Electron Dynamics in the Electron Current Sheet During Strong Guide-Field Reconnection

In this study, we investigate detailed electron dynamics in strong guide-field reconnection (the normalized guide field is similar to 1.5). This reconnection event is observed by the Magnetospheric Multiscale (MMS) spacecraft at the center of a flux rope in the magnetotail. With the presence of a large parallel electric field (E-||) in the electron current sheet, electrons are accelerated when streaming into this E-|| region from one direction, and decelerated from the other direction. Some decelerated electrons can reduce the parallel speed to similar to 0 to form relatively isotropic electron distributions at one side of the electron current sheet, as the estimated acceleration potential satisfies the relation e Phi(||) >= kT(e,||), where T-e,T-|| is the electron temperature parallel to the magnetic field. Therefore, a large E-|| is generated to balance the parallel electron pressure gradient across the electron current sheet, since electrons at the other side of the current sheet are still anisotropic. Based on these observations, we further show that the electron beta is an important parameter in guide-field reconnection, providing a new perspective to solve the large parallel electric field puzzle in guide-field reconnection. Plain Language Summary Magnetic reconnection is a universal process that rapidly converts energy from the magnetic field to plasma. The energy conversion at kinetic scales is of particular interest to researchers, as it is directly related to reconnection process in the central diffusion region. In general, the reconnecting magnetic fields do not have to be antiparallel, and an additional magnetic component known as the guide field (B-g) can appear in the direction perpendicular to the reconnecting plane. Recently, observations from Magnetospheric Multiscale (MMS) mission show a large electric field parallel to the local magnetic field, which is several times larger than the reconnection electric field, can appear in guide-field reconnection, and impact electrons significantly. However, the generation of this large parallel electric field in strong guide-field reconnection is still not fully understood. In this study, we suggest that the electron beta (ratio of the electron thermal pressure to the magnetic pressure) is an important parameter in guide-field reconnection. Only within some proper electron beta range, a parallel pressure gradient across the electron current sheet can form to balance the large parallel electric field.