Abstract
A detailed analysis of the hmF2 data obtained at the network of ionospheric stations is performed to reveal long-term trends independent of the geomagnetic activity variations during the recent decades (nongeomagnetic trends). The method developed and tested by the author earlier is applied. Unlike the results on foF2 published by the author earlier, the picture of hmF2 trends is not homogeneous. For 17 ionospheric stations positive significant trends are obtained. Six stations give negative trends in hmF2. For three stations no trends were derived. The behavior of the positive trends is considered. It is shown that these trends depend neither on geomagnetic latitude nor on local time, both facts confirming their independence of geomagnetic activity. The averaging of the positive trends over all 17 stations gives a relative trend of 0.0011 per year with the standard deviation of 0.0005. At the average height of the F2 layer of about 300 km this means an increase in hmF2 by 0.33 km per year. Possible interpretations of the obtained trends in foF2 and hmF2 from the point of view of anthropogenic changes in the thermosphere are considered.
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