Effect of magnetic turbulence on the ion dynamics in the distant magnetotail

Abstract

The ion dynamics in the distant Earth's magnetotail is studied in the case that a cross tail electric field E0 and reconnection‐driven magnetic turbulence are present in the neutral sheet. The magnetic turbulence observed by the Geotail spacecraft is modeled numerically by a power law magnetic fluctuation spectrum. The magnetic fluctuations have the tearing mode parity with respect to the neutral sheet and are superimposed on a modified Harris sheet. A test particle simulation is performed for the ions, and the particle density, current density, bulk velocity, temperature, pressure, and heat flux are obtained for every point in the distant tail and as a function of the magnetic fluctuation level, δB/B0. It appears that the magnetic turbulence is very effective in maintaining the stationary structure of the current sheet and in changing the ion acceleration due to the electric field to thermal motion. Also, magnetic turbulence can inflate the current carrying region up to a thick current sheet, in contrast with the often assumed thin current sheet. The values obtained for the ion temperature are consistent with those observed in the distant tail by the Geotail spacecraft. The main results are the following: (1) the thickness of the current sheet increases with the level of fluctuations. The thickness λ corresponding to the average magnetic field current profile is obtained for δB/B0 ≃ 0.3. (2) The magnetic pressure outside the current sheet is balanced by particle pressure for δB/B0 ≃ 0.3. This is obtained mostly by an increase in the temperature, while the density profile is not much peaked in the neutral sheet. (3) For low fluctuation levels, heating is anisotropic, most heating going into the y direction; increasing δB/B0 and making reference to the local magnetic field, more heating goes in the parallel rather than in the perpendicular direction, in agreement with part of the observations. (4) A possible splitting of the bulk velocity and of the current density in two sheets is obtained for δB/B0 ≥ 0.2. In general, a relevant level of magnetic turbulence, like δB/B0 ≃ 0.3, appears to be a basic ingredient of the distant magnetotail equilibrium structure.