WAVE RESONANCES AND INDUCED FLOW DUE TO NONLINEAR ALFVEN WAVES IN A STRATIFIED ATMOSPHERE

This paper presents the results of a self-consistent, nonlinear, time- dependent, magnetohydrodynamic numerical code developed to investigate Alfven wave behavior in a smoothly stratified, isothermal, plane-para llel atmosphere with physical parameters approximating solar coronal h oles. Emphasis is given to studies of resonant wave behavior and the o rigin of flow induced by both freely propagating and partially reflect ed monochromatic Alfven waves. The onset and subsequent development of this induced flow is calculated directly in both the linear and nonli near regimes. The results of these investigations show that a resonanc e behavior exists, primarily because of continuous partial reflection experienced by the Alfven wave as it propagates through the atmosphere into a region where the wavelength approaches the scale height of the medium. Waves having periods corresponding to resonant peaks exert co nsiderably more force on the medium than off-peak period waves, result ing in enhanced flow. For the segment of atmosphere considered, the mo re reflection experienced by the wave, the greater the enhanced flow. If only off-peak periods are considered, the relationship between the wave period and induced longitudinal velocity shows that short-period WKB waves push more on the medium than longer, non-WKB waves. However, the increase in flow because of resonance effects at longer wave peri ods is much greater in magnitude. This enhanced flow at resonant perio ds could contribute to the observationally required acceleration of th e solar wind originating from coronal holes. The resulting wave energy transferred to the longitudinal mode may also provide a source of loc alized heating via longitudinal wave steepening and subsequent shock d issipation.