Penetration of the interplanetary magnetic field By into Earth's plasma sheet

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 .