Past and recent studies of the empirical relations between the solar wind (SW) parameters and geomagnetic activity indices have provided a general understanding of the SW-magnetosphere transfer processes. The present study deals with the relationship between the y-component (GSM coordinate system) of the interplanetary electric field (IEF, Ey) and the low latitude magnetospheric convection electric field EM. This relationship provides an estimate of the field transfer efficiency between SW and magnetosphere. First the EM-field, which has been obtained from electron density profiles measured onboard the GEOS-1 and ISEE-1 spacecraft, is correlated directly with the corresponding Ey-field, as calculated from published values of the interplanetary magnetic field and velocity [King, 1979> in a case-by-case study. Second, a statistical study is made between Ey and the geomagnetic index Km for low and moderate magnetic activity (Km?5+) from which one deduces a 'statistical' relationship between Ey and EM. The transfer coefficients obtained by both methods are identical: ΔEM/ΔEy = 0.13. This result is discussed in the framework of present theories of SW-magnetosphere transfer processes.