Formation of Alfvénic resonance layers in magnetic reconnection

Abstract

In the framework of two-dimensional incompressible MHD, we investigate the formation of Alfvénic resonance layers with different super-Alfvénic shear flows. It is found that Alfvénic resonance layers are formed in the inflow region for the cases with the shear flow thickness larger than the current sheet thickness. The Alfvénic layers exist at where the flow velocity is equal to the local Alfvén speed and slowly drift away from the current sheet region as a magnetic island develops. The ratio (D) between the separation of the Alfvénic resonance layers and the current sheet thickness plays a crucial role on magnetic reconnection. It is found that D ∼ 3 is a critical value, which is about the saturated size of a magnetic island in magnetic reconnection without super-Alfvénic shear flow. For D < 3, the super-Alfvénic shear flow shows mainly a suppressing effect on magnetic reconnection and the peaked reconnection rate drops below the rate without a super-Alfvénic shear flow. When D > 3, the boosting effect of Kelvin-Helmholtz instability surpasses the suppressing effect by Alfvénic resonance and the peaked reconnection rate is larger than that without a super-Alfvénic shear flow. For D ∼ 5, the super-Alfvénic shear flow gives rise to a strongest boosting effect on magnetic reconnection. Possible applications are briefly discussed.