A 10-YR ANALYSIS ON THE PLANETARY SCALE OF THE VIRTUAL HEIGHT OF THE F-LAYER

The trend is investigated for 10 yr (1967 1976) of a 24-variate planet ary ionospheric index that is derived from the monthly medians of the virtual height of the F-layer (h'F) simultaneously recorded within an array of 137 ionospheric observatories. The ionosphere appears to vary generally smoothly, with regular oscillations superimposed over a sol ar cycle trend, except that, on isolated occasions, when some anomalou s features occur within the plane perpendicular to the Sun-Earth line, that is, either to the 'bulge' that is observed in the equatorial bel t at approximately 18 LT, or to either one of the two 'cupolas' observ ed over the polar caps. Such events ought to be investigated as isolat ed case histories, apparently uncorrelated with the quasi-periodical s mooth oscillations of the normal condition. The periodicities of the r egular oscillations include a strong seasonal component, plus periods at approximately 38, approximately 44, approximately 20 and approximat ely 31-34 months. The strong seasonal variation envisages that h'F is more sensitive to thermodynamic than to electrodynamic phenomena (unli ke geomagnetic records which show a much weaker yearly variation, owin g to the large disturbances of the background, thus indicating mostly electrodynamic control). The physical meaning of all other periodiciti es cannot be suitably investigated by means of the present analysis ow ing to the possible aliasing caused by tidal oscillations. This cannot be studied when dealing only with monthly median values: for such a p urpose, the same analysis ought to be carried out directly using the r aw hourly records.