The roles played by temperature gradients and velocity shears in causing turbulence in the plasma sheet boundary layer are examined. Both are sources of free energy that can be tapped by perturbations, which can then grow and result in turbulence, thus generating anomalous particle and energy transport. It is found that ion temperature gradients and ion velocity shear play parallel roles in destabilizing a unique low-frequency mode, the ion acoustic wave which may be termed a Kelvin-Helmholtz or temperature gradient instability, depending on which free energy source is dominant. There never is more than one unstable electrostatic mode present, driven by these two sources. A general expression for the critical velocity shear is presented, and the critical value(s) of the temperature gradient is (are) discussed. The finite-&bgr; modifications to these instabilities of the ion acoustic wave and their quasi-linear feedback on the plasma are discussed. ¿ American Geophysical Union 1988