Fundamental Understanding of Magnetic Reconnection via Spiral CT Scan. II. Ion-velocity Pattern and Electron-velocity Pattern

Abstract For the first time, we reveal the ion-velocity pattern and electron-velocity pattern around a reconnection X-line by using the spiral CT scan technique, which is a novel technique analogous to that in the hospital. This X-line was detected by the MMS spacecraft and has been widely believed to host a non-guide-field reconnection. With the help of such an advanced technique, we find two interesting features of ion velocities: (1) in the reconnection plane, ion velocities are diverged to form a “saddle-shape” pattern surrounding the X-point at scale above 0.5 d i (ion inertial length), but are stochastic near the X-point at scale below 0.5 d i ; (2) in the out-of-plane direction, ion velocities are large in the magnetosphere, but are small in the magnetosheath. Also, we successfully identify six features of electron velocities: (1) out-of-plane electron jet with scale of 0.1 d i near the X-point, (2) coupled electron and ion velocities at >0.8 d i but decoupling of them at <0.8 d i , (3) weak electron inflow and strong electron outflow at 0.1 d i < Δ < 0.8 d i , (4) stochastic electron velocities at <0.1 d i , (5) intense electron stream at 0.1 d i < Δ < 0.8 d i at the separatrix, and (6) field-aligned electron beam in the inflow region. Among these features, the first seven greatly meet theoretical expectations if we define the ion diffusion region as Δ < 0.8 d i and electron diffusion region as Δ < 0.1 d i ; the last one is a surprise, indicating that only part of the electrons participate in the reconnection process around the X-line.