Enhancing MHD model accuracy and CME forecasting by constraining coronal plasma properties with Faraday rotation

¹ Istituto Nazionale di Astrofisica, Osservatorio Astrofisico di Torino, via Osservatorio 20, Pino Torinese 10025, Italy

^⋆ Corresponding author.

Received: 13 January 2025
Accepted: 4 April 2025

Abstract

Accurate forecasting and modeling of coronal mass ejections (CMEs) and their associated shocks are pivotal for understanding space weather and its impact on Earth. This requires a detailed understanding of CMEs’ 3D morphology and the properties of the pre-eruption coronal plasma, which are usually inferred from global 3D numerical magnetohydrodynamic (MHD) simulations. Refining MHD models is thus crucial for improving our understanding of CME-driven shocks and their effects on space weather. Faraday rotation measurements of extragalactic radio sources occulted by the solar corona serve as a powerful complementary tool for probing the pre-eruption electron density and magnetic field structure. These measurements thereby allow us to refine predictions from global MHD models. In this paper, we discuss our recent study of the morphological evolution of a CME-driven shock event that occurred on August 3, 2012. Our analysis used white-light coronagraphic observations from three different vantage points in space: the Solar and Heliospheric Observatory (SOHO) and the twin Solar Terrestrial Relations Observatory (STEREO) spacecraft A and B. Obtaining data from these spacecraft, we derived key parameters such as the radius of curvature of the driving flux rope, the shock speed, and the standoff distance from the CMEs’ leading edge. A notable feature of this event was the availability of rare Faraday rotation measurements of a group of extragalactic radio sources occulted by the solar corona, which were obtained a few hours before the eruption. These observations from the Very Large Array (VLA) radio interferometer provide independent information on the integrated product of the line-of-sight (LOS) magnetic field component and electron density. By modeling the shock standoff distance and using constraints from the Faraday rotation measurements, we achieve a high level of agreement between the fast-mode Mach number predicted by the Magnetohydrodynamic Algorithm outside a Sphere (MAS) code in its thermodynamic mode and the value deduced from the analysis of the 3D reconstruction of coronagraphic data, provided that appropriate correction factors (f_b≃2.4 and f_n≃0.5) are applied in advance to scale the simulated magnetic field and electron density, respectively. Our results are consistent with previous estimates and provide critical information for fine-tuning future MHD simulations.