A quantitative study of ionospheric density gradients at midlatitudes

The subauroral ionosphere, at the magnetic latitudes which characterize the northeastern United States, is subject to severe F region ionospheric dens ity structuring due to the space weather effects of magnetospheric disturba nce electric fields. Communications and navigation systems relying on trans ionospheric propagation must be able to compensate for the effects of the s harp changes (> 10X) in total electron content (TEC) associated with the io nospheric trough and storm time disturbance effects at midlatitudes. The Mi llstone Hill incoherent scatter radar database has been used to investigate the spatial extent and temporal evolution of TEC and density altitude/lati tude structure at middle and subauroral latitudes as a function of solar cy cle, local time, and level of geomagnetic activity. More than 11,000 radar elevation scans covering > 20 degrees of latitude and altitudes between 150 and 750 km have been used to identify the characteristics of the density g radient near the equatorward edge of the ionospheric trough in a variety of circumstances spanning 20 years and two solar cycles. Pronounced density g radients can be identified in similar to 35% of the Millstone Hill scans, a nd we present a statistical characterization of average magnitude and locat ion 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 con tribute to our statistics. On most days the trough appears in the radar sca ns between 1600 and 2000 magnetic local time (MLT). Larger TEC gradients oc cur at solar maximum and when the background TEC is higher. The steepest gr adients occur in an environment of high TEC (at solar maximum and adjacent to regions of storm-enhanced density (SED)), when the processes which gener ate the trough are strongest (high Kp). High gradient values occur in the s unlit sector, with maximum values of TEC gradient (similar to 10 TEC/deg la titude, with 1 TEC unit = 10(16) el m(-2)) found in the postnoon ionosphere . Mean solar maximum TEC gradient at 1600 MLT is 3-4 TEC/deg for Kp < 2, in creases linearly with Kp for Kp from 2 to 4, and is nearly constant at a va lue of 7-8 TEC/deg for Kp from 4 to 6. SED, the bulk redistribution of F re gion plasma by disturbance electric fields, can result in TEC > 100 over Ne w England and TEC gradients of similar to 50 TEC/deg.