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Bavassano et al. 1998
Bavassano, B., Pietropaolo, E. and Bruno, R. (1998). Cross-helicity and residual energy in solar wind turbulence: Radial evolution and latitudinal dependence in the region from 1 to 5 AU. Journal of Geophysical Research 103: doi: 10.1029/97JA03029. issn: 0148-0227.

Solar wind plasma and magnetic field measurements by Ulysses have been used to study magnetohydrodynamic turbulence in different heliospheric regions. Four intervals of six solar rotations have been analyzed. Two of them are on the ecliptic around 2 and 5 AU, respectively, one is at midlatitude near 5 AU, and the last one is at high latitude around 3 AU. Conditions on the ecliptic are those typical of high solar activity periods. The midlatitude interval is characterized by very strong gradients in the wind speed, due to an intermittent appearance of the wind coming from the polar coronal hole. In the high-latitude interval, fully inside the polar wind, the speed is steadily high. We investigated at three different scales (1, 4, and 12 hours) the level of correlation between velocity and magnetic field fluctuations, as given by the normalized cross-helicity, and the sharing of the fluctuation energy between its kinetic and magnetic component, as measured by the normalized residual energy. The observations on the ecliptic, while confirming previous findings based on Voyagers data, clearly indicate that the normalized cross-helicity is well different from zero also at distances as large as 5 AU. The midlatitude turbulence, when compared to that at low and high heliographic latitudes, appears much more evolved, with a remarkably lower normalized cross-helicity (in absolute value). This unambiguously highlights that processes at velocity gradients are an important factor in the turbulence evolution. For all the analyzed intervals the residual energy values indicate an imbalance in favor of magnetic fluctuations, in agreement with previous results. The strongest imbalance is observed for the high-latitude sample, where the turbulence is comparatively the least evolved. This is a quite unexpected result, probably related to the presence of interstellar pickup ion populations. In conclusion, our analysis indicates that (1) velocity gradients play a dominant role in driving the turbulence evolution in the solar wind and (2) pickup ion effects might be significant. ¿ 1998 American Geophysical Union

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Abstract

Keywords
Interplanetary Physics, Interplanetary magnetic fields, Interplanetary Physics, Plasma waves and turbulence, Interplanetary Physics, Solar wind plasma, Space Plasma Physics, Turbulence
Journal
Journal of Geophysical Research
http://www.agu.org/journals/jb/
Publisher
American Geophysical Union
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