Long-term trends of the F2-region at mid-latitudes in the Southern Hemisphere

Abstract

The long-term variations in foF2 at Hobart (52.88°S, 147.32°E), Canberra (35.28°S, 149.13° E) and Christchurch (43.53°S, 172.64°E) stations, located in the mid-latitude zone in the Southern Hemisphere were analyzed using 1947–2006 years of the data. The saturation, solar and geomagnetic activity and seasonal effects were removed mainly by using 12-month running mean, linear and multiple regression (twofold regression) methods to find possible signatures of climate change in long-term trends in the foF2. The solar activity proxies, sunspot number, RZ, and F10.7 solar radio flux were used in regression to find the foF2 residuals at midday (12 LT) and midnight (00 LT) of the stations. The long-term trends obtained at 12 LT are more significant and consistent with the model results. The trends estimated with F10.7 solar flux are negative and the trends estimated with RZ are positive (small and not significant). The foF2 decreased by 0.1–0.4 MHz for the 5 solar cycles period which could be mainly due to enhanced CO2 in the troposphere that is cooling the upper atmosphere. Further research is needed to see if the foF2 trends are also affected by other factors such as thermospheric winds, neutral constituents, the secular variation of Earth's magnetic field, long-term changes in stratospheric ozone, solar and geomagnetic activities.