From an analysis of particle and field data obtined on January 19 and 20, 1973, we have determined several important characteristics of the plasma populations in the geomagnetic tail at lunar distance and their relationships to conditions in the solar wind and interplanetary magnetic field (IMF). During an extended period of low geomagnetic activity with the IMF Z component predominantly northward the plasma sheet at lunar distance is observed to cool significantly. Ion temperatures are observed in the range kTi~200--400 eV with electron temperatures of ~100 eV. Number densities are approximately 1 cm-3. During this period the plasma mantle is also present as an extensive but tenuous region of plasma with densities ni=ne~10-2 cm-3, bulk velocity VB~100 km/s, and ion temperature kTi~3 eV. Following a sharp southward turning of the IMF Z component, there is a net transfer of magnetic flux to the tail. At lunar distance the magnetic field in the lobes increases in magnitude, and the density of the plasma mantle increases sharply. The time delay between the southward turning and the observed changes at lunar distance is consistent with the southward IMF being the cause of the increase of the magnetic flux and the plasma mantle density. The electrons in the plasma mantle display a core and halo distribution indicating a solar wind origin. The plasma sheet is observed to become hotter with increasing geomagnetic activity, reaching ion temperatures in excess of 1 keV and electron temperatures up to 900 eV.