Observation of a retreating x line and magnetic islands poleward of the cusp during northward interplanetary magnetic field conditions

Abstract

When the interplanetary magnetic field is northward, reconnection occurs in each hemisphere on lobe field lines, poleward of the cusp. We have identified a case where the Cluster spacecraft crossed the magnetopause and encountered a tailward retreating x line. The x line is identified by the encounter of both a tailward and sunward jet, as well as Hall magnetic field signatures in the out-of-plane direction. Additionally, we find no signatures of electron heating and hypothesize that the spacecraft is too close to the x line to observe the accelerated electrons. Using two spacecraft, we are able to resolve the velocity of the structure, which moves near the magnetosheath speed. The speed of the x line is also consistent with the asymmetric reconnection theory. To our knowledge, this is the first time the speed of a retreating x line has been measured directly. Additionally, we observe ion distribution functions with counterstreaming populations, suggesting that a second x line formed sunward of the original one, leading to a magnetic island.