## 1 Introduction  
The ionosphere is a plasma medium consisting of ions and electrons with also a neutral component that, for most of the cases, are not in thermal equilibrium. In fact, since the early satellites and rockets missions in the 1950s and 1960s [1], a bulk of evidence has been accumulated demonstrating the non-thermal equilibrium between ions, electrons, and neutrals in the ionosphere. Plasma temperature (ions plus electrons) usually exceeds the neutral one by a factor strongly dependent on altitude, time, and location; moreover, large differences among ion and electron temperatures are also observed. As a consequence,when talking about the temperature of the ionosphere, we need to differentiate between ion (\(T_{\rm i}\)), electron (\(T_{\rm e}\)), and neutral (\(T_{\rm n}\)) temperature. The non-thermal equilibrium of the ionosphere has a number of physical and chemical implications for the ionospheric dynamics and composition. For example, the temperature of charged particles deeply influences the distribution of electron density by driving the recombination rate in the bottomside ionosphere, and affects the diffusive equilibrium state in the topside ionosphere [2; 3]. The main source of energy heating the ionosphere is the Sun: either directly, through the EUV and X-ray radiations illuminating the Earth, or indirectly, at high latitudes, through the precipitation of charged particles induced by the interactions between the solar wind and the magnetosphere-ionosphere system and the consequent Joule heating [1; 4; 5; 6]. In the ionization processes occurring in the ionosphere, a certain amount of energy is released as an excess of kinetic energy of photoelectrons. Such photoelectrons dissipate their energy through collisions with charged and neutral particles in a cascade process that first raises the temperature of the electrons (because the transfer of energy via elastic collisions is more efficient between particles of similar mass). In a second stage, the established temperature difference drives the transfer of energy between electrons and the surrounding medium. Above the F2 region, most of the energy transfer is from electrons to ions because ion-electron Coulomb collisions are more efficient than neutral-electron ones; then, the temperature of the ions also increases above the neutrals one. As a further step, the ions lose their excess energy through collisions with neutrals and other ions, thereby raising the temperature of neutrals. As a result of these complex interactions, the entire photoelectron energy dissipates in the neutrals and it is finally transferred down in the lower layers of the atmosphere by conduction and radiation. Among the three mentioned temperatures, \(T_{\rm e}\) is the most variable because of the very low heat capacity of electrons [4]; moreover, electrons are the most affected by the solar radiation energy input and by the interaction with charged particles entering the Earth's ionosphere following the geomagnetic field lines at auroral latitudes. The close coupling with the geomagnetic field lines and the plasmsphere produces very distinct spatial patterns in the \(T_{\rm e}\) distribution [6]. As a consequence, \(T_{\rm e}\) exhibits very distinctive diurnal, seasonal, spatial, and solar activity trends. This large variability makes it difficult to obtain a precise and reliable description of \(T_{\rm e}\) through in-situ or remote sensing measurements. In fact, in-situ measurements taken by Langmuir Probes (LPs) and Retarding Potential Analyzers (RPAs) [1; 7] on-board Low-Earth-Orbit (LEO) satellites provide a global spatial description for different local times, but they are highly dependent on the satellite orbital configuration and altitude and are limited to the satellite mission lifetime. Conversely, ground-based remote sensing techniques such as Incoherent Scatter Radars (ISRs) [7; 8] can better describe the altitudinal, diurnal, and seasonal variability but for a fixed geographic location and for the very few existing ISR facilities.