Modeling the effects of secular variation of geomagnetic field orientation on the ionospheric long term trend over the past century

[1] A middleand low-latitude ionospheric theoretical model is used for the first time to assess the effects of the secular variations of geomagnetic field orientation on ionospheric long-term trends over the past century. It is found that the varied geomagnetic field can produce ionospheric long-term trends in both foF2 and hmF2. Since the amplitudes of geomagnetic field change depend on location, the modeled trends by secular variations of geomagnetic field orientation differ from place to place. The modeled ionospheric trends have obvious seasonal and diurnal variations. These variations show typical regional features. By comparison with existing results, we suggest that the changes of the global geomagnetic field may partly contribute to the inconsistent seasonal and local time variation patterns in ionospheric trends from different observations. The average trends of foF2 and hmF2 are 0.0047 MHz/a and 0.0107 km/a during 1900–2005, which is near or even comparable with those of other trend origins. In comparison with the trend results from worldwide observations, we conclude that the effects of geomagnetic field orientation on the ionospheric long-term trend cannot be ruled out, especially in areas with large geomagnetic field variations.