On the ambiguity between electron diffusion region and magnetospheric separatrix: a revisited MMS event

At the magnetopause, the boundary between the magnetosphere and the shocked solar-wind-dominated region, a fundamental process takes place: magnetic reconnection, which allows part of the solar wind plasma to enter the magnetosphere. In order to investigate this process at the scale of electron dynamics, within a region known as the electron diffusion region (EDR), the Magnetospheric Multiscale (MMS) mission was launched in 2015.Our work focuses on the analysis of a magnetic reconnection event at the magnetopause observed by MMS. This event was initially reported as a crossing of the EDR (Webster et al., 2018).We carried out a detailed investigation of this event to determine the spacecraft trajectories within the reconnection region. The signatures of the electric and magnetic fields, particle velocities and energies, energy dissipation, current analysis, as well as the presence of highly structured whistler and lower-hybrid waves, suggest that the EDR may have been confused with another adjacent region: the magnetospheric separatrix. This region corresponds to the boundary between electrons moving toward the reconnection site and those moving away from it. Both the EDR and the magnetospheric separatrix are electron-scale regions that exhibit a number of similar observational signatures.Our results raise an important question: could some previously reported EDR crossings actually correspond to magnetospheric separatrices? What are the differences in terms of energy conversion and partitioning, wave activity, plasma acceleration and heating between the near-EDR magnetospheric separatrix and the EDR?