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.

Fast-shear-flow generated multiple magnetic islands and Alfvénic resonance layers in magnetic reconnection

“…Influences of fast super-Alfvénic shear flow on the KHIs are investigated with the resistive incompressible MHD simulations. It's found that in the presence of fast super-Alfvénic shear flow, first pair of Alfvén layers can appear in the inflow region away from the reconnection separatrices during magnetic reconnection similarly to our previous work [21], together with the second pair of layers formation when the initial asymptotic flow velocity reaches = v V 2.0 .…”

“…Meanwhile, the strong and close layers (i.e. the layer I) can sufficiently suppress the KHI and the reconnection process, which have already been investigated in our previous research [21]. It is clearly shown that the resonance layers become stronger from figures 1(c) to (d).…”

“…As mentioned in our early works [21], formation of Alfvén resonances can occur in such configuration where the flow velocity matches the local Alfvén speed, and the theory has already been derived in the incompressible MHD model [12,22]. Also, it can be induced from ref.…”

“…As mentioned in our previous work [21], the layers formation condition must be > R 1. The second layer originates only because of the increased asymptotic shear velocity, but not increased parameter R. When the asymptotic shear velocity increased large enough ( )  v V 2.0 , A 0 two pairs of layers are induced.…”

“…In our previous work [21], we investigated the effect of super-Alfvénic shear velocities in the range Alfvén speed, and the ratio between the separation distance of the Alfvénic layers and the thickness of current sheet plays a crucial role on magnetic reconnection.…”