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

Abstract

[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 dB, 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 dB/B 0 . In the relevant range of parameters, B n and dB/B 0 have opposite effects on the current structure and on the ion heating. In particular, for a substantial level of fluctuations, dB/B 0^0 .2, the current splits into two sheets for B n $ 0, but increasing B n requires a higher dB/B 0 in order to have the current splitting. In addition, ion heating increases with the increase of dB/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.

Properties of a bifurcated current sheet observed on 29 August 2001

“…Because no tailward or earthward strong flow was detected, we cannot directly attribute the current sheet bifurcation to the magnetic reconnection process. We discussed alternative mechanisms of a bifurcated sheet formation, based on ion anisotropy (Sitnov et al , 2003) and on magnetic turbulence, concentrated in the central part of the sheet (Greco et al , 2002). Both models give results similar to observations, but the conditions required by the models are not completely satisfied in the considered case.…”

Magnetic turbulence and particle dynamics in the Earth’s magnetotail

“…This suggests that magnetic turbulence is in a steady state, in a statistical sense. We represent these fluctuations as the sum of static magnetic perturbations (Zimbardo et al, 1995;Veltri et al, 1998;Greco et al, 2002) …”

“…Also, Veltri et al (1998) found that the fluctuations enhance ion heating and contribute to the formation of the equilibrium structure. Recently, Greco et al (2002) considered the near-Earth magnetotail, where B n > 0, and found that even in this case the magnetic fluctuations induce a double layer structure in the current sheet, although a stronger δB/B 0 is usually required to achive this effect. In this paper we investigate ion dynamics in the presence of magnetic turbulence in the near Earth and middle magnetotail, where B n is north-ward oriented.…”

“…The structure of the magnetotail current sheet also was studied by hybrid simulations which treat the electrons as a charge neutralizing fluid (Hesse et al, 1995(Hesse et al, , 1996. Further information, like cause and effect relationships, can be obtained by test particle simulations which take into account the two-dimensional (2-D) and three-dimensional (3-D) geometry of the magnetosphere (Ashour-Abdalla et al, 1994Delcourt and Belmont, 1998), the particle acceleration in self-consistent 2-D turbulence (Matthaeus et al, 1984), the electric field morphology (Antonova and Ovchinnikov, 1999), and the presence of magnetic turbulence in the distant (Veltri et al, 1998) and in the near Earth (Greco et al, 2002) magnetotail.…”

“…The considered configuration can be a (local) magnetotail model for the growth phase of magnetospheric substorms. With respect to Greco et al (2002), we extend the turbulence spectrum by including in the model a number of long wavelength modes, which may correspond to the formation of large magnetic islands. We study the influence of B n on the distribution function moments, and we find that it effectively controls the particle dynamics.…”