Evidence for the Operation of the Hanle and Magneto-optical Effects in the Scattering Polarization Signals Observed by CLASP2 across the Mg ii h and k Lines

Abstract

Radiative transfer investigations of the solar Mg ii h and k resonance lines around 280 nm have shown that, while their circular polarization (Stokes V) signals arise from the Zeeman effect, the linear polarization profiles (Stokes Q and U) are dominated by the scattering of anisotropic radiation and the Hanle and magneto-optical (MO) effects. Using the unprecedented observations of the Mg ii and Mn i resonance lines obtained by the Chromospheric LAyer Spectro-Polarimeter (CLASP2), here we investigate how the linear polarization signals at different wavelengths (i.e., at the center, and at the near and far wings of the k line) vary with the longitudinal component of the magnetic field (B L ) at their approximate height of formation. The B L is estimated from the V signals in the aforementioned spectral lines. Particular attention is given to the following quantities that are expected to be influenced by the presence of magnetic fields through the Hanle and MO effects: the sign of the U signals, the total linear polarization amplitude (LP) and its direction (χ) with respect to a reference direction. We find that at the center and near wings of the k line, the behavior of these quantities is significantly different in the observed quiet and plage regions, and that both LP and χ seem to depend on B L . These observational results are indicative of the operation of the Hanle effect at the center of the k line and of the MO effects at the near wings of the k line.

Magnetic Field Information in the Near-ultraviolet Fe ii Lines of the CLASP2 Space Experiment

“…Motivated by several theoretical studies on the polarization of the Mg II h and k lines around 280 nm (Belluzzi & Trujillo Bueno 2012;Alsina Ballester et al 2016;del Pino Alemán et al 2016;del Pino Alemán et al 2020), the Chromospheric LAyer SpectroPolarimeter (CLASP2; Song et al 2022) suborbital mission was launched on 2019 April 11, achieving unprecedented observations of the four Stokes parameters of this near-UV spectral region in an active region plage close to the solar disk center and in a quiet Sun region near the limb. The analysis of the quiet Sun spectral profiles confirmed the theoretical predictions on the scattering polarization signals (Rachmeler et al 2022;Ishikawa et al 2023). The application of the weak-field approximation (WFA) to the circular polarization profiles observed in the plage region allowed for the mapping of the longitudinal component of the magnetic field (B L ) across different layers of the solar chromosphere (Ishikawa et al 2021), extended to the photosphere with simultaneous observations by Hinode/SOT-SP (Kosugi et al 2007).…”