michhaa

• Haahr, Michael; Gudiksen, Boris Vilhelm & Nordlund, Åke (2025). Two-way coupled particle-in-cell and magnetohydrodynamic simulation of 2D fan-spine reconnection. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 701. doi: 10.1051/0004-6361/202555813. Fulltekst i vitenarkiv Vis sammendrag

  Context. Magnetic reconnection is a key mechanism for energy release in the solar atmosphere, but its kinetic-scale microphysics remains difficult to model in large-scale solar geometries. Aims. We investigate whether fully kinetic particle-in-cell (PIC) simulations can be stably and meaningfully embedded within global magnetohydrodynamic (MHD) models of the solar corona using a realistic fan-spine magnetic configuration. Methods. We employed a two-way coupled PIC-MHD scheme implemented in the DISPATCH code framework. The PIC solver is embedded within a reconnecting current sheet in a solar-like topology. A physical adjustment of constants is used to bridge kinetic and fluid scales while maintaining self-consistent plasma ordering. Results. The system evolves stably over more than 45 000 ion plasma periods, exhibiting clear kinetic signatures such as Hall-driven quadrupolar magnetic fields, a reconnection geometry reminiscent of the Petschek configuration, and supra-thermal particle populations. The reconnection rate in the PIC region remains steady and physically consistent, while coupling artefacts are effectively suppressed by fade-in/fade-out boundary weighting. Conclusions. Our results demonstrate that fully kinetic reconnection can be embedded in global solar MHD models with physical fidelity and computational efficiency. This hybrid approach offers a practical pathway to multi-scale kinetic modelling in realistic astrophysical environments.

• Haahr, Michael; Gudiksen, Boris Vilhelm & Nordlund, Åke (2025). Coupling particle-in-cell and magnetohydrodynamics methods for realistic solar flare models. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 696. doi: 10.1051/0004-6361/202452117. Fulltekst i vitenarkiv Vis sammendrag

  Context. Simulating solar flares requires capturing both large-scale magnetohydrodynamic (MHD) evolution and small-scale kinetic processes near reconnection sites. Bridging these scales has been a significant computational challenge. Aims. This study introduces a Particle-In-Cell (PIC) solver integrated within the DISPATCH framework, facilitating seamless embedding within MHD simulations. This development aims to enable self-consistent multi-scale solar flare simulations. Methods. Our PIC solver, inspired by the PhotonPlasma code, addresses the Vlasov-Maxwell equations in collisionless plasma. We validate its accuracy through fundamental plasma tests – including plasma oscillations, two-stream instability, and current sheet reconnection. To make kinetic simulations computationally feasible, we employ physical adjustment of constants (PAC), modifying the speed of light, elementary charge, and electron mass to shift plasma scales. Additionally, we implement and validate a coupling strategy that enables smooth transitions between kinetic and fluid regimes. Results. The PIC solver successfully recovers expected plasma dynamics and electromagnetic field behaviour. Our analysis highlights the effects of PAC on reconnection dynamics, underscoring the importance of transparent and well-documented scaling choices. Test cases involving propagating waves across PIC-MHD interfaces confirm the robustness of our coupling approach. Conclusions. The integration of the PIC solver into the DISPATCH framework makes it possible to run self-consistent, multiscale solar flare simulations. Our approach provides a computationally efficient foundation for investigating reconnection physics in large-scale astrophysical plasmas.

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• Gudiksen, Boris Vilhelm; Haahr, Michael & Øyre, Eilif Sommer (2025). A multi-scale implementation. Fulltekst i vitenarkiv Vis sammendrag

  A description of the work on multi-physics modelling of the solar atmosphere happening at the Rosseland centre for Solar physics, University of Oslo

• Haahr, Michael (2024). Towards Realistic Solar Flare Models: A PIC-MHD hybrid code in DISPATCH. Fulltekst i vitenarkiv
• Haahr, Michael; Gudiksen, Boris Vilhelm & Nordlund, Åke (2025). Multi-Scale Modelling of Magnetic Reconnection in the Solar Corona. A Two-Way Coupled PIC–MHD Model Bridging Microphysics and Global Dynamics in the Exascale Era. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  Solar flares are among the most energetic events in the solar system, powered by magnetic reconnection in the corona where stressed magnetic fields rapidly reorganise, releasing energy as heat, radiation, and accelerated particles. Understanding this multi-scale process is vital for advancing solar and space plasma physics, as well as for forecasting space weather that can disrupt satellites, communications, and power grids on Earth. A central challenge in modelling solar flares lies in the extreme disparity between global and kinetic scales. While magnetohydrodynamic (MHD) models efficiently describe large-scale coronal dynamics, they fail to resolve the microphysics of reconnection and particle acceleration. Fully kinetic methods, such as Particle-in-Cell (PIC) simulations, capture these effects but remain computationally unfeasible at global scales. This thesis presents the development and application of a two-way coupled PIC–MHD model within the DISPATCH framework, enabling localised kinetic resolution embedded within a global MHD domain. The PIC solver, developed and validated within DISPATCH, incorporates a flexible unit-agnostic formulation. To bridge fluid and kinetic scales, we employ the Physical Adjustment of Constants (PAC) method, rescaling key physical constants to enlarge kinetic scales without compromising dimensionless plasma parameters. A key result is the successful two-way coupling by averaging across patch interfaces, enabling stable and self-consistent hybrid simulations. The model is applied to a solar flare scenario involving a 2D fan–spine magnetic topology under solar-like plasma conditions. Simulations show that, even with PAC, the reconnection rate within the PIC region remains comparable to the MHD prediction and consistent with expectations for collisionless reconnection, albeit on the lower end of the range. PAC broadens the current sheet, stabilising it and suppressing secondary plasmoid formation in most cases. However, secondary plasmoids still emerge in the most realistic PAC setup, indicating a partial return of tearing activity. Crucially, the simulations demonstrate sustained acceleration of suprathermal particles, With non-thermal protons dominating the energy budget—an effect rarely explored in solar contexts. Kinetic signatures, notably quadrupolar Hall magnetic fields, extend beyond the PIC region and perturb the global magnetic field topology. These results demonstrate that hybrid PIC–MHD modelling with localised kinetic resolution and physics adjusted by PAC can capture the global dynamics and essential microphysics of solar flare reconnection. The framework developed here enables realistic, multiscale simulations that reveal how kinetic processes influence the release of magnetic energy in the solar corona.

• Haahr, Michael; Gudiksen, Boris Vilhelm & Nordlund, Aake (2025). Multi-Scale Modelling of Magnetic Reconnection in the Solar Corona: A Two-Way Coupled PIC-MHD Model Bridging Microphysics and Global Dynamics in the Exascale Era. Universitetet i Oslo. Fulltekst i vitenarkiv

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