COCONUT MHD coronal model as a basis for EUHFORIA 2.0 space weather forecast

B. Kuźma¹ - M. Brchnelova¹ - B. Perri¹ - T. Baratashvili¹ - F. Zhang¹ - A. Lani¹ - S. Poedts^1,2

¹ Center for Mathematical Plasma Astrophysics, Department of Mathematics, KU Leuven, Belgium
² Institute of Physics, University of Maria Curie-Skłodowska, Poland

Abstract
We have developed COCONUT -- a novel MHD solar corona model as a basis for EUHFORIA 2.0 space weather forecasting. The steady-state solution in a numerical domain extending from the solar surface up to 0.1 AU is computed. Here the pre-processed magnetic maps prescribed at the inner boundary determine the magnetic topology. These magnetic maps consist of radial components of Helioseismic and Magnetic Imager magnetograms projected on spherical harmonics with a selected maximum frequency. Within the numerical domain a set of ideal MHD equations with gravity is solved implicitly on an unstructured mesh. This choice of grid allows us to omit the issue of singularities presented at the poles in typically used structured grids. Through heavy parallelization, an optimized CFL profile and other numerical acceleration techniques, we achieved superior convergence times with high robustness and reliability even for numerically challenging cases of maximum solar activity. The MHD solution is generally obtained within a few hours of computation, which is crucial for space weather forecasting systems. Our model passed a series of tests and was further validated using detailed comparison with observations. It successfully recreated magnetic structures (streamers, pseudo streamers, coronal holes) observed during the solar eclipses in March 2015 and in July 2019, thus corresponding to maximum and minimum of solar activity. The COCONUT code will be next extended to include the heating and radiation terms for a more realistic physical description. In the future, a multispecies formulation will be implemented allowing us to consider also the impact of electrons as well as of neutrals on the solution. Figs 3, Refs 10.