17:15 〜 19:15
[PEM10-P06] Development of Multi-Hierarchy Simulation Code KAMMUY and its Application to Understanding Cross-Scale Coupling in Magnetic Reconnection
Magnetic reconnection plays an important role in various space weather phenomena, including CME, flare, and substorms. It converts magnetic field energy into plasma energy by changing the topology of magnetic field lines and has a multi-scale nature. An open question in magnetic reconnection is the connection between the MHD and kinetic scales. Plasmoid instability, first proposed in 2007, appears to explain the fast magnetic reconnection in the framework of MHD with uniform resistivity, and it is expected to be applicable to solar flares. On the other hand, the solar coronal plasma should be treated as a collisionless plasma when the current sheet becomes thin. In collisionless plasma, fast magnetic reconnection is realized by Hall effect. Specifically, the size of a solar flare is about 10 Mm, while the dissipation region (~ ion inertial length) is about 1 m. Therefore, it is not obvious whether the fast magnetic reconnection due to the Hall effect holds up to the solar flare scale, or the plasmoid instability arises at certain scales. Solving this problem by PIC simulation which describes the plasma kinetics is impossible because of the significant scale gap between the grid size and the system size. MHD simulation is also inappropriate because it cannot describe the resistivity from first principles, which means that the magnetic field dissipation due to the resistivity is artificially given that the system can be either Petschek-type reconnection or plasmoid-mediated reconnection.
In this study, we adopted the multi-hierarchy simulation to overcome this large scale gap and developed simulation code from scratch. This is a method to reduce computational cost by using PIC simulation in the region where plasma kinetics is important, while other regions are treated by MHD simulation. Although the actual solar flare scale cannot be handled at this stage, we set the largest possible box size (100 ~ 1000 ion inertial lengths, with NVIDIA A100 GPUs) and connected MHD and PIC in the inflow region. As a result, only one secondary plasmoid was generated in the electron diffusion region. It is different from the picture of plasmoid instability which many plasmoids are generated in the diffusion region. Our results suggest that the conventional picture of the plasmoid-mediated reconnection does not apply to the solar flare.
In this study, we adopted the multi-hierarchy simulation to overcome this large scale gap and developed simulation code from scratch. This is a method to reduce computational cost by using PIC simulation in the region where plasma kinetics is important, while other regions are treated by MHD simulation. Although the actual solar flare scale cannot be handled at this stage, we set the largest possible box size (100 ~ 1000 ion inertial lengths, with NVIDIA A100 GPUs) and connected MHD and PIC in the inflow region. As a result, only one secondary plasmoid was generated in the electron diffusion region. It is different from the picture of plasmoid instability which many plasmoids are generated in the diffusion region. Our results suggest that the conventional picture of the plasmoid-mediated reconnection does not apply to the solar flare.