Long-term changes in the Delta foF2 parameter based on the data of two European ionospheric stations

Analysis of changes in the critical frequency foF2 in recent decades has been performed by determination of "Delta foF2" parameter. These values determine the mean change of foF2 values from the "etalon period" (1958-1980) to later periods. The results are compared with the determination of foF2 trends, which was performed in a series of previous publications of the authors. The data of two most reliable ionospheric stations of the European region, Slough and Juliusruh, are analyzed. The results confirm all principal conclusions obtained earlier, which were based on analysis of the trends. A systematic decrease of foF2 with time occurs (which corresponds to a negative trend), and the character of changes in the Delta values with season and local time on the whole agrees with the character of changes in the trend. It is shown that the results based on the data of both considered stations show good agreement.