Forced Reconnection Research Articles

Symmetry breaking bifurcations of a current sheet

Using a time evolution code with periodic boundary conditions, the viscoresistive hydromagnetic equations describing an initially static, planar current sheet with large Lundquist number have been evolved for times long enough to reach a steady state. A cosh2 x resistivity model was used. For long periodicity lengths Lp, the resistivity gradient drives flows that cause forced reconnection at X point current sheets. Using Lp as a bifurcation parameter, two new symmetry breaking bifurcations were found: a transition to an asymmetric island chain with nonzero, positive, or negative phase velocity, and a transition to a static state with alternating large and small islands. These states are reached after a complex transient behavior, which involves a competition between secondary current sheet instability and coalescence.

Current sheets and nonlinear growth of the m=1 kink-tearing mode

A calculation is presented that accounts for rapid nonlinear growth of the m=1 kink-tearing instability. The equilibrium analysis contained in the Rutherford theory [Phys. Fluids 16, 1903 (1973)] of nonlinear tearing-mode growth is generalized to islands for which the constant-ψ approximation is not valid. Applying the helicity-conservation assumption introduced by Kadomtsev [Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Vienna, 1977), Vol. I, p. 555], the presence of a current-sheet singularity is shown that gives rise to a narrow tearing layer and rapid reconnection. This rapid reconnection, in turn, justifies the use of the helicity conservation assumption. The existence of a family of self-similar m=1 equilibrium islands is demonstrated. The formalism introduced here is shown to apply both to the case of the m=1 kink-tearing mode and to the case of forced reconnection. These two cases are compared and contrasted.

A simulation study of forced reconnection processes and magnetospheric storms and substorms

We simulate, on the basis of a two‐dimensional incompressible MHD code, the plasma dynamics of a long magnetotail (∼200 RE) under a constant and time‐varying driving force delivered from the solar wind. It is found that under a constant driving force the magnetic fields in the magnetotail tend to reconnect impulsively and the formation of X line and plasmoids occurs intermittently and repeatedly every 2–4 hours. Under a time‐varying driving force the post‐driven phase of a plasmoid formation and the spontaneous reconnection process are also studied. Our results show that magnetic reconnection in the magnetotail during magnetospheric substorms and storms is basically a driven process. A complete sequence of the event, going from the gradual pile‐up of magnetic flux to the formation and the antiearthward convection of the new X line and to the ejection of plasmoids from the antiearthward end of the magnetotail, requires the presence of the driving force. The simulation results may also be useful in unifying the different interpretations (1) of the plasmoid formation and plasma sheet thinning and (2) of the relative importance of magnetotail processes with respect to the direct solar wind effects during substorms.

Forced reconnection by nonlinear magnetohydrodynamic waves

Forced magnetic reconnection induced by magnetohydrodynamic (MHD) waves may account for the triggering of explosive solar activities such as flares. Reconnection in a neutral sheet plasma can be driven by the ponderomotive force associated with nonlinear MHD waves accompanying plasma vortex motion. The nonlinear stage of forced reconnection by MHD waves is simulated with a MHD particle-code: Some conditions for fast reconnection are discussed with applications to solar flares.

Reconnection rates of magnetic fields including the effects of viscosity

The Sweet–Parker and Petschek scalings of the magnetic reconnection rate are modified to include the effect of the viscosity. The modified scalings show that the viscous effect can be important in high-β plasmas. The theoretical reconnection scalings are compared with numerical simulation results in a tokamak geometry for three different cases: a forced reconnection driven by external coils, the nonlinear m=1 resistive internal kink, and the nonlinear m=2 tearing mode. In the first two cases, the numerical reconnection rate agrees well with the modified Sweet–Parker scaling when the viscosity is sufficiently large. When the viscosity is negligible, a steady state which was assumed in the derivation of the reconnection scalings is not reached and the current sheet in the reconnection layer either remains stable through sloshing motions of the plasma or breaks up to higher m modes. When the current sheet remains stable, a rough comparison with the Sweet–Parker scaling is obtained. In the nonlinear m=2 tearing mode case where the instability is purely resistive, the reconnection occurs on the slower dissipation time scale (ψ̇s∼η). In addition, experimental data of the nonlinear m=1 resistive internal kink in tokamak discharges are analyzed and are found to give reasonable agreement with the modified Sweet–Parker scaling.

Reconnection Phenomena during the Formation Phase of Field-Reversal Experiments

The results of hybrid simulations of the formation and the evolution to equilibrium of field-reversal experiment (FRX) configurations are presented. The observed rapid reconnection is explained in terms of forced reconnection driven by a Kruskal-Schwarzschild instability and the self-consistent production of toroidal magnetic field.