Tearing-mediated reconnection in magnetohydrodynamic poorly ionized plasmas

A seminar by Libby Tolman

Many plasma environments, such as star-forming molecular clouds, the solar chromosphere, and the diffuse interstellar medium, are poorly ionized and threaded by dynamically important magnetic fields. We use theory and computation to study tearing-mediated reconnection in such poorly ionized systems. In poorly ionized plasmas, a magnetic null on a scale below ​​vA,n0/ni0, with vA,n0 the neutral Alfvén speed and ni0 the neutral–ion collision frequency, will self-sharpen via ambipolar diffusion into a current sheet. This sharpening occurs at an increasing rate, inhibiting the onset of reconnection. Once the current sheet becomes thin enough, however, ions decouple from neutrals and the thinning of the current sheet slows, allowing the onset of tearing in a time of order ni0 . We find that the wavelength and growth rate of the mode that first disrupts the forming sheet can be predicted from a poorly ionized tearing dispersion relation; as the plasma recombination rate increases and ionization fraction decreases, the growth rate becomes an increasing multiple of ni0 and the wavelength becomes a decreasing fraction of vA,n0/ni0. After reconnection onsets in a current sheet, the system enters a nonlinear phase characterized by a stochastic plasmoid chain, but the characteristics of this chain differ from those of a stochastic plasmoid chain in fully ionized plasma. The plasma in the plasmoids is characterized by an ionization fraction which is much larger than that of the background plasma. Our results could have significant implications for understanding of several important astrophysical processes, including the transport of cosmic rays in the interstellar medium.

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