ON THE INVERSION OF THE SCATTERING POLARIZATION AND THE HANLE EFFECT SIGNALS IN THE HYDROGEN Lyα LINE

Abstract

Magnetic field measurements in the upper chromosphere and above, where the gas-to-magnetic pressure ratio β is lower than unity, are essential for understanding the thermal structure and dynamical activity of the solar atmosphere. Recent developments in the theory and numerical modeling of polarization in spectral lines have suggested that information on the magnetic field of the chromospherecorona transition region could be obtained by measuring the linear polarization of the solar disk radiation at the core of the hydrogen Lyman-α line at 121.6 nm, which is produced by scattering processes and the Hanle effect. The Chromospheric Lyman-α Spectropolarimeter (CLASP) sounding rocket experiment aims to measure the intensity (Stokes I) and the linear polarization profiles (Q/I and U/I) of the hydrogen Lyman-α line. In this paper we clarify the information that the Hanle effect can provide by applying a Stokes inversion technique based on a database search. The database contains all theoretical Q/I and U/I profiles calculated in a one-dimensional semi-empirical model of the solar atmosphere for all possible values of the strength, inclination, and azimuth of the magnetic field vector, though this atmospheric region is highly inhomogeneous and dynamic.We focus on understanding the sensitivity of the inversion results to the noise and spectral resolution of the synthetic observations as well as the ambiguities and limitation inherent to the Hanle effect when only the hydrogen Lyman-α is used. We conclude that spectropolarimetric observations with CLASP can indeed be a suitable diagnostic tool for probing the magnetism of the transition region, especially when complemented with information on the magnetic field azimuth that can be obtained from other instruments.

Indication of the Hanle Effect by Comparing the Scattering Polarization Observed by CLASP in the Lyα and Si iii 120.65 nm Lines

“…Using the parameters listed in Table 1 , since it is insensitive to the Hanle effect. The Lyα line starts to approach the Hanle saturation regime (i.e., the Q/I and U/I signals are only sensitive to the magnetic-field direction) above ∼50G (see Ishikawa et al 2014a). The simultaneous measurements of the scattering polarization in the Lyα and in the Si III line with different sensitivities to the Hanle effect, provide more information to constrain the magnetic field with a larger sensitivity range than that only with the Lyα line.…”

Prospects of Solar Magnetometry—From Ground and in Space

“…This setup maximizes the photon efficiency and allows for high sensitivity polarimetry. To this end, all polarimetric error sources must be controlled by a thorough calibration concept and a rigorous polarimetric error budget tracking (Ishikawa et al 2014a). The linear polarization is modulated by a rotating wave plate made from MgF 2 with a retardance of half a wave, rotating at a period of 4.8 seconds.…”

Three-dimensional simulations of scattering polarization and the Hanle effect in MHD chromospheric models

“…However, to this end, a reliable model atmosphere of the observed region would be needed. Given an observation and a model atmosphere, one can solve the NLTE problem for possible magnetic field configurations and to constrain the solution (e.g., Manso Sainz & Trujillo Bueno 2010, Ishikawa et al 2014.…”