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.

Magnetic Pulsations: Their Sources and Relation to Solar Wind and Geomagnetic Activity

“…In addition, its dimensions are continually changing in response to the solar wind so the cavity properties are always changing. These facts have made it difficult to demonstrate conclusively the existence of cavity modes experimentally (Radoski, 1971;Crowley et al, 1987;Samson et al, 1991;Kivelson et al, 1997). There is, however, some indirect evidence discussed next that they exist and are an important factor in determining the properties of ground ULF waves.…”

Detection of ultralow‐frequency cavity modes using spacecraft data

“…The results of such a test are reported in the present paper. One possible signature of a fast mode resonance was presented by Kivelson et al [1997] who observed a constant tone at 24 mHz during a spacecraft radial pass through the plasma trough. This event is discussed later in the present paper.…”

ULF waves from the ionosphere to the outer planets

“…However, because the standing resonance decays rapidly, this may not rule out some global mode contribution. As noted above, in the case examined by Kivelson et al [1997], the putative cavity mode amplitude died away quickly unless the IMF cone angle remained "quasi-parallel" (i.e., < 45°) and thus satisfied the condition for transmission of wave power in the Pc3-4 band from the solar wind to the magnetosphere. Intermittent bursts of input power, which need not be coherent, could then account for the temporal variability reported by Chi and Russell.…”

“…Waters et al [2002] identified characteristics of the fast mode resonances in simulations (frequency independent of L over a range of L in the plasmasphere or plasma trough, 90° phase difference between azimuthal electric and fieldaligned magnetic perturbations, expected nodal structure of the compressional magnetic field) and looked for these characteristics in several cases of putative fast mode resonances in the magnetosphere. In the event described by Kivelson et al [1997], they find no significant maxima in cross phase of the parallel field component evaluated from ISEE 1 and 2 magnetometer data for separations of 400 to 620 km. They acknowledge that it is unlikely that the signature would be detectable for such small separations for the low values of Q that apply in the magnetosphere.…”

“…However, on the time scale of hours, external conditions are rarely steady. In an example from ISEE 1 measurements, Kivelson et al [1997] identified compressional power at a roughly constant frequency of 24 mHz over an interval of several hours during which the spacecraft moved through the outer dayside magnetosphere. The spectrum of the azimuthal component displayed peaks at several harmonics whose frequency increased as L decreased, typical of field line resonances (see Figure 7).…”