10:45 〜 11:00
[PEM10-06] Study of slow-mode shock formation and particle acceleration in the symmetric magnetic reconnection based on hybrid simulations
キーワード：magnetic reconnection, slow-mode shocks, hybrid simulations
Magnetic reconnection is a ubiquitous process which breaks the magnetic field lines, and transforms the energy stored in them from contortion, into kinetic energy. Petschek’s model of reconnection [Petschek, 1964] has reconnection rate comparable to in-situ observations, and it has a small diffusion region which is flanked by two slow-mode shocks on each side of the exhaust region. While slow-mode shocks are clearly detected in various Magnetohydrodynamic (MHD) simulations [e.g., Heyn et al., 1985; Biernat et al., 1989; Lin & Lee, 1993], there is no clear consensus on existence of slow-mode shocks in hybrid and Particle In Cell (PIC) simulations. Some simulations [e.g., Lin and Swift, 1996; Lottermoser et al. 1998; Higashimori and Hoshino, 2012] do report the presence of slow-mode shocks far from the X-point (80 λi – 120 λi), others [e.g., Scholer et al., 2000; Drake et al., 2009] do not observe slow-mode shocks. In the Earth’s magnetotail, Saito et al. [1995] reported that about 10% of the crossings from upstream to exhaust had a slow-mode shock signature. And, in the Earth’s magnetopause the slow-mode shock detection percentage was reported to be 20% by Walia et al. [2018]. Our focus in this study is on mechanisms that involve upstream to exhaust boundary. In particular, we explore the existence of slow-mode shocks in magnetic reconnection, the reasons behind the low observation rates, and their role in the energization of particles.
We studied the structure of the magnetic reconnection boundary by using 2.5D hybrid simulations. We used the six Rankine-Hugoniot conditions and the six specific conditions for slow-mode shocks, which are also used by in-situ satellite observations, to study the presence of slow-mode shocks. We observe that the reconnection boundary can be interpreted as a slow-mode shock from as close as ~9 λi (λi = ion inertial length) from the X-point. The detection of slow-mode shocks increases with the increasing distance from the X-point and with the increasing ion plasma beta. Additionally, it is observed that if the slow-mode shocks are analyzed by taking artificial satellite cuts at various angles, the detection percentage of slow-mode shocks can decrease to ~10% for very oblique crossings. The detection percentage of slow-mode shocks is the percentage of points out of the total points studied, where the slow-mode shocks were observed. At the point of incidence into the slow-mode shocks, the cold particles get accelerated and they gain energy. However, as we move further from the X-point, a non-classical picture of slow-mode shocks emerges, where the slow-mode shock downstream region has accelerated particles but they are not accelerated by the shock in its immediate vicinity. This accelerated crescent-shaped beam population is first accelerated at the slow-mode shock much closer to the X-point, and then it travels to the slow-mode shock downstream region further away from the X-point, gaining energy on the way by reflecting between the two slow-mode shocks. It was speculated by Wygant et al. [2005] that the electric field structure which causes the bounce of ions and leads to gain in energy could be slow-mode shock-like structure. Our simulations support that, since we observe that along with the reconnection electric field, the cross-shock electric field and thus, the slow-mode shocks play a role in reflection and acceleration of particles.
We studied the structure of the magnetic reconnection boundary by using 2.5D hybrid simulations. We used the six Rankine-Hugoniot conditions and the six specific conditions for slow-mode shocks, which are also used by in-situ satellite observations, to study the presence of slow-mode shocks. We observe that the reconnection boundary can be interpreted as a slow-mode shock from as close as ~9 λi (λi = ion inertial length) from the X-point. The detection of slow-mode shocks increases with the increasing distance from the X-point and with the increasing ion plasma beta. Additionally, it is observed that if the slow-mode shocks are analyzed by taking artificial satellite cuts at various angles, the detection percentage of slow-mode shocks can decrease to ~10% for very oblique crossings. The detection percentage of slow-mode shocks is the percentage of points out of the total points studied, where the slow-mode shocks were observed. At the point of incidence into the slow-mode shocks, the cold particles get accelerated and they gain energy. However, as we move further from the X-point, a non-classical picture of slow-mode shocks emerges, where the slow-mode shock downstream region has accelerated particles but they are not accelerated by the shock in its immediate vicinity. This accelerated crescent-shaped beam population is first accelerated at the slow-mode shock much closer to the X-point, and then it travels to the slow-mode shock downstream region further away from the X-point, gaining energy on the way by reflecting between the two slow-mode shocks. It was speculated by Wygant et al. [2005] that the electric field structure which causes the bounce of ions and leads to gain in energy could be slow-mode shock-like structure. Our simulations support that, since we observe that along with the reconnection electric field, the cross-shock electric field and thus, the slow-mode shocks play a role in reflection and acceleration of particles.