Collisionless magnetic reconnection in the magnetosphere

Abstract

Magnetic reconnection underlies the physical mechanism of explosive phenomena in the solar atmosphere and planetary magnetospheres, where plasma is usually collisionless. In the standard model of collisionless magnetic reconnection, the diffusion region consists of two substructures: an electron diffusion region is embedded in an ion diffusion region, whose their scales are based on the electron and ion inertial lengths. In the ion diffusion region, ions are unfrozen in the magnetic fields while electrons are magnetized. The resulted Hall effect from the different motions between ions and electrons leads to the production of the in-plane currents, and then generates the quadrupolar structure of out-of-plane magnetic field. In the electron diffusion region, even electrons become unfrozen in the magnetic fields, and the reconnection electric field is contributed by the off-diagonal electron pressure terms in the generalized Ohm’s law. The reconnection rate is insensitive to the specific mechanism to break the frozen-in condition, and is on the order of 0.1. In recent years, the launching of Cluster, THEMIS, MMS and other spacecraft has provided us opportunities to study collisionless magnetic reconnection in the Earth’s magnetosphere, and to verify and extend more insights on the standard model of collisionless magnetic reconnection. In this paper, we will review what we have learned beyond the standard model with the help of observations from these spacecraft as well as kinetic simulations.

Kinetic Alfven wave (KAW) eigenmode in magnetosphere magnetic reconnection

“…PIC simulations show that the Hall electric field are mostly electrostatic but with some magnetic induced component (Lu et al 2021). For the Hall electric field, Fig.…”

“…For the Hall electric field, Fig. 8 shows the composition of its electrostatic component and its electromagnetic component (Lu et al 2021). The electrostatic component is defined as E 1 = −∇ and thus ∇ × E 1 = 0 .…”

“…But the induced electromagnetic component is finite. The electromagnetic component is due to the time variations of the Hall B y (Lu et al 2021).…”

“…On the ion-motion scale, ions start to become demagnetized whereas electrons are still coupled to the magnetic field. The difference in the ion and electron motion produces charge separation, Hall fields and the associated electric current (Sonnerup 1979;Lu et al 2022). On the even smaller electron-scale, electrons are demagnetized and correspond to the 'breaking' of the field line (Hesse et al 1999;Lu et al 2011).…”

“…The difference in the ion and electron motion produces charge separation, Hall fields and the associated electric current (Sonnerup 1979;Lu et al 2022). On the even smaller electron-scale, electrons are demagnetized and correspond to the 'breaking' of the field line (Hesse et al 1999;Lu et al 2011). Kinetic physics is considered vital to explain the fast rate of collisionless reconnection.…”