Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsh43c1961p&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #SH43C-1961
Physics
[2159] Interplanetary Physics / Plasma Waves And Turbulence
Scientific paper
This year, for the first time, the reduced normalized magnetic helicity spectrum has been analyzed as a function of the angle θ between the local mean magnetic field and the flow direction of the solar wind using wavelet techniques. In fast wind, at scales localized near kρp = 1 and kc/ωpp = 1, where ρp is the thermal proton gyro-radius and c/ωpp is the proton inertial length, the analysis reveals two distinct populations of fluctuations. There is a population of fluctuations at oblique angles, centered about an angle of 90 degrees, which are right hand polarized in the spacecraft frame and are believed to be associated with kinetic Alfven waves although the signal covers a wide range of oblique angles and a satisfactory interpretation of their spectrum through comparison with theory has not yet been obtained. A second population of fluctuations is found at angles near zero degrees which are left-hand polarized in the spacecraft frame. The data indicates that these are parallel propagating electromagnetic waves consisting either of left-hand polarized ion cyclotron waves propagating predominantly away from the sun or right-hand polarized whistler waves propagating predominantly toward the sun along the local mean magnetic field. As a consequence of the Doppler shift, both types of waves have the same polarization in the spacecraft frame. Unfortunately, the wave polarization in the plasma frame is difficult to determine using magnetic field data alone. Whether the observed waves are right- or left hand polarized in the plasma frame is a fundamental problem for future investigations. The analyses of spacecraft data performed so far have assumed that the solar wind velocity is directed radially outward from the sun. However, in the ecliptic plane at 1 AU, the flow direction typically deviates from the radial direction by a few degrees, sometimes more, and this adversely affects measurements of the angular helicity spectrum. To correct this, new measurements obtained using data from the Wind spacecraft use the scale dependent local mean solar wind velocity when computing the angle
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