ANGLE BETWEEN THE AVERAGE INTER-PLANETARY MAGNETIC-FIELD AND THE PROPAGATION DIRECTION OF PLANE LARGE-AMPLITUDE ALFVEN WAVES

The direction of the time averaged magnetic field in a large amplitude Alfvén wave in the solar wind can be significantly different than the direction of the ambient interplanetary magnetic field which is not directly observable. The theoretical properties of plane large amplitude Alfvén waves in the MHD approximation constrain the time averaged magnetic field to cluster around the direction of minimum variance which is aligned with the wave normal . Thusspacecraft magnetometer observations in the solar wind of minimum variance directions strongly peaked about the average magnetic field directionparticularly in the leading edges of fast streamsare entirely consistent with plane large amplitude Alfvén waves which have wave normals aligned with the directions of minimum variance. These observations also do not necessarily imply that geometrical hydromagnetic calculations for Alfvén wave propagation direction in the solar wind are incorrect. Howeverthere is a discrepancy between geometrical hydromagnetics theory and observations that IMF minimum variance directions tend to be aligned with the ideal Parker spiral instead of the radial direction. © 1979 by the Chinese Geophysical Society