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Pulsed cross ~nodulation, arising in the region from 60 to 80 km, has been observed during undisturbed days in the spring, summer, and fall of 1960. The manner in which these observations were obtained and the uncertainties associated with the electron-density profiles determined from then1 are discussed.Average electron-density profiles for each hour of quiet spring days have been deduced. These show that the most rapid changes in electron density occur within one hour of ground sunrise and sunset. Around sunrise a rather uniform layer of about 100 electrons/cc is created almost sin~ultaneously throughout the entire region from 60 to 80 km. As the day progresses, the average electron density between 70 and 80 km changes by a factor of about 10, while the average density between 60 and 70 k111 changes by only a factor of 2.These features of the U region are discussed in the light of theoretical predictions which assume that cos~nic rays and solar Lyman-alpha radiation account for the nor~nal ionizatio~~ in this region.

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