Day to day variability of h′F and foF2 during some solar cycle epochs

Comparison of the diurnal and seasonal changes of the variability (VR) of the virtual height of reflection (h′F) and those of critical frequency of the F layer (foF2) is carried out at Ibadan (7.4°N, 3.9°E, 6°S dip) in the African sector. The effect of latitude on both characteristics is investigated by combining data from Singapore (1.3°N,103.8°E, 17.6°S dip) in the East Asian sector and Slough (51.5°N, 359.4°E, 66.5°S dip) in the European sector. The variability of foF2 is found greater than that of h′F except during high solar activity when night-time h′F VR is about the same as night-time foF2 VR possibly due to high post-sunset rise in F region heights at this epoch. Both characteristics have pre-midnight and post-midnight peaks with the latter being the greater one at all epochs with the exception of all the seasons of 1958 but June Solstice, very likely because of the same greater F region height during maximum solar activity. Night-time foF2 VR is greater in September Equinox and June Solstice at all epochs while night-time h′F VR is greater in June Solstice during the three epochs, on a general note. No seasonal trend is observed in the daytime variability of both characteristics during the three epochs except for h′F VR of December Solstice during high solar activity, which is greater than those of other seasons. VR of both parameters are found to increase and decrease alternately with Zurich sunspot number (Rz) at Ibadan and Singapore throughout the day. VR of foF2 at Slough is found to increase from midnight to 1300 h after which it alternates. Generally, post-midnight VR of both characteristics are greater than those of pre-midnight at low latitudes during all epochs while daytime VR are the least. At the mid-latitude station of Slough, foF2 VR is about the same throughout the hours of the day during 1968 and 1971, its post-midnight value is however greater than those of pre-midnight and daytime during 1970. Daytime h′F VR is greater than post- and pre-midnight values during 1968, but about the same throughout the hours of the day during 1971.

► Comparison of foF2 VR and h′F VR shows post-midnight and pre-midnight peaks.
► foF2 VR is greater than h′F VR, generally.
► Only night-time VR of both parameters shows seasonal trends.
► While night-time VR is greater at low-latitude than mid-latitude, the reverse is the case for day-time VR.
► foF2 VR and h′F VR increases and decreases alternately with Rz.