Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2012
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012apj...745l...8h&link_type=abstract
The Astrophysical Journal Letters, Volume 745, Issue 1, article id. L8 (2012).
Astronomy and Astrophysics
Astronomy
2
Solar Wind, Turbulence, Waves
Scientific paper
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, \mathbf {V}_{SW}) and analyze its orientation with respect to the local background magnetic field \mathbf {B}_{0,\,local}. As an example, we take only measurements made in an outward magnetic sector. When \mathbf {B}_{0,\,local} is quasi-perpendicular to \mathbf {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 \mathbf {B}_{0,\,local}, a property that is characteristic of an oblique Alfvén wave rather than oblique whistler wave. For an oblique whistler wave, the major axis of the magnetic ellipse is expected to be aligned with \mathbf {B}_{0,\,local}, thus indicating significant magnetic compressibility, and the polarization turns from right to left handedness as the wave propagation angle (θkB) increases toward 90°. Therefore, we conclude that the observation of a right-handed polarization ellipse with orientation perpendicular to \mathbf {B}_{0,\,local} seems to indicate that oblique Alfvén/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.
He Jiansen
Marsch Eckart
Tu Chuanyi
Yao Shuo
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