There has been considerable recent interest in the relationship between the cross-tail magnetic field component B-y and tail dynamics. The purpose of this paper is to give an overall description of the penetration of the interplanetary magnetic field (IMF) B-y into the near-Earth plasma sheet. We show that plasma sheet B-y may be generated by the differential shear motion of field lines and enhanced by the flux tube compression. The latter mechanism leads to a B-y analogue of the pressure-balance inconsistency (Erickson and Wolf, 1980) as flux tubes move from the far tail toward the Earth, The growth of B-y, however, may be limited by the dawn-dusk asymmetry in the shear velocity as a result of plasma sheet tilting. B-y penetration into the plasma sheet implies field-aligned currents flowing between hemispheres. These currents together with the IMF B-y related mantle field-aligned currents effectively shield the lobe from the IMF B-y.
