Parallel-Kinetic-Perpendicular-Moment Simulations of Magnetic Reconnection Between Non-Equilibrium Flux Ropes

Magnetic flux ropes are ubiquitous structures in both astrophysical and laboratory plasmas whose formation and coalescence are essential processes in magnetic reconnection. However, accurately capturing these dynamics with realistic parameters remains challenging due to their multiscale nature. Here, we simulate magnetic reconnection between flux ropes with line-tied geometry in a strong guide field using a novel Parallel-Kinetic-Perpendicular-Moment (PKPM) plasma model. This model retains the kinetic dynamics parallel to the magnetic field, while approximating the motions perpendicular to the field in a spectral expansion analogous to hybrid fluid-kinetic approaches. Using non-equilibrium initial conditions, our approach accurately captures the kinetic-scale relaxation of each flux rope before coalescence occurs. We identify a quasi-seperatrix layer between the merging islands and compute the relative importance of kinetic terms in supporting the reconnection rate. Our model shows strong agreement with recent laboratory experiments at the UCLA Large Plasma Device and properly accounts for an important diamagnetic current observed in experiments, which does not develop when using standard theoretical solutions like the Fadeev equilibrium as initial conditions.