Effects of a guide field on the evolution of a current sheet
- Institute of Space Science, National Central University, Jhongli, 320 Taiwan (China)
The evolution of an initial current sheet is studied by using the set of one-dimensional (1D) magnetohydrodynamic equations. In a simulation of the 1D Riemann problem along the z direction, the initial magnetic field is chosen as B(z)=-B{sub x0} tanh(z/{delta})x+B{sub y}y+B{sub z}z, where B{sub x0} denotes the antiparallel component and B{sub y} is called the guide field of current sheet. For the B{sub y}=0 case, a pair of slow shocks is formed and propagates away from the current sheet. For B{sub y}{ne}0 (even for a very small B{sub 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{sub y}=0 case, indicating that the case with B{sub y}=0 is a singular case. In this paper, an attempt is made to reconcile the B{sub y}=0 case with small nonzero B{sub y} cases by examining the early time evolution of TDISs and slow shocks. The early current evolutions for the B{sub y}=0 case and for small B{sub y} cases are found to be very similar. For B{sub y}{ne}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{sub y} (or {phi}{sub {infinity}}) and the plasma beta {beta}{sub {infinity}} and also examined, where {phi}{sub {infinity}}=tan{sup -1}(B{sub y}/B{sub x0}) is the initial angle between the tangential magnetic field and the x axis, and the subscript {infinity} denotes the location far from the current sheet. The rotation angle ({delta}{phi}) of tangential magnetic field across TDIS develops gradually with time and reaches the final value {delta}{phi}{sub final}=90 degree sign -{phi}{sub {infinity}}. For small B{sub y} ({phi}{sub {infinity}}{yields}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 {phi}{sub {infinity}}, and increase with plasma beta {beta}{sub {infinity}}. With a decreasing {phi}{sub {infinity}} or an increasing {beta}{sub {infinity}}, the Alfven Mach number (M{sub A}=V{sub n1}/V{sub A}) associated with slow shocks increases, and the amount of magnetic energy converted into kinetic energy also increases.
- OSTI ID:
- 20860316
- Journal Information:
- Physics of Plasmas, Vol. 13, Issue 10; Other Information: DOI: 10.1063/1.2358496; (c) 2006 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); ISSN 1070-664X
- Country of Publication:
- United States
- Language:
- English
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