A shear BY field in the Earth's magnetotail and its variations in the current sheet
We use Cluster and THEMIS simultaneous observations to study the spatial distributions of a shear BY field in the Plasma Sheet (PS) of the Earth's magnetotail at -31 RE < X < -9 RE. The best correlation between the BY field in the PS (BY_PS) and the Y-component of the Interplanetary Magnetic Field (IMF) (BY_IMF) was observed during the quiet PS periods when high speed plasma flows were not detected. During active PS periods the correlation between the BY_PS and BY_ IMF was poor. The analysis of spatial distribution of the BY field along the direction perpendicular to the Current Sheet (CS) plane showed the presence of one of the following configurations, which can be self-consistently generated in the CS: 1) the "quadrupole" distribution of the BY field usually associated with the Hall current system in the vicinity of X-line and 2) the symmetrical "bell-shaped" distribution formed due to the BY amplification near the neutral plane of the CS. Multipoint observations revealed the transient appearance of the "quadrupole" BY distribution during the periods of X-line formation in the mid-tail. This distribution was observed during a few minutes within, at least, 12 RE from the estimated X-line position. On the contrary, the symmetrical "bell-shaped" distribution is more localized in the radial direction and, generally, has a larger observation time (up to ∼10 min). Thus, the internal CS perturbations caused either by the Hall currents related to reconnection or by the peculiarities of the local quasi-adiabatic ion dynamics sufficiently affect the shear BY field existing in the magnetotail due to the partial IMF penetration.