Dipolarization fronts as a consequence of transient reconnection: In situ evidence

Dipolarization fronts (DFs) are frequently detected in the Earth's magnetotail from X_GSM = -30 R_E to X_GSM = -7 R_E. How these DFs are formed is still poorly understood. Three possible mechanisms have been suggested in previous simulations: (1) jet braking, (2) transient reconnection, and (3) spontaneous formation. Among these three mechanisms, the first has been verified by using spacecraft observation, while the second and third have not. In this study, we show Cluster observation of DFs inside reconnection diffusion region. This observation provides in situ evidence of the second mechanism: Transient reconnection can produce DFs. We suggest that the DFs detected in the near-Earth region (X_GSM > -10 R_E) are primarily attributed to jet braking, while the DFs detected in the mid- or far-tail region (X_GSM < -15 R_E) are primarily attributed to transient reconnection or spontaneous formation. In the jet-braking mechanism, the high-speed flow "pushes" the preexisting plasmas to produce the DF so that there is causality between high-speed flow and DF. In the transient-reconnection mechanism, there is no causality between high-speed flow and DF, because the frozen-in condition is violated. Key Points DFs are observed inside reconnection diffusion region Three formation mechanisms of DF are compared Causality between flow and DF is discussed