ON THE ROLE OF INTERPLANETARY SOURCES IN THE EVOLUTION OF LOW-FREQUENCY ALFVENIC TURBULENCE IN THE SOLAR-WIND

The Alfvenic turbulence observed in the interplanetary space is mainly made up of outward traveling waves of solar origin. Inward waves, although a minor component of the turbulence, play an important role in transferring energy between different scales through nonlinear interactions with outward modes. Inward waves can only have an interplanetary origin. A good understanding of how inward waves are generated in the interplanetary space is crucial in modeling the Alfvenic turbulence evolution. Solar wind regions with high differential kinetic energy, which can be converted into turbulent energy, are the best places to search for local sources of Alfvenic turbulence. We have studied one of these regions at different heliocentric distances inside 1 AU, focusing on large-scale fluctuations (periods approximately between 1 and 12 hours). Such a region was indicated by previous investigations as one of the best candidates for turbulence generation. In fact, fluctuations with an inward sense of correlation are observed, but a detailed analysis leads us to favor the hypothesis that they are mainly due to variations in plasma velocity, density, and magnetic field related to structures convected by the solar wind and not to real inward propagating waves. At least in the inner heliosphere, a view in which local sources of waves play a relevant role in driving the Alfvenic turbulence evolution does not seem realistic. We propose that such evolution, especially at the larger scales, is mainly governed not by interplanetary sources but rather by interactions of the outward waves of solar origin with the wind structure.