Three-dimensional magnetohydrodynamic wave behavior in active regions: individual loop density structure

We present the numerical results from a three-dimensional (3D) nonlinear MHD simulation of wave activity in an idealized active region in which individual, realistic loop density structure is included. The active region is modeled by an initially force-free, dipole magnetic configuration with gravitationally stratified density and contains a loop with a higher density than its surroundings. This study represents an extension to the model of Ofman & Thompson. As found in their work, we see that fast wave propagation is distorted by the Alfvén speed profile and that the wave propagation generates field line oscillations, which are rapidly damped. We find that the addition of a high-density loop significantly changes the behavior inside that loop, specifically in that the loop can support trapped waves. We also find that the impact of the fast wave impulsively excites both horizontal and vertical loop oscillations. From a parametric study of the oscillations, we find that the amplitude of the oscillations decreases with increasing density contrast, whereas the period and damping time increase. This is one of the key results presented here: that individual loop density structure can influence the damping rate, and specifically that the damping time increases with increasing density contrast. All these results were compared with an additional study performed on a straight coronal loop with similar parameters. Through comparison with the straight loop, we find that the damping mechanism in our curved loop is wave leakage due to curvature. The work performed here highlights the importance of including individual loop density structure in the modeling of active regions and illustrates the need for obtaining accurate density measurements for coronal seismology.