Thermal birth below the sonic point of turbulent streams in the solar wind

Astron. Astrophys. 317, L31, 1997

R. Grappin J. Léorat E. Cavillier et G. Prigent

Abstract

We study the loss of stability of streams in the region of acceleration of the solar wind. We solve the Euler equations in spherical coordinates with open boundaries. We assume here axisymmetry, temperature conservation during advection, and neglect the magnetic field. We show that any initial velocity modulation vanishes with distance when the inner boundary temperature is uniform, while a persistent stream structure obtains in presence of a temperature modulation at the inner boundary. The stream structure is shown to be unstable in the region where the expansion rate is significantly smaller than the nonlinear time, i.e., below the sonic point. Two observational features are recovered: the correlation between temperature and radial velocity fluctuations (which persits in spite of the turbulence at stream interface), and a trend towards pressure equilibrium (Marsch and Tu, 1993).