Evidence of an extended electron current sheet and its neighboring magnetic island during magnetotail reconnection

We have identified a spatially extended electron current sheet (ECS) and its adjacent magnetic island during a magnetotail reconnection event with no appreciable guide field. This finding is based on data from the four Cluster spacecraft and is enabled by detailed maps of electron distribution functions and DC electric fields within the diffusion region. The maps are developed using two‐dimensional particle‐in‐cell simulations with a mass ratio m i / m e = 800. One spacecraft crossed the ECS earthward of the reconnection null and, together with the other three spacecraft, registered the following properties: (1) The ECS is colocated with a layer of bipolar electric fields normal to the ECS, pointing toward the ECS, and with a half width less than 8 electron skin depths. (2) In the inflow region up to the ECS and separatrices, electrons have a temperature anisotropy ( T e ∥ / T e ⊥ > 1), and the anisotropy increases toward the ECS. (3) Within about 1 ion skin depth ( d i ) above and below the ECS, the electron density decreases toward the ECS by a factor of 3–4, reaching a minimum at edges of the ECS, and has a local distinct maximum at the ECS center. (4) A d i ‐scale magnetic island is attached to the ECS, separating it from another reconnection layer. Our simulations established that the electric field normal to the ECS is due to charge imbalance and is of the ECS scale, and ions exhibit electron‐scale structures in response to this electric field.