Current sheet structure and kinetic properties of plasma flows during a near-Earth magnetic reconnection under the presence of a guide field

Fortunate positioning of Cluster and TC-1 in the plasma sheet (PS) of the Earth's magnetotail has allowed studies of the current sheet (CS) structure and particle dynamics in mesoscale and microscale in both sides of the near-Earth reconnection, which took place between 03:42 and 03:55 UT on 22 September 2004. The distinctive feature of this event was the presence of a strong negative B-Y field forming a bell-like spatial profile with the maximum absolute value near the neutral plane. The magnitude of this B-Y field was almost two times larger than the interplanetary magnetic field (IMF) and therefore could not be explained solely by the IMF penetration into the magnetotail. We propose a possible intrinsic mechanism of the B-Y field enhancement near the neutral plane based on peculiarities of the nonadiabatic ion interaction with the thin CS. An analysis of test particle trajectories shows that in the presence of a guide field with the bell-like spatial profile, a pronounced north-south asymmetry appears in the refraction/reflection properties of nonadiabatic ions from the CS. In a region tailward of the reconnection (B-Z<0), this asymmetry results in an increase of the density of the keV ions ejected into the northern PS and moving tailward. These ions can carry the tailward current which may be responsible for the strong negative B-Y near the neutral plane, i.e., self-consistent enhancement of a B-Y field could occur near the neutral plane.