| Summary: | This study focuses on the open question of the electron temperature (<i>T</i><sub>e</sub>) variation with solar activity in the topside ionosphere at mid- and high latitudes. It takes advantage of in situ observations taken over a decade (2014–2023) from Langmuir probes on board the low-Earth-orbit Swarm B satellite and spanning an altitude range of 500–530 km. The study also includes a comparison with <i>T</i><sub>e</sub> values modeled using the International Reference Ionosphere (IRI) model and with Millstone Hill (42.6° N. 71.5° W) incoherent scatter radar observations. The largest <i>T</i><sub>e</sub> variation with solar activity was found at high latitudes in the winter season, where <i>T</i><sub>e</sub> shows a marked decreasing trend with solar activity in the polar cusp and auroral regions and, more importantly, at sub-auroral latitudes in the nightside sector. Differently, in the summer season, <i>T</i><sub>e</sub> increases with solar activity in the polar cusp and auroral regions, while for equinoxes, variations are smaller and less clear. Mid-latitudes generally show negligible <i>T</i><sub>e</sub> variations with solar activity, which are mostly within the natural dispersion of <i>T</i><sub>e</sub> observations. The comparison between measured and modeled values highlighted that future implementations of the IRI model would benefit from an improved description of the <i>T</i><sub>e</sub> dependence on solar activity, especially at high latitudes.
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