Do phase portraits resist current sheet bifurcation?

We investigate the non-linear dynamics of charged particles in the double-humped current sheets that develop in the Earth's magnetotail. Using Poincaré surfaces of section, we examine the various dynamical regimes that occur in a sharp field reversal and compare them to those obtained in single-humped current sheets. We demonstrate that bifurcation of the magnetotail current sheet does not alter the overall phase portrait structure which is characterized by three distinct regimes, viz., trapped, quasi-trapped, and transient. The phase portraits in single- and double-humped current sheets are not strictly identical though. A greater abundance of trapped particles and a distortion of the transient orbit domain are obtained in the latter case. In contrast, numerical simulations performed with an asymmetrical double-humped current distribution reveal considerable modification of the phase portrait with an expansion of the stochastic regime and a drastic reduction of the transient one. The degree of asymmetry in double-humped current sheets accordingly appears as a primary parameter to characterize the nonadiabatic behavior of charged particles.