Quasi-adiabaticity in bifurcated current sheets.

We examine the non-linear dynamics of charged particles in “bifurcated” (double-humped) current sheets. We show that, when the adiabaticity parameter κ is of the order unity, particles experience a negligible change of magnetic moment. Such a behavior contrasts with the prominent magnetic moment scattering that is achieved in single-humped current sheets at such κ values. It resembles the quasi-adiabatic (Speiser-type) behaviors that is obtained at κ ≪ 1 as a result of resonance between the fast oscillation about the midplane and the slow gyromotion near this plane. We demonstrate that beams of κ ≈ 1 particles may form downstream of bifurcated current sheets, which propagate along the magnetic field lines up to high latitudes. Quasi-adiabaticity and associated beam formation thus occur at significantly larger κ values in double-humped current sheets than in single-humped ones, or equivalently for lower particle energy and/or weaker field reversal