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A. G. Kosovichev, A. C. Birch (Stanford University)
Analysis of p-mode frequency splittings as measured by the Solar Oscillations Investigation-Michelson Doppler Imager (SOI-MDI) on board the Solar and Heliospheric Observatory (SOHO) and the ground-based Global Oscillations Network Group (GONG) experiment reveals that the symmetric component of the solar rotation rate, radially averaged over the upper 28 Mm of the convection zone and closer than 15 deg to the poles, is roughly 310 nHz (corresponding to a period of 37.3 days), which is slower than the 320 nHz (corresponding to a period of 36.2 days) estimated from surface measurements. The slow polar rotation is sometimes interpreted as a polar vortex and is important for understanding solar dynamics. In addition, zonal flows, previously inferred from the SOI-MDI f-mode splittings, are seen with both SOI-MDI and GONG p-mode splittings. The GONG data provide strong support for the findings of Schou and colleagues. We discuss possible physical mechanisms for the slow rotation of the polar regions.