The Solar Meridional Circulation and Sunspot Cycle Variability

TL;DR

This study analyzes solar surface magnetic element motions from 1996 onward, revealing systematic variations in meridional flow that influence sunspot cycle strength and timing, with implications for understanding solar magnetic activity.

Contribution

It provides the first long-term measurements of meridional flow variations and links these to cycle-to-cycle differences in solar magnetic activity.

Findings

Meridional flow is faster at cycle minima and slower at maxima.

Flow weakening near sunspot zones correlates with cycle strength.

Variations in flow contributed to the weak polar fields and cycle 24.

Abstract

We have measured the meridional motions of the magnetic elements in the Sun's surface layers since 1996 and find systematic and substantial variations. In general the meridional flow speed is fast at cycle minima and slow at cycle maxima. We find that these systematic variations are characterized by a weakening of the meridional flow on the poleward sides of the active (sunspot) latitudes. This can be interpreted as a inflow toward the sunspot zones superimposed on a more general poleward meridional flow profile. We also find variations in the meridional flow which vary from cycle-to-cycle. The meridional flow was slower at both the minimum and maximum of cycle 23 compared to similar phases of cycles 21, 22, and 24. Models of the magnetic flux transport by a variable meridional flow suggest that it can significantly modulate the size and timing of the following sunspot cycle through its impact on the Sun's polar magnetic fields. We suggest that the meridional flow variations observed in cycle 23 contributed to the weak polar fields at the end of the cycle which then produced a weak cycle 24 and the extraordinary cycle 23/24 minimum.