R. Horiuchi

T. Sato

Horiuchi,  R.

Sato,  T.

NIFS-Repository (National Institute for Fusion Science)

§17. Triggering Mechanism of Collisionless Driven Reconnection in Three Dimensions Horiuchi, R., Sato, T. We have clarified from three-dimensional parti-cle simulation that two different types of plasma instabilities are excited in a collision less current layer in the absence of an external driving source. The lower hybrid drift instability (LHDI) is ob-served to grow in the periphery of current layer in an early period, while a drift kink instability (DKI) is triggered at the neutral sheet in a late period as a result of the nonlinear deformation of the current sheet by the LH-DI. A reconnect ion electric field grows at the neutral sheet in accor-dance with the excitation of the DKI. In order to examine how collisionless reconnec-tion evolve in three dimensions in the presence of an external driving source, we carry out the full particle simulation on a 128 X 64 X 64 point grid by making use of 24 million particles. Figure 1 shows the simulation result for the case of the driv-ing field Eo = -0.02Bo. The Poynting flux, which is supplied into the simulation domain from the exterior region, moves towards the current layer together with the convergent plasma flow, while compressing the current profile. The compres-sion by the convergent plasma flow increases the growth rate of the DKI. Because most of ions in-side the current layer is unmagnetized, the elec-tric field or the Poynting flux, which is carried by the convergent plasma motion, penetrates into the current layer due to the particle kinetic effect. When the Poynting flux reaches the neutral sheet, collisionless reconnect ion is triggered by the con-vecti ve electric field. Once magnetic reconnect ion takes place, a gen-erated fast plasma flow carries away the plasma near the reconnect ion point towards the down-stream. After the reconnection electric field reaches its saturation level, the amplitude of the DKI decreases and the width of the current layer increases (see Fig. 1). These phenomena indicate that the reconnect ion flow carries away the plas-mas faster than the flux is accumulated at the re-connection point by the input flow. Consequently, the current profile is flattened in the vicinity of the reconnection point and the DKI becomes sta-326 4 Q) (ij () Cf) (ij .~ 0.. Cf) djz o 200 40~ t (() ce Figure 1: Time-evolutions of the drift kink mode (solid), the reconnection electric field (dashed), and the half-width of current layer (dotted). ble there. Thus, we cannot see any modification of the current sheet by the DKI in the vicinity of reconnection point (midline), although there ap-pears the n = 1 modification of the current sheet along the vertical axis in the downstream region (see Fig.2). It is concluded that the drift kink in-stability is not a primary cause of collisionless re-connection in the presence of an external driving source. 100 ~ 0 Figure 2: Perspective views of the current sheet at tWce = 408 for the same case as Fig. 1 where the current sheet is defined by the condition Bx = O. References 1) R. Horiuchi and T. Sato, Phys. Plasma 1, 3587 (1994); 4, 277 (1997); 6, 4565 (1999). 

Triggering Mechanism of Collisionless Driven Reconnection in Three Dimensions

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