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

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.

Accepted by ApJ, 13 pages, 7 Figures

References in corpus (2)

• The crucial role of surface magnetic fields for the solar dynamo
• Variations in High Degree Acoustic Mode Frequencies of the Sun during Solar Cycle 23 and 24

Cited by in corpus (8)

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• Collective Study of Polar Crown Filaments in the Past Four Solar Cycles
• Seismic differences between solar magnetic cycles 23 and 24 for low-degree modes
• What Seismic Minimum Reveals About Solar Magnetism Below the Surface?
• Causal Interaction between the subsurface rotation rate residuals and radial magnetic field in different timescales
• 22 Year Solar Magnetic Cycle and its relation to Convection Zone Dynamics