Extreme F-region gradients generated by patch-arc interactions in the polar cap

Tuesday, 16 December 2014: 5:24 PM
Joshua L Semeter1, Hanna Dahlgren2, Matthew D Zettergren3, John Swoboda2, Gareth W Perry4, Jean-Pierre St-Maurice4, Keisuke Hosokawa5, Kazuo Shiokawa6 and Michael J Nicolls7, (1)Boston Univ, Boston, MA, United States, (2)Boston University, Boston, MA, United States, (3)Embry-Riddle Aeronautical Univ, Daytona Beach, FL, United States, (4)University of Saskatchewan, Saskatoon, SK, Canada, (5)University of Electro-Communications, Tokyo, Japan, (6)Nagoya University, Solar terrestrial Environment Laboratory, Nagoya, Japan, (7)SRI International Menlo Park, Menlo Park, CA, United States
We report observations of electrodyamic interactions between drifting F-region plasma structure and discrete polar cap arcs. Three-dimensional time-dependent images of ionospheric state variables (Ne, Te, Ti, Vi) are produced using multi-beam measurements by the Resolute Bay Incoherent Scatter Radar (RISR). The resulting parameter maps are registered with all-sky images of 630-nm and 557-nm emissions acquired by the collocated OMTI imager. The combined analysis allows us to disambiguate spatial and temporal effects, revealing the formation of a deep density depletion between the arc and the plasma patch, formed by the combined action of electrodynamic evacuation and enhanced chemical recombination in the auroral downward current region. This mechanism results in a steep density gradient (gradient scale length <5-km) extending for at least 800-km in a direction tangential to the arc. This region should be highly unstable to gradient drift instability, and a likely source of enhanced HF scatter. Interpretations are supported through three-dimensional transport modeling.