Formation Of Magnetic Flux Tubes Research Articles

Formation of magnetic flux tubes in cylindrical wedge geometry

Three-dimensional (3D) MHD numerical simulations have not been able to demonstrate convincingly the spontaneous formation of large vertical flux tubes. Two-dimensional (2D) magnetoconvection in axisymmetric cylinders forms a central magnetic flux tube surrounded by annular convection rings. To study the robustness of this type of solution in three dimensions, the nonlinear resistive MHD equations are solved numerically in a 3D cylindrical wedge from an initially uniform vertical magnetic field. It is shown that the 2D result is retrieved for small domain radii. However, for larger radii the central axis loses its importance and in this case many convection cells form in the numerical domain. Magnetic flux is captured between cells where flow converges and the reduced amount of flux that congregates at the central axis is eroded by the surrounding convection.

Time-dependent magnetic reconnection: Two- and three-dimensional MHD simulations

Observations of flux transfer events (FTEs) at the earth magnetospause and plasmoids in the magnetotail indicate that magnetic reconnections in the earth magnetosphere are time-dependent. Motivated by the observed flux transfer events, Lee and Fu proposed a multiple X-line reconnection (MXR) model for the dayside magnetopause. The multiple X-line reconnection is intrinsically a time-dependent process and is different from the classical single X-line reconnection (SXR) process. As a consequence of MXR, plasmoids or magnetic flux ropes are intermittently formed and convected out of the reconnection region. Two- and three-dimensional MHD simulations have been carried out to study the time-dependent magnetic reconnection process. The 2-D simulation results show that the time-dependent multiple X-line process tends to occur when the system length is long and the resistivity is small. Otherwise, the quasi-steady single X-line process takes place. It is found that the occurrence of multiple reconnection sites in the current sheet may be due to either (1) the development of tearing mode instability in the local slab model, or (2) the nonuniformity of the current sheet in the global model. In the three-dimensional case, the multiple X-line reconnection process leads to the formation of magnetic flux tubes or magnetic ropes. In the three-dimensional case, the field line topology at the ends of the magnetic rope is very complicated. The simulation shows that magnetic ropes formed through the MXR process usually have “frayed” ends. In addition, a strong tube-aligned plasma flow is observed during the formation of magnetic flux tubes in the 3-D simulation.

A magnetohydrodynamic simulation of the formation of magnetic flux tubes at the Earth's dayside magnetopause

We have studied dayside magnetic reconnection by using a three‐dimensional global magnetohydrodynamic simulation of the interaction between the solar wind and the magnetosphere. We found two different mechanisms for the formation of magnetic flux tubes at the dayside magnetopause which depend on the orientation of the interplanetary magnetic field (IMF). The dayside magnetic flux tubes occur only when the IMF has a southward component. A strongly twisted and localized magnetic flux tube similar to magnetic flux ropes appears at the subsolar magnetopause when the IMF has a large By component. When the By component is small, twin flux tubes appear at the dayside magnetopause. Both types of magnetic flux tube are consistent with several observational features of flux transfer events and are generated by antiparallel magnetic reconnection.

Simulation of multiple X-line reconnection at the dayside magnetopause

The multiple X-line reconnection process, which was proposed by Lee and Fu [1985] for the dayside magnetopause, is simulated based on a two-dimensional incompressible MHD model. It is found that when the system length is much larger than the thickness of the current layer, magnetic flux tubes are repeatedly formed and convected out of the system. Impulsive magnetic reconnections associated with the formation of magnetic flux tubes are observed. In the case of the dayside magnetopause, magnetic flux tubes with a cross-section of ∼ 1 RE² are found to be formed repeatedly every 5-10 minutes, consistent with satellite observations.