A large magnetosphere database has been used to study spatial and temporal variations of the geomagnetic tail lobe field strength. The study uses 12,388 average tail lobe measurements from the region -15>X>-70 RE obtained from 11 different spacecraft over a 20-year period. The average radial gradient is found to be represented by B=1659.2/R1.46+7.47 which can be used to eliminate the dependence of individual measurements on distance down the tail. A set of 5562 of the resulting normalized data points is associated with simultaneously measured solar wind dynamic pressure and interplanetary magnetic field (IMF) values which permit the study of relationships of Blobe with geomagnetic activity and with solar wind parameters on various timescales. Variations of annual averages of Blobe and AE exhibit a solar cycle dependence and are very similar to each other and to solar wind dynamic pressure variations. On an hourly time scale, tail lobe field strength normalized to a distance of 30 RE is found to depend on both solar wind dynamic pressure, Psw, and IMF Bz. This dependence can be represented as B2n=-96.5+294.5√Psw+2.36BIMF2 sin2(&thgr;/2) where &thgr; varies from 0¿ for northward to 180¿ for southward. The ability to quantitatively predict average Blobe given solar wind pressure and IMF B provides a baseline which should be useful in the study of more rapid substorm variations which are due to internal magnetospheric dynamics. ¿ American Geophysical Union 1996