Dynamics of HF radar backscatter in the middle and high latitude ionospheres

The Super Dual Auroral Radar Network (SuperDARN) radars have been extensively used to study the high latitude ionosphere for more than 20 years. Geomagnetic conditions are known to influence ionospheric dynamics and the propagation of high frequency (HF) signals. It has been noted in the literature that SuperDARN radars experience a loss of backscatter during storm main and recovery phase and this is often attributed to D region absorption. This work uses a superposed epoch analysis that normalises the duration of each storm phase in order to determine the backscatter occurrence during 25 intense geomagnetic storms. The results show that an increase in backscatter from the E region is a feature of storm main phase. This increase in E region backscatter is shown to be due to increased ionisation at E region altitudes and ray tracing shows that the increase in E region ionisation provides an alternative explanation for the loss of F region backscatter during storm main and recovery phases. It is also shown using a log-linear regression analysis that the SYM-H index is a good indicator of the duration of the (normalised) storm recovery phase. This can be used during storm recovery to predict the return of non-storm HF propagation conditions. The classification of backscatter as ionospheric or ground scatter currently relies on the magnitude of the spectral width and Doppler velocity of the radar backscatter. However, the effect of space weather on HF spectral broadening is not well understood. Based on an analysis of six years of data, the relationship between spectral width magnitude and field aligned currents from the AMPERE project were investigated. The spatial distribution of spectral width is shown to depend on the orientation of the interplanetary magnetic field, and therefore on the field aligned current distribution. The spectral width data are shown to regularly contain a sharp boundary between small and large spectral width values. This boundary, called the spectral width boundary, has previously been suggested as a proxy for the location of the open-closed magnetic field boundary. However, an analysis of the field aligned current data coinciding with spectral width boundaries suggests that the spectral width boundary is a signature of the ionosphere between the Region 1 and Region 2 field aligned currents. Furthermore, the correlation between the spectral width boundary and the open-closed magnetic field boundary is shown to be due to data selection techniques used in previous studies.