THE TEMPORAL VARIATION OF THE FREQUENCY OF HIGH-LATITUDE FIELD LINE RESONANCES

The diurnal variation in the frequencies of the continuum of ULF field fine resonances has been calculated by using the cross-spectral phase of the north-south components of data from latitudinally spaced groun d magnetometers in the Canadian Auroral Network for the OPEN Program U nified Study (CANOPUS) array. On most days the continuum is seen only during the local daytime, and only a single harmonic with an inverted U-shaped temporal variation in frequency is seen. At 67 degrees geomag netic latitude (L = 6.6) the general trend is a resonant frequency aro und 2 mHz near local damn, increasing up to similar to 5 mHz by 0600-0 700 local time, followed by a decrease in frequency to 2 mHz by 1500-1 600 local time. Near local noon, the fundamental resonant frequency is similar to 3 mHz at 71 degrees (L = 11.3), increasing monotonically t o 7 mHz at 65 degrees (L = 6.1). The waves appear to be a part of the resonant Alfven mode continuum as opposed to the single-frequency, dri ven magnetic field line resonances often seen at high latitudes. The c ross-phase spectra show evidence of impulsively driven resonances that energize the continuum over the latitudinal range of the CANOPUS magn etometers. The temporal variation in the resonant frequency is modeled by using the Tsyganenko (1987) magnetic field model and cold plasma M HD theory. With the use of the observed resonant frequencies, the plas ma density for June 1, 1990 was 4.2x10(6) H+/m(3) at L = 6.6 while the data for June 7, 1990, showed densities up to 100x10(6) H+/m(3). Thes e results suggest that observations of the magnetohydrodynamic continu um in the magnetometer data may give a very effective method for groun d-based time-dependent mapping of the equatorial plasma density.