Kinetic Simulation of Cold Ion Heating in Asymmetric Reconnection

Abstract

Cold ions from the plasmasphere can reach the Earth's magnetopause through the plasma wind and plume, affecting the reconnection at the magnetopause, such as changing the reconnection rate and influencing the energy budget. Recent observations found that cold ions were substantially heated by magnetopause reconnection. Although a few mechanisms have been proposed to account for the cold ion acceleration at the magnetopause, the dominant mechanism remains unclear. In this paper, we perform 2.5‐dimensional fully kinetic simulations to study cold ion heating in asymmetric reconnection at Earth's magnetopause. We find that cold ions absorb about 10%–25% of the total released magnetic energy, which is predominantly converted into the thermal energy of cold ions. The proportion of the energy gained by cold ions increases with the increment of the initial density ratio of the cold ions to hot ions in the magnetosphere. Cold ions are primarily accelerated by the Hall electric field Ez ${E}_{z}$ at the magnetospheric separatrix region, while the out‐of‐plane electric field Ey ${E}_{y}$ does negative work. The increase in thermal energy of cold ions is mainly caused by stochastic heating, that is, the viscous heating associated with the non‐gyrotropic pressure tensor, −()P⃡ng’⋅∇⋅trueV→ ${-}\left({\overleftrightarrow{P}}_{ng}^{\mbox{'}}\cdot \nabla \right)\cdot \overrightarrow{V}$, contributes the most. The work done by pressure is most significant around the magnetosheath separatrix region. These results can significantly deepen our understanding of the cold ion dynamics at Earth's magnetopause.

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).…”

“…While the reconnection electric field E y decelerates type 1 particles ( t < 28), it accelerates particle 6 after switching to type 2 (Figure 8j). Given that the number of type 1 particles is significantly larger than that of type 2, E y does negative work on the cold ions overall (Song et al., 2023a). Notably, the reconnection electric field E y is negative around the X‐line (i.e., E y < 0), whereas the type 1 particles have a positive velocity v icy > 0 due to their gyromotion (Figure 7g).…”

“…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.…”

“…The crescent-shaped signatures of cold ions appear when cold ions can cross the magnetospheric Hall electric field layer in less than one gyration and reach the side with few cold ions. 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.…”