Fig. 3
From: Ionospheric electron density modelling using B-splines and constraint optimization
\(F_2\)-layer electron density profile with the lower bound functions \(\kappa _{r;l}(\varphi ,\lambda ) = N_{m;l}^{F_2}(\varphi ,\lambda ) = 0.2\) EDU and \(\kappa _{r;l}(\varphi ,\lambda ) = h_{m;l}^{F_2}(\varphi ,\lambda ) = 220\) km as well as the upper bound functions \(\kappa _{r;u}(\varphi ,\lambda ) = N_{m;u}^{F_2}(\varphi ,\lambda ) = 2.5\) EDU and \(\kappa _{r;u}(\varphi ,\lambda ) = h_{m;u}^{F_2}(\varphi ,\lambda ) = 500\) km of the 2 key parameters \(N_m^{F_2}(\varphi ,\lambda )\) and \(h_m^{F_2}(\varphi ,\lambda )\) at the spatial position \((\varphi ,\lambda )\); the region within the bounds—highlighted in yellow colour—is denoted as the feasible region \(\mathcal{F_R}\). The figure shows alongside also the day and night side electron density profiles, indicating that the bound functions are actually time-dependent