Physics – Geophysics
Scientific paper
May 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995jgr...100.7987w&link_type=abstract
Journal of Geophysical Research (ISSN 0148-0227), vol. 100, no. A5, p. 7987-7996
Physics
Geophysics
44
Diurnal Variations, Earth Magnetosphere, Extremely Low Frequencies, Geomagnetism, Geophysics, Magnetohydrodynamic Waves, Resonance Lines, Resonant Frequencies, Geomagnetic Latitude, Magnetic Resonance, Magnetohydrodynamics, Magnetometers, Plasma Density
Scientific paper
The diurnal variation in the frequencies of the continuum of ULF field line resonances has been calculated by using the cross-spectral phase of the north-south components of data from latitudinally space ground magnetometers in the Canadian Auroral Network for the OPEN Program Unified 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 deg geomagnetic latitude (L = 6.6) the general trend is a resonant frequency around 2 mHz near local dawn, increasing up to approximately 5 mHz by 0600-0700 local time, followed by a decrease in frequency to 2 mHz by 1500-1600 local time. Near local noon, the fundamental resonant frequency is approximately 3 mHz at 71 deg (L = 11.3), increasing monotonically to 7 mHZ at 65 deg (L = 6.1). The waves appear to be a part of the resonant Alfven mode continuum as opposed to the single-frequency, driven magnetic field line resonances often seen at high latitudes. The cross-phase spectra show evidence of impulsively driven resonances that energize the continuum over the latitudinal range of the CANOPUS magnetometers. The temporal variation in the resonant frequency is modeled by using the Tsyganenko (1987) magnetic field model and cold plasma MHD theory. With the use of the observed resonant frequencies, the plasma density for June 1, 1990, was 4.2 x 10(exp 6) H(+)/m(exp 3) at L = 6.6 while the data for June 7, 1990, showed densities up to 100 x 10(exp 6 H(+)/m(exp 3). These results suggest that observations of the magnetohydrodynamic continuum in the magnetometer data may give a very effective method for ground-based time-dependent mapping of the equatorial plasma density.
Donovan Eric F.
Samson John C.
Waters Colin L.
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