Evidence of a Magnetic Null in Electron-Only Reconnection

Magnetic nulls are traditionally suggested as main points where energy dissipation occurs during reconnection. Such nulls have been widely observed in the diffusion region of standard reconnection following a two-fluid picture, but have never been investigated in the thin current sheet of electron-only reconnection. Recently, an electron-only reconnection event was observed by the Magnetospheric Multiscale (MMS) mission in Earth's magnetosheath. The small separation of MMS spacecraft (∼7 km) provides a good opportunity to examine whether magnetic nulls exist in electron-only reconnection. By utilizing the First-Order Taylor Expansion method, we find a radial null in this event. Moreover, such nulls degenerate into X-type nulls inside a reconnecting current sheet, suggesting that it's quasi 2D structure. By reconstructing the topology of this null, we can also estimate the reconnection rate of this electron-only reconnection event and find it's time-varying, suggesting it's an unsteady process. Our results are useful to further understand the process of electron-only reconnection.