Latitude-Dependent Time Variations of the Solar Tachocline

TL;DR

This study analyzes 30 years of helioseismic data to investigate how the solar tachocline's properties vary with time and latitude, revealing significant changes in its characteristics that relate to solar magnetic activity and cycles.

Contribution

It provides the first detailed analysis of the temporal and latitudinal variations of the tachocline's properties using long-term helioseismic data, highlighting magnetic field influences.

Findings

The tachocline's jump varies significantly over time without simple correlation to solar activity.

The tachocline's width is larger when solar activity is lower, indicating magnetic confinement effects.

The tachocline has been moving closer to the convection zone base at low latitudes over recent decades.

Abstract

We have examined how the characteristics of the tachocline -- i.e., the change in rotation rate $\delta\Omega$, 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.