7.5 Polar rotation

Another interesting feature revealed by the early GONG and MDI observations (Schou et al., 1998Jump To The Next Citation PointBirch and Kosovichev, 1998) was that, while the surface rotation rate was mostly well described by the usual three-term expansion in the cosine of the colatitude 𝜃, Ω(𝜃) = A + B cos2𝜃 + C cos4𝜃, (e.g., Snodgrass 1984Jump To The Next Citation Point) the rotation rate close to the poles was significantly slower than that. The authors speculated that this might be a result of drag from the solar wind, and that the effect might therefore disappear or become less marked at epochs of higher activity. In fact, though the inferred high-latitude rate did speed up during solar maximum – as seen, for example, in Howe et al. (2005Jump To The Next Citation Point) and in Figure 26View Image – it remained at all times lower than the extrapolation of the three-term fit.
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