Onset of collisionless magnetic reconnection in thin current sheets: Three-dimensional particle simulations

Abstract

Three-dimensional (3D) particle-in-cell simulations of collisionless magnetic reconnection are presented. The initial equilibrium is a double Harris-sheet equilibrium and periodic boundary conditions are assumed in all three directions. No magnetic seed island is imposed initially, and no flow conditions are imposed. The current sheet width is assumed to be one ion inertial length calculated with the density in the center of the current sheet. The ion to electron mass ratio is mi/me=150, which suppresses the growth of the drift kink instability. Two different runs have been performed: a simulation with exactly antiparallel magnetic field and a simulation with a constant guide field of the same magnitude as the antiparallel field superimposed. In the antiparallel case the inductive field of the waves excited by the lower hybrid drift instability (LHDI) leads to rapid acceleration of the electrons in the center of the current sheet and subsequently to a current sheet thinning. The current increase in the center is balanced by reverse currents in the gradient region. In the thin current sheet rapid reconnection sets in which self-organizes into a two-dimensional structure with a single X line. However, ∼15% of the total flux is reconnected while reconnection is still patchy and 3D. In the guide field case the growth rate of the LHDI is reduced, but leads nevertheless after a considerably longer time to electron acceleration in the current sheet center and to a thinning of the current layer, followed by single X line reconnection. It is suggested that electron acceleration due to LHDI in current sheets of the order of the ion scale results in rapid onset of reconnection.

Nonlinear Evolution of the Lower-Hybrid Drift Instability in a Current Sheet

“…The ion flow is also modified, but to a much smaller degree due to the large inertia. The essential physics of this electron acceleration process is very different from recent results [14,15] in which an inductive electric field plays the dominant role while the electrostatic field is negligible [14]. In addition, these low mass ratio simulations do not observe the strong current bifurcation.…”

“…However, new results from both theory and simulation are beginning to challenge this conclusion. In a number of simulations, a strong enhancement of the central current density associated with the LHDI is observed [12 -16] and it has been suggested this effect gives rise to the rapid onset of reconnection [14,15]. Most of these simulations were performed with artificial ion to electron mass ratios m i =m e 100-400 and very thin layers i =L 1:7-2:2, where L is the half thickness of the layer.…”

New role of the lower-hybrid drift instability in the magnetic reconnection

“…For the thinnest case, i / L = 1.828, the LHDI causes a peaking of the current profile. This effect has been documented in previous works [12][13][14][16][17][18]28 and the alteration is primary due to electron acceleration: the ion and electron density modifications are weaker. 12,13,16 Within a fluid model, current peaking due to LHDI was also predicted in Ref.…”

“…4 Recently, the effects of the LHDI have been revisited. [12][13][14][15][16][17][18] In the present paper, it is shown that the LHDI may play an important role in the onset of magnetic reconnection, but through a mechanism very different in nature than anomalous resistivity. Although the LHDI is a microscopic instability, it is also responsible for macroscopic changes in the current sheet.…”

Dissipation in relativistic pair-plasma reconnection

“…2 Although some questions remain in turbulent plasmas, most recent investigations have found that, in the central magnetic reconnection region, the relevant additional term is provided by the thermal inertia. [2][3][4][5][6][7][8][9][10] The physics behind this is associated with the transient nature of particle orbits in the reconnection region. Pre-acceleration particles are continuously transported into the reconnection region, whereas accelerated particles tend to leave.…”