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°N, 139.5°E) and Wakkanai (45.4°N, 141.7°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.

中文翻译：

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将太阳活动周期 24 纳入计算两个日本电离层站 foF2 长期趋势的影响

地球电离层 F2 区域的主要电离源之一是太阳 EUV 辐照度，占其安静时间变化的约 90%。因此，在长期趋势估计之前，必须过滤掉太阳活动。最后两个太阳活动周期呈现出低活动水平，特别是第 24 个太阳周期是最近十个太阳周期中最低的。针对两个中纬度电离层站：国分寺（35.7°N，139.5°E）和稚内（45.4°N，141.7°E），分析了包含最后一个太阳周期对 foF2 趋势估计的影响。过滤是考虑 foF2 和 Rz 之间以及 foF2 和 F10.7 之间不同回归的残差来完成的。在这两种情况下，当太阳周期 24 包含在趋势估计中时，foF2 趋势变得不那么负，因为 foF2 残差系统地超过了 foF2 和 F10.7 或 Rz 之间的线性、二次或三次拟合从 2008 年起的预测值。此外，两个站点的地球磁场长期变化将在白天引起正的foF2趋势，可以抵消温室气体减少的趋势。有趣的是，包括最新的太阳周期并不一定意味着数据统计分析中的结果不正确，而只是太阳活动的平均下降以及趋势下降。两个站点的地球磁场长期变化将在白天引起正的 foF2 趋势，这可以抵消温室气体减少的趋势。有趣的是，包括最新的太阳周期并不一定意味着数据统计分析中的结果不正确，而只是太阳活动的平均下降以及趋势下降。两个站点的地球磁场长期变化将在白天引起正的 foF2 趋势，这可以抵消温室气体减少的趋势。有趣的是，包括最新的太阳周期并不一定意味着数据统计分析中的结果不正确，而只是太阳活动的平均下降以及趋势下降。