Trends in the F2 layer parameters at the end of the 1990s and the beginning of the 2000s

Abstract

The behavior of the critical frequency foF 2 of the ionospheric F2 layer is considered for the period 1990-2010. Various available databanks of ionospheric vertical sounding data are described and compared. The analysis is performed in terms of comparison of the foF 2 data for the period 1958-1980 to the data for 1990-2010. Two moments of the day (14:00 LT and the after-sunset moment) and two seasons (winter and summer) are considered for 12 stations for which the necessary data were available. The scatter of the foF 2 values (in terms of the standard deviation, SD) relative to the dependence on the solar activity index F 10.7 is considered. It is demonstrated that the values of SD for the period 1998-2010 are much higher than for the period 1958-1979. This increase in the SD includes two factors: real increase in the foF 2 scatter and systematic decrease (negative trend) in foF 2. This systematic decrease makes it possible to provide an independent evaluation of the trend in foF 2 : -0.03 MHz per year. Analysis of foF 2 behavior for each of all 12 stations is performed. The obtained linear trends are negative for all stations, but the trend magnitude varies from one station to another. The trends for two moments of time are found of the same order of magnitude: -0.024 and -0.054 MHz per year for the summer and winter seasons, respectively. Some conclusions on the behavior of the hmF 2 trends for the same period are presented. Possible causes of the changes in the trends in the F2 layer parameters are discussed. Key Points Negative trends in critical frequency are derived for 1990-2010. Negative trends in F2-layer height are derived for 1990-2010 It is found that scatter of foF2 and hmF2 increases with time © 2013. American Geophysical Union. All Rights Reserved.