Gradual reduction in ionospheric F2 region before the 2013 Lushan earthquake: Contributed to the forthcoming earthquake or solar activity?

Ionospheric anomalies before earthquakes during geomagnetic quiet conditions have been extensively discussed due to the importance of earthquake prediction. However, because of the high variability of ionosphere, a real caution should be adopted before attributing the observations to seismo-ionospheric precursors. In an investigation of contradictory results in ionospheric variation before the 2013 Lushan earthquake, we examine the ionospheric variation versus solar activity. In order to assess the influence of solar activity of 27day periodicity on ionospheric variation, a quadratic regression model is developed by using the data of F-2 layer critical frequency (f(o)F(2)) in April of 2008-2014. Using the model, ionospheric variation before the 2013 Lushan earthquake is analyzed. The results show that f(o)F(2) was strongly correlated with solar activity index, F-10.7, and had a low correlation with the remaining by eliminating the contribution of solar radiation, which indicates that the gradual reduction in f(o)F(2) before the earthquake was contributed to the variation of solar radiation rather than the forthcoming earthquake.