Abstract
Electron concentrations in the D and E regions of the ionosphere have been measured by incoherent scatter radar from 130‐km altitude down to the D region ledge with an altitude resolution of 900 m. The results are displayed as consecutive 5‐min altitude profiles including sunrise, afternoon, and sunset periods. The ledge is found to undergo systematic variations between 80 and 90 km. A comparison with theoretical models for electron concentrations at 90 and 110 km reveals generally good agreement. However, at 90 km for χ ≲60° the model values are high by a factor of 2, and at 110 km for χ=90° (sunrise and sunset) they are high by a factor of 4. The incoherent scatter results are consistent with rocket measurements and display well‐known patterns of sporadic E as well as the postsunset valley in the E region.
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