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

L. A. Hajkowicz (1)

1 Department of Physics [Brisbane, QLD]

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 spread-F (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 spread-F 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 sub-auroral 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.

Domaines: Océan, Atmosphère; Sciences de la Terre

L. A. Hajkowicz. Morphology of quantified ionospheric range spread-F over a wide range of midlatitudes in the Australian longitudinal sector. Annales Geophysicae, 2007, 25 (5), pp.1125-1130. ⟨hal-00318321⟩