arXiv subjects

Keita Akutagawa

Publications and source records attributed to Keita Akutagawa.

4 recordsLinked to original sources

arXiv2026

A generalized energy-consistent finite difference scheme for 10-moment magnetohydrodynamics

Pressure anisotropy and off-diagonal pressure stresses are ubiquitous and play important roles in collisionless/weakly collisional plasmas. The Chew-Goldberger-Low (CGL) MHD model is often used; however, it can lose hyperbolicity when the pressure anisotropy or plasma beta becomes large, making it hard to develop approximate Riemann solvers. An alternative approach is to use the 10-moment MHD equations, but their eigenmode analysis is also difficult, which similarly hinders the development of less-diffusive Riemann solvers. This paper presents a new energy-consistent finite difference scheme for 10-moment MHD designed to operate over a broad range of plasma beta. The proposed scheme extends the 10-moment MHD model using the energy-consistent finite-difference approach developed for conventional MHD. Nonlinear filtering is applied to all six independent components of the pressure tensor, and the kinetic and magnetic energies dissipated by the filtering are explicitly transferred to the diagonal pressure components under an equipartition assumption to maintain consistency with the total energy balance. The proposed scheme is validated against seven test problems in the isotropic limit, the gyrotropic limit, and without isotropization/gyrotropization. The results demonstrate the expected spatial convergence and total energy behavior, reproduce the linear growth rate, and yield pressure tensor structures qualitatively consistent with theoretical expectations and previous simulations, spanning plasma beta values from $10^{-10}$ to $10^{10}$. The proposed scheme provides a promising framework for large-scale simulations of collisionless plasmas across widely separated plasma beta regimes and opens a path toward applications such as solar wind turbulence and plasmoid-mediated reconnection.

physics.plasm-ph↗

arXiv2026

Multi-hierarchy simulation of Riemann problem for reconnection exhausts

Magnetic reconnection drives a wide range of astrophysical plasma phenomena, including solar flares, by converting magnetic energy into plasma energy through changes in magnetic field topology. Petschek reconnection is a magnetohydrodynamic (MHD) model in which magnetic field lines reconnect within a localized diffusion region, and a pair of switch-off slow shocks forms outside this region, enabling efficient energy conversion. Whether this picture remains valid when kinetic effects are included remains an open question. In this study, we examine the formation and properties of slow shocks associated with reconnection exhausts by solving a two-dimensional Riemann problem using a multi-hierarchy framework that couples MHD and particle-in-cell (PIC) simulations. We find that a slow shock close to the switch-off limit forms in the MHD domain even when slow shock formation is suppressed in the PIC domain, and that this behavior is insensitive to the size of the PIC domain. The formation of the slow shock further promotes plasma isotropization within the PIC domain. These results suggest that Petschek-like reconnection remains viable in collisionless-collisional systems, such as solar flares, where temperature anisotropy appears to be relaxed far from the reconnection region.

astro-ph.SR↗

arXiv2025

Influence of kinetic effects in large-scale magnetic reconnection with multi-hierarchy simulation code KAMMUY

Magnetic reconnection is a multiscale phenomenon where fluid- and particle-scale processes interact. The particle-in-cell (PIC) method, capable of resolving kinetic (particle-scale) physics, is extensively used to study the kinetic effects in magnetic reconnection. Meanwhile, because of the high computational cost, PIC simulations cannot capture the interaction between kinetic and fluid dynamics, which poses a major obstacle to understanding magnetic reconnection in large-scale phenomena such as solar flares. A multi-hierarchy simulation that combines Magnetohydrodynamics (MHD) and PIC provides a promising means to overcome these spatial and temporal scale gaps. We developed a multi-hierarchy simulation code KAMMUY (Kinetic And Magnetohydrodynamic MUlti-hierarchY simulation code), in which an ideal MHD simulation for a large domain and a PIC simulation for a smaller domain are solved in parallel with mutual information exchange. To validate the code, we conducted test simulations of MHD wave propagation and the shock tube problem. The results demonstrate that short-wavelength, high-frequency waves generated in the PIC region do not propagate into the MHD region, whereas MHD-scale structures propagate smoothly into the PIC region, highlighting the capability of our code for numerical studies of magnetic reconnection. By applying the KAMMUY code to magnetic reconnection while varying the PIC domain size, we find that the reconnection rate remains unchanged, regardless of the extent of the PIC region where the Hall magnetic field is present. It suggests that the spatial extension of the Hall magnetic field on the scale of $10 \sim 100 λ_i$ does not influence the reconnection rate.

astro-ph.SR↗

arXiv2025

Can plasmoid-mediated reconnection occur in collisionless systems?

Magnetic reconnection is a process that converts magnetic energy into plasma energy by changing the magnetic field line topology. The outstanding question is why the reconnection rate is $\mathcal{O}(0.01 - 0.1)$ in many astrophysical phenomena, for example solar flares and terrestrial substorms. Previous studies have shown two ideas of Hall reconnection and plasmoid instability. However, there is no consensus on which process is the reason for the fast reconnection. In this paper, we discuss the formation of secondary plasmoids in \rewrite{2D antiparallel collisionless reconnection} using 2.5-dimensional particle-in-cell simulations and discuss whether plasmoid-mediated reconnection occur in collisionless systems by comparing with plasmoid instability in resistive MHD simulations. We find that in collisionless systems secondary plasmoids can indeed form. However, the mass ratio has a strong effect on the formation of secondary plasmoids, and it indicates that secondary plasmoids do not emerge using realistic ion-electron mass ratio ($m_i/m_e = 1836$). Furthermore, we find that there is no enhancement of the reconnection rate due to the secondary plasmoid in the collisionless system, as discussed in the plasmoid-mediated reconnection. Although our simulation $\mathcal{O}(100λ_i)$ box is not large enough to discuss astrophysical phenomena such as solar flares, it can reflect a relatively small plasma system such as the Earth's magnetotail.

astro-ph.SR↗