Part of the present understanding of the phenomena governing the radial evolution of the solar wind MHD turbulence can be summarized in the following scenario; outward propagating Alfv¿n waves are generated near the sun and, in addition, more waves of the same type, with mixed sense of propagation, are freshly generated at interplanetary stream shears, through plasma instabilities. The nonlinear interaction between outward propagating (majority modes) and inward propagating (minority modes) Alfv¿n waves is then crucial for the dynamical evolution of the MHD turbulence, which is reflected in the radial dependence of the normalized cross-helicity (&sgr;C). The evolution of this parameter toward lower values, with increasing heliocentric distance, is interpreted as a continuous production of minority modes. However, since the same effect on &sgr;C can be obtained simply by decreasing the presence of the outward modes alone, we have separated the ''inward'' from the ''outward'' contribution by using Els¿sser variables (z?¿) and investigated the possibility that this phenomenon could be statistically related to the presence of nonpropagating field and/or density fluctuations able to reduce the v?,b? correlation of the majority modes. Our results support this point of view and show that while fluctuations of field intensity produce a depletion of the outward modes, fluctuations in density reinforce the inward modes.

The conclusions that we draw from this analysis refer to time scales longer than 1 hour: (1) local generation of Alfv¿n modes is not so common in the solar wind, (2) the radial evolution of &sgr;C is mainly due to the continuous destruction of the outward modes rather than to the presence of inward modes, and (3) inward modes are likely to be representative of plasma structures convected by the solar wind rather than propagating Alfv¿nic fluctuations. These observations pose new theoretical problems for any model which attributes to local generation and as a consequence to e- modes, a role that they do not seem to have in the evolution of the large-scale solar wind turbulence. ¿1991 American Geophysical Union