Abstract
The width of the spectra of signals incoherently scattered from 85 to 93 km at Arecibo shows significant variations during sunrise and sunset periods. After sunset, the width decreases substantially for all these heights, indicating a 20–30% decrease in the quotient of neutral temperature to neutral number density T N ). Some increase in effective positive ion mass could also be possible. Somewhat opposite situations are evidenced during sunrise for heights above 87.5 km. At 85 km, on the other hand, the width decreases during sunrise. This is due to the disappearance of negative ions, which were formed during the night hours.
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