A NEW CHALLENGE TO SOLAR DYNAMO MODELS FROM HELIOSEISMIC OBSERVATIONS: THE LATITUDINAL DEPENDENCE OF THE PROGRESSION OF THE SOLAR CYCLE

• Published in: The Astrophysical journal Vol. 828; no. 1; p. 41
• Main Authors: Simoniello, R., Tripathy, S. C., Jain, K., Hill, F.
• Format: Journal Article
• Language: English
• Published: The American Astronomical Society 01.09.2016
• Subjects: Constraint modelling; dynamo; Dynamo theory; Hemispheres; Latitude; methods: data analysis; Progressions; Rotating generators; Solar cycles; Sun: helioseismology; Sunspots
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/0004-637X/828/1/41

ABSTRACT The onset of the solar cycle at mid-latitudes, the slowdown in the drift of sunspots toward the equator, the tail-like attachment, and the overlap of successive cycles at the time of minimum activity are delicate issues in models of the dynamo wave and the flux transport dynamo. Very different parameter values produce similar results, making it difficult to understand the origin of the properties of these solar cycles. We use helioseismic data from the Global Oscillation Network Group 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 highlights differences in the progression of the cycle below 15° 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° latitude. This prolonged activity causes a delay in the onset of the cycle and an overlap of successive cycles, whose extent differs in the two hemispheres. Then the activity level rises faster, reaching a maximum characterized by a single-peak structure as opposed to the double peak at higher latitudes. Afterwards the descending phase shows up with a slower decay rate. The latitudinal properties of the progression of the solar cycle highlighted in this study provide useful constraints for discerning among the multitude of solar dynamo models.