ACOUSTIC-WAVES IN ISOTHERMAL WINDS IN THE VICINITY OF THE SONIC POINT

We study the propagation of acoustic waves incident on the base of a s tellar wind and the back-reaction on the mean flow in the spherically symmetric, isothermal case, both analytically and via direct simulatio ns of the Navier-Stokes equations. We consider successively the quasi- linear inviscid case and the nonlinear dissipative case (shocks). We s how that wave reflection is small everywhere even when the WKB approxi mation breaks down, and conjecture that the same result could hold for radial Alfven waves in a spherically symmetric wind. We show that, af ter a transient acceleration, outward propagating waves lead to a lowe r mean wind velocity than in the unperturbed wind, so that the average velocity may become negative below the sonic point, the difference wi th the standard result that Lagrangian-mean velocities are higher in p resence of waves being explained by the drift between reference frames . We propose that negative average velocities might provide a test for the presence of compressive waves close to the sun. We conjecture tha t, for MHD fluctuations, the net effect of the wave pressure on the wi nd velocity depends on the importance of compressive components, and t hat this might play a role in the observed correlation between the mea n solar wind velocity and the level of the compressive component in th e wave spectrum.