The evolution of magnetic helicity in driven reconnection processes and the flux rope structures at the dayside magnetopause

A two-dimensional magnetohydrodynamics simulation with three components is carried out to explore the driven reconnection processes at the dayside magnetopause. The numerical results show significant enhancements of the magnetic field strength B and the B-y component at the centers of the recurrent plasmoid-like structures. Such features are in line with the signatures of flux transfer events which are envisaged as passages of magnetic flux tubes. The evolution of the topological structure in reconnected flux tubes is investigated in terms of the principle of magnetic helicity. It is found that the total magnetic helicity in the domain increases as time elapses due to the transportation of magnetic helicity flux through the system boundaries and a peak value of the magnetic helicity density h always occurs in the central region of the reconnected flux tube. Furthermore, the gradual accumulation of h in the current sheet is associated with the accumulated magnetic flux which is carried into this region by the plasma inflow. The concentration of the helicity density h in the core of a flux tube arises from the enhancement of the axial field B-y, which is associated with the continuous addition of B-y flux to the domain. The present results indicate that the occurrence of the flux rope structure in the vicinity of the magnetopause might be related to the solar wind plasma flow. (C) 2004 American Institute of Physics.