On the angle between the average interplanetary magnetic field and the propagation direction of plane large amplitude Alfvén waves
Abstract
The direction of the time averaged magnetic field <B→> in a large amplitude Alfvén wave in the solar wind can be significantly different than the direction of the ambient interplanetary magnetic field B→A, 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 mˆ, which is aligned with the wave normal nˆ. Thus, spacecraft magnetometer observations in the solar wind of minimum variance directions strongly peaked about the average magnetic field direction, particularly in the leading edges of fast streams, are 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. However, there 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.
 Publication:

Geophysical Research Letters
 Pub Date:
 September 1979
 DOI:
 10.1029/GL006i009p00713
 Bibcode:
 1979GeoRL...6..713L
 Keywords:

 Interplanetary Magnetic Fields;
 Magnetic Effects;
 Magnetohydrodynamic Waves;
 Plane Waves;
 Solar Wind;
 Angles (Geometry);
 Magnetohydrodynamic Stability;
 PlasmaElectromagnetic Interaction;
 Solar Terrestrial Interactions;
 Spaceborne Astronomy;
 Variance (Statistics);
 Solar Physics