Effects of Meridional Flow Variations on Solar Cycles 23 and 24

TL;DR

This study uses a surface flux transport model to analyze how variations in meridional flow influence solar cycle characteristics, revealing that flow variations significantly affect polar fields but are not the primary cause of cycle 24's weakness.

Contribution

The paper introduces a new simulation approach to assess the impact of meridional flow variations on solar magnetic fields and cycle strength, highlighting the dominant role of cycle 23's inherent weakness.

Findings

Flow variations impact polar fields by about 20%.

Observed flow variations produced stronger polar fields.

Weakness of cycle 23 primarily caused the weak cycle 24.

Abstract

The faster meridional flow that preceded the solar cycle 23/24 minimum is thought to have led to weaker polar field strengths, producing the extended solar minimum and the unusually weak cycle 24. To determine the impact of meridional flow variations on the sunspot cycle, we have simulated the Sun's surface magnetic field evolution with our newly developed surface flux transport model. We investigate three different cases: a constant average meridional flow, the observed time-varying meridional flow, and a time-varying meridional flow in which the observed variations from the average have been doubled. Comparison of these simulations shows that the variations in the meridional flow over cycle 23 have a significant impact (~20%) on the polar fields. However, the variations produced polar fields that were stronger than they would have been otherwise. We propose that the primary cause of the extended cycle 23/24 minimum and weak cycle 24 was the weakness of cycle 23 itself - with fewer sunspots, there was insufficient flux to build a big cycle. We also find that any polar counter-cells in the meridional flow (equatorward flow at high latitudes) produce flux concentrations at mid-to-high latitudes that are not consistent with observations.