The morphology of lunar semi-diurnal variation in ƒ<SUB>0</SUB>F2 near solar noon

The variations of the midday critical frequency of the F2-layer, &#402;0F2, with the phase of the moon are computed for eighteen stations within about &#177;40&#176; latitude during a period of low sunspot activity. The results show that the phase of the lunar variation of &#402;0F2 is determined by the magnetic and not by the geomagnetic or the geographic latitude. The reversal from the equatorial type of variation with a maximum near 04 lunar hour to the higher latitude type with a maximum near 10 lunar hour occurs at about &#177;11&#176; magnetic latitude. The variation of amplitude with latitude shows a sharp maximum on the magnetic equator, associated with the equatorial electro-jet, and two broad maxima at about &#177;20&#176; magnetic latitude. The latter are closely associated with the well-known maxima in the variation of noon &#402;0F2 with latitude