Spectral characteristics of hydromagnetic waves in the magnetosphere

Abstract

This work is intended to explain why the resonant response of the magnetosphere prefers to have discrete frequencies. Using a cylindrical model for the outer magnetosphere with a plasma density profile proportional to 1/r, we show that the eigenequation characterizing the eigenmodes of the hydromagnetic waves in this model has two turning points along the radial axis. The locations of the turning points depend upon the values of the eigenperiod and the associated east-west wavenumber of the eigenmode. The energy spectrum of the excited cavity modes is seen to have sharp peaks at discrete frequencies when the surface perturbations have a uniform spectrum in the frequency range of interest. We, therefore, have also shown that only the discrete set of the magnetospheric cavity eigenmodes can efficiently couple the perturbations excited on the boundary of the magnetosphere to the field-line resonant mode excited inside the inner turning point of the cavity eigenmode. The most likely values of east-west wavenumbers and wave period range are determined.

Distribution of ULF energy (ƒ < 80 mHz) in the inner magnetosphere: A statistical analysis of AMPTE CCE magnetic field data

“…Simulation results such as those in Figure 1 of Lee and Lysak [1990] show similarity to the compressional Pc 3 oscillations in our data: fundamental frequency near 30 mHz and penetration of the wave energy into the deep magnetosphere. The strong local time dependence of the observed compressional oscillations implies that most theoretical models, which assume an azimuthally symmeffic system, are not yet adequate for quantitative description of the energy flow associated with the global cavity mode [Kuo et al, 1987]. The strong local time dependence of the observed compressional oscillations implies that most theoretical models, which assume an azimuthally symmeffic system, are not yet adequate for quantitative description of the energy flow associated with the global cavity mode [Kuo et al, 1987].…”

“…Whether or not the theoretical results are applicable to the real magnetosphere depends on how strongly the cavity modes are excited and how long the energy can be trapped in the magnetosphere before it is lost to the tail through propagation or to the ionosphere by Joule heating. The strong local time dependence of the observed compressional oscillations implies that most theoretical models, which assume an azimuthally symmeffic system, are not yet adequate for quantitative description of the energy flow associated with the global cavity mode [Kuo et al, 1987].…”

ULF Waves in the Magnetosphere

“…It is also possible that the current theoretical models lack important aspects of boundary conditions and MHD wave propagations in the real magnetosphere. For example, the global mode energy may be quickly lost into the tail so the global mode may not be readily observable [ Kuo et al , 1987]. Lee and Lysak [1990 b ] point out that magnetospheric ULF spectra could still be broad band even when the magnetopause is subjected to a monochromatic forced oscillation, because interference occurs between the forced oscillation and the internal reflected waves.…”