F. Malara et al., Nonlinear evolution of the parametric instability: numerical predictions versus observations in the heliosphere, NONL PR GEO, 8(3), 2001, pp. 159-166
Low-frequency turbulence in the solar wind is characterized by a high degre
e of Alfvenicity close to the Sun. Cross-helicity, which is a measure of Al
fvenic correlation, tends to decrease with increasing distance from the Sun
at high latitudes as well as in slow-speed streams at low latitudes. In th
e latter case, large scale inhomogeneities (velocity shears, the heliospher
ic current sheet) are present, which are sources of decorrelation; yet at h
igh latitudes, the wind is much more homogeneous, and a possible evolution
mechanism is represented by the parametric instability. The parametric deca
y of an circularly polarized broadband Alfven wave is then investigated, as
a source of decorrelation. The time evolution is followed by numerically i
ntegrating the full set of nonlinear MHD equations, up to instability satur
ation. We find that, for beta similar to 1, the final cross-helicity is sim
ilar to 0.5, corresponding to a partial depletion of the initial correlatio
n. Compressive fluctuations at a moderate level are also present. Most of t
he spectrum is dominated by forward propagating Alfvenic fluctuations, whil
e backscattered fluctuations dominate large scales. With increasing time, t
he 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.