A current sheet, or reconnection layer, often manifests the essential physics of magnetic reconnection. The dynamics of this small region can set the rate for reconnection to occur, and thereby influence magnetic self-organization on the larger scale. Fig.1 presents two theoretical models for magnetic field patterns that might occur when magnetic fields of opposite polarity approach a region where they merge (from vertical plasma motion) and reconnect. In both 2D models, newly reconnected field lines emerge from the reconnection region and move away horizontally. The traditional Sweet-Parker model assumes the resistive MHD description, but gives very slow predictions for the reconnection rates due to the narrow current sheet shown by the dotted red lines in Fig.1(a). Alternatively, Petschek introduced shocks which open up the currrent sheet to a wedge shape as shown in Fig.1 (b), leading to more rapid reconnection by eliminating the slow flow of the Sweet-Parker model. While the Petschek reconnection rate is more consistent with the observed fast reconnection rate observed in space (e.g., solar flares) and has become popularly cited, it is not consistent with detailed resistive MHD calculations. However, the MHD framework breaks down within the current sheet when its thickness is sufficiently large. Then ions become demagnetized while electrons stay magnetized, leading to various two-fluid effects including the so-called Hall effect due to separation between ion and electron motions. One of the theoretical predictions of the Hall effect is the presence of a quadrupolar out-of-plane magnetic field.

Fig.1 (a) Sweet-Parker Model; (b)Petschek Model

In the Magnetic Reconnection Experiment (MRX), a well-controlled laboratory experiment at Princeton Plasma Physics Laboratory, these predicted two-fluid effects have been clearly observed during fast reconnection. Figure 2 shows how the profile of the MRX current sheet changes with plasma collisionality by comparing the sheet configuration described by the measured magnetic field vectors and flux contours for high (collisional) and low density (nearly collsionless) cases. In the high density case ( Fig. 2a), where the mean free path is much shorter than the sheet thickness, the current sheet is rectangular, as in the Sweet-Parker, and the classical, relatively slow reconnection rate is measured. There is no quadrupolar out-of-plane magnetic field. At low plasma density (Fig. 2b), where the electron mean free path is longer than the sheet thickness, the Hall MHD effects become dominant as indicated by the out-of-plane field depicted by the color code). The current sheet is of the shape of the Petschek model and a fast reconnection rate is measured. However, a slow shock, a signature of the Petschek model, has not been identified even in this regime to date. The conclusion from these measurements is that two-fluid Hall effects enhance the rate of reconnection in the regime of weak collisionality.

Fig.2: Comparison of neutral sheet configuration described by measured magnetic field vectors and flux counters for high (collisional) and low density cases; (a) Collisional regime (l_mfpsub> ~1mm << δ)); (b) Nearly collisionless regime (l_mfp ~1cm ~δ ). Out-of plane fields are depicted by the color codes ranged -50 G < Bt <50 G.

References: M. Yamada, et al, Phys. Plasmas v. 13, 052119 (2006); Y. Ren et al, PRL, vol.95, 055003 (2005)

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