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We introduce a new approach,using the International Reference Ionosphere 2007(IRI-2007)model and observations from the Moscow ionosonde station,to analyze the long-term trends of ionospheric foF2.Possible origins,whether natural or manmade, are discussed.A small but significant residual foF2 trend is found,with-0.76 MHz over the past 50 years,but-0.2 MHz after eliminating the most variations caused by solar and geomagnetic activities and the variational earth’s magnetic field.We find that this slope depends on local time and seasons.There is a pronounced diurnal negative variation in k(annual mean slope of diurnal variations),with much larger absolute values in the morning than during afternoon or evening;the largest k is about-0.0018 per year in spring,and the smallest trend of about-0.0012 per year occurs in winter.Finally,the evidence of crucial influences of geomagnetic activity and the earth’s magnetic field on the observed trends,as emphasized by the IRI-2007 empirical model,is confirmed,and the effect of CO2 contributions to the trend is also discussed,by cooling the thermospheric temperature or recombining the electron density.
We introduce a new approach, using the International Reference Ionosphere 2007 (IRI-2007) model and observations from the Moscow ionosonde station, to analyze the long-term trends of ionospheric foF2.Possible origins, whether natural or manmade, are discussed. A small but significant residual foF2 trend is found, with-0.76 MHz over the past 50 years, but-0.2 MHz after eliminating the most variations caused by solar and geomagnetic activities and the variational earth’s magnetic field. We find that this slope depends on local time and seasons. where is a pronounced diurnal negative variation in k (annual mean slope of diurnal variations), with much larger absolute values in the morning than during afternoon or evening; the largest k is about-0.0018 per year in spring, and the smallest trend of about-0.0012 per year occurs in winter. Finally, the evidence of crucial influences of geomagnetic activity and the earth’s magnetic field on the observed trends, as emphasized by the IRI-2007 empirical model, is c onfirmed, and the effect of CO2 contributions to the trend is also discussed, by cooling the thermospheric temperature or recombining the electron density.