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Abstract

Solar extreme ultraviolet radiation is continuously absorbed in the dayside upper atmosphere and controls the creation of ionospheric plasma and its variability via complex interactions. Through these coupled processes, solar signatures such as the 27-day solar rotation period are introduced to the ionospheric state and are observed via various measurements. For example, time-delayed 27-day signatures with varying amplitudes are found for the main ion species and electrons, whose distributions in turn also vary in longitude, latitude, and altitude. Other changes are driven by space weather events such as geomagnetic storms or solar flares. Therefore, a full understanding of the chemical and physical processes is required to correctly describe the delayed ionospheric response. In a series of studies, we investigated and characterized these variations with different satellite- and ground-based measurements (e.g. recent studies with various NASA GOLD data products). Simulations with physics-based models were applied to reproduce the observed variations and to identify the driving processes. While the occurrence of the 27-day signatures is mainly controlled by photoionization and recombination, the delay in particular is significantly driven by photodissociation and transport across different ionospheric layers. In this presentation, we provide an overview of the main results of our studies and preview future work. We also highlight those results that are of interest for the improvement of ionospheric modelling.