Solar polarimetry in the K I D₂ line : A novel possibility for a stratospheric balloon

¹ Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa 252-5210, Japan
e-mail: carlos@solar.isas.jaxa.jp
² NASA Goddard Space Flight Center, Planetary Systems Laboratory (Code 693), Greenbelt, MD, USA
³ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁴ Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁵ School of Space Research, Kyung Hee University, Yongin, Gyeonggi 446-701, Republic of Korea
⁶ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía, 18008 Granada, Spain
⁷ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
⁸ Departamento de Astrofísica, Univ. de La Laguna, La Laguna, Tenerife 38205, Spain
⁹ SOKENDAI (The Graduate University for Advanced Studies), Sagamihara, Kanagawa 252-5210, Japan
¹⁰ Kwasan and Hida Observatories, Kyoto University, Kurabashira Kamitakara-cho, Takayama-city, 506-1314 Gifu, Japan

Received: 16 October 2017
Accepted: 22 December 2017

Abstract

Of the two solar lines, K I D₁ and D₂, almost all attention so far has been devoted to the D₁ line, as D₂ is severely affected by an O₂ 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 O₂ band on the K I D₂ 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 O₂ is still present, albeit much weaker, at the expected balloon altitude. We applied the obtained O₂ transmittance to K I D₂ synthetic polarimetric spectra and found that in the absence of line-of-sight motions, the residual O₂ has a negligible effect on the K I D₂ line. On the other hand, for Doppler-shifted K I D₂ data, the residual O₂ might alter the shape of the Stokes profiles. However, the residual O₂ 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 D₂ 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.