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Ann. Geophys., 25, 1125-1130, 2007
www.ann-geophys.net/25/1125/2007/
© European Geosciences Union 2007
Morphology of quantified ionospheric range spread-F over a wide range of midlatitudes in the Australian longitudinal sector
L. A. Hajkowicz
Department of Physics, University of Queensland, Qld.4072, Australia
Abstract. 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.
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