Formation of the slow solar wind in streamers

Abstract

We have investigated a magnetohydrodynamic mechanism which accounts self-consistently for the variability, latitudinal extent, and bulk acceleration of the slow solar wind. Our model represents a streamer beyond the underlying coronal helmet as a neutral sheet embedded in a plane fluid wake, characterized by two parameters which vary with distance from the Sun: the ratio of the cross-stream velocity scale to the neutral sheet width (δ), and the ratio of the typical Alfven velocity to the typical flow speed far from the neutral sheet (A). Depending on the values of these parameters, our linear theory predicts that this system responds to perturbations with three kinds of instability: a streaming tearing instability, and two ideal fluid instabilities with different cross-stream symmetries (varicose and sinuous). In the magnetically-dominated region near the helmet cusp, the steamer is resistively and ideally unstable, evolving from tearing-type reconnection in the linear regime to a nonlinear varicose fluid instability. Travelling magnetic islands are formed which are similar to “blobs” recently revealed by the Large-Angle Spectroscopic COronagraph (LASCO) on the joint ESA/NASA Solar and Heliospheric Observatory (SOHO). Past the Alfven point, the tearing mode is suppressed but an ideal sinuous fluid mode can develop, producing additional acceleration up to typical slow-wind speeds and substantial broadening of the wake. Farther from the Sun, the streamer becomes highly turbulent, thus slowing the acceleration and producing strong filamentation throughout the core of the wake.