10.1 The 1.3 year signal

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Figure 31: Rotation-rate residuals at the equator at 0.72 R ⊙ (top) and 0.63 R⊙ (bottom), for RLS (filled) and OLA (open) inversions of MDI (red triangles) and GONG (black circles) data.

Howe et al. (2000bJump To The Next Citation Point) reported finding variations of the equatorial rotation rate close to the tachocline with a 1.3 year period during the early years (1995 – 1999) of GONG and MDI observations. The strongest signal was seen at 0.72 R⊙, with a weaker anticorrelated signal below the tachocline at 0.63 R ⊙. At higher latitudes, there was also an apparent 1-year periodicity. The signal was more clearly seen in the GONG data, and due to the different temporal sample of the MDI data it was difficult to make a quantitative comparison, but the visual appearance of similar variations in both data sets was quite persuasive. Figure 31View Image extends the data up to the present for the equatorial locations just above and below the tachocline.

Because of the role of the tachocline region in the dynamo, as well as the coincidence of the period with that seen in some heliospheric and geomagnetic observations (Silverman and Shapiro, 1983Jump To The Next Citation PointRichardson et al., 1994Paularena et al., 1995), this claim attracted considerable interest, inspiring modelers such as Covas et al. (2001a) to try to build models in which different periods could exist at the top and bottom of the convection zone. However, Antia and Basu (2000) and Basu and Antia (2001Jump To The Next Citation Point), with a slightly different analysis of the same MDI and GONG data, reported finding no significant variations (Basu and Antia (2001) did see a signal somewhat similar to that reported by Howe et al. (2000bJump To The Next Citation Point) but did not consider it significant).

Moreover, the periodic signal disappears in the post-2001 data even in the original authors’ analysis (Toomre et al., 2003Howe et al., 2007), as shown in Figure 32View Image, and it seems likely that the high-latitude 1-year period was an artifact. Intermittency in short-period variations is a known phenomenon in the geomagnetic-index data (Silverman and Shapiro, 1983) and does not in itself imply that the phenomenon was not real. It will be interesting to see whether the oscillation will reappear in the new solar cycle.

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Figure 32: Sine-wave power in the rotation rate residuals from RLS inversions of GONG data, at 0.72 R⊙, 0°, plotted as a function of frequency for a) 1995 – 2000, b) 1995 – 2003, c) 1995 – 2005, d) 2000 – 2005.

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