Looptop Density Enhancement By Nonlinear Magnetohydrodynamic Waves

We discuss the possibility that disturbances generated in flares can produce the emission measure enhancement at the top of coronal loops observed by the Transition Region and Coronal Explorer ({TRACE}). The mechanism involved is the ponderomotive force of standing waves in the loops. We study the effect of large amplitude waves in loops using first a simple one-dimensional model for which analytical expressions are available, and second by solving the time dependent nonlinear magnetohydrodynamic (MHD) equations in a more realistic three-dimensional configuration. Waves are launched by an initial transverse velocity profile in the loop with footpoints fixed in the photosphere. We find that large initial disturbances can provide an imbalance along the loop, which results in an upflow from the legs of the loop. The accumulation of mass at the top of the loop produces a strong density enhancement. In a later stage, the pressure gradient becomes dominant and inhibits the concentration of mass at the loop apex.