Time-evolving coronal modelling of solar maximum around the May 2024 storm by COCONUT

TL;DR

To address instability and inefficiency of time-evolving coronal models at solar maximum, this paper enhances the COCONUT model's positivity-preserving properties and drives it with hourly magnetograms over two Carrington rotations surrounding the May 2024 solar storm. It compares time-evolving and quasi-steady-state approaches and investigates how grid resolution affects fidelity, finding that inner-boundary evolution markedly improves realism and that a 6th-level mesh with a 5-minute step balances accuracy and efficiency. The results show that refined grids can boost 0.1 AU magnetic-field strength by more than 40% and enable capture of small dipoles, supporting real-time forecasting; the study outlines future directions, including synchronized magnetograms and higher-resolution local refinements, to establish a robust Sun-to-Earth forecasting capability.

Abstract

Coronal simulations of the solar maximum struggle with poor numerical stability and low computational efficiency since the magnetic field is more complex and stronger and coronal structures evolve more rapidly. This paper aims to enhance the numerical stability of the time-evolving COCONUT coronal model to mitigate these issues, to evaluate differences between the time-evolving and quasi-steady-state coronal simulation results, and to assess the impact of spatial resolution on global MHD coronal modelling of solar maximum.After enhancing the positivity-preserving property of the time-evolving COCONUT, we employ it to simulate the evolution of coronal structures from the solar surface to 0.1 AU over two CRs around the May 2024 solar storm event. These simulations are performed on unstructured meshes containing 6.06, 1.52, and 0.38 M cells to assess the impact of grid resolution. We also conduct a quasi-steady-state coronal simulation, treating the solar surface as a rigidly rotating spherical shell, to demonstrate the impact of magnetic flux emergence and cancellation in global coronal simulations. Comparison with observations further validates the reliability of this model.This paper demonstrates that incorporating magnetic field evolution in inner-boundary conditions can significantly improve the fidelity of global MHD coronal simulations around solar maximum. The simulated magnetic field strength using a refined mesh with 6.06 M cells can be more than 40% stronger than that in the coarser mesh with 0.38 M cells. A time step of 5 minutes and the mesh containing 1.5 M cells can effectively capture the evolution of large-scale coronal structures and small-sized dipoles. Thus, this model shows promise for accurately conducting real-time global coronal simulations of solar maximum, making it suitable for practical applications.

Figures (7)

• Figure 1: Distribution of the radial magnetic field used as the inner boundary condition at the solar surface, shown in a co-rotating coordinate system.
• Figure 2: Distributions of open- and closed-field regions derived from the time-evolving (contours) and quasi-steady-state (orange lines) simulation results. All shown in a co-rotating coordinate system, and the white and black patches represent open-field regions with magnetic field lines pointing outward and inward relative to the Sun, respectively, while the grey patches indicate closed-field regions. The orange solid lines overlaid on these contours denote the edge of close-field regions derived from the quasi-steady-state simulation results.
• Figure 3: White-light pB images observed from COR2/STEREO-A (top) and synthesized from quasi-steady-state (middle) and time-evolving (bottom) coronal simulation results ranging from 2.5 to 15 $R_s$ on the meridian planes in the STEREO-A view. Orange lines highlight magnetic field lines on these selected meridional planes.
• Figure 4: Synoptic maps of east-limb (left) and west-limb (right) white-light pB observations from the SOHO instrument LASCO C2 at 3 $R_s$ for CRs 2284 and 2283, respectively. The dashed orange line and solid gray line represent the corresponding magnetic neutral lines (MNLs) derived from the time-evolving and quasi-steady-state simulations, respectively.
• Figure 5: Timing diagrams of simulated radial velocity $V_r$ ($\rm km~s^{-1}$, top), plasma number density ($\rm 10^3 ~ cm^{-3}$, middle), and temperature ($\rm 10^{5}~K$, bottom) along the latitudes intersected by the Sun–Earth line at 0.1 AU. The dashed orange line and solid gray line represent the corresponding magnetic neutral lines derived from the time-evolving and quasi-steady-state simulations, respectively.