Enhancement of oxygen in the magnetic island associated with dipolarization fronts

Abstract

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 0.03 cm^-3, makes the O⁺ flux inside the magnetic island 20 times larger than that outside the magnetic island. In the meantime, the H⁺ density is nearly a constant, 0.1 cm^-3, during the magnetic-island encounter. This results in a dramatic increase of the density ratio, nO<mo form="prefix">+>/nH<mo form="prefix">+, 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 Alfvén speed from V_A ≈ 770 km/s to V_A ≈ 430 km/s inside the magnetic island. The decrease of Alfvén 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.