Ion dynamics in the near-Earth magnetotail: Magnetic turbulence versus normal component of the average magnetic field

[1] The influence on ion motion of magnetic turbulence observed in the near- Earth magnetotail is investigated by numerical simulation. The magnetotail current sheet is modeled as a magnetic field reversal with a normal magnetic field component B-n, plus a three- dimensional spectrum of magnetic fluctuations deltaB, which represents the observed magnetic turbulence. A cross tail electric field E-y is included. A test particle simulation is performed assuming an anisotropic plasma source at the magnetospheric lobes, and using different values of B-n and of the fluctuation level deltaB/ B-0. In the relevant range of parameters, B-n and deltaB/ B-0 have opposite effects on the current structure and on the ion heating. In particular, for a substantial level of fluctuations, deltaB/ B-0 greater than or similar to 0.2, the current splits into two sheets for B-n similar to 0 but increasing B-n requires a higher deltaB/ B-0 in order to have the current splitting. In addition, ion heating increases with the increase of deltaB/ B-0 and with the decrease of B-n, as these changes favor motion perpendicular to the average magnetic field and along the potential drop. When the magnetic fluctuations are small, the diamagnetic current wings and the magnetic field overshoots at the boundary of the current sheet are recovered for weakly anisotropic plasma distributions. Implications for the substorm growth phase and onset are discussed.