Bifurcated Thin Current Sheets in the Earth's Magnetosphere: Comparison of Model and “In Situ” Observations

Abstract

A self-consistent analytical theory of thin current sheets (TCSs) is used to investigate their fine structure in the Earth's magnetotail at various stages of temporal evolution. The model is based on the solution of Grad-Shafranov type equations under quasi-adiabatic (QA) approximation. Quasi-adiabaticity allows the construction of an average equilibrium, assuming the conservation of QA invariant of motion Iz, and then the investigation of its slower evolution due to lz- diffusion. 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 cB near the midplane. As a result profile Bz(z) acquires a complex concave shape instead of a simple linear one, a characteristic of Harris equilibrium. The corresponding TCS cross-tail current profile attains a “double humped” shape, which can be a typical characteristic of TCS during a major part of its “life cycle”. This process of current sheet deterioration by quasi-trapped plasma may finally lead to TCS disruption. The results of numerical modeling are compared with Geotail and Cluster observations of double-humped (also referred to as bifurcated) current sheets in the Earth's magnetotail. The observables predicted by our QA model and the conditions under which they are expected to be observed by Geotail, Cluster and other spacecraft are discussed.