Time-Adaptive PIROCK Method with Error Control for Multi-Fluid and Single-Fluid MHD Systems

Abstract: The solar atmosphere is a complex environment with diverse species and varying ionization states, especially in the chromosphere, where significant ionization variations occur. This region transitions from highly collisional to weakly collisional states, leading to complex plasma state transitions influenced by magnetic strengths and collisional properties. These processes introduce numerical stiffness in multi-fluid models, imposing severe timestep restrictions on standard time integration methods. New numerical methods are essential to address these computational challenges, effectively managing the diverse timescales in multi-fluid and multi-physics models. The widely used time operator splitting technique offers a straightforward approach but requires careful timestep management to avoid stability issues and errors. Despite some studies on splitting errors, their impact on solar and stellar astrophysics is often overlooked. We focus on a Multi-Fluid Multi-Species (MFMS) model, which presents significant challenges for time integration. We propose a second-order Partitioned Implicit-Explicit Runge-Kutta (PIROCK) method that combines efficient explicit and implicit integration techniques with variable time-stepping and error control. Compared to a standard third-order explicit method and a first-order Lie splitting approach, the PIROCK method shows robust advantages in accuracy, stability, and computational efficiency. Our results reveal PIROCK's capability to solve multi-fluid problems with unprecedented efficiency. Preliminary results on chemical fractionation represent a significant step toward understanding the First-Ionization-Potential (FIP) effect in the solar atmosphere.