Comparison between the values of V_zp obtained over Campo Grande. Ionosonde measured values are presented in blue (CG-COPEX), and theoretical V_zp values are presented in red [CG-COPEX(δ)]. The theoretical V_zp values were obtained using the values from ionosonde data over Cachimbo equatorial station and the expression for δ. The gray dashed curve corresponds to the V_zp obtained from ionosonde over Cachimbo and is provided for reference.