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| Paper: |
Could Torsional Oscillations Excite Rossby Waves in the Photosphere over a Sunspot? A Toy Model… |
| Volume: |
462, Progress in Solar/Stellar Physics with Helio- and Asteroseismology |
| Page: |
358 |
| Authors: |
Couvidat, S. |
| Abstract: |
With a simple model we study whether Rossby waves could be excited in the solar photosphere when a large-scale and long-lived zonal flow, e.g., the torsional oscillations, crosses the atmosphere over a sunspot. Indeed the change in vertical stratification produced by strong magnetic fields impacts the absolute vorticity of a fluid parcel advected by the flow, possibly resulting in the excitation of Rossby waves.
Following Spruit (2003), we assume that the solar torsional oscillations are in geostrophic balance. Following Lou (2000) we solve the equations of motion in the shallow-water approximation for the thin photospheric layer. We neglect dynamical effects like flows in sunspots, except for turbulent motions included in a kinematic-viscosity parameter. These approximations allow the use of the quasi-geostrophic potential-vorticity equation with damping and forcing terms. A somewhat far-fetched assumption is that a slow zonal flow can indeed cross the umbral photosphere, i.e., the magnetic-field lines are not perfectly frozen-in. As a result, we characterize the existence of a forced response to the interaction of torsional oscillations with a sunspot and present the fundamental properties of this response. |
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