A 10 yr analysis on the planetary scale of the virtual height of the f-layer

Abstract

The trend is investigated for 10 yr (1967–1976) of a 24-variate planetary 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 solar cycle trend, except that, on isolated occasions, when some anomalous features occur within the plane perpendicular to the Sun-Earth line, that is, either to the ‘bulge’ that is observed in the equatorial belt at ~ 18 LT, or to either one of the two ‘cupolas’ observed over the polar caps. Such events ought to be investigated as isolated case histories, apparently uncorrelated with the quasi-periodical smooth oscillations of the normal condition. The periodicities of the regular oscillations include a strong seasonal component, plus periods at ~38, ~44, ~20 and ~ 31–34 months. The strong seasonal variation envisages that h′ F is more sensitive to thermodynamic than to electrodynamic phenomena (unlike geomagnetic records which show a much weaker yearly variation, owing to the large disturbances of the background, thus indicating mostly electrodynamic control). The physical meaning of all other periodicities cannot be suitably investigated by means of the present analysis owing to the possible aliasing caused by tidal oscillations. This cannot be studied when dealing only with monthly median values: for such a purpose, the same analysis ought to be carried out directly using the raw hourly records.