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Of the two solar lines, K I D and D, almost all attention so far has been devoted to the D line, as D is severely affected by an O atmospheric band. This, however, makes the latter appealing for balloon and space observations from above (most of) the Earth's atmosphere. We estimate the residual effect of the O band on the K I D line at altitudes typical for stratospheric balloons. Our aim is to study the feasibility of observing the 770 nm window. Specifically, this paper serves as a preparation for the third flight of the Sunrise balloon-borne observatory. The results indicate that the absorption by O is still present, albeit much weaker, at the expected balloon altitude. We applied the obtained O transmittance to K I D synthetic polarimetric spectra and found that in the absence of line-of-sight motions, the residual O has a negligible effect on the K I D line. On the other hand, for Doppler-shifted K I D data, the residual O might alter the shape of the Stokes profiles. However, the residual O absorption is sufficiently weak at stratospheric levels that it can be divided out if appropriate measurements are made, something that is impossible at ground level. Therefore, for the first time with Sunrise III, we will be able to perform polarimetric observations of the K I D line and, consequently, we will have improved access to the thermodynamics and magnetic properties of the upper photosphere from observations of the K I lines.© ESO 2018. ; C. Quintero Noda acknowledges the support of the ISAS/JAXA International Top Young Fellowship (ITYF). This work was supported by the funding for the international collaboration mission (SUNRISE-3) of ISAS/JAXA. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 695075) and has been supported by the BK21 plus programme through the National Research Foundation (NRF) funded by the Ministry of Education of Korea. This work has also been supported by Spanish Ministry of Economy and Competitiveness through the project ESP-2016-77548-C5-1-R. ; Peer Reviewed

We investigate the diagnostic potential of the spectral lines at 850 nm for understanding the magnetism of the lower atmosphere. For that purpose, we use a newly developed 3D simulation of a chromospheric jet to check the sensitivity of the spectral lines to this phenomenon as well as our ability to infer the atmospheric information through spectropolarimetric inversions of noisy synthetic data. We start comparing the benefits of inverting the entire spectrum at 850 nm versus only the Ca ii 8542 Å spectral line. We found a better match of the input atmosphere for the former case, mainly at lower heights. However, the results at higher layers were not accurate. After several tests, we determined that we need to weight more the chromospheric lines than the photospheric ones in the computation of the goodness of the fit. The new inversion configuration allows us to obtain better fits and consequently more accurate physical parameters. Therefore, to extract the most from multiline inversions, a proper set of weights needs to be estimated. Besides that, we conclude again that the lines at 850 nm, or a similar arrangement with Ca ii 8542 Å plus Zeeman-sensitive photospheric lines, pose the best-observing configuration for examining the thermal and magnetic properties of the lower solar atmosphere. © 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. ; CQN acknowledges the support of the ISAS/JAXA International Top Young Fellowship (ITYF) and the JSPS KAKENHI Grant Number 18K13596. HI is supported by MEXT/JSPS KAKENHI Grant Number JP15H05816. This work was supported by the funding for the international collaboration mission (SUNRISE-3) of the ISAS/JAXA and the JSPS KAKENHI Grant Numbers 18H03723 and 18H05234, respectively, and by the Research Council of Norway through its Centres of Excellence scheme, project number 262622. JCR is supported by grants from the Swedish Research Council (2015-03994), the Swedish National Space Board (128/15), and the Swedish Civil Contingencies Agency (MSB). This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (SUNMAG, grant agreement 759548). This work has also been supported by Spanish Ministry of Economy and Competitiveness through the project ESP-2016-77548-C5-1-R. DOS also acknowledges financial support through the Ramon y Cajal fellowships. ; Peer Reviewed