Storm-time E-region radar backscatter observed by the SuperDARN HF radars

Storm-time coherent echoes observed by the SuperDARN HF radars in the short ranges corresponding to the E-region backscatter are investigated. Analysis of the spatial distribution of the echoes shows that during storms most of the echoes are detected at locations that are close to the expected locations calculated using a straight-line propagation model (i.e. assuming no radar wave refraction). Analysis of the E layer critical frequency, foE, from one subauroral ionosonde station confirmed that during storms foE typically decreases to lower values as compared with normal conditions. This result suggests that the electron density in the E region is often depleted during storms. The progression of depletion with storm-time and MLT is analysed using several SuperDARN radars separated in longitude. The types of short-range echoes detected during storms are also examined. A new storm-time population of echoes is identified; the echoes exhibit small negative Doppler velocities for a large range of spectral widths. Local ionospheric plasma convection velocity measurements are then used to study the conditions favorable for the generation of these echoes.