9.2 Early helioseismic measurements

The first hints of the signature of the migrating flow bands in helioseismic data can be seen in the BBSO data (Woodard and Libbrecht, 1993), as was pointed out by Howe et al. (2000cJump To The Next Citation Point), but these measurements do not give much information on the radial extent of the flows. Kosovichev and Schou (1997) found evidence of the flows, a few meters per second faster than the general rotation profile, in f-mode measurements from early MDI data; Giles et al. (1998) found a similar pattern using the time-distance technique of local helioseismology, while Schou (1998) and Schou (1999Jump To The Next Citation Point) clearly showed that these flows were migrating in a manner consistent with the Mount Wilson Doppler observations. The first radially-resolved evidence of zonal flow migration was reported by Howe et al. (2000c) for GONG and by Toomre et al. (2000) for MDI, while Howe et al. (2000aJump To The Next Citation Point) combined MDI and GONG data and concluded that the equatorward-migrating part of the flow pattern (at latitudes below about 40°) penetrated to at least 0.92R ⊙ (56 Mm below the surface). Antia and Basu (2000Jump To The Next Citation Point) also reported similar findings. Antia and Basu (2001) studied the evolution of the variations poleward of 50°, which had much higher amplitudes than the equatorward-moving flows and which showed signs of propagating poleward over time. The larger amplitude of the high-latitude signal may be related to the smaller angular momentum closer to the rotation axis.
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