Long-term trend of the ionospheric E-layer response to solar flares

Using data from ground-based vertical sounding of the ionosphere (VS) at the station Moscow and five Japanese stations for the period from 1969 to 2015, we have examined the long-term response of the E layer to solar X-ray flares. The analysis relies on the previously developed method for estimating the ratio between rates of ionization by X-rays qx and ultraviolet radiation qu during flares. We confirmed the existence of a long-term (at least covering the 45-year observation period) increase in the proportion of X-rays in the total ionization rate of the ionospheric E layer, characterized by the ratio qx/q, where q=qx+q. The qx/q ratio is shown to increase throughout the period of interest at a rate independent of the solar activity cycle. There is also no dependence of the qx/q trend rate on the season, latitude (26°–56° N), and longitude (37°–28° E).