Eric Lawrence

Three‐dimensional, impulsive magnetic reconnection in a laboratory plasma

Impulsive, local, 3-D reconnection is identified for the first time in a laboratory current sheet. The events observed in the Magnetic Reconnection Experiment (MRX) are characterized by large local gradients in the third direction and cannot be explained by 2-D models. Detailed measurements show that the ejection of flux rope structures from the current sheet plays a key role in these events. By contrast, even though electromagnetic fluctuations in the lower hybrid frequency range are also observed concurrently with the impulsive behavior, they are not the key physics responsible. A qualitative, 3-D, two-fluid model is proposed to explain the observations. The experimental results may be particularly applicable to space and astrophysical plasmas where impulsive reconnection occurs.

Magnetic reconnection in partially ionized plasmas

We review the theory of magnetic reconnection in weakly ionized gases. The theory is relevant to reconnection in the interstellar medium, protostellar and protoplanetary disks, the outer envelopes of cool stars, and a new laboratory experiment. In general, partial ionization introduces three effects beyond the obvious one: increased resistivity due to electron-neutral collisions. First, magnetic neutral sheets are steepened by plasma-neutral drift, setting up the conditions for reconnection. Second, when ion-neutral friction is strong, the effective ion mass is increased by ρ/ρi, the ratio of total to plasma mass density. This reduces the Alfven speed vA by a factor of ρ/ρi and increases the ion skin depth δi by ρ/ρi. As a result, entrainment of neutrals slows MHD reconnection but permits the onset of fast collisionless reconnection at a larger Lundquist number S, or for a longer current sheet, than in the fully ionized plasma case. These effects, taken together, promote fast collisionless reconnection wh...

Experimental observation of 3-D, impulsive reconnection events in a laboratory plasma

Fast, impulsive reconnection is commonly observed in laboratory, space, and astrophysical plasmas. In this work, impulsive, local, 3-D reconnection is identified for the first time in a laboratory current sheet. The two-fluid, impulsive reconnection events observed on the Magnetic Reconnection Experiment (MRX) [Yamada et al., Phys Plasmas 4, 1936 (1997)] cannot be explained by 2-D models and are therefore fundamentally three-dimensional. Several signatures of flux ropes are identified with these events; 3-D high current density regions with O-point structure form during a slow buildup period that precedes a fast disruption of the reconnecting current layer. The observed drop in the reconnection current and spike in the reconnection rate during the disruption are due to ejection of these flux ropes from the layer. Underscoring the 3-D nature of the events, strong out-of-plane gradients in both the density and reconnecting magnetic field are found to play a key role in this process. Electromagnetic fluctuat...

Study of the effects of guide field on Hall reconnection

The results from guide field studies on the Magnetic Reconnection Experiment (MRX) are compared with results from Hall magnetohydrodynamic (HMHD) reconnection simulation with guide field. The quadrupole field, a signature of two-fluid reconnection at zero guide field, is modified by the presence of a finite guide field in a manner consistent with HMHD simulation. The modified Hall current profile contains reduced electron flows in the reconnection plane, which quantitatively explains the observed reduction of the reconnection rate. The present results are consistent with the hypothesis that the local reconnection dynamics is dominated by Hall effects in the collisionless regime of the MRX plasmas. While very good agreement is seen between experiment and simulations, we note that an important global feature of the experiments, a compression of the guide field by the reconnecting plasma, is not represented in the simulations.