Model of bifurcated current sheets in the Earth's magnetotail: Equilibrium and stability

Abstract

[1] Recent observations of CLUSTER have reconfirmed the recurring presence of bifurcated current sheets. The present paper revisits the model of Schindler and Birn (2002) to describe analytically bifurcated current sheets. Our contributions are in order. First, we describe a number of analytical velocity distribution functions that lead to bifurcated current sheets. Second, we derive necessary and sufficient conditions that determine when current sheets can be produced within the mathematical model of Schindler and Birn (2002). Third, we present a class of bifurcated current sheets, and we describe their properties. Fourth, we study the stability of bifurcated current sheets to the tearing instability finding that bifurcated current sheets tend to be more stable. Finally, we investigate the stability of bifurcated current sheets to the lower hybrid drift instability (LHDI) and kinking instability proving their presence. The work reported here is intended to extend and investigate the properties of instabilities from the typical but academic case of the Harris current sheet to current sheet equilibria that are more realisticly representative of the real magnetotail.Citation: Camporeale, E., and G. Lapenta (2005), Model of bifurcated current sheets in the Earth's magnetotail: Equilibrium and stability,

Comparison of multi-point measurements of current sheet structure and analytical models

“…Bifurcations often observed in satellite experiments were in some cases attributed to dynamic configurations Thompson et al, 2006). Different mechanisms of bifurcation such as trapped plasma (Zelenyi et al, 2002), influence of anisotropy (Sitnov et al, 2003) and specific velocity distributions (Camporeale and Lapenta, 2005) were suggested by theorists, but this issue is left for the future publications.…”

“…The simplest model is the 1-D kinetic Harris model (Harris et al, 1962), which is an exact solution of Vlasov equation. Recently it was generalized for kappa velocity distribution (Fu and Hau, 2005;Hau and Fu, 2007), ion and electron dominated current sheets (Yoon and Lui, 2004) as well as bifurcated sheets (Camporeale and Lapenta, 2005). Constant plasma drift velocity is common feature of these models, and therefore current density is proportional to plasma density j y (z) ∼n(z).…”

New role of the lower-hybrid drift instability in the magnetic reconnection

“…Although it is clear that the peaking of a monotonic current profile will enhance the growth rate of collisionless tearing, the influence of a bifurcated current profile on the stability of the tearing mode deserves a careful investigation that recent simulations performed with CELESTE3D have started to approach. 48 With a favorable anisotropy and a thin enough current sheet, the tearing instability grows large enough to decouple electrons and ions and reconnection can encompass the whole domain. The scaling study indicates that the effect of the LHDI is reduced with the thickness of the current sheet: when the spatial extent of the crossing ion orbits intersect with the region of strong LHDI activity, the onset mechanism is triggered leading to a bifurcated current profile and anisotropic heating.…”

Cluster observations on the thin current sheet in the magnetotail

“…In one category, an isotropic pressure is assumed. 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). Therefore, it implies that in all of them, the pressure is isotropic, and the equilibrium is maintained by a balance between the field line tension and the plasma pressure gradient.…”