We are concerned in this article with the curious finding of Huang et al. [1989> which suggests a polytropic index of plasma compression of about 0.7 in the quiet time central plasma sheet. We show that the reduction of this index from its adiabatic value (=5/3) can be achieved through an ionospheric outflow of plasma which lowers the plasma temperature by virtue of energy partition. The observed correlation between the plasma temperature and density is best reproduced by an ionospheric source of ~5¿1025 ions/s, consistent with independent experimental estimates and smaller than the average solar wind source by a factor of two or more. Despite the change in the polytropic relationship, we find that the global energy equation PV5/3=constant (V=volume of a unit flux tube) still holds approximately (in the absence of other energy-loss mechanisms). ¿ American Geophysical Union 1990