Compressive fluctuations in solar wind slow speed streams are studied by me
ans of a magnetohydrodynamics (MHD) model, which represents the plasma in t
he vicinity of the heliospheric current sheet. The model contains a current
sheet, as well as density and temperature variations, corresponding to a l
arge scale modulation of the specific entropy. Alfvenic fluctuations are in
itially superimposed on the background equilibrium and compressive fluctuat
ions are consequently generated during the time evolution. The resulting co
rrelation between density and temperature fluctuations at various spatial s
cales is interpreted in terms of both generation of magnetosonic fluctuatio
ns and of an "entropy cascade." The latter phenomenon arises as a consequen
ce of the interaction between the MHD turbulence and the underlying large s
cale entropy structure. In particular, it is responsible for anticorrelated
density and temperature fluctuations detected at various scales. The resul
ts of the model are compared with the proton density-temperature correlatio
n calculated during several crossings of solar wind slow speed streams by t
he Helios spacecraft. The model reproduces to a good extent the main observ
ed features, in particular the dependence of the correlation coefficient on
location (close to or far from the current sheet) and on the fluctuation s
cale. The results show that large scale inhomogeneities, in particular, tha
t of specific entropy, are important ingredients in the dynamics of the MHD
turbulence in slow speed streams. [S1063-651X(99)03505-9].