Particle-in-Cell Simulations of Magnetic Reconnection in Collisionless Force-Free Current Sheets
Magnetic reconnection is one of the most fundamental physical processes in plasmas. It is especially important for activity processes in space and astrophysical plasmas. We investigate magnetic reconnection in collisionless plasmas, focussing on current sheet configurations for which the current density is parallel to the magnetic field, i.e. force-free current sheets.
We study the stability and dynamics of the force-free Harris sheet, for which an exact equilibrium distribution function is known
(Harrison and Neukirch, 2009 - below). The force-free Harris sheet has fundamentally different physical properties than the standard Harris sheet with or without guide field. In particular, for the force-free Harris sheet the plasma density and pressure are constant, whereas the mean velocity of the particle species varies with position. Furthermore, it has been shown that the distribution function of the force-free Harris sheet can have unusual structures in velocity space
(Neukirch et al., 2009 - below). We use particle-in-cell (PIC) simulations to study the nonlinear evolution of the force-free Harris sheet when it undergoes magnetic reconnection
(see Figure 1 - below). We intend to study in particular the transition between guide-field reconnection and anti-parallel reconnection, which is expected to happen during the reconnection process of the force-free Harris sheet. We plan to compare the results for the force-free Harris sheet with simulations of the normal Harris sheet with guide field.
Figure 1: Snapshots of a PIC simulation of magnetic reconnection in a force-free Harris sheet. The figures show the magnetic field lines projected onto the x-z-plane (white) and the y-component of the current density (colour contours). This particular simulation uses a total of 1.5 x 109 particles, with a proton to electron mass ratio of 4. The times shown are in units of the (inverse) ion cyclotron frequency and the length scales are in units of the ion inertial length.
References:
- M. G. Harrison and
T. Neukirch,
A one-dimensional Vlasov-Maxwell equilibrium for the force-free Harris sheet,
Physical Review Letters,
102, 135003-1 - 135003-4 (2009).
-
T. Neukirch, F. Wilson and M. G. Harrison,
A detailed investigation of the properties of a Vlasov-Maxwell equilibrium for the force-free Harris sheet,
Physics of Plasmas,
16, 122102-1 - 122102-10 (2009).