A New Challenge to Solar Dynamo Models from Helioseismic Observations: The Latitudinal Dependence of the Progression of the Solar Cycle

TL;DR

This study uses helioseismic data to analyze the latitudinal progression of the solar cycle, revealing differences below 15° latitude that challenge existing solar dynamo models.

Contribution

It provides new observational constraints on the latitudinal dependence of solar cycle progression, aiding the refinement of solar dynamo theories.

Findings

Cycle begins at mid-latitudes and migrates equatorward or poleward.

Prolonged activity below 15° causes delays and overlaps in cycle onset.

Activity peaks faster at low latitudes with a single peak structure.

Abstract

The solar cycle onset at mid-latitudes, the slow down of the sunspot drift toward the equator, the tail-like attachment and the overlap of successive cycles at the time of activity minimum are delicate issues in $\alpha\Omega$ dynamo wave and flux transport dynamo models. Very different parameter values produce similar results, making it difficult to understand the origin of these solar cycle properties. We use GONG helioseismic data to investigate the progression of the solar cycle as observed in intermediate-degree global $p$-mode frequency shifts at different latitudes and subsurface layers, from the beginning of solar cycle 23 up to the maximum of the current solar cycle. We also analyze those for high-degree modes in each hemisphere obtained through the ring-diagram technique of local helioseismology. The analysis highlighted differences in the progression of the cycle below 15\degr\ compared to higher latitudes. While the cycle starts at mid-latitudes and then migrates equatorward/poleward, the sunspot eruptions of the old cycle are still ongoing below 15\degr\ latitude. This prolonged activity causes a delay in the cycle onset and an overlap of successive cycles, whose extension differs in the two hemispheres. Then the activity level rises faster reaching a maximum characterized by a single peak structure compared to the double peak at higher latitudes. Afterwards the descending phase shows up with a slower decay rate. The latitudinal properties of the solar cycle progression highlighted in this study provide useful constraints to discern among the multitude of solar dynamo models.