In-situ Observation of Secondary Magnetic Reconnection Region Beside Ion-Scale Flux Rope at the Magnetopause

Abstract:

Flux transfer events (FTEs) are the flux ropes caused by unsteady reconnection at the magnetopause region, allowing the transportation of solar wind into the terrestrial magnetosphere. Due to the high-resolution plasma data and small separation of the four spacecraft Magnetospheric Multiscale (MMS) mission, the detailed plasma and field structure of ion-scale flux rope FTEs have been investigated. The formation mechanism of these ion-scale flux rope and its relationship with typical size FTEs are still outstanding questions. The results from 3D simulations show that flux ropes can be created and evolve by secondary reconnection in the exhaust of the primary reconnection site. A series of evolving flux ropes in the reconnection exhaust region with size increasing from several ion-scales to the mesoscale have been observed while the secondary reconnection required for their magnetic flux growth has not been directly observed. Using high-resolution data from MMS, we report an in situ observation of a secondary reconnection electron diffusion region (EDR) beside the leading edge of an ion-scale flux rope at the magnetopause. This reconnection EDR is characterized by strong electron current and electron jet reversal, a normal Hall electric field, unmagnetized electrons with non-gyrotropic distributions, a large parallel electric field and strong energy dissipation. Our observation of secondary reconnection beside the flux rope gives direct evidence that flux ropes can be created and evolved in the reconnection exhaust region.