Time Variation of the Solar Tachocline

Abstract We have used solar oscillation frequencies and frequency splittings obtained over solar cycles 23 and 24 and the rising phase of solar cycle 25 to investigate whether the tachocline properties (the change in the rotation rate across the tachocline. i.e., the jump, the width, and the position) show any time variation. We confirm that the change in rotation rate across the tachocline changes substantially; however, the change does not show a simple correlation with solar cycle unlike, for instance, changes in mode frequencies. The change during the ascending phase of solar cycle 25 is almost a mirror image of the change during the descending part of solar cycle 24, tempting us to speculate that the tachocline has a much longer period than either the sunspot or the magnetic cycle. We also find that the position of the tachocline, defined as the midpoint of the change in rotation rate, showed significant changes during solar cycle 24. The width of the tachocline, on the other hand, has shown significant changes during solar cycle 23 but not later. The change in the tachocline becomes more visible if we look at the upper and lower extents of the tachocline, defined as (position ± width). We find that for epochs around solar maxima and minima, the extent decreases before increasing again—a few more years of data should clarify this trend. Our results reinforce the need to continue helioseismic monitoring of the Sun to understand solar activity and its evolution.