The relationship of low-height ionosonde echoes to auroral-zone absorption and VHF D scatter

Weak diffuse HF reflections at heights between 75 and 95 km in the auroral zone are designated E8d. Diurnally, E8d occurs most frequently at noon at all seasons, and shows a pronounced minimum of occurrence around 2000 hours local time. There is little evidence for a seasonal variation from the year's data presented here. It is suggested that E8d is related to the anomalies of auroral-zone absorption and that it is the layer responsible for VHF forward scatter.