpus (18)
- Magneto-convection in a sunspot umbra
- Numerical sunspot models: Robustness of photospheric velocity and magnetic field structure
- Spatially coupled inversion of spectro-polarimetric image data I: Method and first results
- Peripheral downflows in sunspot penumbrae
- Structure of sunspot penumbral filaments: a remarkable uniformity of properties
- Numerical simulations of sunspot decay: On the penumbra -- Evershed flow -- moat flow connection
- Brightness, distribution, and evolution of sunspot umbral dots
- Depth-dependent global properties of a sunspot observed by Hinode using the Solar Optical Telescope/Spectropolarimeter
- Stratification of sunspot umbral dots from inversion of Stokes profiles recorded by Hinode
- Detection of the strongest magnetic field in a sunspot light bridge
- Characteristic Dependence of Umbral Dots on their Magnetic Structure
- Temporal Evolution of Velocity and Magnetic Field in and around Umbral Dots
- Experimental Ti I oscillator strengths and their application to cool star analysis
- Vertical flows and mass flux balance of sunspot umbral dots
- Evidence for convection in Sunspot penumbrae
- Probing photospheric magnetic fields with new spectral line pairs
- Using the infrared iron lines to probe solar subsurface convection
- Investigating the relation between sunspots and umbral dots