Magnetohydrodynamic simulations of Alfvenic pulse propagation in solar magnetic flux tubes: Two-dimensional slab geometries

Two-dimensional magnetohydrodynamic simulations are presented of the evolution of a nonlinear Alfven wave pulse in the region between the solar photosphere and corona. A magnetic field profile that incorporates the characteristic field spreading expected in flux tubes is used. The pulse is chosen initially to have a purely Alfvenic polarization and to extend over a limited horizontal distance. It is shown that as this pulse rises in the atmosphere, it becomes wedge-shaped. The part of the pulse at the center of the flux tube reaches the transition region first, with other parts arriving at a time that is determined by the history of the Alfven speed along the path of the wave. Since field lines that spread out from the center of the flux tube spend longer in the high-density photosphere and chromosphere, and also have a smaller total field strength, waves that travel along them will take longer to reach the corona. The nonlinearity of the Alfvenic pulse drives a plasma flow both parallel to the ambient magnetic field and in a direction normal to the field, owing to transverse modulation of the Alfvenic pulse. The pulse associated with this plasma flow is also wedge-shaped, but the actual shape is different from that of the Alfvenic pulse. Since these plasma flows are compressible, they propagate at a different characteristic speed from the Alfven waves, and so can reach the transition region either before or after the Alfven pulse, the precise result depending on the plasma parameters. As the compressible pulse moves upward, a finite-sized blob of chromospheric material is injected into the corona. The relevance of this to spicules and jets is discussed.