Probing chromospheric magnetic field through multi-line spectropolarimetry

Abstract

The chromosphere, situated between the bright photosphere and the remarkably thin transition region that precedes the million-degrees hot corona, is a highly dynamic and intricate layer within the solar atmosphere. By unraveling the magnetic coupling between the chromosphere, photosphere, transition region, and corona, we may gain valuable insights into the mechanisms through which mass and energy are transferred to the corona as well as the generation of the solar wind that expands outward into the surrounding regions of the solar atmosphere. Hence, it is crucial to carry out simultaneous magnetic field measurements at various heights within the solar atmosphere. In this context, multiline spectropolarimetry is a powerful observational technique, allowing for simultaneous inference of magnetic fields at various heights in the solar atmosphere. The most widely used lines to probe the magnetic field of the solar chromosphere are the Ca ii 8542 ˚A and He i 10830 ˚A lines. However, these lines have their limitations. The formation of the He i 10830 ˚A line occurs within a limited range of heights in the upper chromosphere, and its formation relies on incoming EUV radiation from the coronal and transition region. In contrast, the Ca ii 8542 ˚A line forms from the upper photosphere to the mid-chromosphere. However, in flaring active regions, the Ca ii ion gets ionized to the Ca iii ion, potentially causing the Ca ii 8542 ˚A line to sample deeper layers of the solar atmosphere. The Hα line could be an excellent chromospheric diagnostic. Using 3D radiative transfer modeling, theoretical investigations about the formation of the Hα line intensity profiles have shown that the line core images of the Hα line trace chromospheric magnetic features, and the line also retains opacity during heating events such as flares. However, the diagnostic potential of the Hα line to probe the chromospheric magnetic field from the polarization profiles of the Hα line has been investigated, neither theoretically nor observationally. This thesis addresses this challenge through spectropolarimetric observations of the Hα line recorded simultaneously with other widely used chromospheric spectral lines, such as the Ca ii IR triplet. In our first project, we utilized spectropolarimetric observations of a small pore recorded simultaneously in the Hα and Ca ii 8542 ˚A lines from the Dunn Solar Telescope SPINOR instrument. We inferred the stratification of the line-of-sight magnetic field from the non-LTE inversions of the Ca ii 8542 ˚A line and compared it with that inferred from the weak field approximation over the Hα line. The results of this study supported the conclusion that the Hα line probes the chromospheric magnetic field. To carry out further simultaneous multiline spectropolarimetric observations, we have upgraded the Kodaikanal Tower Tunnel Telescope to record three spectral lines simultaneously. Additionally, we discuss the development and installation of an image stabilization system, incorporating a tip-tilt and autoguider system, designed to reduce seeing-induced cross-talk of the polarimetric signal. Our findings demonstrate the tip-tilt system operates with a cut-off frequency of 80 Hz and reduces the seeinginduced cross-talk in the polarization measurements at least by a factor of 2. We used this enhanced setup to conduct simultaneous spectropolarimetric scans in the Hα and Ca ii 8662 ˚A lines. We analyzed the stratification of the magnetic field inferred from a large sunspot, NOAA 13315, observed on 27th May 2023. We concluded that the line core of the Hα line consistently probes the chromospheric magnetic field at higher heights than that probed by the Ca ii IR triplet lines, and in case of localized heating events, the full spectral range of the Hα line becomes sensitive to the chromospheric magnetic field instead of just the line core