09:30 〜 09:45
[PEM17-13] 磁気リコネクションにおける微視的スケールから巨視的スケールへの遷移について
キーワード：磁気リコネクション、プラズマ運動論シミュレーション、スローショック
Magnetic reconnection is a fundamental process in magnetized plasma, wherein stored magnetic energy is explosively converted into kinetic and thermal energies of plasma. This process is widely acknowledged as a primary mechanism to drive solar and stellar flares, as well as magnetospheric substorms, causing disturbance in the solar-terrestrial environment. In dilute plasmas, fast magnetic reconnection can be initiated by microscopic processes within the diffusion region, where the frozen-in condition is violated and kinetic effects dominate the dissipation of magnetic fields. However, the effects of reconnection extend far beyond the plasma kinetic scale, as evidenced by observations in X-ray images of solar flares, exhibiting a hierarchical nature inherent to magnetic reconnection. In the case of solar flares, we can observe the macroscopic aspects of reconnection, while it is probably initiated at an unobservable microscopic scale. Thus, understanding the transition from microscopic to macroscopic scales in magnetic reconnection is of critical importance for comprehending reconnection-driven phenomena.
A direct approach to unravel the transition from microscopic to macroscopic scales in magnetic reconnection would be very large-scale computer simulations of first-principle, fully-kinetic plasmas. However, currently-available computer resources are still not enough to fully resolve kinetic plasma at realistic macroscopic scales. As an alternative, intermediate models such as two fluid and hybrid models are utilized to cover a broader range than fully-kinetic simulations while incorporating some significant physics beyond magnetohydrodynamics (MHD). Higashimori and Hoshino (2012) conducted hybrid simulations of magnetic reconnection and argued that large temperature anisotropy in association with reconnection inhibits the formation of slow shocks anticipated in fast reconnection at the MHD scale. The temperature anisotropy will be eventually relaxed and the system is expected to approach MHD-scale reconnection.
To study the transition from microscopic to macroscopic scales in magnetic reconnection, we conduct fully-kinetic plasma simulations including artificial relaxation. Depending on simulation parameters, the velocity distribution is isotropized to mimic physical relaxation processes. By comparing simulations with and without the relaxation, we investigate its effect on the evolution of magnetic reconnection. When the relaxation is activated after the saturation of kinetic-scale reconnection, a localized diffusion region is maintained by particle inertia instead of anisotropic pressure, and a bifurcated current structure appears. This structure is identified as switch-off slow shocks, suggesting efficient energy conversion there rather than within the diffusion region. The reconnection rate remains fast even after the relaxation is activated, indicating the transition from kinetic-scale reconnection to Petscheck-type MHD-scale reconnection as a consequence of the relaxation of the velocity distribution.
A direct approach to unravel the transition from microscopic to macroscopic scales in magnetic reconnection would be very large-scale computer simulations of first-principle, fully-kinetic plasmas. However, currently-available computer resources are still not enough to fully resolve kinetic plasma at realistic macroscopic scales. As an alternative, intermediate models such as two fluid and hybrid models are utilized to cover a broader range than fully-kinetic simulations while incorporating some significant physics beyond magnetohydrodynamics (MHD). Higashimori and Hoshino (2012) conducted hybrid simulations of magnetic reconnection and argued that large temperature anisotropy in association with reconnection inhibits the formation of slow shocks anticipated in fast reconnection at the MHD scale. The temperature anisotropy will be eventually relaxed and the system is expected to approach MHD-scale reconnection.
To study the transition from microscopic to macroscopic scales in magnetic reconnection, we conduct fully-kinetic plasma simulations including artificial relaxation. Depending on simulation parameters, the velocity distribution is isotropized to mimic physical relaxation processes. By comparing simulations with and without the relaxation, we investigate its effect on the evolution of magnetic reconnection. When the relaxation is activated after the saturation of kinetic-scale reconnection, a localized diffusion region is maintained by particle inertia instead of anisotropic pressure, and a bifurcated current structure appears. This structure is identified as switch-off slow shocks, suggesting efficient energy conversion there rather than within the diffusion region. The reconnection rate remains fast even after the relaxation is activated, indicating the transition from kinetic-scale reconnection to Petscheck-type MHD-scale reconnection as a consequence of the relaxation of the velocity distribution.