Energization of Cold Ions in Magnetic Reconnection: Particle‐in‐Cell Simulation

Abstract

Cold ions in the ionosphere can reach the nightside magnetotail along the magnetic field lines, which may substantially affect the magnetotail reconnection and reconnection‐driven phenomena. Although reconnection has been intensively studied in the past decades, the dynamics of cold ions in reconnection are poorly known. We performed a 2.5‐D full kinetic simulation to study the energization of cold ions in a symmetric anti‐parallel reconnection. We find that the cold ions can be considerably energized during reconnection. The average energy gain per cold ion is smaller than that per hot ion but larger than that per electron. The cold ions with the highest energy are firstly accelerated around the X‐line, and further accelerated around the reconnection front in the outflow region. They perform the meandering motion at the X‐line, while simultaneously accelerated by the reconnection out‐of‐plane electric field. The velocity distribution of cold ions around the X‐line is characterized by the counter‐streaming inflow component and the accelerated component in the +y quadrant. The cold ions are picked up at the reconnection front and trapped by the secondary island, resulting in a bump in the energy spectrum. The energy of the bump is close to the energy of the propagating speed of the reconnection front and the secondary island. Furthermore, a crescent‐shaped velocity distribution is observed at the reconnection front, caused by the finite Larmor radius effect of the high‐energy cold ions in the flux pileup region. Our result presents an important building block toward a fully understanding of the magnetosphere‐ionosphere coupling.

Kinetics of Cold Ions in Asymmetric Reconnection: Particle‐In‐Cell Simulation

“…In our simulation, we observe that the dimensionless reconnection rate peaks with E r ∼ 0.09 at t ≈ 18 and becomes quasi‐steady after t ≈ 30 with E r ∼ 0.07 (Song et al., 2023a). However, at t ≈ 55, the reconnection rate begins to decline due to the saturation of the tearing mode caused by the ideal conducting boundary condition.…”

“…As depicted in Figures 1a and 1b, a robust electric field E z is present at the magnetospheric separatrix region ( z ∼ −1) throughout the reconnection, pointing from the magnetosphere to the magnetosheath ( E z > 0). Previous simulations of asymmetric reconnection have revealed the presence of this electric field (Burch et al., 2016; Dargent et al., 2019; Malakit et al., 2013; Shay et al., 2016; Wang et al., 2017), which is balanced by the Hall term in generalized Ohm's law (Malakit et al., 2013; Song et al., 2023a), indicating that it is a Hall electric field. The magnitude of the Hall electric field E z at t = 50 (Figure 1b) is smaller than that at t = 30 (Figure 1a) and is weaker in the vicinity of the X‐line than in the magnetospheric separatrix region (Figures 1a and 1b).…”

“…Employing particle‐in‐cell (PIC) simulations, Song et al. (2023a) showed that the temperature of cold ions can increase by approximately 20 times compared to the initial cold ion temperature in both the outflow and the separatrix region of asymmetric reconnection.…”

Kinetic Simulation of Cold Ion Heating in Asymmetric Reconnection

“…Figure 3c shows that the cold ions obtain their energy mainly from the electric field $${E}_{z}$$. Interestingly, the out‐of‐plane electric field $${E}_{y}$$ does negative work, which is different from symmetric reconnection where cold ions gain energy from the out‐of‐plane electric field $${E}_{y}$$ (Divin et al., 2016; Hesse & Birn, 2004; Song et al., 2022). Additionally, the electric field $${E}_{x}$$ always does positive work, but its contribution to the cold ion acceleration is much smaller than that by $${E}_{z}$$ (Figures 3b and 3c).…”

“…Magnetic flux rope is another source of ion acceleration, which can energize ions through the Fermi mechanism when the flux ropes are contracting or merging (Drake et al., 2009). Furthermore, recent simulations have also demonstrated that cold ions can be subject to acceleration through the reconnection electric field in the vicinity of the X‐line (Song et al., 2022). Magnetospheric Multiscale (MMS) observations show that cold ions can be preheated before crossing the Hall electric field layer and heated at the separatrix region at the dayside magnetopause (Toledo‐Redondo et al., 2016).…”