Abstract
Torsional motions are ubiquitous in the solar atmosphere. In this work, we perform 3D numerical simulations which mimic a vortex-type photospheric driver with a Gaussian spatial profile. This driver is implemented to excite MHD waves in an axially symmetric, 3D magnetic flux tube embedded in a realistic solar atmosphere. The Gaussian width of the driver is varied and the resulting perturbations are compared. Velocity vectors were decomposed into parallel, perpendicular and azimuthal components with respect to pre-defined magnetic flux surfaces. These components correspond broadly to the fast, slow and Alfven modes, respectively. From these velocities the corresponding wave energy fluxes are calculated, allowing us to estimate the contribution of each mode to the energy flux. For the narrowest driver ($0.15$ Mm) the parallel component accounts for $\sim 55 - 65\%$ of the flux. This contribution increases smoothly with driver width up to nearly $90\%$ for the widest driver ($0.35$ Mm). The relative importance of the perpendicular and azimuthal components decrease at similar rates. The azimuthal energy flux varied between $\sim 35\%$ for the narrowest driver and $< 10\%$ for the widest one. Similarly, the perpendicular flux was $\sim 25 - 10\%$. We also demonstrate that the fast mode corresponds to the sausage wave in our simulations. Our results therefore show that the fast sausage wave is easily excited by this driver and that it carries the majority of the energy transported. For this vortex-type driver the Alfven wave does not contribute a significant amount of energy.
Highlights
Magnetohydrodynamic (MHD) waves are ubiquitous in the solar atmosphere, and it is considered likely by many that they contribute to solar atmospheric heating by transporting energy from the photosphere up through the lower solar atmosphere and into the low corona
The radial and azimuthal axes indicate the position of each point with respect to the flux tube axis, and the colour scale shows the displacement of the flux surface at that point from its original position
Velocity profiles with a range of different widths between 0.15 Mm (1.67 full width at half-maximum (FWHM)) and 0.35 Mm (3.89 FWHM) were implemented to excite perturbations in a localized magnetic flux tube similar to the one that might be found above an magnetic bright points (MBPs)
Summary
Magnetohydrodynamic (MHD) waves are ubiquitous in the solar atmosphere, and it is considered likely by many that they contribute to solar atmospheric heating by transporting energy from the photosphere up through the lower solar atmosphere and into the low corona. There have been numerous observations in various magnetic structures of each of the MHD wave modes – fast, slow, and Alfven. Perturbations to the background atmosphere are driven by introducing a velocity field in the horizontal plane close to the footpoint of the flux tube. These drivers are intended to mimic different kinds of velocity fields that may be found in the photosphere, as a result of granulation. The logarithmic spiral shape is based on observations by Bonet et al (2008) and others of vortex flows in intergranular lanes (see Section 1) The velocity at a given point and at time t is described by cos(θ + φ)
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