HF radar observations of the F region ionospheric plasma response to Storm Sudden Commencements

Performance of the Super Dual Auroral Radar Network (SuperDARN) HF radars during geomagnetic storms is investigated by analyzing the data collected during storm events over a 5-year period. Changes in the occurrence of F region HF backscatter observed by the 6 most equatorward radars are analyzed statistically using a superposed epoch analysis method with respect to a Storm Sudden Commencement (SSC). Regular diurnal variations of the echo occurrence during geomagnetically quiet days are produced and the amount of detected backscatter during storms is adjusted using quiet time curves. All radars considered in this study show a significant decrease in the number of detected echoes approximately 24 hours following SSC. Unexpected significant changes in occurrence levels are also present within a few hours of SSC, with most radars observing an increase in the amount of backscatter detected. The typical time evolution of F region echo occurrence is highly reminiscent of that of the electron density reported previously. Also considered is the ionospheric convection response to SSC observed by the zonally looking SuperDARN Unwin radar in New Zealand. It is shown that the initial response to SSC is instantaneous within uncertainty and appears to be independent of the magnetic latitude and local time. The observed convection response timing and morphology are discussed in the context of possible ionospheric propagation mechanisms.