Effects of a guide field on the evolution of a current sheet

Abstract

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)=−Bx0tanh(z∕δ)x̂+Byŷ+Bzẑ, where Bx0 denotes the antiparallel component and By is called the guide field of current sheet. For the By=0 case, a pair of slow shocks is formed and propagates away from the current sheet. For By≠0 (even for a very small By), 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 By=0 case, indicating that the case with By=0 is a singular case. In this paper, an attempt is made to reconcile the By=0 case with small nonzero By cases by examining the early time evolution of TDISs and slow shocks. The early current evolutions for the By=0 case and for small By cases are found to be very similar. For By≠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 By (or ϕ∞) and the plasma beta β∞ and also examined, where ϕ∞=tan−1(By∕Bx0) is the initial angle beteween the tangential magnetic field and the x axis, 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 By (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 (MA=Vn1∕VA) associated with slow shocks increases, and the amount of magnetic energy converted into kinetic energy also increases.

Magnetic reconnection under anisotropic magnetohydrodynamic approximation

“…Furthermore, the acceleration of reconnection exhaust is realized not only by the weak slow shocks, but also by the rotational discontinuities, which is striking especially for relatively small guide field reconnection. Note that the similar results have been obtained in the isotropic MHD 27 .…”

“…If the steady-state reconnection can be realized in two-dimensional space, we may discuss the structure of reconnection as a solution of the one-dimensional Riemann problem 27,28 .…”

Effect of guide field on lower-hybrid drift instabilities in current sheet containing energetic particles

“…Even in this case, processes such as formation of transient intermediate shocks can appear, which can modify the plasma density, pressure, and magnetic field. 16 For fixed k y and k z , Eq. (18) has multiple harmonic solutions corresponding to LHD modes with different frequencies and different structures in the x direction.…”

“…The latter plays an important role in magnetic reconnection since it validates the magnetohydrodynamic approximation as well as affects the onset and evolution of the reconnection process, the dynamic properties of the current sheet, as well as the occurrence of transient shocks, secondary instabilities, etc. [15][16][17][18][19][20][21][22] For this reason, the stability of current sheets with a finite guide field has received much attention. [17][18][19][20][21][22] In this paper, we consider the LHDI in a current sheet containing a finite guide field and energetic particles by solving the Vlasov-Maxwell system.…”

Evolution of symmetric reconnection layer in the presence of parallel shear flow

“…9 In addition, small separatrix angle for Petschek-like reconnection configuration makes it possible to simplify multidimensional problem to one-dimensional (1D) magnetohydrodynamic (MHD) Riemann problem, which studies the time evolution of the reconnection layer configuration after the onset of magnetic reconnection. [10][11][12][13][14][15] It is clearly recognized that due to the effects of electromagnetic dissipation, the intermediate shocks (ISs) can be formed from continuous waves. 16 Shi and Lee studied the structures of reconnection layer in different symmetric conditions and found that the IS may occur in the two-dimensional reconnection layer.…”