Responses of Solar Irradiance and the Ionosphere to an Intense Activity Region

Solar rotation (SR) variation dominates solar extremely ultraviolet (EUV) changes on the timescale of days. The F 10.7 index is usually used as an indicator for solar EUV. The SR variation of F 10.7 significantly enhanced during the 2008th–2009th Carrington rotations (CRs) owing to an intense active region; F 10.7 increased about 180 units during that SR period. That was the most prominent SR variation of F 10.7 during solar cycle 23. In this paper, global electron content (GEC) is used to investigate ionospheric response to that strong variation of solar irradiance indicated by F 10.7 . The variation of GEC with F 10.7 was anomalous (GEC‐ F 10.7 slope significantly decreased) during the 2008th–2009th CRs; however, GEC versus EUV variation during that period was consistent with that during adjacent time intervals when using Solar Heliospheric Observatory/Solar EUV Monitor 26–34 nm EUV measurements. The reason is that F 10.7 response to that intense active region was much stronger than EUV response; thus, the EUV‐ F 10.7 slope decreased. We confirmed decreased EUV‐ F 10.7 slope during the 2008th–2009th CRs for different wavelengths within 27–120 nm using Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics/Solar EUV Experiment high spectral resolution EUV measurements. And on the basis of Solar Heliospheric Observatory/Solar EUV Monitor EUV measurements during solar cycle 23, we further presented that EUV‐ F 10.7 slope statistically tends to decrease when the SR variation of F 10.7 significantly enhances. Moreover, we found that ionospheric time lag effect to EUV is exaggerated when using F 10.7 , owing to the time lag effect of EUV to F 10.7 .