Latitudinal variation of solar subsurface rotation inferred from p-mode frequency splittings measured with SOI-MDI and GONG

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 and the ground-based Global Oscillations Network Group (GONG) experiment reveal that the symmetric component of the solar rotation rate in the upper 4% by radius and closer than 20 degrees to the poles is approximately 10 nHz slower than would be expected from a three-term fit, of the form traditionally used to express the main components of latitudinal differential rotation, to the inferred subsurface rotation rate. The slow polar rotation is important for understanding solar dynamics. In addition, zonal flows, previously inferred from the SOI-MDI f-mode splittings by Kosovichev & Schou, are seen with both SOI-MDI and GONG p-mode frequency splittings. These results were obtained with a one-dimensional latitudinal inversion technique that provides better angular resolution near the poles, at the expense of decreased radial resolution, than the standard two-dimensional inversions. Both of these findings confirm and extend the findings of Schou and colleagues (1998) from SOI-MDI data. The agreement found between the different helioseismic experiments, SOI-MDI and GONG, gives confidence in the current inferences of the differential rotation in the Sun's subsurface layers. The physics of the slow polar rotation is not understood yet.