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-parallel atmosphere with physical parameters approximating solar coronal holes. Emphasis is given to studies of resonant wave behavior and the origin of flow induced by both freely propagating and partially reflected monochromatic Alfven waves. The onset and subsequent development of this induced flow is calculated directly in both the linear and nonlinear regimes. The results of these investigations show that a resonance 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 considerably more force on the medium than off-peak period waves, resulting in enhanced flow. For the segment of atmosphere considered, the more 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 periods is much greater in magnitude. This enhanced flow at resonant periods could contribute to the observationally required acceleration of the solar wind originating from coronal holes. The resulting wave energy transferred to the longitudinal mode may also provide a source of localized heating via longitudinal wave steepening and subsequent shock dissipation.