On the ''inward'' component of the Alfvenic turbulence in the solar wind

Although the turbulent nature of solar wind fluctuations has been recognized almost 30 years ago, its governing mechanism is not yet completely understood. In situ observations by spacecraft and numerical simulations have all contributed to show that MHD turbulence observed within fast streams is mainly made of Alfvenic fluctuations whose spectral signature experiences a dramatic evolution during the wind expansion into the interplanetary space. Within the frame of incompressible MHD, this spectral evolution can be partly achieved only if we allow for nonlinear interactions between Alfvenic fluctuations with an ''outward'' (delta Z(+)) and an ''inward'' (delta Z(-)) sense of propagation with respect to the Sun. It has been shown that these fluctuations have a quite different radial evolution and it has been suggested that also their intrinsic nature might not be the same. This topic has been studied by several authors, but a definite answer has not been reached yet, because of the absence of multiple-point observations from spacecraft is a major limitation to distinguish spatial from temporal events, that is, convected from propagating disturbances. The main goal of this paper is to bring additional evidence in favor of the Alfvenic nature of (delta Z(-)) fluctuations at frequencies higher than a given limit which we estimate being around 1/30' at about 60 solar radii.