Quantitative Investigation of Ionospheric Density Gradients at Mid Latitudes

The sub-auroral 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 trans-ionospheric 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 mid latitudes. An accurate characterization of these gradients and their variability is needed in the design and evaluation of navigation systems utilizing trans-ionospheric propagation in this regime. A study is underway at the MIT Haystack Observatory using the Millstone Hill incoherent scatter radar database to investigate the spatial extent and temporal evolution of TEC and density altitude/latitude structure at mid and sub-auroral latitudes as a function of solar cycle, season, and level of geomagnetic activity. The Massachusetts Institute of Technology maintains an extensive radar research facility at Millstone Hill, located 35 miles northwest of metropolitan Boston at 42.6 N latitude, 288.5 E longitude. Multi-megawatt UHF transmitters and a fully-steerable 46-m antenna provide wide-ranging spatial coverage, spanning >30 of latitude and 4+ hours of local time at F-region heights. The facility is situated at 54 geomagnetic latitude such that its extensive field-of-view for ionospheric observations encompasses the full extent of mid-latitude, sub-auroral, and auroral features and processes. The Millstone Hill incoherent scatter radar has been in operation through three solar cycles and its data characterize ionospheric features and response over the altitude range 100 km to 1000 km with a typical altitude/spatial resolution of 50 km. Longterm operations as a part of the international Incoherent Scatter World Day program have built up a database of radar elevation scans which sample a span of 15 25 latitude with better than 1 latitude resolution and ~35 km altitude resolution. Data from each radar elevation scan is used to produce vertical TEC in the altitude range 150 km 650 km with better than 1 latitude resolution. Density gradients associated with the deep, narrow ionospheric trough which forms during disturbed conditions are ~10 TEC units / degree latitude. Storm enhanced density (SED), the bulk redistribution of F-region plasma by disturbance electric fields, can result in TEC > 100 over New England