MHD waves generated by high-frequency photospheric vortex motions

Download the paper, Annales Geophysicae, 2011.

Figure 1: Initial magnetic flux tube configuration. Selected colored lines correspond to the magnetic field lines. Also, color indicates the value of the magnetic field strength at different regions of the flux tube. The iso-contours of the initially constant magnetic field are overplotted at the top of the simulation box. Note, only central part (i.e. x, y=[0.8,1.2] Mm ) of the full computational domain (i.e x, y =[0,2] Mm) is shown.

Figure 2a, 2b: Rendering of the magnetic field (br, bφ, bz) and velocity (Vr, Vφ, Vz) perturbation components generated by the vortex-type periodic driver at the footpoint of the magnetic flux tube. Two different times are shown. The top and bottom sets of six horizontal slices at hight h=0.12Mm correspond to simulation times t=1.5 s and t=120.02 s, respectively.

Figure 3a, 3b: Snapshots showing the iso-surfaces of the horizontal velocity component Vx during numerical simulation. From left to the right the snapshots are at t= 7.2, 312 s. At the centre of the numerical domain we have over-plotted the 2D vertical slice of the background Bz component of the magnetic field. Rendering of the 3D numerical data has been constructed using the VAPOR visualisation package.

Figure 4: Time-distance diagrams of the radial (Vr) and vertical (Vz) components of the velocity rendered approximately at the axis of the magnetic flux tube are shown Figure. Two types of waves with different phase speeds can be resolved. The SMAW and FMAW propagate with phase speeds Vph = 3.8 km/s and Vph = 8.2 km/s, respectively. The dark horizontal regions on the time-distance diagram of the radial component (Vr) (see left panel) occur due to wave interference (both SMAW and FMAW) in the radial direction.

Figure 5: A time series of horizontal cuts at height h=0.75 Mm of the total magnetic field. The strength of the magnetic field is shown as iso-contours. The difference in time between the snapshots is approximately equal to 30 seconds, ie, the period of footpoint driver. There is clear evidence of the sausage and kink oscillations which propagate along the magnetic flux tube.