Relationships between phase structure and energy flux in magneto hydro dynamic waves in the magnetosphere

Phase motion in waves as indicated by a measured phase delay between two po ints in a particular field component does not in general imply that energy is being carried in the direction of the phase delay so determined. We inve stigate here the relation between phase variation and energy flow in hydrom agnetic wave signals in a model magnetosphere and identify when phase measu rements between separated sites in space can indicate unambiguously the dir ection of the wave energy flux. For energy transfer parallel to the field, the plasma velocity (or electric field) is the most reliable guide. Perpend icular to the field, measurement of phase delays in the compressional magne tic field in space is the best guide, the phenomenon of field line resonanc e substantially complicating the behavior of the transverse components. In this case, however, although commonly the compressional component phase del ay will be aligned with the direction of energy flow, there are important c ircumstances where the energy flow is antiparallel to the phase gradient an d thus to the direction of the phase propagation. The analysis presented he re will be especially useful in interpreting measurements from multispacecr aft missions such as Cluster 2, a joint mission of the European Space Agenc y and NASA.