Plasma irregularities adjacent to auroral patches in the postmidnight sector

[1] We demonstrate a close association between decameter-scale plasma irregularities in the E region ionosphere and auroral patches in the postmidnight sector. In September 2009, campaign-based measurements of the aurora were conducted in Iceland with a white light all-sky camera (ASC) at Tjörnes (66.20°N, 17.12°W) and the SuperDARN radar at þykkvibaer (63.77°N, 20.54°W). On one night during the campaign period, the ASC observed the successive passage of auroral patches in the postmidnight sector after a small substorm-like activity. The patches were drifting predominantly eastward across the field-of-view of the ASC with a speed of approximately 360–450 m s−1, which is consistent with the sunward convection in the postmidnight westward electrojet. The simultaneous radar measurements recorded strong radar backscatter echoes (>15 dB) within the gaps between adjacent auroral patches, while such echoes were not observed or were very weak in the region of the aurora. The Doppler velocity estimation showed that the electric field was clearly reduced within the patches, which was probably the result of the enhanced conductance associated with auroral precipitation. Thus, this reduction in the electric field suppressed the generation of irregularities (i.e., radar echoes) in the regions of auroral patches. This suggests that the conductance enhancement associated with precipitating electrons not only modified the electric field within the aurora but also affected the generation of small-scale plasma structures in the vicinity of the patch-type optical auroral forms.