DO OBLIQUE ALFVEN/ION-CYCLOTRON OR FAST-MODE/WHISTLER WAVES DOMINATE THE DISSIPATION OF SOLAR WIND TURBULENCE NEAR THE PROTON INERTIAL LENGTH?

To determine the wave modes prevailing in solar wind turbulence at kinetic scales, we study the magnetic polarization of small-scale fluctuations in the plane perpendicular to the data sampling direction (namely, the solar wind flow direction, V-SW) and analyze its orientation with respect to the local background magnetic field B-0,B- local. As an example, we take only measurements made in an outward magnetic sector. When B-0,B- local is quasi-perpendicular to V-SW, we find that the small-scale magnetic-field fluctuations, which have periods from about 1 to 3 s and are extracted from a wavelet decomposition of the original time series, show a polarization ellipse with right-handed orientation. This is consistent with a positive reduced magnetic helicity, as previously reported. Moreover, for the first time we find that the major axis of the ellipse is perpendicular to B-0,B- local, a property that is characteristic of an oblique Alfven wave rather than oblique whistler wave. For an oblique whistler wave, the major axis of the magnetic ellipse is expected to be aligned with B-0,B- local, thus indicating significant magnetic compressibility, and the polarization turns from right to left handedness as the wave propagation angle (theta(kB)) increases toward 90 degrees. Therefore, we conclude that the observation of a right-handed polarization ellipse with orientation perpendicular to B-0,B- local seems to indicate that oblique Alfven/ion-cyclotron waves rather than oblique fast-mode/whistler waves dominate in the dissipation range near the break of solar wind turbulence spectra occurring around the proton inertial length.