Evidence of Solar-Cycle Related Structural Changes in the Solar Convection Zone

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Finding changes in the structure of the convection zone as a function of solar activity has proved to be very challenging, with uncertainties overwhelming any signatures of change. However, we now have two solar cycle’s worth of helioseismic data, from the ground as well as from space, and this helps in teasing out small changes in structure. We have investigated activity-related changes in the Sun by performing helioseismic inversions of the different data sets with respect to a reference data set that covered the minimum between cycles 23 and 24; this was done independently for the ground-based GONG data and the space-based data from MDI and HMI. We sacrificed resolution in order to decrease the uncertainties in the inversion results.

We found statistically significant changes for both GONG and MDI+HMI data sets, and we show the results centered at 0.75 R_⊙ in Figure 1. We also directly inverted for the sound-speed difference below and above the base of the convection zone, and we find that the difference change with solar activity, and that the sound-speed within the convection zone decreases with respect to that below the convection-zone base as solar activity increases. Inversions for the change of the sound-speed gradient hint at changes in the position of the convection-zone base, but we are unable, with the current data sets, to quantify the changes.