Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsm53a1424t&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #SM53A-1424
Physics
2109 Discontinuities (7811), 2752 Mhd Waves And Instabilities (2149, 6050, 7836), 7524 Magnetic Fields, 7835 Magnetic Reconnection (2723, 7526), 7851 Shock Waves (4455)
Scientific paper
The evolution of an initial current sheet is studied by using the set of one-dimensional (1D) MHD equations. In a simulation of the 1D Riemann problem along the z direction, the initial magnetic field is chosen as B(z)=- Bx0\tanh(z/δ)\hat{x}+By\hat{y}+Bz\hat{z}, where Bx0 denotes the antiparallel component and B_y is called the guide field of current sheet. For the B_y=0 case, a pair of slow shocks is formed and propagates away from the current sheet. For B_y≠0 (even for a very small B_y), it is found that a pair of slow shocks and an additional pair of time-dependent intermediate shocks (TDISs) are formed. TDISs are not present in the B_y=0 case, indicating that the case with B_y=0 is a singular case. In this paper, an attempt is made to reconcile the B_y=0 case with small non-zero B_y cases by examing the early time evolution of TDISs and slow shocks. The early current evolutions for the B_y=0 case and for small B_y cases are found to be very similar. For B_y≠0 cases, the plasma density and pressure are found to increase and the magnetic field decreases across TDISs. The dependence of current sheet evolution on initial B_y (or φ_∞) and the plasma beta β_∞ is also examined, where φ_∞=\tan-1(B_y/B_x) is the initial rotation angle of tangential magnetic field, and the subscript ∞ denotes the location far from the current sheet. The rotation angle (Δφ) of tangential magnetic field across TDIS develops gradually with time and reaches the final value Δφfinal=90°-φ_∞. For small B_y (φ_∞→ 0), it is found that the time reaching the final state is very long. Both pressure and temperature downstream of the slow shock decrease with φ_∞, and increase with plasma beta β_∞. With a decreasing φ_∞ or an increasing β_∞, the Alfvén Mach number (M_A=Vn1/VA) associated with slow shocks increases, and the amount of magnetic energy converted into kinetic energy also increases.
Lee Lillian
Tsai Chung-Chin
Wu Baofeng
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