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Helioseismic measurement of the radial gradient of rotation in the near-surface shear layer

Presentation #502.04 in the session Solar Interior Flow and Dynamics.

Published onSep 18, 2023
Helioseismic measurement of the radial gradient of rotation in the near-surface shear layer

We conducted an analysis of the logarithmic radial gradient (dlnΩ/dlnr) of the solar rotation rate, specifically focusing on the near-surface shear layer of the Sun, using ring-diagram analysis applied to 15-degree and 30-degree regions observed by the Helioseismic and Magnetic Image from May 2010 to April 2023. Random noise in the observed mode frequencies can introduce spurious oscillatory behavior in the solutions due to error correlations. These oscillatory patterns are noticeable and pervasive in the inversion results. To mitigate these effects, we have fit the mode parameters of power spectra averaged at each location over full Carrington rotations and annual averages as well. We have used both Optimized Local Averaging and Regularized Least Squares inversions to obtain more reliable solutions from which appropriate inversion tradeoff parameters could be selected. Averaging over 12 years, based on annual averages of power spectra, we estimated that at the equator, within the depth range of 25 to 14 Mm, we identified a linear increase in the logarithmic radial gradient, in absolute value, from -0.2 to -0.7, which corresponds to -0.045 per Mm. The radial gradient becomes less steep away from the equator, varying approximately with the cosine of the latitude. The gradient remains nearly constant up to a depth of 7 Mm, where it experiences a sudden shift towards greater negativity at shallower layers. This shift reaches a minimum around 3.5 Mm, where the logarithmic radial gradient is equal to -1.8 at the equator and decreases by approximately -0.02 per degree until at least 60° in latitude. For layers shallower than 2 Mm, the gradient is either -1 or larger. We also present evidence of its variation over the solar cycle.

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