EFFECTS OF MAGNETIC RECONNECTION IN THE KELVIN-HELMHOLTZ INSTABILITY AT THE MAGNETOSPHERIC BOUNDARY

MHD simulation study is performed to investigate the evolution of the Kelvin-Helmholtz (KH) instability and the effects of magnetic reconnec tion at the magnetopause boundary. The long-term evolution is markedly different according to the initial configuration of the terrestrial a nd interplanetary magnetic fields. Highly distorted magnetic field lin es due to the KH instability become reconnected and flattened so that they resume the straight field line structure when the terrestrial mag netic field and the interplanetary magnetic field are parallel to each other and the intensity of the involved magnetic field is weak. The d etached magnetic islands are transported toward the magnetosheath regi on and suffer local reconnections. On the other hand, when the intensi ty of the magnetic field is increased but the Kelvin-Helmholtz instabi lity condition is still satisfied, the wavy field lines due to the KH instability straighten without involving magnetic reconnection. When t he terrestrial and interplanetary magnetic fields are anti-parallel to each other, magnetic reconnection occurs early but the strong plasma flow makes the magnetic islands flatter as time develops. No steady re connection is observed in this case. Instead, neighboring magnetic isl ands merge together. Momentum transport is most effective in the paral lel field case with magnetic reconnection among the three cases consid ered here. Reduction of the velocity shear is also most prominent in t his case. (C) 1997 Elsevier Science Ltd.