ALBERT

Description: The GEOTAIL spacecraft has previously observed cold ion beams of apparent ionospheric origin in the Earth's magnetotail lobes at large geocentric distances (r 〉 50 RE). It is proposed here that ion beams originating in certain sections of the polar ionosphere may be on magnetic flux lubes that are not closely linked to the magnetosheath and solar wind beyond. Rather, these flux tubes extend far down the tail and have very weak convective electric fields on them. In other parts of the tail cross-section, it is suggested that there are recently reconnected field lines that are open and connected directly out to the solar wind. These are the flux tubes that represent rotational discontinuities at the magnetopause. ISEE-3, in earlier data, clearly observed parts of the lobe that were relatively devoid of plasma and parts that were densely loaded with plasma, probably due to local magnetosheath plasma entry. We suggest that the cold ionospheric ion streams seen by GEOTAIL could drift very slowly toward the plasma sheet in the regions of weak E-fields. Thus, the beams at large geocentric distances should tend to be seen in relatively “pristine” lobe conditions and would be on flux tubes that are connected to ionospheric regions of low convection (E×B) electric fields. © 1996, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.

Description: 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 Alfvén spectrum. The discrete spectrum refers to compressional eigenmodes with spatially constant eigenfrequencies. The continuous shear Alfvén 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 Alfvén 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 Alfvén 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.