HR: 0800h
AN: SM21B-0378    [Abstracts]
TI: Density Depletion and Hall Effect in Magnetic Reconnection
AU: * Jin, S
EM: spjin@ustc.edu.cn
AF: School of Earth and Space Sciences, University of Science and Technology of China, No. 1, Jinzhai Road, Hefei, 230026 China
AU: Yang, H
SM21B-0378 AF: School of Earth and Space Sciences, University of Science and Technology of China, No. 1, Jinzhai Road, Hefei, 230026 China
AB: We investigate the layers of density depletion in magnetic reconnection using a 2.5 dimensional Hall MHD code developed from a multi-step implicit scheme. The numerical results at the quasi-steady state of Hall MHD reconnection with di/L0 greater than 0.1 show not only the density depletions along the magnetic separatrices but also a dip in the density in the vicinity of the X neutral line. We examine a series of cases with di/L0 ranging from 0.25 to 0.01 and find the depleting layers of density along the separatrices become more and more slight as the Hall effect weakens with decreasing di/L0, and they are hardly distinguishable for the cases with di/L0 less than 1/20. What­_s more, the profile of the density Ý¥* at the central point of the diffusion region as the function of di/L0 illustrates that Ý¥* decreases as Hall effect strengthens with increasing di/L0. As seen in the profile of Ý¥* versus di/L0, Ý¥* is approximate to 2.0 in the cases with di/L0 less than 1/50 where the reconnection dynamics are controlled by the resistivity. However, Ý¥* is less than 1.0 in the cases with di/L0 greater than 0.1 where the reconnection dynamics are dominated by Hall effect. It demonstrates that the density depletion in the magnetic reconnection is a peculiar feature to the case with a strong Hall term. The Cluster spacecraft observations confirmed the existence of a density dip at the current sheet following the shape of the separatrix. We take a cut line closed to Cluster trajectory along z and plot the spatial profiles of three components Bx, By, Bz in magnetic field, the density Ý¥ and the y component (JxB)y of Hall term along this cut line. The patterns of the profile Bx, By and Bz are similar to the Cluster observations. In addition, the profiles of density Ý¥ exhibit a significant increase of density in the central region of plasma outflow and the obvious dips in density near the locations of the separatrices. Such a spatial variation in density is in qualitative agreement with four-spacecraft measurement. And the density dips located at the magnetic separatrices coincide with the peaks in the spatial profile of (JxB)y, that demonstrates the major role of Hall term in the formation of the density depletion layers near the separatrices. In the serendipitous encounter of Wind spacecraft with an active reconnection diffusion region, a dip in the density was detected just preceding the time of the flow reversal. Aiming at this observation by Wind, the spatial profiles of the density Ý¥, the x component Vix of ion velocity and the out-of-plane component By of magnetic field along x are plotted for the case of di/L0 = 0.2. The curve Ý¥ versus x exhibits a clear dip in the density corresponding to the reversal of ion velocity Vix and By bipolar signature associated with By quadrupolar structure around the X line. Images of the overall profiles behave like the detection by Wind. However, the observed density depletion is not centered at the time of the flow reversal, but just preceding this time. The case with an initial guide field By0=0.5B0 is explored. It is very interesting to note that the depletion region of density around the X-line has a shape distortion in comparison with the case of By0 = 0. The curve Ý¥ vs x also exhibits a clear dip in the density around the X line. It is worthy of note that this density dip in the case with By0=0.5B0 is not in coincides with the reversal of ion velocity Vix but is located at left side of the Vix reversal point. Our argument here is that the observed density depletion would correspond to a dip in the vicinity of the X line rather than along the separatrices.
DE: 2700 MAGNETOSPHERIC PHYSICS (6939)
DE: 2723 Magnetic reconnection (7526, 7835)
DE: 2724 Magnetopause and boundary layers
SC: SPA-Magnetospheric Physics [SM]
MN: Fall Meeting 2005