Characteristics of solar meridional flows during solar cycle 23

TL;DR

This study analyzes solar meridional flows during cycle 23 using MDI data, revealing variations in flow velocities, migrating patterns, and their relation to solar activity, with differences observed between solar maximum and minima.

Contribution

It provides detailed insights into the behavior and temporal evolution of meridional flows during solar cycle 23, highlighting their connection to solar activity and zonal flows.

Findings

Meridional flow speeds are lower during solar maximum than minima.

Migrating flow patterns align with sunspot butterfly diagrams.

Flow components exhibit different time-dependencies at various depths.

Abstract

We have analyzed available full-disc data from the Michelson Doppler Imager (MDI) on board SoHO using the "ring diagram" technique to determine the behavior of solar meridional flows over solar cycle 23 in the outer 2% of the solar radius. We find that the dominant component of meridional flows during solar maximum was much lower than that during the minima at the beginning of cycles 23 and 24. There were differences in the flow velocities even between the two minima. The meridional flows show a migrating pattern with higher-velocity flows migrating towards the equator as activity increases. Additionally, we find that the migrating pattern of the meridional flow matches those of sunspot butterfly diagram and the zonal flows in the shallow layers. A high latitude band in meridional flow appears around 2004, well before the current activity minimum. A Legendre polynomial decomposition of the meridional flows shows that the latitudinal pattern of the flow was also different during the maximum as compared to that during the two minima. The different components of the flow have different time-dependences, and the dependence is different at different depths.