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CLASP Constraints on the Magnetization and Geometrical Complexity of the Chromosphere-Corona Transition Region

Author/s: J. Trujillo Bueno, J. Štěpán, L. Belluzzi, A. et al.

Reference: 2018 ApJ 866 L15 | Link

Statistical determination of the 3D model of the solar atmosphere (characterized by the degree of magnetization and corrugation of its transition region) that produces theoretical polarization signals in agreement with the CLASP observations. The white point corresponds to a state-of-the-art 3D model, while the 3D models that better explain the CLASP data (see the region inside the white ellipse) have a significantly larger geometrical complexity and a smaller degree of magnetization. For more details see Trujillo Bueno et al. (2018).
Statistical determination of the 3D model of the solar atmosphere (characterized by the degree of magnetization and corrugation of its transition region) that produces theoretical polarization signals in agreement with the CLASP observations. The white point corresponds to a state-of-the-art 3D model, while the 3D models that better explain the CLASP data (see the region inside the white ellipse) have a significantly larger geometrical complexity and a smaller degree of magnetization. For more details see Trujillo Bueno et al. (2018).

The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a suborbital rocket experiment that on 2015 September 3 measured the linear polarization produced by scattering processes in the hydrogen Lyman-alpha line of the solar disk radiation. The line-center photons of this spectral line radiation mostly stem from the chromosphere-corona transition region (TR). These unprecedented spectropolarimetric observations revealed an interesting surprise, namely that there is practically no center-to-limb variation (CLV) in the Q/I line-center signals. Using an analytical model, we first show that the geometrical complexity of the corrugated surface that delineates the TR has a crucial impact on the CLV of the Q/I and U/I line-center signals. Secondly, we introduce a statistical description of the solar atmosphere based on a three-dimensional (3D) model derived from a state-of-the-art radiation magneto-hydrodynamic simulation. Each realization of the statistical ensemble is a 3D model characterized by a given degree of magnetization and corrugation of the TR, and for each such realization we solve the full 3D radiative transfer problem taking into account the impact of the CLASP instrument degradation on the calculated polarization signals. Finally, we apply the statistical inference method presented in a previous paper to show that the TR of the 3D model that produces the best agreement with the CLASP observations has a relatively weak magnetic field and a relatively high degree of corrugation. We emphasize that a suitable way to validate or refute numerical models of the upper solar chromosphere is by confronting calculations and observations of the scattering polarization in ultraviolet lines sensitive to the Hanle effect.

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