Effect of the Inclusion of Solar Cycle 24 in the Calculation of foF2 Long-Term Trend for Two Japanese Ionospheric Stations

One of the main ionization sources of the F2 region of the Earth's ionosphere is the solar EUV irradiance accounting for ~ 90% of its variability during quiet time. Consequently, prior to long-term trend estimations solar activity must be filtered out. The last two solar activity cycles present low activity levels, and particularly solar cycle 24 is the lowest in the last ten solar cycles. The effect of the inclusion of this last solar cycle on foF2 trend estimation is analyzed for two mid-latitude ionospheric stations: Kokubunji (35.7 degrees N, 139.5 degrees E) and Wakkanai (45.4 degrees N, 141.7 degrees E). Filtering is done considering the residuals of different regressions between foF2 and Rz and also between foF2 and F10.7. In both cases, foF2 trends become less negative when solar cycle 24 is included in trend estimations since foF2 residuals systematically exceeds the values predicted by a linear, quadratic or cubic fit between foF2 and F10.7 or Rz from 2008 onwards. In addition, the Earth's magnetic field secular variation at both stations would induce a positive foF2 trend during daytime that could counteract the greenhouse gases decreasing trend. It is interesting to think that including the latest solar cycles does not necessarily imply incorrect results in the statistical analysis of the data, but simply that solar activity is decreasing on average and also the trend.