Parametric instability of Alfvenic fluctuations in high-latitude solar wind.

Low-frequency turbulence in the solar wind is characterized by a high degree of Alfvenicity close to the Sun. Cross-helicity tends to decrease with increasing the distance from the Sun at high latitudes, as well as, in slow-speed streams at low latitudes. While in the latter case large scale inhomogeneities (velocity shears, the heliospheric current sheet) are present, which are sources of decorrelation, at high latitudes the wind is much more homogeneous, and a possible evolution mechanism is represented by the parametric instability. The parametric decay of an circularly polarized broad-band Alfvenic turbulence is then investigated, as a source of decorrelation. The initial spectrum is similar to that observed close to the Sun. The time evolution is followed by numerically integrating the full set of nonlinear MHD equations. We find that, at relevant wavelenglits, the final ratio e(-)/e(+) < 0.5, corresponding to a partial depletion of the initial correlation. Most of the spectrum is dominated by forward propagating Alfvenic fluctuations, while backscattered fluctuations dominate large scales. With increasing time the spectra of Elsasser variables tend to approach each other. Some results concerning quantities measured in the high-latitude wind are reviewed, and a qualitative agreement with the results of the numerical model is found.