Effects of entropy inhomogeneity on density-temperature correlation in solar wind

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].