Fabio Bacchini

Context. Although magnetic reconnection in coronal current sheets is a fundamental energy release mechanism in the solar corona, the role of the guide field in governing its formation and self-consistent dynamical and thermal evolution is not fully understood. Aims. We investigate how varying the guide field strength affects the formation and evolution of a localized reconnecting coronal current sheet, as well as its thinning, reconnection rate, and plasma properties. Methods. We performed 2.5D resistive magnetohydrodynamic (MHD) simulations of the formation and subsequent reconnection of coronal current sheets across a diverse range of guide field strengths, tracking the temporal evolution of the reconnection rate, current-sheet geometry, temperature, and outflow velocities. Results. A stronger guide field prevents oppositely directed field lines from approaching sufficiently to form the current sheet, delaying and suppressing its subsequent evolution. The reconnection rate ( ηJ _max ) exhibits a rising phase resembling a power law, with exponent δ ₁ decreasing with increasing guide field, indicating slower temporal growth of the reconnection rate with increasing guide fields. The aspect ratio, which characterizes the thinning of the current sheet, follows a similar power-law trend with the exponent α ₁ also decreasing with the strength of the guide field. A subsequent slower growth phase of ηJ _max shows a power law exponent δ ₂ that increases with guide field, as does the corresponding aspect-ratio exponent α ₂ . During the evolution of the current sheet, the latter phase is achieved due to the sharp decremental transition of the guide field within the current sheet after a time when the increasing inflows are inefficiently resisted by the guide field itself. In the reconnection regime during prior phases, the power law exponents of both the maximum temperature and the outflow velocities also decrease with an increasing guide field demonstrating that stronger guide fields suppress the growth of these observational signatures of reconnection. Conclusions. The present study suggests that in the presence of nonzero guide fields, a self-consistently formed current sheet displays phases of efficient and inefficient quenching of the reconnection mechanism, promoted by the aforementioned guide fields. With increasing guide field strengths, we further quantified this behavior for various indicators of the reconnection process. We also discuss the applicability of the obtained results to observations of apparent current sheets as well as probing background coronal conditions without any knowledge of guide field observations.

We present 2D and 3D particle-in-cell simulations of driven collisionless reconnection triggered by the compression and merging of two force-free flux tubes in a strongly magnetized pair plasma. During current-sheet formation, the evolution remains quasi-2D. Reconnection onset is systematically delayed in 3D, especially for stronger guide fields. Increasing the imposed drive accelerates both tearing and drift-kink growth, while the guide field suppresses coherent drift-kink activity. All runs enter a fast-merging phase with normalized reconnection rates of order 0.08–0.1. The 3D runs show slightly stronger in-plane magnetic-energy depletion and particle energization than their 2D counterparts, indicating a modestly enhanced conversion of reconnecting magnetic energy into particle energy. Faster driving increases the fraction of particles at injection and the nonthermal energy fraction, while a stronger guide field decreases it. In contrast, the spectral slopes of the energy distributions remain similar, p ≃ 1.6–2.0, and the cutoff converges to γ _cut / σ _in ≃ 50 with only a weak dependence on dimensionality, drive strength, or guide field.

Using fully kinetic simulations that capture unprecedentedly large (from electron to ion) scales, we study magnetogenesis driven by continuous large-scale forcing until nonlinear dynamo saturation. We uncover a two-stage mechanism in collisionless ion–electron plasmas whose dynamics diverge dramatically from the pair-plasma case. In the first phase, electron pressure anisotropy triggers electron-Weibel modes, seeding small-scale magnetic fields. Then, a second growth phase emerges when the more massive ions develop their own strong anisotropy and drive ion-Weibel-type modes; concurrently, a Biermann-battery mechanism contributes to amplifying the magnetic field. This combined dynamics provides a tenfold amplification of the magnetic field in comparison to the pair-plasma case. Over long times, dynamo action continues until the system reaches a statistical steady state. This self-consistent kinetic mechanism provides a plausible explanation for robust magnetogenesis wherever an external forcing continuously stirs the plasma.

The Kelvin–Helmholtz instability (KHI) is a major driver of multiscale plasma dynamics at velocity shear layers, where it can promote the formation of current sheets and the onset of magnetic reconnection and drive plasma energization. While recent kinetic studies have shown efficient electron heating during nonlinear KH evolution, the connection between reconnection dynamics and localized electron energization is still not fully understood. We investigate this link using 2D fully kinetic simulations of KHI developing in a double-periodic system with two velocity shear layers and a uniform guide field, initialized from a finite Larmor radius equilibrium. During the nonlinear stage, initially coherent vortices evolve into layers populated by fragmented current sheets containing active magnetic reconnection sites. The global energetics reveal distinct species-dependent energization pathways. Ions act as the primary energy reservoir, transferring energy to the electromagnetic fields, while electrons receive the dominant net positive energy input. Electron energization is strongly anisotropic ( T _{∥, e} > T _{⊥, e} ) and localized within intermittent reconnecting current sheets associated with enhanced field–particle energy exchange and elevated agyrotropy. These regions also show the development of suprathermal tails in the electron energy distributions, providing evidence for nonthermal electron energization. Despite opposite vorticity orientations, the two shear layers exhibit similar statistical behavior. Together, these results establish a direct connection between reconnection-associated current structures and localized electron energization in collisionless KHI dynamics.

The understanding and modeling of high-energy particle transport in turbulent magnetic fields is an important open challenge in space physics and astrophysics. The multiscale, nonlinear nature of turbulence, and the high variability of turbulence properties across different environments, make it particularly challenging to reach a full understanding of the interactions between particles and turbulent fluctuations. Using synthetic, realistic looking turbulent magnetic field realizations generated by the BxC toolkit, we investigate how the scattering of particles is affected by anisotropic fluctuations in strongly turbulent fields. We find evidence that, in the absence of a uniform background or guide magnetic field, the scattering process is not governed by the turbulence correlation length. We then further verify this hypothesis by studying particle transport in the presence of a guide field. We find evidence of a different scattering mechanism from the usual pitch-angle diffusion used to describe scattering in strong-guide-field settings.

Nonthermal features are ubiquitously observed in electron velocity distribution functions in the solar wind, yet their origin in the collisionless, turbulent, expanding solar-wind plasma remains unclear. We investigate how solar-wind expansion and Alfvénic turbulence jointly generate and regulate these features using the first fully kinetic particle-in-cell simulation of an expanding turbulent plasma under heliospheric conditions. In our setup, expansion-driven weakening of the magnetic field adiabatically cools the plasma perpendicularly to the mean field while leaving the parallel temperature largely unchanged, driving the system toward the firehose-instability threshold. Concurrently, strongly anisotropic turbulence leads to perpendicular heating and the development of nonthermal features. Subsequently, we find that suprathermal electron populations preferentially develop in the parallel direction, forming pronounced power-law tails even under weakly compressive, highly Alfvénic conditions, and persist despite anisotropy regulation by the firehose instability. The preferentially parallel energization suggests the involvement of parallel electric fields or resonant wave–particle interactions, rather than simple velocity-space redistribution. These results provide the first direct evidence of the emergence of nonthermal-electron features in a unified kinetic framework linking expansion, turbulence, and instabilities in the solar wind.

Numerical models are essential for advancing fundamental research in heliophysics and for operational forecasting of space weather events. Open science can accelerate progress, but applying an open science approach to numerical models requires careful consideration; models are not data analysis software and often demand substantial domain expertise, experience, and computational resources to be used correctly. This commentary distills the recommendations from Corti et al. (2026), https://doi.org/10.22541/essoar.176824639.92354528/v2 and proposes a pathway for advancing heliophysics and space weather modeling through open science. Rather than focusing on open science policies and practices, we recommend concrete actions to make models usable by the broader community (open use), increase confidence and transparency throughout the Research‐to‐Operations‐to‐Research (R2O2R) pipeline (open validation), enable collaborative model improvement (open development), and strengthen partnerships (open collaboration). Together, these recommendations address several of the open science challenges experienced by contributing authors in developing, validating, using, and hosting advanced models, as identified during the 2024 Open Science Workshop held at College Park, USA. The recommendations also highlight the importance of a culture of shared responsibility, adherence to best practices, and proper recognition of prior work. The Heliophysics Open Modeling Environment will serve as a platform for modelers and model users to work together, facilitate community modeling, and increase the scientific return on modeling investment.

We present a community‐wide effort to develop a strategy and action plan to advance heliophysics and space weather modeling through open science. While open science has the potential to enhance the quality and pace of scientific discovery, its application to scientific modeling requires more careful consideration regarding open data and open software guidelines, as scientific models differ significantly from data analysis software. We gathered feedback from modeling teams worldwide through a living survey and discussion sessions at the 2024 Open Science Workshop in College Park, USA, and at the 2025 COSPAR ISWAT Working Meeting in Cape Canaveral, USA. We complement these findings with lessons learned from almost 25 years of experience at the Community Coordinated Modeling Center in enabling open use of models. We identify key roadblocks in current open science practices and guidelines and offer recommendations for future progress across four overlapping themes: open use of models and simulation results, open validation, open development, and open collaboration. An essential outcome of the discussion is the need for model developers and model users to speak with a united voice and promote the role of models in future open science efforts. We introduce a new cross‐domain community initiative called the Heliophysics Open Modeling Environment (HOME), which will be integrated as an overarching activity within COSPAR ISWAT. HOME will serve as a platform for modelers and model users to work together, facilitate community modeling, improve the scientific return on modeling investment, and advance understanding, modeling, and forecasting in heliophysics and space weather.

We present a community effort to assess how open science can advance heliophysics and space weather modeling. Open science has the potential to enhance the quality and pace of scientific discovery, but its application to scientific modeling requires more careful consideration with respect to open data and open software guidelines, as complex scientific models are not ordinary software. We gathered feedback from modeling teams worldwide through a living survey and discussion sessions at the Open Science Workshop in College Park, USA, in 2024, and the COSPAR ISWAT Initiative Working Meeting in Cape Canaveral, USA, in 2025. We complement these findings with lessons learned from almost 25 years of experience at the Community Coordinated Modeling Center in enabling open use of models. We identify key roadblocks in current open science practices and guidelines and offer recommendations for future progress. Our findings are organized into four overlapping themes: open use of models and simulation results, open validation, open development, and open collaboration. An essential outcome of the discussion is the need for model developers and users to speak with a united voice and promote the role of models in future open science efforts. We introduce a new cross-domain community initiative called Heliophysics Open Modeling Environment (HOME), which will be integrated as an overarching activity within the COSPAR ISWAT Initiative. HOME will serve as a platform for modelers and model users to work together, facilitate community modeling, improve the scientific return on modeling investment, and advance innovation in heliophysics and space weather.