Physics and Chemistry of the Earth, 27, 589-594, 2002.

Published estimates of the trend in hmF2 using data from ionosondes over the last 30-40 years range from +0.8 km y^-1 to -0.6 km y^-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 y^-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 y^-1; neglecting any trends in geomagnetic activity can produce variations of +0.03 to +0.2 km y^-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 y^-1 variability; the phase of the 11-year solar cycle, and its harmonics, captured by the datasets can cause variability of +/- 0.5 km y^-1; and the neglect of local time variations in thermospheric wind conditions could result in +0.2 km y^-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 y^-1, which are in close agreement with the satellite drag trend estimates.