On April 22, 1979, from 0840 to 1018 UT, ISEE 1 and ISEE 2 stayed in the tail current sheet at 17 RE, crossing the current sheet center several times while the initially northward interplanetary magnetic field Bz turned southward under conditions of constant solar wind velocity and dynamic pressure. An overview of the period from 0840 to 1050 UT, discussing the solar wind conditions, the tail magnetic field response at ISEE 1, ISEE 2 and IMP 8, and the ground electrojet activities has been presented in an accompanying paper. Herein under the assumption that the magnetic field is antisymmetric about the current sheet center, the hyperbolic tangent current sheet model of Harris has been used to calculate the current sheet thickness from these two close spacecraft measurements of the tail magnetic field. The thickness varied from 2.5 to 0.5 RE during this period. The current sheet thinned just prior to a rapid poleward expansion of the auroral electrojet and just after the arrival of a southward turning of the IMF Bz and apparent tail field reconnection at ISEE 1 and 2. The plasma density and temperature distribution in the vertical direction appears to be close to adiabatic at this time and the total pressure is roughly constant through the lobe. In this case study, the polytropic index of the tail plasma is found to be 5/3 on the average. This implies that the plasma convection at 17 RE in tail was steadily adiabatic over the period of about 100 min.¿ 1997 American Geophysical Union