Solar polarimetry in the K I $D_2$ line: A novel possibility for a stratospheric balloon

Abstract

Of the two solar lines, K I $D_1$ and $D_2$, almost all attention so far has been devoted to the $D_1$ line, as $D_2$ is severely affected by an O${}_2$ 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${}_2$ band on the K I $D_2$ 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${}_2$ is still present, albeit much weaker, at the expected balloon altitude. We applied the obtained O${}_2$ transmittance to K I $D_2$ synthetic polarimetric spectra and found that in the absence of line-of-sight motions, the residual O${}_2$ has a negligible effect on the K I $D_2$ line. On the other hand, for Doppler-shifted K I $D_2$ data, the residual O${}_2$ might alter the shape of the Stokes profiles. However, the residual O${}_2$ 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 $D_2$ 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.

8 pages, 9 figures, main journal publication

References in corpus (3)

• The second flight of the SUNRISE balloon-borne solar observatory: overview of instrument updates, the flight, the data and first results
• Solar magnetoconvection and small-scale dynamo: Recent developments in observation and simulation
• Solar polarimetry through the K I lines at 770 nm

Cited by in corpus (7)

• DeSIRe: Departure coefficient aided Stokes Inversion based on Response functions
• Sunrise III: Overview of Observatory and Instruments
• Study of the polarization produced by the Zeeman effect in the solar Mg I b lines
• Understanding Sun-as-a-star variability of solar Balmer lines
• The Sunrise Chromospheric Infrared Spectro-Polarimeter SCIP: an instrument for SUNRISE III
• The IAG Solar Flux Atlas: Telluric Correction With a Semi-Empirical Model
• Development of Fast and Precise Scan Mirror Mechanism for an Airborne Solar Telescope