Observations of ULF field line resonances detected by seven magnetometers in the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) array were used to estimate equatorial cold plasma mass densities in and around the plasmatrough region as a function of both radial distance and time of day. The magnetometers ranged in latitude from Pinawa (L=4.3) to Rankin Inlet (L=12.4). The equatorial cold plasma mass densities were estimated using the T89 magnetic field model, an R-4 plasma density distribution function and the wave equation from cold plasma MHD theory. These ground based plasma mass density estimates were compared to those obtained from the CRRES spacecraft. Typical values obtained from the ground based measurements were ~2 H+ cm-3 at L~10 to ~10 H+ cm-3 at L~7. The data recorded on 3 October, 1990 showed excellent agreement with the spacecraft observations for 8.5 It was found that the differences between ground based estimates of plasma mass density and the values derived from spacecraft observations increased in regions of large plasma mass density spatial gradients. This result may be explained in terms of the spacing of magnetometers in the ground based array. ¿ 1999 American Geophysical Union