High Lundquist Number Research Articles

Dynamics of seed magnetic island formation due to geometrically coupled perturbations

Seed magnetic island formation due to a dynamically growing external source in toroidal confinement devices is modeled as an initial value, forced reconnection problem. For an external source whose amplitude grows on a time scale quickly compared to the Sweet–Parker time of resistive magnetohydrodynamics, the induced reconnection is characterized by a current sheet and a reconnected flux amplitude that lags in time the source amplitude. This suggests that neoclassical tearing modes, whose excitation requires a seed magnetic island, are more difficult to cause in high Lundquist number plasmas.

Dynamics of thin current sheets and their disruption by ballooning instabilities: A mechanism for magnetospheric substorms

Multipoint satellite observations indicate the development of thin current sheets and an impulsive intensification of the cross-tail current density in the growth phase at near-earth distances during a short interval (<1 min) just before onset, after a period of sluggish growth (∼0.5–1.5 h). These multiple time scales are accounted for by analysis and two-dimensional magnetohydrodynamic simulation of the magnetotail in the high-Lundquist number regime, including the earth’s dipole field. In the slow growth phase, a thin current sheet develops spanning Y points that stretch from the midtail region (∼30RE) to the near-earth region (∼10RE). This is followed by an impulsive enhancement in the current sheet amplitude due to flux pileup, consistent with observations. The stretched magnetotail with an embedded thin current sheet is found to be unstable to an ideal compressible ballooning instability with rapid spatial variation in the dawn–dusk direction. The linear instability is demonstrated by numerical solutions of the ideal ballooning eigenmode equation for a sequence of two-dimensional magnetotail configurations containing a thin current sheet, realized during the impulsive growth phase. Line-tied boundary conditions are imposed at the ionosphere, and shown to have a strong influence on the linear stability of ballooning modes at near-earth distances. It is suggested that the ideal ballooning instability provides a possible mechanism for disrupting the cross-tail current at substorm onset.

Nonlinear Phenomena Due to anm=1 Non-Resonant Current-Driven Mode in RFP with Relatively HighSNumber

In an RFP plasma with relatively high Lundquist number S , nonlinear phenomena due to a single non-resonant current-driven mode with poloidal mode number m = 1 are examined by using a numerical simulation of an incompressible plasma. In contrast to the low S number case, a new phenomenon, i.e., a nonlinear oscillation is observed, which is brought about by the nonlinear interaction of the m = 1 mode with itself. This nonlinear oscillation occurs around a helical equilibrium in close vicinity to the initial axisymmetric equilibrium and is damped by the energy cascade to the higher harmonics and dissipation. The higher harmonics convert the global flow pattern into the local one, which creates anti-parallel magnetic field lines. Consequently, a nonlinear reconnection occurs after the nonlinear oscillation is completely or almost completely damped.