Monitoring spatial and temporal variations in the dayside plasmasphere using geomagnetic field line resonances

fIt is well known that the resonant frequency of geomagnetic field lines is determined by the magnetic field and plasma density. We used cross-phase a nd related methods to determine the field line resonance frequency across 2 .4 less than or equal to <L less than or equal to 4.5 in the Northern Hemis phere at 78 degrees-106 degrees magnetic longitude and centered on L=2.8 in the Southern Hemisphere at 226 degrees magnetic longitude, for several day s in October and November 1990. The temporal and spatial variation in plasm a mass density was thus determined and compared with VLF whistler measureme nts of electron densities at similar times and locations. The plasma mass l oading was estimated and found to be low, corresponding to 5-10% He+ on the days examined. The plasma mass density is described by a law of the form ( R/R-eq)(-p), where p is in the range 3-6 but shows considerable temporal va riation, for example, in response to changes in magnetic activity. Other fe atures that were observed include diurnal trends such as the sunrise enhanc ement in plasma density at low latitudes, latitude-dependent substorm refil ling effects, shelves in the plasma density versus L profile, and a longitu dinal asymmetry in plasma density. We can also monitor motion of the plasma pause across the station array. Properties of the resonance were examined, including the resonance site, Q, and damping. Finally, we note the appearan ce of fine structure in power spectra at these latitudes, suggesting that m agnetospheric waveguide or cavity modes may be important in selecting wave frequencies.