Trends in the critical frequency foF2 after 2009

Abstract

The problem of how the deep minimum of solar activity of 2008–2009 influenced trends in the critical frequency of the F2 layer and whether the negative trends in foF2 are conserved in the following five years is considered in the paper. The initial data series used in previous publications by the authors are completed by data to 2014 (2015). Methods of searching for trends that were described in the past several times by the authors were applied. It was found that, in the vast majority of cases, the monotonous change in the curves used to derive the foF2 trend was distorted in years close to the anomalously low minimum of solar activity in 2008–2009. That decreased the obtained trend in the critical frequency as compared with the results based on the data to 2009. However, subsequent years again lead to a tendency toward monotonous decrease in foF2. The causes of the anomalies indicated above lie apparently in the inadequacy of the solar activity index F10.7 for the description of changes in the aeronomical parameters determining the foF2 behavior in the period of a very deep minimum of activity that is widely described in some publications.