Ionospheric long-term trends for South American mid-latitudes

Using ionosonde made observations at Concepción (36.8°S; 73.0°W) for the 1958–1994 interval, long-term trends of critical frequency (foF2) and peak height (hmF2) of the ionospheric F2-layer are analysed. The trends found for different times-of-day and all seasons are consistent with an increasing diurnal-variation amplitude of both foF2 and hmF2. An increasing hmF2 trend of up to 1.5 km/year found between midnight and dawn during winter has no precedent. It is suggested that these long-term amplitude changes may be associated with changes in the prevailing thermospheric meridional neutral winds.