Enhancement of oxygen in the magnetic island associated with dipolarization fronts

A significant enhancement of O+ is observed by Cluster inside an earthward propagating magnetic island behind a dipolarization front (DF). Such enhancement, from 0.005 to 0.03cm(-3), makes the O+ flux inside the magnetic island similar to 20 times larger than that outside the magnetic island. In the meantime, the H+ density is nearly a constant, 0.1cm(-3), during the magnetic-island encounter. This results in a dramatic increase of the density ratio, n(O+)/n(H+), from 0.05 to 0.3 (about 10 times as large as the average value in the plasma sheet) and a dramatic decrease of the local Alfven speed from V-A approximate to 770km/s to V-A approximate to 430km/s inside the magnetic island. The decrease of Alfven speed indicates an asymmetric reconnection and a slow magnetic reconnection rate near the secondary X line. Since the reconnection rates at the primary X line and secondary X line are imbalanced, the DFs and magnetic islands are pushed to propagate earthward by the outflow of the primary reconnection, as demonstrated in recent simulations.