Summary
AB: We investigate the storm-time E-region backscatter observed by the Tasman International Geospace Environment Radar (TIGER) HF radars. In the afternoon sector during the negative geomagnetic storm phase, HF echoes form a curved occurrence band joining to the F-region band at ranges ~700 km. The echo band location and curvature during these periods is close to that given by a straight-line propagation model which assumes no ionospheric refraction. This is an unexpected result in view of generally large refraction found at HF during geomagnetic storms. It is argued that this result may be due to electron density depletions taking place in the local ionosphere later in the storm development cycle when negative phase occurs. This interpretation is further supported by ionosonde observations taken at Hobart and Macquarie Island. The types of echoes present in the datasets of both TIGER radars are also investigated. A new storm-time echo population is found exhibiting some characteristics similar to those observed for echoes at abnormally large aspect angles reported in a previous HF study. We find that this new population can be distinguished from normal Eregion echoes due to a characteristic elevation in their measured spectral width. We also investigate the local ionospheric conditions favourable for the detection of these echoes for a number of different storm events using other Super Dual Auroral Radar Network (SuperDARN) HF radars. In particular, the Doppler velocity of the echoes is compared with the strength of the local ionospheric plasma convection. It is proposed that high electric fields and low electron densities are required in order for these high-aspect-angle echoes to be generated and detected using HF radars.