The cross-polar potential tends to be saturated when the value of the merging electric field (Em) is high. Using data from the Wind and ACE satellites and the northern polar cap index (PCN) as a proxy for the cross-polar potential, Nagatsuma <2002> has shown that the degree of saturation does not depend on the intensity of the interplanetary magnetic field on the GSM-YZ plane (BT) but depends on that of Em. This result supports the idea that the enhancement of the region 1 current plays a significant role in regulating the efficiency of the solar wind-magnetosphere interaction, since the intensity of the region 1 current is a function of Em. If the region 1 current controls the efficiency of the interaction, ionospheric conductivity also controls the degree of saturation, since the region 1 current system is a voltage generator. We have examined the solar zenith angle (SZA) dependence of the PCN saturation and have compared the SZA dependences of ionospheric conductivity models. Our statistical results show that the degree of saturation increases with decreasing solar zenith angle at the northern corrected geomagnetic pole under sunlit conditions and that the opposite trend occurs during darkness. These characteristics are explained by the SZA dependence of the total Pedersen conductivity estimated from the northern and southern polar caps. This result confirms that the development of the region 1 current plays a significant role in regulating magnetospheric convection.