Effects of ion-neutral collisions on Alfvén waves: The presence of forbidden zone and heavy damping zone

Abstract

Alfven waves are low-frequency transverse waves propagating in a magnetized plasma. We define the Alfven frequency ω0 as ω0=kVAcosθ, where k is the wave number, VA is the Alfven speed, and θ is the angle between the wave vector and the ambient magnetic field. There are partially ionized plasmas in laboratory, space, and astrophysical plasma systems, such as in the solar chromosphere, interstellar clouds, and the earth ionosphere. The presence of neutral particles may modify the wave frequency and cause damping of Alfven waves. The effects on Alfven waves depend on two parameters: (1) α=nn/ni, the ratio of neutral density (nn), and ion density (ni); (2) β=νni/ω0, the ratio of neutral collisional frequency by ions νni to the Alfven frequency ω0. Most of the previous studies examined only the limiting case with a relatively large neutral collisional frequency or β≫1. In the present paper, the dispersion relation for Alfven waves is solved for all values of α and β. Approximate solutions in the limit β≫1 as w...