The subauroral ionosphere, at the magnetic latitudes which characterize the northeastern United States, is subject to severe F region ionospheric density structuring due to the space weather effects of magnetospheric disturbance electric fields. Communications and navigation systems relying on transionospheric propagation must be able to compensate for the effects of the sharp changes (>10X) in total electron content (TEC) associated with the ionospheric trough and storm time disturbance effects at midlatitudes. The Millstone Hill incoherent scatter radar database has been used to investigate the spatial extent and temporal evolution of TEC and density altitude/latitude structure at middle and subauroral latitudes as a function of solar cycle, local time, and level of geomagnetic activity. More than 11,000 radar elevation scans covering >20¿ of latitude and altitudes between 150 and 750 km have been used to identify the characteristics of the density gradient near the equatorward edge of the ionospheric trough in a variety of circumstances spanning 20 years and two solar cycles. Pronounced density gradients can be identified in ~35% of the Millstone Hill scans, and we present a statistical characterization of average magnitude and location for these steepest TEC gradients. In some cases (especially near noon) the equatorward edge of the trough lies poleward of our observational field of view, and gradients associated with phenomena other than the trough contribute to our statistics. On most days the trough appears in the radar scans between 1600 and 2000 magnetic local time (MLT). Larger TEC gradients occur at solar maximum and when the background TEC is higher. The steepest gradients occur in an environment of high TEC (at solar maximum and adjacent to regions of storm-enhanced density (SED)), when the processes which generate the trough are strongest (high Kp). High gradient values occur in the sunlit sector, with maximum values of TEC gradient (~10 TEC/deg latitude, with 1 TEC unit=1016 el m-2) found in the postnoon ionosphere. Mean solar maximum TEC gradient at 1600 MLT is 3--4 TEC/deg for Kp100 over New England and TEC gradients of ~50 TEC/deg. ¿ 2001 American Geophysical Union