Geometry of low-frequency solar wind magnetic turbulence: Evidence for radially aligned Alfvénic fluctuations

• Published in: Journal of Geophysical Research 1999-05, Vol.104 (A5), p.9975-9988
• Main Authors: Saur, Joachim, Bieber, John W.
• Format: Article
• Language: English
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; Interplanetary space; Solar system; Solar wind plasma
• ISSN: 0148-0227; 2156-2202
• DOI: 10.1029/1998JA900077

We employ “omnitape” magnetic field data to determine the geometry of low‐frequency (5‐ to 12‐hour timescales) magnetic fluctuations in the solar wind. We consider three axisymmetric geometries, slab, two‐dimensional (2‐D), and isotropic, as well as binary combinations of them. Both the radial direction and the mean magnetic field direction are considered as candidate symmetry axes. We apply simultaneously three different tests for distinguishing these geometries. Our analysis decisively favors a binary geometry composed of 2‐D turbulence symmetric with respect to the mean field direction and slab turbulence symmetric with respect to the radial direction. The presence of a slab component with radial symmetry provides observational support for a long‐standing theoretical prediction of radially aligned Alfvénic fluctuations at 1 AU. We also find a variation of turbulence properties with solar wind speed, such that magnetic fluctuations in slow wind are more energetic and possess a greater proportion of slab modes than those in fast wind, and the ratio of longitudinal to transverse power in the 2‐D component increases with wind speed.