Abstract
Abstract. A magnetotail event on 15 September 2001 is analyzed, during which the Cluster spacecraft observed both embedding and bifurcation of the Thin Current Sheet (TCS). It is indicated that the ion anisotropy and nongyrotropy are responsible for those new equilibrium features that represent deviations from the conventional Harris model. Measurements show that an embedded proton TCS manifests a pressure anisotropy with p||>p⊥, simultaneously associated with a density embedding; while a bifurcated oxygen ion TCS exhibits a pressure anisotropy mainly with p||<p⊥ and nongrotropy, except at the edges where p||>p⊥ is the necessary marginal firehose stability condition. The local flapping motion of the TCS was observed, and some particular features such as the solitary wave-like behavior and the kink motion in the plane perpendicular to the Sun-Earth direction were revealed. The present investigation intimates the complexity of equilibria of the magnetotail current sheet and declares the crucial role played by the ion kinetics in the dynamics of TCSs.
Highlights
The understanding of the dynamics of the magnetotail current sheet is important to many space phenomena
The conventional current sheet model is based on the Harris equilibrium, which has the well-known tanh-type magnetic field profile and the bell-shaped density and current profile, that is, current and plasma densities have a simple profile with a single peak in the center of the sheet where the magnetic field has a minimum (Harris, 1962)
The observations presented here reveal the crucial role played by the ion kinetics in the dynamics of Thin Current Sheet (TCS)
Summary
The understanding of the dynamics of the magnetotail current sheet is important to many space phenomena. These models either choose the particle distributions to be a function of two invariants of motion, namely the total particle energy and the component of the canonical momentum along the current direction, as in the Harris model but in their generalized non-Maxwellian forms, or adopt equivalently the Grad-Shafranov equation, in which the current density is a function of the electromagnetic field vector potential (Schindler et al, 2002; Mottez, 2003; Birn et al, 2004a; Genot et al, 2005; Camporeale et al, 2005). The wave properties do not match any local excitation mechanism previously discussed so far in the literature (Zhu et al, 1996; Daughton, 1999, 2002, 2003; Lapenta et al, 2002) It raises another question to understand the dynamics of TCSs. Here, TCS dynamics is reported from observations during the Cluster spacecraft crossing of the magnetotail on 15.
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