Numerical simulations of impulsively generated vertical oscillations in a solar coronal arcade loop

Abstract

Aims. The main aims of the paper are to carry out numerical simulations of the vertical oscillations in a coronal loop in order to determine their dependence on various parameters and to compare them with recent TRACE observations. Methods. We consider impulsively generated oscillations in a solar coronal arcade loop. The two-dimensional numerical model we implement in the ideal MHD regime includes the effects of nonlinearity and line curvature. We perform parametric studies by varying both the position and the width/strength of the pulse. Results. A pulse launched below a loop is in general found to excite multiple wave modes, in particular a vertical oscillation with many properties of a kink mode, fast mode oscillations and a slow mode pulse (or two slow mode pulses, depending on the location of the original pulse). From our parametric studies we deduce that wave periods and attenuation times of the excited waves depend on the position below the loop summit, as well as on the width of the pulse. Wider pulses launched closer to a foot-point and to the loop's apex trigger wave packets of longer period waves which are more strongly attenuated. A perturbed loop does not return to its initial state but is instead stretched, with its apex shifted upwards. As a result the perturbations propagate along the stretched loop and consequently stronger and wider pulses which stretch a loop more lead to longer period oscillations. A pulse located near one of the foot-points is found to excite a distortion mode leading to asymmetric oscillations which are distinct from the vertical or horizontal kink modes that have been identified in TRACE data.

Excitation of vertical kink waves in a solar coronal arcade loop by a periodic driver

“…We adopt and modify the coronal arcade model described in Selwa et al (2005Selwa et al ( , 2006Selwa et al ( , 2007a. In this model, the coronal arcade is embedded in a two-dimensional motionless environment (V e = 0; all variables referring to the environment are denoted by subscript "e") in which mass density e = const and gas pressure p e = e c 2 se /γ has to be constant.…”

“…In this model, the Alfvén speed in the environment is given by V Ae = |B e |/ √ μ e . Following Selwa et al (2005Selwa et al ( , 2006Selwa et al ( , 2007a we model a denser loop that is embedded in the arcade in such a way that its edges follow two specific magnetic field lines. We choose x f = ±0.7 L as footpoints of the loop and w = 0.0125 L as its half-width at the footpoints.…”

“…Instead of adopting the line-tying boundary conditions at the bottom of the simulation region implemented by Selwa et al (2005Selwa et al ( , 2006Selwa et al ( , 2007a, we model wave reflection from the bottom boundary by implementing a dense photosphere-like layer for z < 0 region given by…”

“…The current work is motivated by the rapid attenuation of magnetohydrodynamic waves through leakage found by Selwa et al (2005Selwa et al ( , 2006Selwa et al ( , 2007a and Gruszecki et al (2006Gruszecki et al ( , 2008 in earlier 2D simulations. The simulated oscillations are damped considerably more rapidly than the observed oscillations.…”

“…They found that the maximum wave period, which can successfully be excited by the footpoint driving mechanism, corresponds to the fifth harmonic of the loop. In this paper, we extend the approaches of both Selwa et al (2005Selwa et al ( , 2006Selwa et al ( , 2007a and Brady & Arber (2005) by perturbing the loop with an oscillatory pressure that acts on the whole loop from the ambient medium. Ballai et al (2008) studied kink waves in straight cylindrical loop excited by EIT waves.…”