Bifurcated Thin Current Sheets in the Magnetotail: Comparison of Model and Spacecraft Observations

The evolution of the Earth's magnetotail current sheet through different stages is studied using a self-consistent theory of thin current sheets. The model solves a Grad-Shafranov type equation for the current sheet under quasi-adiabatic approximation. Quasi-adiabaticity allows the construction of an average equilibrium, assuming the conservation of quasi-adiabatic invariant of motion Iz. However the adiabatic invariant evolves in time and this is represented as a diffusion process, leading to a slow change of the equilibrium. This diffusion leads to a gradual trapping of transient and unbounded (or the so called Speiser orbit) particles into orbits trapped in the vicinity of the sheet midplane. It is found that the cross-tail current of such newly trapped population is opposite to the one from transient Speiser orbits and eventually flattens the profile of the magnetic field near the midplane. As a result profile Bx(z) acquires a complex concave shape instead of a simple linear one, a characteristic of Harris equilibrium. The corresponding cross-tail current profile attains a "double humped" shape, which can be a typical characteristic of a thin current sheet during a major part of its "life cycle". This process of current sheet deterioration by quasi-trapped plasma may finally lead to the disruption of the current sheet. The results of numerical modeling are compared with Geotail and Cluster observations of bifurcated or double-humped current sheets in the Earth's magnetotail. The role of electron currents under these conditions are studied. The observables predicted by our model and the conditions under which they are expected to be observed by Geotail, Cluster and other spacecraft will be discussed.