Variation of plasmatrough density derived from magnetospheric field line resonances

Abstract

The diurnal variation of ULF field line resonant frequencies has been calculated using the cross phase of data from the north‐south components 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 plasma density function, and the T87 magnetic field model, the variation of plasma density in the equatorial region has been calculated from the experimentally determined resonant frequencies. Consecutive, adjacent magnetometer pairs provide six local daytime spatial estimates of the variation in plasma mass density between 4 and 11 RE. Typical values 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 afternoon increase in density near geosynchronous orbit in agreement with convection models of the magnetosphere. The ground‐based measurements of plasma mass density have been compared with data from the Los Alamos Magnetospheric Plasma Analyser on board the 1989‐046 geosynchronous spacecraft. These comparisons show that the ground‐based technique should allow a robust procedure for calculating dayside, time‐dependent mappings of the equatorial plasma mass density from the plasmapause to the magnetopause in near real time.

Optical signatures of auroral arcs produced by field line resonances: comparison with satellite observations and modeling

“…Correspondingly, the calculations of Lotko et al (1998) give a frequency for the FLR near 11 mHz, assuming the plasma density in the equatorial plane is about 1 particle per c.c. Nevertheless, numerous observations (Samson et al, 1992a;Waters et al, 1996) indicate that the FLR frequency at this latitude is much lower, at times below 2 mHz. Chan et al (1994) have clearly shown that a stretched field topology in the magnetotail, associated with near-Earth (8−12 R E tailward) pressure gradients, can lead to FLR eigenmodes with much lower frequencies.…”

Polarization properties of standing shear Alfvén waves in non-axisymmetric background magnetic fields

“…For simplicity, we choose the density profile to be a power law (e.g. Waters et al, 1996) ρ=ρ eq (r/5) −4 , where r is measured in Earth radii, and ρ eq =7 amu/cm 3 . Detailed studies of the effect of the density distribution on the ULF wave frequencies have been conducted in the past (e.g.…”

The frequency variation of Pc5 ULF waves during a magnetic storm

“…The distribution and time variation of plasma density in the magnetosphere can be estimated from ground-based observations of ULF waves as standing waves of the Alfven mode (Obayashi and Jacobs, 1958;Waters et al, 1991Waters et al, , 1996. Theoretical work suggested the development of the field line resonance model of ULF waves (Tamao, 1966;Chen and Hasegawa, 1974;Southwood, 1974).…”