Latitude-Dependent Time Variations of the Solar Tachocline

Abstract

We have examined how the characteristics of the tachocline -- i.e., the change in rotation rate $δΩ$, or the "jump", the position of the midpoint of the tachocline, $r_d$, and the width of the tachocline, $w_d$, -- change as a function of time at different latitudes using 30 years of helioseismic data obtained by the GONG network. We find a statistically significant change in the jump, however, these changes do not have a simple correlation with solar activity. The dependence is different for solar Cycles 23 and 24, and for Cycle 25, it is more similar to that of Cycle 24. While our measured changes of the tachocline's width with time are marginally statistically significant, {the cross correlation is statistically significant and implies that the width is larger when the solar activity is smaller, suggesting that magnetic fields play a role in confining the tachocline. The position of the tachocline shows a significant secular change at low latitudes ($< \simeq 50^\circ$).} At these latitudes, the tachocline has been moving steadily closer to the base of the convection zone. This is consistent with other measurements that have shown that the overall complexity of solar activity has been decreasing over the last few decades. It leads us to speculate that strong magnetic fields tend to push the tachocline deeper into the radiative zone.

Figures (10)

• Figure 1: The $\delta\Omega_3$ and $\delta\Omega_5$ components of the change in the rotation rate across the tachocline plotted as a function of time, in panels labeled (a) and (b), respectively. The greenish-blue points are results for 144-day data sets using the SGK pipeline, the red points are results for 720-day data sets also using the SGK pipeline, while the blue points correspond to results for 720-day data sets using the GONG project pipeline. Values are plotted at the midpoint of the respective time series. The light blue line shows the 10.7 cm radio flux averaged over the 144 days corresponding to each data set. The radio flux magnitude can be read from the axes on the right-hand side of the panels. The solid and dashed vertical lines are epochs of solar maxima and minima, respectively.
• Figure 2: The $\delta\Omega_3$ and $\delta\Omega_5$ components of the change in the rotation rate across the tachocline plotted as a function of the 10.7 cm radio flux in panels labeled (a) and (b), respectively. Only results using the 144-day data sets are shown. The points are color-coded by the phase of the solar cycle, as described by the legend in panel (a), where the upward and downward arrows correspond respectively to the ascending and descending phases of the cycle.
• Figure 3: The change in the rotation rate across the tachocline at different latitudes plotted as a function of time. Panels (a)--(e) show the results at latitudes 0, 15, 30, 45, and $60^\circ$. Only 144-day results are shown. The light blue line shows the 10.7 cm radio flux averaged over 144 days; the magnitude of the radio flux can be read from the axes on the right-hand side of the panels. The solid and dashed vertical lines are epochs of solar maxima and minima, respectively.
• Figure 4: The change in the rotation rate across the tachocline at different latitudes plotted as a function of the 10.7 cm radio flux. Panels (a)--(e) show the results at latitudes 0, 15, 30, 45, and $60^\circ$. Only 144-day results are shown. The points are color-coded by the phase of the solar cycle using the same convention as in Fig. \ref{['fig:j1j2rf']}, i.e., black points for the descending phase of Cycle 22, dark blue empty squares for ascending Cycle 23, light blue filled squares for descending Cycle 23, dark green empty triangles for ascending Cycle 24, light green filled triangles for descending Cycle 24, and orange points for ascending Cycle 25.
• Figure 5: The latitude at which $\delta\Omega=0$, i.e., $\theta_{\rm v}$ plotted as a function of time. Greenish-blue points are results for 144-day data sets using the SGK pipeline, red points are for 720-day data sets using the SGK pipeline, and blue points are for the 720-day data sets obtained with the GONG project pipeline. The background light blue line shows the 10.7 cm radio flux averaged over 144 days; the magnitude of the radio flux can be read from the axes on the right-hand side of the panel. The solid and dashed vertical lines are epochs of solar maxima and minima, respectively.