Trends in the F2-layer parameters to 2023

Abstract

Diurnal variations in the long-term trends in the F2-layer critical frequency, foF2, are studied in detail based on the observations to 2022 and use of several solar activity (SA) indices to remove the SA effects in the foF2 variations. The new results confirm the conclusions derived earlier based on the data only to 2014. In the winter months, two well pronounced periods are observed: the nighttime hours when the trends are small by their magnitude and the daytime hours when the negative trends are maximal. In March, the picture is close to that in winter months, however the daytime negative trends are weaker than in winter. No pronounced diurnal variation in the foF2 trends is found in the summer month June. A search for long-term trends in foF2 is performed based on the observations at three stations in the Northern Hemisphere (Juliusruh, Boulder, and Moscow) and three stations in the Southern Hemisphere (Townsville, Hobart, and Canberra). The data for two winter months in each hemisphere for five near-noon LT moments are analyzed. Three solar activity proxies (F30, Ly-α, and MgII) are used to get rid of the SA effects. Negative trends are obtained for all considered situations (station, month, LT moment, SA proxy). The trends for different stations agree well with each other. A tendency in intensification in the negative trends during the recent years is seen in all months.