## 2 Materials and Methods
### Electron Temperature Description by the IRI Model  
In the last version of IRI (IRI-2016, [9]), two different models for \(T_{\rm e}\) are implemented; they are conventionally named BIL-1995, and TBT-2012 ([[http://irimodel.org/](http://irimodel.org/)]([http://irimodel.org/](http://irimodel.org/)), accessed on 24 September 2021). The BIL-1995 is the oldest option and is based on the works by [PERSON] [31; 32; 33] who, through the application of the Booker approach [34] to the [PERSON] [20] and [PERSON] and Pluge [35] models, describes the \(T_{\rm e}\) vertical profile for the ionospheric altitude range between 60 and 2000 km. Since the IRI-2001 version [36], IRI has included an additional option based on Intercosmos data [37], that since the IRI-2012 version [38] was updated with the most recent TBT-2012 model [26] which incorporates all available satellite \(T_{\rm e}\) data. The solar activity dependence, also described in [PERSON] et al. [26], is mainly based on results from [PERSON] et al. [39]. In the current IRI version, this additional option is called TBT-2012+SA and represents the option related to the TBT-2012 model with the solar activity term switched on. Since the TBT-2012+SA is the current default option for the \(T_{\rm e}\) modeling in IRI, it has been applied in this work to perform a comparison with Swarm and ISRs measured data and will be briefly described here. In the TBT-2012 model [26], \(T_{\rm e}\) values at five fixed anchor points at 350, 550, 850, 1400, and 2000 km of altitude are modeled through a spherical harmonic expansion in a system of associated Legendre polynomials (up to the 8 th order) in terms of the magnetic local time and a latitudinal coordinate based on a combination of invariant and dip latitude (involip, [40]). Through the use of \(T_{\rm e}\) measurements from LPs on-board several satellite missions (for a detailed description of the dataset see [PERSON] et al. [23] and [PERSON] et al. [26]), the coefficients of the spherical harmonic expansions were calculated for summer and winter solstices, and for the combined equinoxes (independently of the hemisphere); then, interhemispheric differences in magnetic coordinates were not considered in the model. The solar activity dependence was then included by selecting three solar activity ranges and describing the corresponding variability as a function of the \(PF_{\rm 10.7}\) solar index [26]. The complete vertical \(T_{\rm e}\) profile was obtained by applying the Booker profile function formalism [33; 34]. First, a piece-wise function was built making a linear interpolation of modeled \(T_{\rm e}\) values between anchor points, which produced a vertical profile divided in regions with a constant \(T_{\rm e}\) gradient. Second, Epstein-step functions were applied to realize the transition from regions with different gradients and obtain the skeleton profile, which produced a continuous analytical representation of the \(T_{\rm e}\) vertical profile. Finally, the skeleton profile was integrated to obtain the Booker profile function of \(T_{\rm e}\), which output a continuous and smooth description of the entire modeled \(T_{\rm e}\) vertical profile. For a detailed mathematical description of the Booker profile function formalism, refer to the Appendix section of [PERSON] et al. [30]. In IRI, \(T_{\rm e}\) is constrained to be equal to \(T_{\rm n}\) described by the NRLMSISE-00 model [41] for altitudes lower than 120 km. An example of IRI (TBT-2012+SA default option) modeled vertical \(T_{\rm e}\) profile is given in Figure 1 (green curve) where the anchor points that IRI uses to describe the entire profile are highlighted. An anchor point at a non-fixed altitude around 270 km (\(T_{\rm e,m}\) in Figure 1) was introduced on the basis of Jicamarca and Arecibo ISRs data to describe a local daytime maximum in \(T_{\rm e}\) characterizing mostly the low latitudes [33].