Incidence of solar cycle 24 in nighttime foF2 long-term trends for two Japanese ionospheric stations

Ionospheric long-term trend studies are of great scientific interest since they contribute to the more general and controversial climatic change issue. In this paper we analyze the effect of the inclusion of solar cycle 24 on estimation of the nighttime trend of the critical frequency of ionospheric F2 layer (foF2) for Kokubunji (35.7°N, 139.5°E) and Wakkanai (45.4°N, 141.7°E), two mid-latitude Japanese stations. Even though during the night recombination and transport processes prevail, ionization still depends directly on solar activity, so it must be filtered out before any long-term variation assessment. As usual, filtering is done considering the residuals of the regressions between foF2 with the solar radio flux at 10.7 cm (F10.7) and a ratio between the core and wing line intensities of the emitted ionized Magnesium doublet (Mg II index). Similar to the case of daytime foF2 values, night trends become less negative when solar cycle 24 is included since foF2 residuals systematically exceed the values predicted by F10.7 or Mg II from 2009 onwards. This effect is weaker in the case of Mg II, which is expected to be a better solar extreme ultraviolet (EUV) proxy than F10.7 in the case of the solar radiation involved in the F2 layer ionization. A plausible cause for cycle 24 incidence in the trend may be due to the use of F10.7 or Mg II as EUV proxy to filter solar activity. It is because they both seem to be inaccurate for filtering purposes since the last deep minimum in about 2008. Otherwise, there could be a real physical cause for this observation.