Princeton University Astroplasmas Seminar

In high-Lundquist-number plasmas, reconnection proceeds via onset of tearing, followed by a non-linear phase when plasmoids continuously form, merge, and are ejected from the current sheet (CS). This process is understood in fully ionized, magnetohydrodynamic plasmas. However, many plasma environments, such as star-forming molecular clouds and the solar chromosphere, are poorly ionized. We use theory and computation to study tearing-initiated reconnection in such poorly ionized systems. In this talk, we focus on the onset and linear evolution of this process. In poorly ionized plasmas, magnetic field nulls on scales below v_{An0}/\nu_{ni0}, with v_{An0} the neutral Alfven speed and \ni_{ni0} the neutral-ion collision frequency, will self-sharpen via ambipolar diffusion into CSs which tear. This nonlinear sharpening occurs at an increasing rate, delaying the onset of reconnection by decreasing the CS width at which the tearing growth rate becomes larger than the CS formation rate. Once the CS becomes thin enough, however, ions decouple from neutrals and thinning of the CS slows, allowing tearing onset. We find that the wavelength of the mode that first disrupts the forming sheet decreases as ionization fraction decreases and that onset occurs in a time of order \nu_{ni0}^{-1}.