VARIATION OF PLASMATROUGH DENSITY DERIVED FROM MAGNETOSPHERIC FIELD LINE RESONANCES

The diurnal variation of ULF field line resonant frequencies has been calculated using the cross phase of data from the north-south componen ts 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) pl asma density function, and the T87 magnetic field model, the variation of plasma density in the equatorial region has been calculated from t he experimentally determined resonant frequencies. Consecutive, adjace nt magnetometer pairs provide six local daytime spatial estimates of t he variation in plasma mass density between 4 and 11 R(E). Typical val ues 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 afterno on increase in density near geosynchronous orbit in agreement with con vection models of the magnetosphere. The groundbased measurements of p lasma mass density have been compared with data from the Los Alamos Ma gnetospheric Plasma Analyser on board the 1989-046 geosynchronous spac ecraft. These comparisons show that the ground-based technique should allow a robust procedure for calculating dayside, time-dependent mappi ngs of the equatorial plasma mass density from the plasmapause to the magnetopause in near real time.