Quantitative analysis of a Hall system in the exhaust of asymmetric magnetic reconnection

Taking advantage of high-resolution measurements from the MMS mission, we find evidence for a complete Hall system in the exhaust of asymmetric magnetic reconnection 40 Di downstream of the X line. The investigation of the fine structure of the Hall system reveals that it displays features in the exhaust similar to those reported previously in the ion diffusion region by simulations and observations. This finding confirms the importance of particle-scale processes in the reconnection exhaust as well. On the magnetospheric side of the exhaust, electrons are strongly accelerated by parallel electric fields. This process significantly contributes to feed the Hall current system, resulting in a nonnegligible Hall magnetic field signature on this side despite an otherwise lower density. Calculation of the induced out-of-plane magnetic field by in-plane currents (based on Biot-Savart law) provides direct quantitative evidence for the process of Hall magnetic field generation by the Hall current system. A strong normal Hall electric field is present only on the magnetospheric side of the exhaust region, consistent with previous works. Multipoint data analysis shows that the ion pressure gradient in the ion momentum equation produces this Hall electric field. This global pattern of the Hall system can be explained by kinetic Alfvén wave theory.
Funding details: 41231066, NSFC, National Natural Science Foundation of China; Funding details: 41574159, NSFC, National Natural Science Foundation of China; Funding details: 41574161, NSFC, National Natural Science Foundation of China; Funding details: 41574163, NSFC, National Natural Science Foundation of China; Funding text: MMS data can be accessed at https://lasp.colorado.edu/mms/sdc/public/. We acknowledge all the MMS teams for their wonderful work. This work was supported by the National Natural Science Foundation of China (grants 41574163, 41574159, 41574161 and 41231066) and the Specialized Research Fund for State Key Laboratories. Work on MMS at IRAP is supported by CNRS and CNES.QC 20171201