MAGNETOHYDRODYNAMIC FLUCTUATIONS IN THE EARTHS MAGNETOSHEATH AT 1500 LT-ISEE-1 AND ISEE-2

In the first study to employ dual-spacecraft cross-spectral techniques in the magnetosheath, we have analyzed ISEE 1 and ISEE 2 magnetic field and plasma data from an afternoon magnetosheath crossing to determine dominant MHD wave modes and directions. Our principal discovery is of guided MHD waves traveling along the flow-modified wave characteristics (the curves along which wave energy is propagated in a flowing medium - see, for example, Akhiezer et al. [1975]). Wave polarizations were determined by transforming the magnetic field to a field- and boundary-aligned coordinate system. We have inferred wave propagation directions using the time delays between the passage of coherent structures across the spacecraft. Weaker broadband coherent oscillations in the direction radial from the planet could be associated with large scale motions of the shock and/or magnetopause, but the dominant signals appear not to be from such a source. Separate broadband Alfven and slow magnetoacoustic waves have been identified in the inner sheath, propagating across the field and flow toward the magnetopause. In the outer magnetosheath the most prominent disturbances were narrow-band Alfven waves propagating downstream and along the field. The phase propagation directions detected for the largest amplitude signals were closely aligned with the flow-modified wave characteristics, the group propagation directions. From this result, we infer that the sources of the dominant wave activity are localized MHD disturbances on the bow shock and not spatially coherent distributed sources such as bulk boundary motion. We suggest that the flow-modified characteristics of the two field-guided MHD modes may play an important role within the magnetosheath, determining the directions along which energy is transmitted between the solar wind and the magnetosphere.