PHYSICAL PROCESSES IN THE HELIOSHEATH: THEORETICAL PREDICTIONS

摘要

The venerable Voyager 1 spacecraft has recently crossed the solar wind termination shock and entered the high temperature low velocity region of the heliosphere separating the solar wind from the local interstellar medium. In this review we discuss recent theoretical model predictions for the dynamical evolution of partially ionized plasma flows and highly energetic particle populations in the heliosheath under the influence of a variety of processes of solar and interstellar origin. We discuss three major drivers of dynamical evolution of the region: the 22-year solar cycle, large-scale disturbances transmitted through the termination shock, and macroscopic instabilities at shear and density interface layers. Sunspot cycle variations of the solar wind dynamic pressure force the entire heliosphere to "breath", where the termination shock and the heliopause move asymmetrically toward or away from the Sun. Global heliospheric disturbances propagating outward with the solar wind collide with the termination shock and inject waves and discontinuities into the downstream region adding to the turbulent content of the heliosheath flow. Possible effects of the termination shock oscillations on anomalous cosmic ray intensities upstream of the shock are briefly addressed. Solar cycle driven heliospheric magnetic field reversals are expected to generate a complicated magnetic field topology in the heliosheath, with important consequences for galactic cosmic ray modulation. In this context we discuss the possibility of enhanced attenuation of galactic radiation by the "modulation wall" bordering the heliopause. Finally, the heliopause itself, as a tangential discontinuity separating a dense and cold interstellar flow from the tenuous hot heliosheath plasma is subject to hy-drodynamic instabilities. Using theoretical analysis and computer simulations we predict that charge transfer between the plasma and interstellar hydrogen atoms would drive the interface unstable. The role of secondary atom populations and magnetic fields on linear and nonlinear instability development is also discussed.