Radial evolution of cross helicity in the solar wind at high latitudes: Ulysses observations and turbulence modeling results

We employ a turbulence transport theory to explain the high latitude radial evolution of cross helicity, or Alfvénicity, observed by the Ulysses spacecraft. Although evolution is slower than at low latitudes, the plasma evolves towards a low cross helicity state under influence of shear driving, which is weaker at high latitudes owing to the absence of stream interaction regions. The flattening of the cross helicity versus radius has been previously observed and attributed to a saturation effect; here this result emerges from the turbulence equations as a consequence of weakened shear. It is of potential interest that very small or vanishing shear driving might in principle allow the cross helicity to again increase with radius, since it is mainly shear that opposes dynamic alignment, the tendency of freely evolving MHD turbulence to increase the Alfvenic correlation. Here we compare the theory, including weakened but non-zero shear driving, with Ulysses observations during solar minimum conditions. We analyze hourly averages of velocity and magnetic field data along with temperature data. Cross helicity, turbulence energy and correlation length are computed. We find that the observations significantly constrain combinations of model parameters and initial conditions that allow agreement with theory. Many parameter combinations are thus eliminated, leaving ranges of parameters that ``span'' the statistical spread of the observed data. Notably the analysis suggests that the turbulence energy at 0.3 AU is higher at high latitudes than at low latitudes. This may have important consequences, for example, for cosmic ray modulation. This work supported in part by NASA NAG5-11603 and NSF ATM-0105254