A possible signature of magnetic cavity mode oscillations in ISEE spacecraft observations

Abstract

The structure of magnetohydrodynamic waves in the terrestrial magnetosphere is controlled by the dispersion relations of the wave modes, the inhomogeneities of the system and the boundary conditions. If the waves are confined within the magnetospheric cavity with proper boundary conditions, two types of spectra are predicted by MHD theory. One is a discrete compressional spectrum and the other is a continuous shear Alfven spectrum. The discrete spectrum refers to compressional eigenmodes with spatially constant eigenfrequencies. The continuous shear Alfven spectrum corresponds to field line resonances with spatially varying resonant frequencies. There have been many reports of observations consistent with the theoretical predictions for shear Alfven waves. However, only a few spacecraft observations of the global cavity eigenmodes or the coupling between the two modes have been reported. Here we report an observation using ISEE 1 and 2 spacecraft magnetometer data. On a quiet day with low solar wind dynamic pressure and low geomagnetic activity, the ISEE 1 and 2 spacecraft identified a compressional wave with constant frequency throughout most of its inbound orbit in the outer magnetosphere. Multiple harmonics of shear Alfven waves with spatially varying frequencies were observed in the azimuthal component. Evidence of the coupling between these two waves in found. Comparing our observations with the results of a computer simulation in a dipole field, we find qualitative (not quantitative) agreement. We also consider why so few examples of radially-extended monochromatic compressional oscillations have been found. We conclude that the exceptional circumstances required for development and identification of these wave structures occur very rarely.