Chromospheric polarimetry through multiline observations of the 850-nm spectral region

Abstract

Future solar missions and ground-based telescopes aim to understand the magnetism of the solar chromosphere. We performed a supporting study in Quintero Noda et al. (2016) focused on the infrared Ca ii 8542Å line and we concluded that is one of the best candidates because it is sensitive to a large range of atmospheric heights, from the photosphere to the middle chromosphere. However, we believe that it is worth to try improving the results produced by this line observing additional spectral lines. In that regard, we examined the neighbour solar spectrum looking for spectral lines that could increase the sensitivity to the atmospheric parameters. Interestingly, we discovered several photospheric lines that greatly improve the photospheric sensitivity to the magnetic field vector. Moreover, they are located close to a second chromospheric line that also belongs to the Ca ii infrared triplet, i.e. the Ca ii 8498Å line, and enhances the sensitivity to the atmospheric parameters at chromospheric layers. We conclude that the lines in the vicinity of the Ca ii 8542Å line not only increase its sensitivity to the atmospheric parameters at all layers, but also they constitute an excellent spectral window for chromospheric polarimetry.

Solar polarimetry through the K i lines at 770 nm

“…We complement the previous study computing the maximum polarization signals using the Bifrost simulation. We focus on the region enclosed by the square in Figure 2 and we perform column by column computations (similar to what we did in Quintero Noda et al 2016Noda et al , 2017 using the one dimensional geometry package of RH. The results are presented in Figure 8 where we also include the line core intensity (top row) to check if the results discovered in the RF to temperature are also found in a more complex atmosphere.…”

Chromospheric polarimetry through multiline observations of the 850-nm spectral region – II. A magnetic flux tube scenario

“…In order to confirm this, we plot in Figure 9 the maximum of the absolute value of the Stokes V RF for the wavelength range λc − 0.6Å ≤ λ ≤ λc + 0.6Å (with λc the line core wavelength), at a given height for both Ca ii lines. This plot is similar to that of Figure 4 in Quintero Noda et al (2017) and allows the visualization of the RF in a one dimensional format. We can see that, in general, the results are similar to those found in the mentioned work where the Ca ii 8498Å forms slightly lower in the atmosphere.…”

“…However, we also closed the work presented in Quintero Noda et al (2017) explaining that there are several studies that we should perform in order to fully understand the capabilities of the 850 nm spectral region and, most important, its limitations. Among these studies we mentioned the inversion of noisy synthetic profiles from realistic simulations, comparing the advantages of inverting a single chromospheric line, for instance the Ca ii 8542Å, versus inverting simultaneously all the spectral lines that fall in the 850 nm window (with different height of formation in the solar atmosphere) or observations with ground-based telescopes of these lines pointing to different magnetic regions.…”

“…Here we summarize the method we use in this work as it is basically the same as in Quintero Noda et al (2017). We synthesise with the nicole code (Socas-Navarro et al 2000, 2015 the full spectrum shown in Figure 1 of the former publication.…”

“…This points out that the horizontal component of the magnetic field (just Bx in this case as it is a 2D simulation) is probably changing in a height scale shorter than the difference in the height of formation between these spectral lines. Although we know that the atmospheric parameters from realistic MHD simulations largely change with height in short scales, it is noteworthy that the spectral lines are sensitive to those changes, something that we could not foresee through the response functions (RF) study in Quintero Noda et al (2017).…”