Evidence of a Magnetic Null in Electron-Only Reconnection

Abstract

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.