We compare te rms ion concentration fluctuations ΔN1/N1 at Ogo 6 altitudes with the critical frequency structures on ionograms from nearby ionosondes. For detectable spread F, (ΔN1/N)rms is found to be statistically proportional to th frequency 'spread' 2Δf/f throughout the range 0.1--80%. The satellite probe can resolve ΔN over scales smaller than th ionogram radio wavelengths, but it does not necessarily traverse the largest density fluctuation visible to the ionosonde: thus the minimum percentage density fluctuations observed in some cases by the satellite are less than the minimum percentage frequency spread. We conclude that small-scale irregularities (<40 km) account for 'frequency spreading' although still larger scale irregularities may explain ionogram 'range spreading.' The relative intensity of the small-scale density fluctuations is independent of altitude in the interval 0--5 scale heights above the F region peak. These properties are consistent with a plasma instability process for spread F.