Relationships between phase structure and energy flux in magnetohydrodynamic waves in the magnetosphere

Phase motion in waves as indicated by a measured phase delay between two points 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 investigate here the relation between phase variation and energy flow in hydromagnetic wave signals in a model magnetosphere and identify when phase measurements between separated sites in space can indicate unambiguously the direction of the wave energy flux. For energy transfer parallel to the field, the plasma velocity (or electric field) is the most reliable guide. Perpendicular to the field, measurement of phase delays in the compressional magnetic field in space is the best guide, the phenomenon of field line resonance substantially complicating the behavior of the transverse components. In this case, however, although commonly the compressional component phase delay will be aligned with the direction of energy flow, there are important circumstances where the energy flow is antiparallel to the phase gradient and thus to the direction of the phase propagation. The analysis presented here will be especially useful in interpreting measurements from multispacecraft missions such as Cluster 2, a joint mission of the European Space Agency and NASA.