The Hanle effect of Ly$\alpha$ in an MHD model of the Solar Transition Region

Abstract

In order to understand the heating of the solar corona it is crucial to obtain empirical information on the magnetic field in its lower boundary (the transition region). To this end, we need to measure and model the linear polarization produced by scattering processes in strong UV lines, such as the hydrogen Ly$\alpha$ line. The interpretation of the observed Stokes profiles will require taking into account that the outer solar atmosphere is highly structured and dynamic, and that the height of the transition region may well vary from one place in the atmosphere to another. Here we report on the Ly$\alpha$ scattering polarization signals we have calculated in a realistic model of an enhanced network region, resulting from a state-of-the-art radiation MHD simulation. This model is characterized by spatially complex variations of the physical quantities at transition region heights. The results of our investigation lead us to emphasize that scattering processes in the upper solar chromosphere should indeed produce measurable linear polarization in Ly$\alpha$. More importantly, we show that via the Hanle effect the model's magnetic field produces significant changes in the emergent $Q/I$ and $U/I$ profiles. Therefore, we argue that by measuring the polarization signals produced by scattering processes and the Hanle effect in Ly$\alpha$ and contrasting them with those computed in increasingly realistic atmospheric models, we should be able to decipher the magnetic, thermal and dynamic structure of the upper chromosphere and transition region of the Sun.

Accepted for publication in The Astrophysical Journal Letters

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• The scattering polarization of the Ly-alpha lines of H I and He II taking into account PRD and J-state interference effects
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• Millimeter radiation from a 3D model of the solar atmosphere I. Diagnosing chromospheric thermal structure
• The magnetic sensitivity of the Mg ii k line to the joint action of Hanle, Zeeman and magneto-optical effects
• The formation of IRIS diagnostics VI. The Diagnostic Potential of the C II Lines at 133.5 nm in the Solar Atmosphere
• Three-dimensional radiative transfer simulations of the scattering polarization of the hydrogen Ly$\alpha$ line in a MHD model of the chromosphere-corona transition region
• The Formation of IRIS Diagnostics. VII. The Formation of the OI 135.56 nm Line in the Solar Atmosphere
• Casting the Coronal Magnetic Field Reconstruction Tools in 3D Using MHD Bifrost Model
• Millimeter radiation from a 3D model of the solar atmosphere II. Chromospheric magnetic field
• Force-Free Field Reconstructions Enhanced by Chromospheric Magnetic Field Data
• On the inversion of the scattering polarization and the Hanle effect signals in the hydrogen Lyman-$\alpha$ line
• Magnetic field diagnostics and spatio-temporal variability of the solar transition region
• Diagnostic potential of wavelength-integrated scattering polarisation signals of the solar He II Ly-alpha line