Published estimates of the trend in hmF2 using data from ionosondes over the last 30-40 years range from +0.8 to -0.6 km yr(-1) and are subject to the influence of several factors. These are considered here based upon an analysis of two southern hemisphere geomagnetically mid-latitude stations, Argentine Islands and Port Stanley. The influence of the equation used to calculate hmF2 at these stations can result in variations of +/-0.2 km yr(-1); choice of solar proxy has a small influence on the end result, where using E10.7 instead of F10.7 produces changes of -0.04 km yr(-1); neglecting any trends in geomagnetic activity can produce variations of +0.03 to +0.2 km yr(-1) at the two mid-latitude stations considered in this paper; for datasets of 30-40 years length ringing due to long memory processes can produce +/-0.2 km yr(-1) variability; the phase of the 11-year solar cycle, and its harmonics, captured by the datasets can cause variability of +/-0.5 km yr(-1); and the neglect of local time variations in thermospheric wind conditions could result in +0.2 km yr(-1) for analysis which only considers local midday data. The Argentine Islands and Port Stanley datasets show ringing terms that are still converging towards trend results of -0.25 to -0.30 km yr(-1), which are in close agreement with the satellite drag trend estimates
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