Particle orbits in model current sheets with a nonzero By component

The problem of charged particle motions in magnetotaillike model current sheets (Speiser, 1965) is revisited with the inclusion of a nonzero dawn-dusk magnetic field component By. In a zero By current sheet, particle orbits in the phase space (z, z) are symmetric about the z = 0 plane for most models as long as Bz is z-independent and antisymmetry of Bx about z = 0 is assumed, i.e., Bx(z) = −Bx(−z). This can be clearly seen from the fact that the equations of motion for charged particles are unchanged when z is changed to −z. In a nonzero By current sheet, this symmetric character in the phase space does not exist. Assuming a small By (same order as Bz), three cases are examined in this paper: case 1: Bx ≠ 0, By ≠ 0, Bz = 0, and Ey = 0; case 2: Bx ≠ 0, By ≠ 0, Bz = 0, and Ey ≠ 0 for “trapped” particles; and case 3: Bx ≠ 0, By ≠ 0, Bz ≠ 0, and Ey ≠ 0 for “escaped” particles. Our study shows that a nonzero By component can disturb particle orbits, change their bounce frequency, and destroy the symmetry of the orbits about the z = 0 plane. For the escaped particles, acceleration and ejection of particles from the current sheet are mainly controlled by the zero By current sheet system. However, with the injection of low energy particles, positively (negatively) charged particles, after acceleration by the current sheet, tend to be ejected toward the northern (southern) boundary of the current sheet for a positive By component (i.e., dawn to dusk), and toward the opposite direction for a negative By component. We present several examples of test particle computations for a parabolic model.