Formation of dipolarization fronts after current sheet thinning

Abstract

Dipolarization front (DF)—a sharp boundary separating hot tenuous plasmas from cold dense plasmas—is a key structure responsible for particle acceleration and energy transport in the magnetotail. How such a structure is formed has been unclear so far. Two possible mechanisms suggested in previous studies are magnetic reconnection and spontaneous formation. Both of them require current sheet thinning as a prerequisite. However, observational evidence of the DF formation associated with current sheet thinning has not been reported. In this study, we present such an observation, showing the DF formation after current sheet thinning. We estimate the half thickness of the current sheet to be ∼1000 km and the rate of current sheet thinning as ∼38 km/s. We find that the DF is likely formed at XGSM ≈ −20 RE. During the current sheet thinning, the plasma becomes cold and dense; during DF formation, many magnetic islands are produced. Although current sheet thinning and DF formation have been individually analyzed in the previous studies, this study, for the first time, links the two transient processes in the magnetotail.