Example of exponentially enhanced magnetic reconnection driven by a spatially-bounded and laminar ideal flow

Abstract

In laboratory and natural plasmas of practical interest, the spatial scale ${\Delta }_d$ at which magnetic field lines lose distinguishability differs enormously from the scale $a$ of magnetic reconnection across the field lines. In the solar corona, plasma resistivity gives $a/{\Delta }_d\sim 10^{12}$, which is the magnetic Reynold number $R_m$. The traditional resolution of the paradox of disparate scales is for the current density $j$ associated with the reconnecting field $B_{rec}$ to be concentrated by a factor of $R_m$ by the ideal evolution, so $j\sim B_{rec}/{\mu }_0{\Delta }_d$. A second resolution is for the ideal evolution to increase the ratio of the maximum to minimum separation between pairs of arbitrarily chosen magnetic field lines, ${\Delta }_{max}/{\Delta }_{min}$, when calculated at various points in time. Reconnection becomes inevitable where ${\Delta }_{max}/{\Delta }_{min}\sim R_m$. A simple model of the solar corona will be used for a numerical illustration that the natural rate of increase in time is linear for the current density but exponential for ${\Delta }_{max}/{\Delta }_{min}$. Reconnection occurs on a time scale and with a current density enhanced by only $\ln (a/{\Delta }_d)$ from the ideal evolution time and from the current density $B_{rec}/{\mu }_{0}a$. In both resolutions, once a sufficiently wide region, ${\Delta }_r$, has undergone reconnection, the magnetic field loses static force balance and evolves on an Alfvénic time scale. The Alfvénic evolution is intrinsically ideal but expands the region in which ${\Delta }_{max}/{\Delta }_{min}$ is large.

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