Space missions have traditionally focused on performing spectropolarimetric observations measuring the four Stokes parameters in a narrow spectral window where one or two photospheric absorption lines of interest are present. For instance, see Hinode/SP (Tsuneta et al. 2008; Lites et al. 2013), SDO/HMI (Pesnell, Thompson & Chamberlin 2012; Schou et al. 2012), and Solar Orbiter/PHI (Gandorfer et al. 2011; Solanki et al. 2015). On the contrary, ground-based telescopes usually have instruments which can cover several spectral lines simultaneously, e.g. THEMIS/MTR (López Ariste, Rayrole & Semel 2000; Paletou & Molodij 2001), SST/CRISP (Scharmer et al. 2003, 2008), DST/IBIS (Cavallini 2006) and DST/SPINOR (Socas-Navarro et al. 2006), Gregor/GRIS (Collados et al. 2012; Schmidt et al. 2012), or ZIMPOL (Povel 2001), among others. In most of the mentioned cases, the light beam occupies almost the full length of the camera in one of its directions due to the larger spectral coverage which directly increases the amount of data generated. Although the data rate is not usually a crucial factor for ground-based telescopes, space-based missions have an extremely limited telemetry. Thus, unless there is a strong reason to expand the wavelength coverage it is highly recommendable to keep focusing on narrow spectral windows which contain a high density of useful lines. In this regard, we find it extremely helpful to perform theoretical studies and observations to define the optimum spectral window for the purposes of a given mission. For example, Hinode/SP was strongly supported by ASP (Elmore et al. 1992) observations and SDO/HMI benefited from ground-based observations but also from specific supporting works like Norton et al. (2006). We performed a similar study in Quintero Noda et al. (2016) aiming to support future missions with chromospheric polarimetry as the main target, for instance Solar-C (Katsukawa & Solar-C Working Groups 2011; Katsukawa et al. 2012; Watanabe 2014; Suematsu & Solar-C Working Group 2016). We concluded that the Ca ii 8542 Å is a unique spectral line which is sensitive to a large range of heights, from the photosphere to the chromosphere (see figs 4 and 6 of the mentioned work). However, its sensitivity to the magnetic field at photospheric layers is low which precludes examining quiet Sun magnetic fields at these heights. Therefore, there is still room for improvement and, for this reason, we decided to examine the solar spectrum at the vicinity of the Ca ii 8542 Å line. In this regard, if we observe additional spectral lines we will increase the number of spectral points with valuable information, particularly if these lines have different heights of formation, which will enhance the accuracy of the inferred atmospheric parameters (for example Asensio Ramos et al. 2007). However, in order to achieve this purpose we have to expand the spectral window which will increase the data rate if we maintain the same spectral sampling. Therefore, it is essential that these additional lines provide valuable information complementing the Ca ii 8542 Å spectral line.
Citation: Schmidt et al. 2012
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