KINETIC PLASMA TURBULENCE IN THE FAST SOLAR WIND MEASURED BY CLUSTER

The k-filtering technique and wave polarization analysis are applied to Cluster magnetic field data to study plasma turbulence at the scale of the ion gyroradius in the fast solar wind. Waves are found propagating in directions nearly perpendicular to the background magnetic field at such scales. The frequencies of these waves in the solar wind frame are much smaller than the proton gyrofrequency. After the wavevector k is determined at each spacecraft frequency f(sc), wave polarization property is analyzed in the plane perpendicular to k. Magnetic fluctuations have delta B-perpendicular to > delta B-parallel to (here the parallel to and perpendicular to refer to the background magnetic field B-0). The wave magnetic field has right-handed polarization at propagation angles theta(kB) < 90 degrees and > 90 degrees. The magnetic field in the plane perpendicular to B-0, however, has no clear sense of a dominant polarization but local rotations. We discuss the merits and limitations of linear kinetic Alfven waves (KAWs) and coherent Alfven vortices in the interpretation of the data. We suggest that the fast solar wind turbulence may be populated with KAWs, small-scale current sheets, and Alfven vortices at ion kinetic scales.