An assessment of theLshell fitting beam-swinging technique for measuring ionosphericEregion irregularity drift patterns
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The majority of current ground‐based radar experiments measuring ionospheric convection flows derive the ionospheric flow vectors from line‐of‐sight (l‐o‐s) velocity measurements made from a single radar site. In order to deduce vector information from such a set of scalar measurements, assumptions have to be made about the spatial and temporal structure of the flow pattern. These assumptions involve an actual or effective beam‐swinging technique. In this study the flow direction estimates from the application of sophisticated beam‐swinging software, developed by the Applied Physics Laboratory of the Johns Hopkins University, to over 20,000 estimates of the l‐o‐s ionospheric E region mean irregularity drift velocity from the Wick radar of the Sweden And Britain Radar‐auroral Experiment (SABRE) system, are compared with merged velocity vectors derived from data from both SABRE radar sites. The application of the beam‐swinging software to VHF E region backscatter data is found to lead to flow direction estimations which are in error a significant amount of the time, with beam‐swinging‐derived drift velocities lying outside a cone of half width 30° around the merged SABRE drift velocities in 30% or more of cases. Both curvatures and gradients in the ionospheric flow pattern measured by SABRE cause distortions of the beam‐swinging‐evaluated flow patterns. Moreover, from an examination of the Wick l‐o‐s velocity data and the data fitting calculations alone, it is often not possible to distinguish between accurate and inaccurate beam‐swinging flow direction estimates. Criteria are discussed which can be employed to increase confidence in the beam swinging results. Although the present comparison is confined to E region irregularity drift velocity measurements, these criteria probably have implications for all monostatic experiments employing a beam‐swinging technique (e.g., coherent and incoherent radars and optical instruments). It is concluded that it is highly desirable for the next generation of ionospheric radars to be capable of making common‐volume measurements from two well‐separated sites.