34th Solar Physics Division Meeting, June 2003
Session 26 The Changing Solar interior II
Oral, Friday, June 20, 2003, 9:00am-12:00noon, Auditorium

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[26.08] Interpretations of Torsional Oscillations

H. Spruit (MPI for Astrophysics)

The processes proposed for the torsional oscillation pattern of solar rotation are reviewed. Helioseismology shows that its amplitude declines with depth from the surface, though significant amplitudes survive over most of the convection zone. Its relation with the solar cycle, with largest positive (negative) amplitudes at the leading (trailing) latitudes of the main activity belt, suggests a magnetic cause. The early start of the oscillation, a few years before the start of the new activity cycle is difficult to interpret in magnetic models and has led to suggestions that the primary cause is not magnetic but a hydrodynamic oscillation somehow synchronized with the cycle. `Direct' magnetic models using the Lorentz force of the cycle's magnetic field (likely located at the base of the convection zone) do not fit the depth dependence easily. The latitude dependence, interpreted as a flow around the active latitudes, suggests a strong influence of rotation.

An `indirect' magnetic model is proposed, in which the flow is driven by the thermal effects of the surface magnetic fields. The additional radiative loss in the small scale magnetic field (which exceeds the reduced radiation from spots) causes a small temperature decrease below active regions. The associated pressure effect causes a geostrophic flow with the correct amplitude, latitude dependence and depth dependence. The early start of the oscillation may indicate that the cycle starts with small scale mixed fields that do not show up on synoptic magnetograms.

It is pointed out that any long-lived temperature fluctuations near the surface must be associated with systematic flows. These flows are much easier to detect with current technology than the surface brightness variations due to the temperature effects themselves.


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