Physics – Plasma Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsh52a..01n&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #SH52A-01
Physics
Plasma Physics
[2134] Interplanetary Physics / Interplanetary Magnetic Fields, [2149] Interplanetary Physics / Mhd Waves And Turbulence, [7863] Space Plasma Physics / Turbulence
Scientific paper
Measuring energy distributions in the four-dimensional frequency and wave vector domain using spacecraft data is one of the most important tasks for better understanding of solar wind turbulence. It answers two fundamental questions about the nature of solar wind: (1) if the fluctuation represents turbulence or a set of linear mode waves, and (2) how anisotropic the fluctuation is, particularly in the plane perpendicular to the flow direction. The CLUSTER mission is suitable for this task, as it performs four-point measurements in space. Its magnetic field observation is analyzed using the wave telescope technique and the 4D energy distribution is determined experimentally in the flow rest frame (co-moving with the flow speed) by correcting the Doppler shift. The distribution exhibits the following properties: (1) no clear dispersion-like characteristics in the frequency and wave number planes, (2) double anisotropy in the wave vector domain imposed by the mean magnetic field and the flow directions, (3) symmetry with respect to the sign of wave vector. These results suggest that the solar wind is in a well developed turbulent state and justifies the picture of two-dimensional turbulence geometry under a mean magnetic field. However, the fluctuations are not axisymmetric around the mean field and exhibit larger amplitudes in the perpendicular direction to the flow. Anisotropy with respect to the flow direction may as well indicate inhomogeneous fluctuations such as flow velocity shear on a large scale in the solar wind.
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