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Session 1 - Chromosphere, Corona, Flares.
Display session, Friday, June 27
Ballroom B, Chair: Charles Kankelborg

[1.13] Effects of MHD Instabilities on the Structure of the Slow Solar Wind

J. Karpen, R. Dahlburg (Naval Research Laboratory)

Recent LASCO observations of the streamer belt reveal a continual outflow of material, often in the form of discrete ``blobs" (Sheeley et al. 1997, ApJ, in press). These features first appear above the cusps of helmet streamers as density enhancements \sim1 R_sun in size, which then expand while accelerating away from the Sun at velocities of \sim50 - 400 km s^-1. Wavy structure along streamers also is observed to evolve with time. We have explored the possibility of explaining the formation of these time-dependent structures through resistive and ideal instabilities occurring in a system comprised of a single current sheet embedded in a wake-type flow. Our linear analysis of this system in both sub- and super-Alfvénic regimes has identified three modes (Dahlburg et al. 1997, Phys. Plasmas, submitted): a varicose, resistive mode; a varicose, ideal mode; and a sinuous, ideal mode. Wang et al. (1988, Solar Phys. 117, 157) used the terms streaming tearing mode, streaming sausage mode, and streaming kink mode, respectively, to describe the same instabilities in a different context. The ideal modes are of particular interest as they grow much more rapidly than the resistive mode (for typical coronal Lundquist numbers) and are driven by the free energy of the surrounding fast solar wind. To study the development of observable structures by this mechanism, we performed 2D and 3D nonlinear simulations initialized with small velocity and magnetic field perturbations defined by the linear results, as well as with random noise. We will discuss the growth and saturation of the unstable modes, and present predictions of growth times and characteristic lengths scaled to the coronal regime for comparison with the LASCO observations of evolving streamer morphologies.

Funding for this work was provided by ONR, the NASA Space Physics Theory Program, DoD and NASA's High Performance Computing and Communications Programs, and the NASA Numerical Aerodynamic Simulation Program.

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