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Comparison of F1 layer critical frequency between recent two solar minimums

This study compares the F1 layer critical frequencies (foF1) between the 22/23 and 23/24 solar minimums to investigate the effects of extremely low solar activities on F1 layer. The foF1 data of 1996 and 2008 at various latitudes are collected and analyzed. Results show that the foF1 value in 2008 is quantitatively identical to that in 1996. This equality of foF1 is contrary to the expectation, since the solar activities are lower in 2008 than in 1996. In order to know the reason for the unexpected results, the production rate (q) of ionization in F1 layer is examined. The EUV intensity and q at the peak of the q profile are calculated, through the integration of atmospheric number densities above the peak. Because the thermospheric densities are anomalously low under extremely low solar activities, the EUV attenuations are unusually small. The EUV intensity and q at the peak in 2008 are almost equal to those in 1996. Further, the equality of foF1 between 1996 and 2008, obtained from the equality of q at the peak, is consistent with the observed results.