Radiance Cascades: A Novel High-Resolution Formal Solution for Multidimensional Non-LTE Radiative Transfer

Abstract

Non-LTE radiative transfer is a key tool for modern astrophysics: it is the means by which many key synthetic observables are produced, thus connecting simulations and observations. Radiative transfer models also inform our understanding of the primary formation layers and parameters of different spectral lines, and serve as the basis of inversion tools used to infer the structure of the solar atmosphere from observations. The default approach for computing the radiation field in multidimensional solar radiative transfer models has long remained the same: a short characteristics, discrete ordinates method, formal solver. In situations with complex atmospheric structure and multiple transitions between optically-thick and -thin regimes these solvers require prohibitively high angular resolution to correctly resolve the radiation field. Here, we present the theory of radiance cascades, a technique designed to exploit structure inherent to the radiation field, allowing for efficient reuse of calculated samples, thus providing a very high-resolution result at a fraction of the computational cost of existing methods. We additionally describe our implementation of this method in the DexRT code, and present initial results of the synthesis of a snapshot of a magnetohydrodynamic model of a solar prominence formed via levitation-condensation. The approach presented here provides a credible route for routinely performing multidimensional radiative transfer calculations free from so-called ray effects, and scaling high-quality non-LTE models to next-generation high-performance computing systems with GPU accelerators.