Detection of Large Guide Field Electron-Only Reconnection in a Filamentary Current Sheet Immersed in a Large-Scale Magnetopause Reconnection Exhaust

We present the Magnetospheric Multiscale (MMS) mission's observations of large-scale reconnection with a laminar ion exhaust containing many electron-scale filamentary current sheets, one of which exhibited a super-ion-Alfv\'enic electron jet flowing opposite to the ion exhaust jet, identified as electron-only reconnection. Despite the close proximity (a few electron inertial lengths) of the four MMS spacecraft, they recorded different electron-only reconnection signatures, including differences in the magnitude and profile of the electron jet, parallel electric fields, and dissipation jE^'+v₄, indicating extremely short-scale spatial and temporal variability. Furthermore, the leading spacecraft did not detect reconnection signatures, suggesting that MMS may have witnessed the onset of electron-only reconnection. Hall magnetic field perturbations associated with an electron-scale current sheet were detected after the onset. The dissipation jE^' in the electron-only current sheet was dominated by the component perpendicular to the magnetic field, contrasting with the dominance of the parallel component in previous large guide field reconnection events.