A magnetofrictional method is used to construct two-dimensional MHD equilibria of the Earth's magnetosphere for a given distribution of entropy function (S=pV&ggr;), where p is the plasma pressure and V is the tube volume per unit magnetic flux. It is found that a very thin current sheet with Bz<0.5nT and thickness <1000km can be formed in the near-earth magnetotail (x~-8 to -20Re) during the growth phase of substorm. The tail current sheets are found to become thinner as the entropy or the entropy gradient increases. We suggest that the new ''entropy anti-diffusion instability'' associated with plasma transport across field lines leads to magnetic field dipolarization and accelerates the formation of thin current sheet, which may explain the observed explosive growth phase of substorms. ¿American Geophysical Union 1995