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

A study of MHD turbulence properties ill the polar wind has been performed with Ulysses data. The parameters under examination are the normalized cros s helicity and the normalized residual energy. The correlation coefficient between velocily and magnetic field fluctuation vectors has also been compu ted. Both southern and northern phases of full immersion in the high-latitu de 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 e volution leads to a decrease of cross helicity and Alfvenic correlation whe n distance increases. Farther out no appreciable variation is observed. It is concluded that in the high-latitude heliosphere the MHD turbulence maint ains a clear Alfvenic character even at large distances. hs regards the res idual energy, a clear imbalance in favor of the magnetic fluctuations is ev erywhere present. This imbalance becomes even more pronounced for increasin g 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 p olar turbulence does not appear too different from that observed inside the trailing edge of low-latitude fast streams.