Scintillation indices for incident plane wave (blue curve) and spherical wave (red curve) approaches. The satellite altitude is set at H_sat = 600 km, the distance L_t between satellite and irregularities varies between 0 and R − R_iono = 598 km. At L_t = 0 km, the ionospheric layer is very close to the satellite; at L_t = 598 km, the ionospheric layer is very close to the receiver.