THERMAL BIRTH BELOW THE SONIC POINT OF TURBULENT STREAMS IN THE SOLAR-WIND

We study the loss of stability of streams in the region of acceleratio n of the solar wind. Mie solve the Euler equations in spherical coordi nates with open boundaries. We assume here axisymmetry, temperature co nservation during advection, and neglect the magnetic field. We show t hat any initial velocity modulation vanishes with distance when the in ner boundary temperature is uniform, while a persistent stream structu re obtains in presence of a temperature modulation at the inner bounda ry. The stream structure is shown to be unstable in the region where t he expansion rate is significantly smaller than the nonlinear time, i. e., below the sonic point. Two observational features are recovered: t he correlation between temperature and radial velocity fluctuations (w hich persits in spite of the turbulence at stream interface), and a tr end towards pressure equilibrium (Marsch and Tu, 1993).