Incoherent scatter (IS) radar measurements of the ionospheric electron density are analyzed for the signature of the magnetic separatrix in the dayside ionosphere by comparison with separatrix locations determined using precipitating electron data from DMSP spacecraft within 1.5 hours in magnetic local time (MLT) of the Sondrestrom radar. A model of photoionization is used to remove its effects from the measured electron density to study the ionization by precipitating particles. The altitude of peak ionization and the peak ionization rate are used to determine whether a magnetic field line is open or closed. We observe that on closed field lines the peak ionization rate is high (>2200 cm-3 s-1) or the peak is lower than 140 km in altitude, and on open field lines the peak ionization is low (<900 cm-3 s-1) or the peak is higher than 140 km. These rules are used to identify the separatrix with an accuracy of 0.36¿ and a precision of ¿0.39¿. This signature of the separatrix is only apparent in the prenoon and noon sectors (0600 to 1300 MLT). In the postnoon sector (1630--1800 MLT) the altitude of peak ionization and the peak ionization rate do not show any systematic difference between the open and closed field line regions. Finally, an example is presented in which the data on the location and motion of the separatrix and IS radar measurements of F region plasma velocity are used to measure the magnetic reconnection rate as a function of the interplanetary magnetic field clock angle. ¿ 2001 American Geophysical Union