http://arxiv.org/abs/2201.01504
Inversion of spectropolarimetric observations of the solar upper atmosphere is one of the most challenging goals in solar physics. If we account for all relevant ingredients of the spectral line formation process such as three-dimensional (3D) radiative transfer out of local thermodynamic equilibrium (NLTE), the task becomes extremely computationally expensive. Instead of generalizing 1D methods to 3D, we develop a new approach to the inverse problem. In our meshfree method we do not consider the requirement of 3D NLTE consistency as an obstacle, but as a natural regularization with respect to the traditional pixel-by-pixel methods. This leads to more robust and less ambiguous solutions. We solve the 3D NLTE inverse problem as an unconstrained global minimization problem avoiding repetitive evaluations of the $\Lambda$~operator. Apart from 3D NLTE consistency, the method allows to easily include additional conditions of physical consistency such as zero divergence of the magnetic field. Stochastic ingredients make the method less prone to ending up in local minima of the loss function. Our method is capable of solving the inverse problem by orders of magnitude faster than it would be possible using grid-based methods. The method can provide accurate and physically consistent results if sufficient computing time is available, but also approximate solutions in case of very complex plasma structures or limited computing time.
J. Stepan, T. Aleman and J. Bueno
Thu, 6 Jan 22
7/56
Comments: 18 pages, 13 figures, accepted for publication in A&A