An investigation of small-scale relationships between optical and HF radar aurora

An investigation is undertaken of the relationship between visual aurora and the occurrence of radar-detectable irregularities in the nightside ionosphere. Understanding how auroral signatures appear in HF radar backscatter could combine the advantages of detailed information about auroral fluxes in optical measurements with extended coverage of HF radars. Auroral particle precipitation ionizes the ionosphere and creates localized plasma density enhancements. Irregularities with various smaller sizes are generated from larger density structures through instability-induced cascading. HF waves are coherently scattered by decameter structures within the ionospheric plasma. Hence aurorally induced irregularities can be seen by the radar in the form of "HF radar aurora.";A statistical treatment of the occurrence of optical and HF radar aurora reveals a high degree of variability in backscatter patterns even under seemingly similar auroral displays. The small-scale correspondence between visual aurora and HF backscatter thus represents a more differentiated picture than the spatially and temporally averaged data of earlier studies. The relationship between the occurrence or the characteristics of aurora and the occurrence of HF echoes can therefore not be quantified.;An analysis of single events isolates processes that lead to the observed variety of backscatter patterns in the presence of aurorally induced irregularities. They involve the ambient ionospheric density and localized enhanced densities at different altitude regimes and locations in the path of the radar signal. Conditions for HF wave propagation are partly determined by the aurora itself, partly they are imposed by ambient ionospheric density levels. It is found that low or high ambient densities have a dominating effect on the success of ionospheric probing. Low densities hamper the return of radar signals despite the presence of irregularities. High ambient densities can overcome some of the adverse effects on HF wave propagation associated with sporadic E. The information contained in the diversity of the relationships between optical and HF backscatter improves thus our knowledge about the nighttime ionosphere. A more detailed specification of ionospheric parameters is necessary to gain better insight into these relationships.