External Reconnection Research Articles

Non-linear simulations of internal reconnection events in spherical tokamaks

Three dimensional MHD simulations areexecuted in a full toroidal geometry to clarify the physicalmechanisms of the internal reconnection event (IRE) which is observedin spherical tokamak experiments. The simulation results reproduceseveral main properties of IREs. A notable property in the lineargrowth of modes is the simultaneous excitation of multiple low nmodes, which grow together with similar growth rates. A spontaneousphase alignment mechanism among these excited modes is found in thenon-linear stage of the development, which causes a bulge-likedeformation of the torus and subsequent expulsion of heat energy tothe external region through the occurrence of external reconnection.

Steady state magnetic reconnection at high magnetic Reynolds number: A boundary layer analysis

A boundary layer analysis of two‐dimensional current sheets appearing in magnetic reconnection is presented in the framework of resistive incompressible two‐dimensional magnetohydrodynamics. The coordinate normal to the current sheet is rescaled by a power of the resistivity in order to obtain inner limit equations which satisfy the symmetry conditions and represent magnetic annihilation to lowest order. They can be matched to the outer expansion of the incompressible resistive MHD equations with a domain of validity smaller than the current sheet length. The magnetic vector potential and the stream function are expanded in powers of the inverse magnetic Reynolds number Rm. For the inner limit equations, four cases are found depending on the boundary conditions in the overlap region. These cases correspond to different scalings of the velocity and the magnetic field which are equivalent to different prescribed boundary conditions of the magnetic field and plasma velocity. For the case of the magnetic field dominating over the flow, inner and outer solutions are obtained up to the second order in the expansion parameter and are matched in the intermediate region. Scaling laws for the width of the current sheet are derived: It decreases for increasing magnetic Reynolds number. The characteristic length for growth of the magnetic field component normal to the current sheet increases with magnetic Reynolds number. A Sweet‐Parker scaling is found for the external reconnection rate.

Eruptive instability of cylindrical prominences

The stability of prominences and the dynamics of an eruption are studied. The prominence is represented by an uniformly twisted, curved, magnetic tube, anchored at both ends in the photosphere. Several stages of the eruption are analyzed, from the pre-eruptive phase and the onset of the instability, up to the late phases of the process. Before the eruption, the prominence evolves through a series of equilibrium states, slowly ascending either due to an increase of the electric current or to mass loss. The eruption starts when the ratio of the current to the total mass attains a critical value after which no neighbouring equilibrium exists. The linearized equation of motion was used to obtain the instability threshold, which is presented in a form enabling comparison with the observations. The height at which the prominence erupts depends on the twist, and is typically comparable with the footpoint half-separation. Low-lying prominences are stable even for large twists. The importance of the external field reconnection below the filament, and the mass loss through the legs in the early phases of the eruption is stressed. The oscillations of stable prominences with periods on the Alfven time-scale are discussed. The results are compared with the observations.