Temporal Evolution of the Sun's Meridional Circulation During Solar Cycle 24

The Sun's meridional circulation plays an important role in solar dynamo, which drives the Sun's magnetic-field variation cycles, and it is curious how the meridional circulation itself evolves with the solar cycle. Now with more than 10 years of observation from SDO/HMI, we're able to explore the temporal variation of the solar meridional flow over different phases of solar cycle 24. In this study, we employ a recently developed comprehensive measurement scheme with time-distance helioseismology method, and make measurements in a series of two-year windows within 2010-2020, to obtain a time-dependent meridional-flow profile. For each two-year measurement, a systematic center-to-limb effect is first removed through solving a set of linear equations, and then the effect-removed travel-time shifts are used to invert for meridional flow in a least-square way. Our results have shown changes in the meridional-circulation pattern over different phases of the solar cycle. In the second part of the work, we also discuss about frequency-dependent measurements of the meridional flow, through some tests with a newly developed frequency-domain measurement scheme. Since the center-to-limb effect has a strong frequency dependence, while the flow does not rely on frequency, measurements in different frequency bands can potentially help validate the effect-removal method and better determine the meridional-circulation profile.