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

Abstract

AbstractSolar rotation (SR) variation dominates solar extremely ultraviolet (EUV) changes on the timescale of days. The F10.7 index is usually used as an indicator for solar EUV. The SR variation of F10.7 significantly enhanced during the 2008th–2009th Carrington rotations (CRs) owing to an intense active region; F10.7 increased about 180 units during that SR period. That was the most prominent SR variation of F10.7 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 F10.7. The variation of GEC with F10.7 was anomalous (GEC‐F10.7 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 F10.7 response to that intense active region was much stronger than EUV response; thus, the EUV‐F10.7 slope decreased. We confirmed decreased EUV‐F10.7 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‐F10.7 slope statistically tends to decrease when the SR variation of F10.7 significantly enhances. Moreover, we found that ionospheric time lag effect to EUV is exaggerated when using F10.7, owing to the time lag effect of EUV to F10.7.