THE INFLUENCE OF THE UPPER-ATMOSPHERE NEUTRAL PARTICLE DENSITY ON THE OCCURRENCE OF EQUATORIAL SPREAD-F

Over extended periods of time spread-F occurrence is considered in equatorial latitudes at stations located in the region between the geomagnetic equator and the equatorial anomaly crest region. Distinct differences are observed in the annual and sunspot-cycle distributions for spread-F occurrence before midnight and after midnight. The paper proposes that before midnight the low upper-atmosphere neutral-particle densities (NPDs) at elevated ionospheric heights (which usually occur after sunset) are responsible for travelling ionospheric disturbance with increased wave amplitudes. These enhanced amplitudes produce the tilted isoionic surfaces responsible for the off-vertical reflection needed for spread-F to be observed on ionograms. After midnight low NPDs at normal ionospheric heights also produce large wave amplitudes with resulting spread-F occurrence. Paradoxically the two sets of distributions (the premidnight and the postmidnight set) are out of phase with each other. For example the postmidnight set shows maximum spread-F occurrence in sunspot-minimum years while the premidnight set has a maximum in sunspot-maximum years when the large postsunset height rises occur.