The diurnal variation of ULF field line resonant frequencies has been calculated using the cross phase of data from the north-south components recorded at seven latitudinally spaced ground magnetometers in the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) array. CANOPUS magnetometers range in latitude from Rankin Inlet (L=12.4) south to Pinawa (L=4.3). Using cold plasma MHD theory, an R-4 plasma density function, and the T87 magnetic field model, the variation of plasma density in the equatorial region has been calculated from the experimentally determined resonant frequencies. Consecutive, adjacent magnetometer pairs provide six local daytime spatial estimates of the variation in plasma mass density between 4 and 11 RE. Typical values are 1--20 H+ cm-3 for the plasmatrough and 50--200 H+ cm-3 for the plasmasphere. The data recorded on June 7, 1990, shows an afternoon increase in density near geosynchronous orbit in agreement with convection models of the magnetosphere. The ground-based measurements of plasma mass density have been compared with data from the Los Alamos Magnetospheric Plasma Analyser on board the 1989-046 geosynchronous spacecraft. These comparisons show that the ground-based technique should allow a robust procedure for calculating dayside, time-dependent mappings of the equatorial plasma mass density from the plasmapause to the magnetopause in near real time. ¿ American Geophysical Union 1996