ATS-6 radio beacon data obtained at Boulder, Colorado, between August and November 1974 are analyzed for effects related to the changing level of geophysical disturbance during four solar rotations. It is found that both the electron content at midnight and the decay pattern during the night vary strongly with the parameter ΣKp. Changes of ionization distribution may be inferred by using the effective longitudinal gyrofrequency, determined in the experiment, and the slab thickness. The consequence of increased activity is seen to be a preferred loss of protonospheric plasma. During the night, net upward shift in the distribution, owing to loss at the lower levels, is clearly evident. On magnetically quiet days a substantial electron content is present all night, presumably maintained by downward flow from a ‘reservoir’ in the distant protonosphere. Some comparisons are made with ionospheric models, but more sophisticated models will be needed to explain the observations in detail.