The Influence of the 27-Day Variation of the Upper Atmosphere Neutral-Particle Density on Ionospheric Spread-F Occurrence

Days of maximum 10.7 cm solar flux associated with solar rotations have been used as controls to investigate, using superposed-epoch methods, spread-F occurrence levels over a range of latitudes from sub-auroral regions to equatorial-anomaly crest regions. For all latitude regions, 27-day periodicities, and other periodicities of lesser significance, were recorded in the spread-F plots, with minima in the oscillations located near the central positions. For these oscillations spread-F occurrence levels changed by 5 percent in sub-auroral regions, about 15 percent in mid-latitude regions and 10 percent in low-latitude regions. It can be seen that, as the sun rotates, solar-activity changes have a significant influence on spread-F occurrence. With some complications, generally the spread-F minima were delayed after the solar-flux maxima by a few days. These and other experimental results suggest an explanation in terms of an hypothesis which considers the so-called 27-day variation of the upper-atmosphere neutral-particle density (neutral density) associated with the solar-rotations. The proposal is that it is the control by the neutral density of the medium-scale travelling ionosphere disturbance wave amplitudes which determines whether or not spread-F is observed on ionograms