A study of MHD turbulence properties in the polar wind has been performed with Ulysses data. The parameters under examination are the normalized cross helicity and the normalized residual energy. The correlation coefficient between velocity and magnetic field fluctuation vectors has also been computed. Both southern and northern phases of full immersion in the high-latitude fast wind have been examined. Observations during the short low-latitude phase around the Ulysses perihelion have been used as a term of comparison. Our results indicate that in the region up to about 2 AU the turbulence evolution leads to a decrease of cross helicity and Alfvénic correlation when distance increases. Farther out no appreciable variation is observed. It is concluded that in the high-latitude heliosphere the MHD turbulence maintains a clear Alfvénic character even at large distances. As regards the residual energy, a clear imbalance in favor of the magnetic fluctuations is everywhere present. This imbalance becomes even more pronounced for increasing distance inside 2 AU, due to turbulence evolution. Other effects lead to an inversion of this trend in the outer region. It is noteworthy that the polar turbulence does not appear too different from that observed inside the trailing edge of low-latitude fast streams.

Alfvenic turbulence in the polar wind: A statistical study on cross helicity and residual energy variations

PIETROPAOLO, Ermanno;
2000-01-01

Abstract

A study of MHD turbulence properties in the polar wind has been performed with Ulysses data. The parameters under examination are the normalized cross helicity and the normalized residual energy. The correlation coefficient between velocity and magnetic field fluctuation vectors has also been computed. Both southern and northern phases of full immersion in the high-latitude fast wind have been examined. Observations during the short low-latitude phase around the Ulysses perihelion have been used as a term of comparison. Our results indicate that in the region up to about 2 AU the turbulence evolution leads to a decrease of cross helicity and Alfvénic correlation when distance increases. Farther out no appreciable variation is observed. It is concluded that in the high-latitude heliosphere the MHD turbulence maintains a clear Alfvénic character even at large distances. As regards the residual energy, a clear imbalance in favor of the magnetic fluctuations is everywhere present. This imbalance becomes even more pronounced for increasing distance inside 2 AU, due to turbulence evolution. Other effects lead to an inversion of this trend in the outer region. It is noteworthy that the polar turbulence does not appear too different from that observed inside the trailing edge of low-latitude fast streams.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11697/580
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