Large-scale electric fields in the magnetosphere, represented by the scalar potentials of corotation and convection fields, are examined in order to interpret both plasmapause positions at different local times and energetic particle penetrations inside the plasmapause observed by the Explorer 45 (S3-A) dc E field measurements and particle detectors. The Volland-Stern type convection electric field is assumed: E↘=-∇&PHgr;, and &PHgr;=AR&ggr; sin ¿. A uniform dawn-dusk convection electric field which corresponds to an exponent (&ggr;) of unity is not consistent with the observational data, but the Volland-Stern model with &ggr;=2 more likely represents the convection electric field during the hours of electric field increases during magnetic activity. In the time-independent case the shape of the plasmapause does not depend on the intensity of the convection electric field, so that by using two plasmapause distances at different local times along the S3 outbound and inbound orbits the shape factor &ggr; of the convection field could be determined. The average value of &ggr; for 42 cases was found to be 2.4.