Variations in foF2 trends with season and local time

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Abstract
The median values of critical frequencies observed at ionospheric stations Slough, Juliusruh, and Rome are analyzed in detail. Long-term trends in foF2 for the period 1985–2010 were found using the method previously described numerous times by the authors. The dependence of the trend value k on local time and month of the year was studied. For all three stations, a similar seasonal and diurnal behavior of k is obtained. The maximum values of the negative trends in foF2 fall on the noontime period of the day (1000–1600 LT). In the seasonal behavior, two maximums of the absolute value of k (in the end of winter-beginning of spring and middle of fall) are observed. The absolute values of the trends are small in the summer months. The conclusion regarding the seasonal behavior of k coincides with the conclusion of the authors in an earlier publication: negative trends in foF2 are better pronounced in winter than in summer. The possible conclusions on the character of trends in the thermospheric parameters, which follow from the obtained conclusions on variations in the foF2 trends, are discussed.

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