Simulation Studies On The Hall Current Effect In The Magnetic Reconnection And Magnetic Flux Rope Structures In The Magnetotail

Magnetic reconnection is an effective mechanism for converting magnetic energy into kinetic and thermal energies of plasma, and plays an essential role in the energy coupling system between the Sun and the Earth. Recent research has indicated that the non-resistivity terms in the generalized Ohm's law have important effects on the reconnection dynamics. In this thesis, the Hall effect in magnetic reconnection is investigated using an USTC Hall MHD scheme with an adequate computational accuracy and numerical stability, which was derived from multi-step implicit scheme.In the driven reconnection process with various scales the effect of Hall current is studied numerically in chapter 3. In the cases with L_c/d_i≤1.0 (L_c is the half-thickness of initial current sheet layer, d_i is the ion inertial length), the features of Hall MHD reconnection are shown as follows: a quasi-steady single X-line reconnection is obtained, the B_y component with a quadrupolar structure is generated and the maximum reconnection rate is larger than 0.11. For the cases with L_c/d_i>1.0, the effect of Hall current on the reconnection dynamics weakens and Hall MHD reconnection is gradually transformed into resistive MHD reconnection as L_c/d_i increases.Magnetic reconnection with various plasma β (the ratio of plasma pressure to the magnetic pressure) is studied numerically in chapter 4. It is demonstrated by the comparing studies that the reconnection dynamics is controlled by the Hall effect and the effect of scalar electron pressure gradient is negligible in the generalized Ohm's law for the cases with a zero guide field (i.e., B_y0=0 at t=0). It is also found that the openness of the magnetic separatrices and associated quadrupolar B_y structure is enlarged as β increases. When β>2.0 fine structures of B_y contours with reversed sign emerge. The numerical results indicate that the variations in electron velocity V_e are greater than those in ion velocity V_i and the decoupling of electron and ion occurs in a larger scale lengths than d_i as β increases. Clearly, the reversed current, which is associated with the relative motion between electrons and ions, generates the fine structures of B_y contours in the outflow region. Then the corresponding profile of B_y component exhibits a wave signature. Enhanced wave activities observed during a Cluster crossing of the high-β exterior cusp region might be related to the Hall effects of magnetic reconnection shown in the present simulation.The density depletions were detected by Wind, Cluster and Polar spacecrafts in the...