Morphology of quantified ionospheric range spread-F over a wide range of midlatitudes in the Australian longitudinal sector

Ionograms from a standard vertical-incidence ionosonde chain (nine stations), obtained over a wide range of southern latitudes (in geom.lat. range: 23–52 S), were digitally scanned at 5-min intervals at nighttime (18:00– 06:00 LT) for 13 months (January 2004–January 2005). An important parameter of the F-region, so-called range spreadF (Sr), was for the first time quantified in km. Maximum in Sr was recorded at a sounding frequency of 1.8 MHz for each night and for each ionosonde station. A distinct pattern in the magnitude (in km) and in the percentage occurrence of the range spread-F was present in southern winter only (the June solstice). The sub-auroral region (geom. lat. ≥52 S) is characterised by consistently high spread-F (average Sr≈100 km) on 80–100 per cent of the observed nights. There is a sharp equatorward boundary in the spreadF activity in a latitudinal range: 52–48 S followed by a midlatitude region (44–48 S) which exhibits a peak in Sr (≈50 km) in winter only, observed on half of the nights. The midlatitude activity reaches its minimum at 42–43 S, with Sr less than 20 km on one third of the nights. The low midlatitudes (23–36 S) are characterised by a strong peak in Sr again in winter, centred at about 30 S (average Sr≈70 km) on 80 per cent of the nights. The pattern becomes largely absent during other seasons particularly in southern summer (the December solstice) when spread-F activity shifts to subauroral latitudes. The pattern in the occurrence of spread-F appears to have a global character as the enhanced spread-F activity is observed in the Japanese sector in local summer (i.e. the June solstice). It appears that the midlatitude spread-F minimum is only apparent but not real. It delineates the boundary between aurorally generated spread-F (due to travelling ionospheric disturbances, TIDs) and low midlatitude spread-F whose origin is not known.