A unified framework for multi-physics simulations of the solar atmosphere

Observations have revealed a wealth of small-scale features in the solar atmosphere that play critical roles in the transport of energy and particles across the photosphere, chromosphere, ands corona (e.g. Wedemeyer et al., 2016). However, despite significant advances over the past few decades, observational tools often lack the sensitivity and resolution needed to resolve small-scale phenomena in three-dimensional space and to measure magnetic fields in the chromosphere and corona. High-resolution numerical simulations have helped mitigate these limitations, yet they still suffer of critical computational constraints, making it necessary to resort to reduced-dimensionality numerical models or simplified atmospheric representations. As a result, fundamental questions in solar physics remain unanswered: What mechanisms drive chromospheric eruptions, solar wind acceleration, and coronal heating? How do non-ideal and multi-fluid effects shape the chromospheric dynamics? What is the magnetic structure of the chromosphere and corona, and how do small-scale phenomena influence it?

One critical missing element for accurately simulating the solar atmosphere is capturing both its highly dynamic small-scale structures and the vastly different plasma conditions across its layers, from the dense, magnetised plasma of the photosphere to the tenuous, million-degree coronal plasma. Crucially, simulating all atmospheric layers within a single physical model while maintaining high accuracy remains computationally unfeasible. Current simulations thus trade physical accuracy for computational feasibility, failing to capture the transition from tightly coupled ion-neutral dynamics in the photosphere to the weakly coupled two-fluid regime of the chromosphere, where ion-neutral interactions play a key role in energy dissipation (Ballester et al., 2018). Understanding this transition is key to elucidate how energy is transported and dissipated across the enigmatic solar atmosphere (Danilovic, 2023).

This project takes a novel approach: coupling multiple physical models, each optimised for a specific atmospheric layer, while ensuring a consistent treatment of their interactions. More precisely, we propose incorporating two state-of-the-art simulation tools — CO5BOLD, which excels at modelling the photosphere and lower chromosphere, and MPI-AMRVAC, which is well suited for simulating the upper chromosphere and corona — into a unified numerical framework. Leveraging modern coupling libraries and exascale computing, we will develop a simulation framework that models the full complexity of the solar atmosphere, including chromospheric two-fluid effects to achieve an unprecedented level of realism. A key innovation is the seamless transition from an MHD description in the lower layers to a two-fluid treatment in the upper chromosphere, enabling a physically consistent representation of energy and particle transport across the solar atmosphere.

The outcomes of this project will provide the solar physics community with an advanced numerical tool for investigating the small-scale processes governing the dynamics of the solar atmosphere with unprecedented realism. This framework will pave the way for high-resolution simulations that offer new insights into energy transport, magnetic structuring, and wave propagation in the solar atmosphere. Beyond solar physics, the methodologies developed here will advance multi-physics coupling strategies for astrophysical and laboratory plasmas. By tackling a fundamental challenge in numerical modelling, this project will provide a crucial step toward a more complete understanding of the Sun’s dynamic atmosphere and its role in shaping the near-Earth space environment.